General Aviation Interior Noise: Noise Control Measure Evaluation - Part 3

TABLE OF CONTENTS Section 1. INTRODUCTION ........................................................................................................... 1-1 1.1 Background .......................................................................................................... 1-1 2. IMPROVED CONDITION RESPONSE ANALYSIS ...................................................... 2-1 3. CESSNA MODEL 182E ................................................................................................ 3-1 3.1 Surface Treatment Evaluation .............................................................................. 3-2 3.1.1 Cabin Window Treatment ......................................................................... 3-3 3.1.2 Under Cowling Treatment ......................................................................... 3-4 3.2 Aircraft Configuration and Operational Effects ..................................................... 3-8 3.3 Tail Cone and Aft Bulkhead Treatments .............................................................. 3-8 3.4 Linear Array Measurements ................................................................................. 3-11 3.5 High Frequency Tone Evaluation ......................................................................... 3-13 3.6 Cabin Active Noise Control Survey ...................................................................... 3-14 4. CESSNA MODEL 206 .................................................................................................. 4-1 4.1 Cabin Noise and Vibration Spectra ...................................................................... 4-1 4.2 Panel Tap Test ..................................................................................................... 4-6 4.3 Linear Array Measurements ................................................................................. 4-6 5. CESSNA MODEL 182F ................................................................................................ 5-1 5.1 Passive Treatment Evaluation ............................................................................. 5-3 5.1.1 Firewall Treatment .................................................................................... 5-3 5.1.2 Distributed Vibration Absorbers ................................................................ 5-5 5.2 Active Structural Acoustic Control ........................................................................ 5-8 6. OBSERVATIONS AND CONCLUSIONS ...................................................................... 6-1 7. REFERENCES .............................................................................................................. 7-1 General Aviation Interior Noise: Page iii Part III- Noise Control Measure Evaluation

LIST OF FIGURES

Paae

1-2 1.1 Typical Single Engine General Aviation Interior Noise Spectra ........................

Figure 3-1 Figure 3.1 Cessna Model 182E Test Aircraft with Three-Bladed Propeller ........................

3.2 Interior Microphone Spectra: Baseline Aircraft @ 2,400 rpm, 75% Power Figure Cruise ................................................................................................................ 3-3 Interior Window Treatment ................................................................................ 3-4 Figure 3.3 Figure 3.4 Effect of Treatment Configuration on Cabin 400 Hz - 5P Tone Level .............. 3-5 Figure 3.5 Effect of Treatment Configuration on Cabin 480 Hz - 4F-6P Tone Level ......... 3-6 Firewall Noise Reduction ................................................................................... 3-7 Figure 3.6 Figure 3.7 Engine Speed and Power Setting Effects on Pilot Microphone AC1 ................ 3-9 Figure 3.8 Engine Speed and Power Setting Effects on Co-Pilot Microphone AC2 ........... 3-10 Standard Aft Bulkhead ..................................................................................... 3-12 Figure 3.9 Tail Cone Fitted with Foam Wedges ................................................................ 3-12 Figure 3.10 Figure 3.11 Array Results for Two-Bladed Propeller- Baseline- 80 Hz ............................ 3-14 Array Results for Two-Bladed Propeller - Baseline - 120 Hz .......................... 3-15 Figure 3.12 Figure 3.13 Array Results for Two-Bladed Propeller - Baseline - 160 Hz .......................... 3-15 Figure 3.14 Array Results for Two-Bladed Propeller - Baseline - 240 Hz .......................... 3-16 Figure 3.15 Array Results for Three-Bladed Propeller- Baseline - 120 Hz ....................... 3-16 Figure 3.16 Array Results for Three-Bladed Propeller - Baseline - 240 Hz ....................... 3-17 Figure 3.17 High Frequency Tone Correlation .................................................................... 3-17 Cessna Model 206 Test Aircraft ........................................................................ 4-1 Figure 4.1 Figure 4.2 Model 206 Window Vibration Spectra ............................................................... 4-3 Figure 4.3 Model 206 Firewall Vibration Spectra ................................................................ 4-4 Figure 4.4 Model 206 Cabin Noise Spectra In Forward Cabin ........................................... 4-5 Figure 4.5 Cessna 206 Microphone Array .......................................................................... 4-7 Narrow Band Spectrum at Microphone A1 ....................................................... 4-7 Figure 4.6 Figure 4.7 Model 206 Microphone Array Measurement - 120 Hz Tone ............................ 4-8 Figure 4.8 Model 206 Microphone Array Measurement - 240 Hz Tone ............................ 4-8 Cessna Model 182F Test Aircraft ..................................................................... 5-1 Figure 5.1 Figure 5.2 Model 182F Bare Cabin Interior Noise Spectra ................................................ 5-2 Effect of Firewall Treatment on Firewall Vibration ............................................ 5-4 Figure 5.3 Figure 5.4 Typical Under Cowling Noise Spectra .............................................................. 5-5 Distributed Vibration Absorbers ........................................................................ 5-6 Figure 5.5 General Aviation Interior Noise: Page iv Part III- Noise Control Measure Evaluation

LIST OF FIGURES

Paae

5-6 Figure 5.6 Typical DVA Installation: Cabin Roof ...............................................................

5-7 Typical DVA Installation: Cabin Sidewall .........................................................

Figure 5.7 5-8 Figure 5.8 Typical Motran Installation: Co-Pilot's Door .....................................................

5-9 Figure 5.9 Typical Motran Installation: Windshield ............................................................

Figure 5.10 Summary of Control at AC1 .............................................................................. 5-12 Figure 5.11 Summary of Control at AC2 .............................................................................. 5-12 Figure 5.12 Summary of Control at AC3 .............................................................................. 5-13 Figure 5.13 Summary of Control at AC4 .............................................................................. 5-13 General Aviation Interior Noise: Page v Part III- Noise Control Measure Evaluation

LIST OF TABLES

Paae

Table2.1 2-4

Conditioned Response Analysis Using All Simulation Vectors ........................

Table2.2 2-5

Conditioned Response Analysis Using Optimum Simulation Vectors ..............

Table3.1 3-1

Instrumentation Layout and Channel Assignment ............................................

Table3.2 3-2

Schedule of Treatment Locations and Applicable Materials ............................

Table3.3 3-2

Description of Materials ....................................................................................

Table3.4 3-3

Baseline Interior Microphone Levels ................................................................

Table3.5 3-11

Overall SPL Difference: Two-Bladed Minus Three-Bladed Propeller ..............

Table3.6 3-12

Summary of Tail Cone and Aft Bulkhead Treatments ......................................

Table4.1 Model 206 Tone Phase Evaluation .................................................................. 4-6

Table5.1 5-2

Instrumentation Schedule During Passive Treatment Evaluation ....................

Table5.2

Passive Treatment Test Configurations ............................................................ 5-3

Table5.3

Passive Treatment Weights .............................................................................. 5-3

Table5.4

Effect of Treatment Configuration on the 120 Hz Tone ..................................... 5-7

Table5.5

Effect of Treatment Configuration on the 240 Hz Tone ..................................... 5-7

Table5.6

Best ASAC Results: 8 Actuators and 8 Error Microphones ............................. 5-10 Table5.7 Best ASAC Plus Passive Treatment ................................................................. 5-11

Table5.8

Summary of Overall Noise Control .................................................................... 5-12 General Aviation Interior Noise: Page vi Part III- Noise Control Measure Evaluation 1. INTRODUCTION 1.1 Background Poor pilot communications with ground control personnel and passengers, and pilot and passenger fatigue during extended duration flights in single engine General Aviation aircraft is attributed to excessive interior noise and vibration. Typical cabin spectra for a single engine, two-bladed and three-bladed, propeller aircraft are given in Figure 1-1. The spectra were recorded in the same aircraft at identical power settings, namely, an engine speed of 2,400 rpm at 75% power cruise at an altitude of 5,000 ft. The two-bladed propeller noise spectrum is rich in harmonics of the fundamental engine rotational speed at 40 Hz with the dominant low frequency responses corresponding to harmonics of the fundamental propeller at 80 Hz and engine firing at 120 Hz, while the three-bladed propeller noise spectrum is dominated by the coincident harmonics of the propeller and engine firing at 120 Hz. Both spectra exhibit a mid- frequency response centered around 520 Hz consisting of several adjacent tones at 40 Hz spacing. The sources of these dominant tones are generally believed to be from airborne propeller, engine exhaust, and engine case radiation and/or from direct structure-borne vibration from engine excitation.

The work reported herein is an extension to the work accomplished under NASA Grant NAG-I-2091 on the development of noise/source/path identification techniques for single engine propeller driven General Aviation aircraft. The previous work developed a Conditioned Response Analysis (CRA) technique to identify potential noise sources that contributed to the dominating tonal responses within the aircraft cabin. The objective of the present effort was to improve and verify the findings of the CRA and develop and demonstrate noise control measures for single engine propeller driven General Aviation aircraft.

An improvement in the CRA procedures, including the generation of a normalized error parameter to guide the selection of simulation vectors, is described is Section 2. During the course of the present research effort, three single engine General Aviation aircraft were employed. In May 2000, the Cessna 182E aircraft, used in the previous noise source/path investigation, was flight tested with various applications of surface treatments to identify major paths of noise transmission into the aircraft. A summary of the results is given in Section 3.

Thereafter, in October 2000, the Cessna 182E aircraft was flight tested at various propeller speed and engine power settings, employing both two- and three-bladed propellers, to determine to what extent aircraft operational effects could be used to reduce cabin noise levels. During the flight tests, the effects of tail cone and aft bulkhead treatments were evaluated and linear array measurements were recorded to determine the characteristics of the wave field within the cabin, also reported in Section 3. Thereafter, the Cessna 182E aircraft was returned from research to operational flight status and was no longer available for the program.

A Cessna Model 206 three bladed propeller single engine aircraft, void of all interior trim, was made available for the program. The bare cabin was believed to be a good test bed for development/demonstration of noise control treatments. Ground and flight tests were conducted on the Model 206 during March 2001 to identify panel resonant response, cabin acoustic wave General Aviation Interior Noise: Page 1-1 Part III- Noise Control Measure Evaluation _2B = 91.3dBA I I < m "4O i > 70 ._1 if) if) _. 60 "4O C if) , , i , , , , i , , , , i , , , , i , , , , i , , , , i , , , , i , , , , i , , , , i , , , , 100 200 300 400 500 600 700 800 900 1000 Frequency- Hz a) Two-Bladed Propeller _3B = 90.7dBA < m "4O i > 70 ._1 if) if) I _. 60 "4O C l --_v if)

I1

i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i 0 100 200 300 400 500 600 700 800 900 1000 Frequency- Hz b) Three-Bladed Propeller Figure 1.1 Typical Single Engine General Aviation Interior Noise Spectra.

General Aviation Interior Noise: Page 1-2 Part III- Noise Control Measure Evaluation characteristics, and cabin noise and vibration levels during normal cruise conditions, as reported in Section 4. Unfortunately, the Cessna Model 206 was removed from the program before noise control measures could be developed. At this point in the program, a Cessna Model 182F was leased from a private individual, which would allow interior removal and application and evaluation of various passive and active noise control measures. The Model 182F was equipped with a three-bladed propeller, supplied to the program by McCauley Propeller Systems, and underwent extensive ground and flight tests, as reported in Section 5.

Hundreds of noise and vibration spectra were recorded and analyzed during the various ground and flight tests conducted during the project. The authors have attempted to extract sample data from which general conclusions can be drawn as to the nature of the noise environment in single engine General Aviation aircraft and potential for noise control measure application. Several general observations and conclusions are summarized in Section 6.

Detailed summaries of all data were transmitted to NASA Langley and were placed into a NASA General Aviation Database along with other research contributions from other organizations.

Reference is made to this database throughout the report by the specific entries in the database contained within square brackets [*], as listed in Section 7.

General Aviation Interior Noise: Page 1-3 Part III- Noise Control Measure Evaluation

2. IMPROVED CONDITION RESPONSE ANALYSIS

The Condition Response Analysis (CRA) conducted on the Cessna Model 182E single engine propeller driven aircraft, as reported in the Research Summary for NASA Grant NAG-l- 2091 dated September 1999 [1], was revised to include error analyses and the simultaneous inclusion of both auxiliary pressure and accelerometer responses during the evaluation.

In the previous CRA analysis, the set of auxiliary accelerometers on the engine and engine mount structure were independently employed to predict the level of structure-borne noise transmission. The level of structure-borne noise transmission due to engine vibration was found to be quite low. Nevertheless, the corresponding acceleration responses at all other auxiliary locations were predicted based on the estimated level of structure-borne engine vibration transmission into the aircraft and these response levels were then removed from the in- flight response vector before the airborne transmission predictions were carried out. The airborne transmission predictions were carried out in two analysis sets. The one analysis set included all accelerometer responses on aircraft panels and lightweight structure, which were all the accelerometer responses not included in the initial structure-borne noise transmission evaluation. The second analysis set included all microphone responses, which consisted of several microphones external to the aircraft and one under the engine cowling adjacent to the firewall. The details of the analysis process are given in Reference [1].

The primary reason for separating the accelerometer and microphone responses in the previous CRA analyses was the large difference in magnitudes between the accelerometer responses in gravity units and the microphone responses in normalized pressures relative to the standard reference pressure of 2 x 10 5 Pascal. To elevate this problem, the accelerometer responses were scaled by the characteristic impedance of the radiation media as shown by the following expression:

toC(±/](ar-, /

(2.1)

t) Lt)ak g J

where, o) is the circular frequency,/9o is the density of the media, c is the speed of sound, g is the acceleration due to gravity (9.8 m/sec), and P,.fis the reference pressure (2 x 10 5Pascal).

The CRA procedure used to relate the ground test response data to the in-flight response data begins with determining the linear sum of ground test airframe response parameter vectors, which best fit the in-flight airframe response parameters measured during flight. Thus, we seek the vector {o_}, such that: {=d} = [A a] {_} (2.2) where, {=d} - is to be a close approximation to {d}, the in-flight airframe response vector, General Aviation Interior Noise: Page 2-1 Part III- Noise Control Measure Evaluation [A a] - is a matrix of selected {a g } response vectors (N x J), and {o_} - the desired source simulation weight vector U x 1).

This being the case, we may then estimate the in-flight structure-borne and airborne noise components from: {_} = [Pal {o_} (2.3) where, {_ } - is an estimate of the in-flight response vector, [pa] _ is a matrix of the {pg} response vectors (N x J), consistent with {o_} and {o_} - is the source simulation weight vector determined from the best fit to the in-flight structural response parameters.

The solution approach taken was to include all the ground simulation information in a single evaluation and to use a Moore-Penrose pseudo inverse of the over determined system of equations to obtain a solution.

{o_k} =pinv [A a] {d} (2.4)

The extent to which the above formulation of CRA facilitates noise source/path identification for the Cessna Model 182E aircraft was evaluated using the ground test source simulation data sets consisting of airborne propeller (ABP), airborne exhaust (ABEX), airborne engine (ABE), structure-borne engine (SBE), and two additional structure-borne simulations via direct excitation of the right forward (SBRM) and left forward engine mounts (SBLM). The latter two simulations were not employed in the previous study; however, they were included in the present analysis for completeness. The above accelerometer response scaling was applied to the corresponding rows of Equation 2.2 before the pseudo inverse process. The normalized error for the fit process was based on the difference between the measured in-flight response vector, {d}, and the predicted response vector, {=d}, normalized by the mean of the in-flight response vector.

A conditioned response analysis for selected tones for the Cessna Model 182E aircraft was carried out initially using all six simulation vectors, and the results are given in Table 2-1.

For each of the tones analyzed, the in-flight measured sound pressure levels at four interior microphones AC1 through AC4 are given in the table under the heading of "In-Flight Levels."

The predicted responses for these interior microphones are given in the adjacent column along with the contribution from each of the simulation vectors used in the analysis. The last two columns give the mean and standard deviation of the normalized error resulting from attempting to match the 27 auxiliary responses measured during the flight. The simulations for the 120 Hz General Aviation Interior Noise: Page 2-2 Part III- Noise Control Measure Evaluation first firing (1F) tone and the 480 Hz fourth firing and sixth propeller (4F-6P) harmonics appear to be worth noting. Results for the other tones are not encouraging, and further evaluation was carried out. Various combinations of the source simulations were used to reduce the normalized error while maintaining cabin levels similar to those measured in flight. The following conclusions were drawn from the optimum CRA solutions given in Table 2-2: .

80 Hz Fundamental Propeller Tone: Clearly the distribution of higher noise levels in the aft of the aircraft (AC3 and AC4) could not be simulated with any combination of the simulation vectors generated during the CRA. The airborne propeller simulation vector resulted in the lowest normalized error (1.15), which indicates a very poor match. The conclusion is that the major noise source associated with the fundamental propeller was not properly simulated. The propeller wake tip vortex impingement on the fuselage may be the missing source.

.

120 Hz Fundamental Engine Firing Tone: This tone appears to be best simulated using only the airborne exhaust simulation source vector. While the predicted cabin noise levels are higher than measured, the general distribution is well represented. When the airborne engine simulation vector was coupled with the exhaust simulation vector, the normalized error was slightly reduced from 0.66 to 0.63; however, the predicted cabin levels were even higher.

.

160 Hz Second Propeller Harmonic: The best simulation for this tone is the airborne propeller source. The distribution of cabin noise levels appears to be reasonable, however, somewhat low. This indicates that propeller airborne noise is a contributor and there may possibly be a source missing which would improve the CRA procedure.

.

240 Hz Second Firing and Third Propeller Harmonics: The airborne propeller and exhaust simulation vectors provided the best fit for this spectral component.

The predicted cabin levels were reasonable, and the normalized error was 0.47.

.

400 Hz Fifth Propeller Harmonic: No combination of available source simulations could be used to improve the high level of normalized error found for this tonal component. Thus, there may be an additional noise source responsible for this spectral component.

.

480 Hz Fourth Firing and Sixth Propeller Harmonics: A very good fit for this spectral component was achieved using all of the airborne simulation vectors, normalized error being 0.22. However, the addition of the structure-borne simulation vectors resulted in only a small decrease in the normalized error to 0.21 with negligible changes in the level of cabin noise transmission. It appears that the major contributor to this component is airborne engine case radiation.

General Aviation Interior Noise: Page 2-3 Part III- Noise Control Measure Evaluation Table 2.1 Conditioned Response Analysis Using All Simulation Vectors.

Error Cabin In- Predicted Response Levels Tone Microphone Flight SBRM SBLM Mean Stdev Levels ALL ABP ABEX ABE SBE AC1 69.9 79.5 82.8 0 76.1 70.1 54.5 38.5 1.84 8.93 AC2 70.4 77.5 80.9 0 76.7 71.6 54.1 56.4 1P AC3 83.3 77 78.8 0 68.9 67.7 53.8 58.9 AC4 82.6 73.2 76.3 0 68.7 72.5 58.5 59.6 AC1 80.4 88.5 0 88.4 93.9 78.9 70.6 53.8 0.56 4.46 AC2 78.1 86.4 0 88.5 94 83.7 79.4 78.3 1F AC3 81.9 92 0 94 87.8 81.8 70.7 68.8 AC4 76 83.6 0 83.2 87.2 76.5 65.3 67.3 AC1 76.2 73.4 65.9 0 70.7 51.8 53.1 19.8 0.99 2.43 AC2 82.8 74.8 71.1 0 62.1 46.1 64.4 35.3 2P AC3 81.1 75.1 73.9 0 71.6 48.6 61.8 47.3 AC4 79.5 77.9 75.5 0 64.7 48.5 65.7 42.4 AC1 75.9 80.7 75.9 78 66 39.9 49.7 32 1.01 4.22 240 AC2 69.5 79.6 73.4 70.5 61.7 39.1 46.5 59.2 2F-3P AC3 85.6 81.3 84 74.6 58.4 43.1 35.9 52.6 AC4 75.4 86.4 84.7 73.8 57.1 49.4 52.2 57.3 AC1 79.4 69.6 63.2 0 69.1 40.8 48.2 32.6 1.12 4.61 AC2 80.5 54 54.6 0 53.9 60 41 53.1 5P AC3 64.8 70.3 65.7 0 64.4 48 39.5 44.6 AC4 66.4 70.3 44 0 67.6 60.5 44.7 48.6 AC1 82.8 77.1 70.9 52.8 76.2 55.3 51.3 46.9 0.21 0.83 AC2 81.1 85 51.9 50.2 84.9 53.5 58.8 63.1 4F-6P AC3 72.2 76.6 60.6 57.3 78.4 54.5 53.8 60.8 AC4 74.8 74.1 63.9 54.6 69.8 50.4 48.8 59.6 General Aviation Interior Noise: Page 2-4 Part III- Noise Control Measure Evaluation Table 2.2 Conditioned Response Analysis Using Optimum Simulation Vectors.

Error Cabin In-Flight Predicted Response Levels Tone Microphone Levels ALL I ABP IABEXl ABE I SeE ISBRM I SBLM Mean IStdev AC1 69.9 78.8 78.8 0 0 0 0 0 1.15 9.47 8O AC2 70.4 77.0 77.0 0 0 0 0 0 1P AC3 83.3 74.9 74.9 0 0 0 0 0 AC4 82.6 72.3 72.3 0 0 0 0 0 AC1 80.4 86.6 0 86.6 0 0 0 0 0.66 5.07 AC2 78.1 86.7 0 86.7 0 0 0 0 1F AC3 81.9 92.2 0 92.2 0 0 0 0 AC4 76 81.4 0 81.4 0 0 0 0 AC1 76.2 68.2 68.2 0 0 0 0 0 0.60 2.90 AC2 82.8 73.4 73.4 0 0 0 0 0 2P AC3 81.1 76.2 76.2 0 0 0 0 0 AC4 79.5 77.8 77.8 0 0 0 0 0 AC1 75.9 77.1 74.7 0 66.6 0 0 0 0.47 5.43 24O AC2 69.5 73.4 72.3 0 62.3 0 0 0 2F- AC3 85.6 82.9 82.9 0 59.0 0 0 0 3P AC4 75.4 83.9 83.6 0 57.7 0 0 0 AC1 79.4 69.6 63.2 0 69.1 40.8 48.2 32.6 1.12 4.61 4OO AC2 80.5 54 54.6 0 53.9 60 41 53.1 5P AC3 64.8 70.3 65.7 0 64.4 48 39.5 44.6 AC4 66.4 70.3 44 0 67.6 60.5 44.7 48.6 AC1 82.8 77.0 70.4 51.9 76.2 0 0 0 0.22 0.96 48O AC2 81.1 84.9 51.5 49.3 84.9 0 0 0 4F- 6P AC3 72.2 76.7 60.1 56.4 78.4 0 0 0 AC4 74.8 73.6 63.4 53.7 69.8 0 0 0 General Aviation Interior Noise: Page 2-5 Part III- Noise Control Measure Evaluation 3. CESSNA MODEL 182E The Cessna Model 182E was an unmodified single engine two-bladed propeller experimental aircraft equipped with a standard interior, as shown, fitted with a three-bladed propeller, in Figure 3- 1. This aircraft was employed in the previous project to develop noise source/path identification techniques [1] and was used in two additional flight test programs, results from which are summarized below. The instrumentation layout used during the flight tests consisted of 9 microphones and 7 accelerometers located under the engine cowling and within the aircraft cabin, as listed in Table 3- Figure 3.1 Cessna Model 182E Test Aircraft with Three- Bladed Propeller.

1. The aircraft was nominally operated in the standard cruise condition at 75% power at a fixed engine speed of 2,400 rpm at an altitude of 5,000 feet, unless otherwise noted.

Table 3.1 Instrumentation Layout and Channel Assignment.

Channel Type- Nomenclature Description 1 Accelerometer - EC2 Engine lateral vibration 2 Accelerometer - EC12 Firewall normal acceleration - mid center 3 Microphone - EC14 Firewall sound pressure level- upper center 4 Microphone - AC1 Above pilot's control column 5 Microphone - AC2 Above co-pilot's control column 6 Microphone - AC3 Near right rear seat passenger's head 7 Microphone - AC4 Near left rear seat passenger's head 8 Microphone - AC20 Between pilot and co-pilot ear height 9 Microphone- AC21 Behind pilot's head 10 Microphone - AC22 Behind co-pilot's head 11 Accelerometer - CB1 On center of aft cabin bulkhead 12 Accelerometer - AC5 Instrument panel right side 13 Accelerometer - AC7 Windshield right side 14 Accelerometer - AC9 Pilot's side window center 15 Accelerometer - AC11 Right rear passenger's window center 16 Microphone - TC1 A/C Tail cone General Aviation Interior Noise: Page 3-1 Part III- Noise Control Measure Evaluation 3.1 Surface Treatment Evaluation Various passive noise treatments were applied to the surfaces of the test aircraft in an attempt to identify the major noise source paths. The areas of the test aircraft, where application of noise absorption or noise blocking materials were used to identity paths of noise propagation, are listed in Table 3-2 along with the material used, namely, the configuration nomenclature, approximate area of coverage, and approximate weight of the material. Table 3-3 gives the make-up of the passive control materials. Twelve flight test configurations were flown with various combinations of applied materials, including a baseline configuration. A composite spectrum of the seven interior microphones recorded during the flight test of the baseline configuration is given in Figure 3-2. The corresponding noise levels are listed in Table 3-4. The highest noise levels are in the forward cabin at AC1 and AC2. Of particular concern are the major tones at the blade passage frequency of 80 Hz and firing frequency of 120 Hz and their harmonics.

Table 3.2 Schedule of Treatment Locations and Applicable Materials.

Area Weight (ft 2) Location to be Treated Config. (Ibs) Applicable Materials Usage Under Cowling: UCT Firewall C2 4.6 4.6 WB10-PSA Add Transmission Loss & Absorption 3.0 3.25 WB10 + Fiberfax Add Transmission Loss Muffler Wrap MW Cowling Surface C3 E-100SM-PSA Add Absorption In Cabin: FW1 2.8R Front Side Windows (2) 2.91R 1) WB10-PSA Add Transmission Loss 2.8L 2.80L FW2 2.8R 2.97R 2) R104-10CM-25PSA Increase Transmission Loss 2.8L 2.70L RW1 1.75R Rear Side Windows (2) 1.80R 1) WB10-PSA Add Transmission Loss 1.75L 1.86L RW2 1.75R 1.70R 2) R104-10CM-25PSA Increase Transmission Loss 1.75L 1.88L Instrument Panel IPS 4.5 4.5 WB10- PSA Add Transmission Loss Windshield WS1 11.5 11.65 1) WB10-PSA Add Transmission Loss WS2 11.5 11.38 2) R104-10CM-25PSA Increase Transmission Loss **80% Coverage of both top and bottom of Cowling with 1-inch absorption material - estimate from photographs.

Table 3.3 Description of Materials.

Weight/Area Material (Ibs/ft 2) Description WB10 1.0 Loaded vinyl with and without PSA E-100SM-PSA 0.17 1-inch absorbing foam with 1 mil aluminized polyester surface + PSA R104-10CM-25PSA 1.04 1.0 Ib/ft 2 loaded vinyl with 0.25" decoupling foam + PSA PSA - Pressure Sensitive Adhesive General Aviation Interior Noise: Page 3-2 Part III- Noise Control Measure Evaluation Table 3.4 Baseline Interior Microphone Levels.

Microphone Un-Weighted A-Weighted AC1 107.2 93.0 AC2 105.4 92.6 AC3 108.4 89.3 AC4 109.7 91.0 AC20 107.4 90.1 AC21 107.8 90.5 AC22 107.1 89.7 ?, '_ 75 i P 70 == == _- 65 P, 50 ' ' -' -' ' ' -J- '- ' '- -' 0 100 200 300 400 500 600 700 800 900 1000 Frequency- Hz Figure 3.2 Interior Microphone Spectra: Baseline Aircraft @ 2,400 rpm, 75% Power Cruise.

Repeated data runs were made with the under cowling treatment to establish flight-to- flight repeatability in the measurements. It was found that repeatability to within 1.5 dB was achieved for all but the fundamental firing tone at 120 Hz, which exhibited a 4 dB variation between flights. Details on the effectiveness of the various noise control treatments on cabin noise reduction are given in Reference [2]. The limited areas where passive treatment appears to warrant further evaluation are discussed below.

3.1.1 Cabin Window Treatment The extent of the cabin window treatment is shown in Figure 3-3. The instrumentation panel is also shown in this figure. Simultaneous coverage of the windshield and side General Aviation Interior Noise: Page 3-3 Part III- Noise Control Measure Evaluation Figure 3.3 Interior Window Treatment.

windows of the cabin could not be accomplished due to safety issues associated with flying the aircraft totally blind. There appears to be some promise that treatment of the windshield will lower the average cabin levels for the 400 Hz tone, as shown in Figure 3-4. Likewise, the tone at 480 Hz appears to be sensitive to nearly all the passive treatments, as shown in Figure 3-5. As one should expect, passive treatment appears to be more effective for higher frequency control.

The only exception being the under cowling treatment discussed below.

3.1.2 Under Cowling Treatment The aircraft Under Cowling Treatment (UCT) consisting of: (1)4.5 sq. ft. (4.5 lbs) of firewall blocking mass, (2) 1-inch thick absorber on 80% of the upper and lower cowling surface, and (3) Muffler Wrap (MW) 3 sq. ft. (3.25 lbs) blocking mass with fiberfax. Flight tests were conducted for the baseline aircraft, the full UTC and with the UTC minus the MW (UCT- MW). The under the cowling microphone (EC14) was used as a reference source indicator to compute what is defined as Firewall Noise Reduction (FNR) at the various interior microphone locations. Firewall Noise Reduction is the difference in noise levels between the under cowling microphone and the cabin microphone of interest at each of the tonal frequencies of interest.

Firewall noise reduction at the pilot's microphone (AC1) and co-pilot's microphone (AC2) positions are given in Figure 3-6. The data shows the Under Cowling Treatment to provide 8 to 13 dB(A) noise reduction at the fundamental blade passage frequency of 80 Hz.

These levels were reduced to 6 dB(A) when the muffler wrap was removed. At the exhaust fundamental of 120 Hz, the Under Cowling Treatment showed a 5-6 dB(A) reduction in cabin noise levels; however, when the muffler wrap was removed, the levels returned to that of the baseline. At the propeller 2 nd harmonic of 160 Hz and the combination 3P - 2F tone of 240 Hz, the noise reduction improved slightly when the muffler wrap was removed. It is to be noted that General Aviation Interior Noise: Page 3-4 Part III- Noise Control Measure Evaluation 85.0 80.0 m "0 | | | m >_ 75.0 ._1 70.0 '10 o m 65.0 • Maximum • Minimum • Average i 60.0 Baseline UCT UCT-MW UCT+IPS UCT+WS1 UCT+WS2 Configuration 85.0 80.0 m qD | m >= 75.0 ._1 "-I 0_ _b 0_ _L _L 70.0 ] '10 • Maximum _ ,L • Minimum o m • Average 65.0 60.0 I I I I I Baseline UTC+FWl UTC+FWl+RW1 UTC+FW2+RW1 UTC+FW2+RW2 UTC+FW2 Configuration Figure 3.4 Effect of Treatment Configuration on Cabin 400 Hz - 5P Tone Level.

General Aviation Interior Noise: Page 3-5 Part III- Noise Control Measure Evaluation 90.0 85.0 ,< IZI "0 i 80.0 > .J 75.0 -I ]!

D.

_L "o 70.0 r- -I o t/) • Maximum 65.0 • Minimum • Average i 60.0 Baseline UCT UCT-MW UCT+IPS UCT+WS1 UCT+WS2 Configuration 90.0 85.0 ,< m "ID 80.0 i == 4' ._1 == 75.0 t_ t_ a.

70.0 • Maximum --I • Minimum o (/) • Average 65.0 60.0 Baseline UTC+FWl UTC+FWl+RWl UTC+FW2+RWl UTC+FW2+RW2 UTC+FW2 Configuration Figure 3.5 Effect of Treatment Configuration on Cabin 480 Hz - 4F-6P Tone Level.

General Aviation Interior Noise: Page 3-6 Part III- Noise Control Measure Evaluation Firewall Noise Reduction: Engine Mic to AC1 • Baseline • UCT UCT-MW v m 20 "o i C

|

o (3 "0 _3 n" _3 .2 15 o z .................. i ................... i ................... i ................... i ................... i ...................

4O 80 120 160 200 240 280 Tonal Frequency- Hz Firewall Noise Reduction: Engine Mic to AC2 • Baseline • UCT UCT-MW v m -o i t- ._.9 (3 25 '-t "O rr .2 O Z • • 4O 80 120 160 200 240 280 Tonal Frequency- Hz Figure 3.6 Firewall Noise Reduction.

General Aviation Interior Noise: Page 3-7 Part III- Noise Control Measure Evaluation the change in the reference microphone EC14 between data runs was less than 1.0 dB and, thus, small compared to the changes in noise reduction. Noise reductions at the rear passenger locations were not as pronounced as in the forward cabin. In general, it appears that the Under Cowling Treatment was most effective in the forward cabin and a viable noise control measure [581.

3.2 Aircraft Configuration and Operational Effects To evaluate the effect of engine speed and power setting on cabin noise levels, the Cessna 182E was flown at an altitude of 5,000 feet at engine speeds of 2,000 rpm, 2,200 rpm, 2,400 rpm, and 2,600 rpm at power settings of 55%, 65%, 75%, and 85%, respectively, of maximum engine power. The speed and power matrix was flown for both the two-bladed and three-bladed propeller configurations to determine the effect of blade loading on the cabin noise levels [34]. Overall, sound pressure levels, out to 1,000 Hz, recorded at the pilot location AC1 and co-pilot location AC2 during the power matrix evaluation for both the two- and three-bladed propeller configurations are given in Figures 3-7 and 3-8, respectively. In general, the propeller cabin noise signatures increased with increasing engine power. For the two-bladed propeller configuration, it appears that increasing engine speed from 2,400 rpm to 2,600 rpm can be used to decrease the forward cabin noise levels by approximately 2 dB at the higher power settings.

The three-bladed propeller exhibited an engine speed tuning effect with marked increases in cabin noise levels at 2,200 rpm for a couple of the engine power settings.

For the same power setting, the two-bladed propeller should have a higher per blade loading than the three-bladed propeller and, therefore, should produce higher noise levels. The difference in cabin noise levels between the two-bladed propeller and three-bladed propeller at each of the power and speed matrix test points were computed and are listed in Table 3-5. The difference in under cowling noise levels given by the data listed under EC14 can be used to indicate the expected differences due to engine noise, which appears to be small compared to several of the cabin noise level differences. There appears to be several engine power and speed points where significant noise reduction was achieved using the three-bladed propeller. The most noted difference in the use of a three-bladed propeller is the reduced number of distinct tones in the spectrum, which can be significant if narrow band noise control measures are required for noise reduction.

3.3 Tail Cone and Aft Bulkhead Treatments The tail cone area of the aircraft represents a rather large volume that could possibly be used for noise control purposes or act as a noise source due to its large surface area on which propeller wake impingement may provide excitation. The bulkhead separating the cabin from the tail cone is a very lightweight molded Kydex® panel (see Figure 3-9) affording little in the way of transmission loss between the two volumes. An evaluation was carried out to determine if the tail cone volume was an active member in the generation or suppression of noise in the cabin area [35]. The aircraft tail cone was fitted with 8-inch deep wedges for a depth of approximately 36 inches to reduce reflections or sources from this area of the aircraft that may propagate energy into the aircraft cabin area, see Figure 3-10. The configuration was denoted as "TC Wedges."

General Aviation Interior Noise: Page 3-8 Part III- Noise Control Measure Evaluation 95.0 -O- 55% 94.0 -O- 65% 75% 93.0 -N- 85% 92.0 "O 91.0 i ._1 D.

Or) 90.0 p 89.0 o 88.0 87.0 86.0 i i i i i i i i i I i i i i i i i i i I i i i i i i i i i I i i i i i i i i i I i i i i i i i i i 85.0 1800 2000 2200 2400 2600 2800 Engine Speed - rpm a) Two-Bladed Propeller 95.0 55% 94.0 -0--65% 75% 93.0 "-!-85% 92.0 m "O 91.0 I ._1 0.

Or) 90.0 a) 89.0 == o 88.0 87.0 86.0 85.0 1800 2000 2200 2400 2600 2800 Engine Speed - rpm b) Three-Bladed Propeller Figure 3.7 Engine Speed and Power Setting Effects on Pilot Microphone ACI.

General Aviation Interior Noise: Page 3-9 Part III- Noise Control Measure Evaluation 95.0 94.0 .-O-65% --A--75% 93.0 -M-85% 92.0 "tO 91.0 l ._1 a.

Or) 90.0 89.0 o 88.0 87.0 86.0 i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i 85.0 1800 2000 2200 2400 2600 2800 Engine Speed - rpm a) Two-Bladed Propeller 95.0 --0--55% 94.0 93.0 .-==-85% 92.0 "O 91.0 = ._1 O.

(/) 90.0 89.0 o 88.0 87.0 86.0 85.0 1800 2000 2200 2400 2600 2800 Engine Speed - rpm b) Three-Bladed Propeller Figure 3.8 Engine Speed and Power Setting Effects on Co-Pilot Microphone AC2.

General Aviation Interior Noise: Page 3-10 Part III- Noise Control Measure Evaluation Table 3.5 Overall SPL Difference: Two-Bladed Minus Three-Bladed Propeller.

AC1 Engine Power AC2 Engine Power Spee¢t 55% 65% 75% 85% Speed 55% 65% 75% 85% 2000 _iii_ 0.8 _ii_ _ii_ 2000 _iiiii_ 1.0 _i_ _ii_ 2200 -1.8 -0.4 1.1 _iii_ 2200 -0.9 0.2 _iii6 _iii_ 2400 0.4 _iiii_ii i_iiii_ _iii_ii 2400 1.2 i_iiiii_ 0.8 i_iiii_ 2600 i_iiiii_ 0.4 1.1 _ii_ 2600 _iii_i_ 1.1 0.7 _ii8 AC3 AC4

Spee_ 55%165% 75% 85% Speed 550/01650/01750/0 850/0

x+x+x+:, x+x+x+: x+x+x+: 2000 -0.6 0.9 0.7 0.5 2000 0.5 _iiii_ _i_ _i8 2200 -1.8 1.2 1.2 0.5 2200 -2.2 0.5 _i_ _ii_ x+x+x+:, x+x+x+: +x+x+x. x+x+x+:, x+x+x+: x+x+x+: 2400 _iiii_ 1.3 _!i_ 1.3 2400 _i9 8ii_ _i_ 3i!_ x+x+x+:. +x+x+x. x+x+x+:, x+x+x+: 2600 _ii_ 0.5 0.1 0.3 2600 _ii_ _ii_ 0.7 _!i6 AC20 A021

Speed 55% I 65% 75% 85% Speed 55o165o175% 85%

x+x+x+: x+x+x+: x+x+x+: x+x+x+: 2000 1.0 0.9 _ii_i_ _iii_i_ 2000 1.1 1.4 _ii_i _i_ 2200 -2.4 -1.5 1.1 1.2 2200 -1.2 -0.3 _i_ _i_i_ 2400 1.1 1.2 0.6 i_i_ii_ 2400 _i_ _i_ _ii_i_i 3ii_ 2600 _iii_ 0.5 0.1 0.3 2600 _iii_ 1.3 0.0 i_iiiii_ AC22 ECl 4 Spee¢t 55% 65% 75% 85% Speed 55% 65% 75% 85% 2000 0.2 i_iiiii6 i_iiiii6 _ii_ 2000 -1.2 -1.2 -1.4 -1.2 2200 -1.4 -0.2 -0.1 -0.1 2200 -0.5 -0.5 -0.6 -0.1 2400 _iii_ i_iiiii8 1.3 i_iiii9 2400 -0.6 -0.7 -0.9 -0.6 2600 _iii_ -0.2 1.1 0.9 2600 -0.6 -0.5 -0.8 -0.9 Flight tests were conducted at the standard cruise condition of 2,400 rpm and 75% power and noise levels recorded at the standard microphone locations. The lightweight bulkhead was then replaced with a ¾-inch thick medium density fiberboard (MDF) to greatly increase the transmission loss at the aft cabin location, denoted as "MDF Blkd," and the flight test repeated.

This configuration was employed to maximize any effects of standing waves within the cabin.

Data were also recorded for the standard Kydex ® trim panel and denoted as "Standard."

Measurements were taken for both the two-bladed and three-bladed propeller configurations. A comparison of the overall sound pressure levels within the cabin and tail cone areas are summarized in Table 3-6. Clearly, there is no difference in cabin noise levels between the three configurations for either propeller configuration. The difference in cabin noise levels between the two- and three-bladed configurations is clearly seen. The tail cone noise level (TC1) does not appear to be as sensitive to the change in propeller configuration.

3.4 Linear Array Measurements The purpose of this effort was to evaluate the acoustic environment of the aircraft interior and characterize the environment as modal standing waves or free-field traveling waves within certain frequency ranges. The array used for this exercise consisted of 14 microphones spaced General Aviation Interior Noise: Page 3-11 Part III- Noise Control Measure Evaluation Figure 3.9 Standard Aft Bulkhead.

Figure 3.10 Tail Cone Fitted with Foam Wedges.

Table 3.6 Summary of Tail Cone and Aft Bulkhead Treatments.

Overall Sound Pressure Level - dBA Microphone Two Bladed Propeller Three Bladed Propeller Standard TC Wedges MDF Blkd Standard TC Wedges MDF Blkd AC1 92.3 91.9 92.2 90.4 90.9 90.5 AC2 92 92.3 92.2 91.2 91.5 91.3 AC3 89.6 88.8 88.8 86.9 86.4 86.5 AC4 92.1 92.1 91.6 88.6 88.4 88.5 AC20 90.4 90.3 90.3 89.8 90.2 89.5 AC21 91.6 91.9 91.5 88.5 88.7 88.4 AC22 90.2 90.6 89.8 88.9 89.3 88.7 TC1 97.2 90 88.2 96.5 N/A 88.2 General Aviation Interior Noise: Page 3-12 Part III- Noise Control Measure Evaluation six inches apart, aligned along the centefline of the aircraft. During flight, measurements were conducted with the forward most microphone in the array placed at the center of the instrument panel shroud, two inches above the edge. Microphones AC1 and AC2 were used as stationary reference microphones. The data was processed using reference microphone AC1 to determine the relative phase of the array microphones. Data was collected for the following three cabin conditions, for both the two-bladed and three-bladed propellers, namely, baseline, tail cone treatment with wedges, and MDF partition installed. Detailed results were generated in terms of sound pressure level and sound pressure phase distribution along the length of the cabin at 80, 120, 160, and 240 Hz for the two-bladed propeller configuration, and 120 and 240 Hz for the three-bladed propeller configuration [48]. A brief summary of the results is given below.

Results from the three test configurations were very consistent at any of the blade passage or engine firing frequencies. This confirms the passive effect of the tail cone and aft bulkhead on overall sound pressure levels within the cabin. Array results for the two-bladed propeller configuration with the baseline standard interior are shown in Figures 3-11 through 3- 14. At 80 Hz, see Figure 3-11, a dip is observed in the sound pressure level in the forward cabin. This sound pressure level dip corresponds to a phase shift. After this phase shift occurs, the phase distribution becomes linear with a positive slope progressing into the middle and aft cabin having the characteristics of a traveling wave. At 120 Hz, the sound pressure level is rather uniform with a slight increase along the cabin and the phase trend is also linear. The sound pressure level distribution at 160 Hz also exhibits traveling wave characteristics, however, not as pronounced as for the lower frequency tones. The characteristics of the 240 Hz tone are much less obvious. Results for the three-bladed propeller, for the 120 Hz and 240 Hz tones, are given in Figures 3-15 and 3-16. The 120 Hz tone displays the linear phase trend typical of a traveling wave while the 240 Hz tone is similar to the two-bladed propeller being much less definitive. However, at 240 Hz, both propeller configurations initially appear as traveling waves. In general, it appears that the primary noise source is radiating from the forward cabin and propagating as a traveling wave. The drop-in sound pressure level in the forward cabin for the 80 Hz propeller tone may be due to phase interference from a secondary source, such as propeller wake impingement.

3.5 High Frequency Tone Evaluation A high frequency tone appears in the cabin spectra of the Cessna 182E for a majority of the engine speed and power settings evaluated during the October flight tests [45]. The tone frequency ranges from 827.5 Hz to 910 Hz and is most dominant on the pilot side of the aircraft (AC1, AC4, and AC21). The high frequency tone is clearly seen in the spectra shown in Figure 3-2 for the two-bladed propeller aircraft. The high frequency tone is also present in the three-bladed propeller aircraft as can be seen in Figure l-lb. At the aircraft standard engine speed and power settings, the tone was not as clear as for other flight configurations. The tone levels and response frequencies do not correlate with engine speed; however, the frequency of the tone appears to correlate well with aircraft speed, as is shown by the data in Figure 3-17.

This data indicates the tone may be generated from a seal leak or aerodynamic disturbance, such as vortex shedding.

General Aviation Interior Noise: Page 3-13 Part III- Noise Control Measure Evaluation 3.6 Cabin Active Noise Control Survey Frequency response functions between nine potential speaker control source locations within the Cessna Model 182E aircraft cabin and the seven potential error microphone locations used during the flight tests were generated to assist in an Active Noise Control (ANC) evaluation of the aircraft [39]. A slow sine sweep (approximately 0.73 octave/minute) input in the frequency range from 40 to 500 Hz was used to drive the speaker to excite the cabin. The speaker cavity pressure was used as a measure of the source strength and, thus, was the reference input for all the frequency response functions. The drive speaker was located at nine different locations within the cabin as documented by photographs. The corresponding frequency response functions, displayed as real and imaginary, and magnitude and phase spectra were placed into the NASA General Aviation Database [39].

Linear Array Measurements - Cessna 182 - October 2000 2 Bladed Prop - Baseline - 80 Hz 110 180 Measured SPL '_,13,,_ Measured Phase 150 --Traveling Wave Phase Slope (Theory) IO0 9O

o

v -30 ._ 80 a.

-60 -90 -120 -150 60 I I I I I -180 10 20 30 40 50 60 70 80 Microphone Axial Position (in) Figure 3.11 Array Results for Two-Bladed Propeller - Baseline - 80 Hz.

General Aviation Interior Noise: Page 3-14 Part III- Noise Control Measure Evaluation Linear Array Measurements - Cessna 182 - October 2000 2 Bladed Prop - Baseline - 120 Hz

110 18O

,=,,,,,,,iu.,,_ Measured S PL Measured Phase -- --Traveling Wave Phase Slope (Theory) 30 _ 6"

o

a.

¢o -30 ._ 8O a.

-60 -90 7O -120 -150 6O I I I I I 1 I -180 10 20 30 40 50 60 70 80 Microphone Axial Position (in) Figure 3.12 Array Results for Two-Bladed Propeller- Baseline- 120 Hz.

Linear Array Measurements - Cessna 182 - October 2000 2 Bladed Prop - Baseline - 160 Hz 110 180 •,,,,,,,,-lU.,,_ M e as u re d SPL _, "_ .,,,,,,,,,,_.,,,_ M e as u re d Phase _ "_ -- --Traveling Wave Phase Slope (Theory) "_ '_ 30 _

o

_ 85 -30 J 8O a.

-60 -90 7O -120 -150 I I I I I I I ' '_' 6O -180 0 10 20 30 40 50 60 70 80 Microphone Axial Position (in) Figure 3.13 Array Results for Two-Bladed Propeller- Baseline- 160 Hz.

General Aviation Interior Noise: Page 3-15 Part III- Noise Control Measure Evaluation Linear Array Measurements - Cessna 182 - October 2000 2 Bladed Prop - Baseline - 240 Hz 110 18O Measured SPL _-"_ Measured Phase

\

mTraveling Wave Phase Slope (Theory) \

\

\

\ 60

\ v 85 0 == ¢o -30 \ 8O o.

\

-60 -90

\

7O -120

\

-150

\

6O I I I I I --'= ' I I -180 10 20 30 40 50 60 70 80 Microphone Axial Position (in) Figure 3.14. Array Results for Two-Bladed Propeller - Baseline - 240 Hz.

Linear Array Measurements - Cessna 182 - October 2000 3 Bladed Prop - Baseline - 120 Hz 110 180 Measured S PL Measured Phase m mTraveling Wave Phase Slope (Theory) 30 _ v

o

.j 85 a.

¢o -30 ._ 80 a.

-60 -90 -120 -150 60 I I I I I 1 I -180 10 20 30 40 50 60 70 80 Microphone Axial Position (in) Figure 3.15 Array Results for Three-Bladed Propeller - Baseline - 120 Hz.

General Aviation Interior Noise: Page 3-16 Part III- Noise Control Measure Evaluation Linear Array Measurements - Cessna 182 - October 2000 3 Bladed Prop - Baseline - 240 Hz 110 180 •,....,u=_ M easu red SPL _Measured Phase

\

mTraveling Wave Phase Slope (Theory) \

\

\

\ 60

\ 30 ==

\

6"

o

-30 ,1= a.

\

-60 -90

\

-120

\

-150 % I I I I I , "_,L , 1 60 -180 0 10 20 30 40 50 60 70 80 Microphone Axial Position (in) Figure 3.16. Array Results for Three-Bladed Propeller - Baseline - 240 Hz.

14O 12O _eO • i H "ID D.

820 830 840 850 860 870 880 890 900 910 920 Tone Frequency - Hz Figure 3.17 High Frequency Tone Correlation.

General Aviation Interior Noise: Page 3-17 Part III- Noise Control Measure Evaluation 4. CESSNA MODEL 206 The Cessna Model 206 was tested at Cessna Aircraft during the last week in March 2001 and the first week in April. The aircraft was equipped with a three-bladed propeller, 6-cylinder engine with dual exhausts. The engine mount was a bed type mount versus the tubular truss type found on the Model 182 aircraft. The Model 206 could accommodate six passengers; however, it was often used to carry additional cargo with only four passengers, such as the Model 182 aircraft. The Model 206 was equipped with a single door in the forward cabin on the pilot's side of the aircraft and a pair of doors aft behind the co-pilot's door (see Figure 4-1). The aircraft was bare of standard interior; however, damping foam was applied on several panels in the forward section of the aircraft cabin. The damping treatment was a standard application by the airframe manufacturer. The Model 206 aircraft was equipped with nine microphones in the cabin interior and two external microphones, one under the cowling and one downstream from the right hand exhaust pipe. In addition to the 11 microphones, the aircraft was equipped with 13 accelerometers on windows and structural panels, which were identified as potential noise radiators [79].

:::::: ::::::::::::::::::::::::::::::::::::::: ...... :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::::::: :: :::::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::::::::::::::::::::: :::: ::::::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ...... :::: :::: ...... ::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ....... :::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: Figure 4.1 Cessna Model 206 Test Aircraft.

4.1 Cabin Noise and Vibration Spectra Response data were acquired for the baseline configuration, a firewall treatment configuration where approximately 8 lbs of surface mass treatment was applied to the firewall, and a muffler configuration where "improved mufflers" were installed in addition to the firewall treatment. Spectral data were generated out to 1,000 Hz, which captured the major aircraft responses relative to cabin noise levels. The flight tests were conducted at an engine speed of 2,400 rpm at 75% power cruise at an altitude of 5,000 feet. Detailed spectra and tabular forms General Aviation Interior Noise: Page 4-1 Part III- Noise Control Measure Evaluation of overall noise and vibration levels with responses at major tones were generated [81]. Several observations were noted from the recorded data and only selected data will be given herein to highlight the Model 206 noise and vibration environment.

.

Windshield vibration is dominated by the 120 tone with forward cabin window and panel vibrations exhibiting high vibration levels at the 60 Hz and 120 Hz tones (see Figure 4-2).

.

Firewall treatment appears to greatly reduce center firewall vibration across the spectrum (see Figure 4-3). Vibration reduction in the lower firewall structure was not as apparent, nor was the corresponding reduction in cabin noise levels.

.

The Model 206 cabin microphones exhibited coincident firing and propeller tones as shown in Figure 4-4. However, the cabin microphones off of the aircraft centerline also exhibited responses at 60, 180, and 300 Hz, which are 1/2 orders of the firing and propeller tones at 120, 240, 360 Hz, etc. These tones are believed to be from the dual exhausts [82]. Consider one side of the dual exhaust seeing two firings and one intake on the first revolution and one firing and two intakes on the second revolution, resulting in three firings per two revolutions on either side of the engine. Thus, a 3/2 order of the engine speed (40 Hz) would generate 60 Hz and higher order harmonics. The phase at microphones AC4 (behind pilot's head) and AC5 (behind co-pilot's head) was evaluated to determine if any conclusions could be drawn about the origin of the tones at 180 Hz and 300 Hz. The magnitude and phase data were extracted from time correlated 0.8-second data traces of the two microphones to look at the phase difference between the microphone responses and are given in Table 4-1. Out-of-phase responses would support the speculation of out-of-phase sources, such as the exhaust ports on either side of the fuselage. It appears that the fundamental propeller and engine exhaust firing tone at 120 Hz is in phase across the cabin, while the second tone at 240 Hz is out-of-phase. The target 180 Hz and 300 Hz tones are both out-of-phase across the cabin. Note that all instrumentation was powered via d.c. batteries and, thus, 60 Hz electrical noise was not present.

.

The high frequency tone just above 900 Hz is clearly present, as was the case for the Model 182 aircraft.

.

Replacing the muffler with the "improved mufflers" made no difference in cabin noise levels. This was verified by the downstream exhaust levels, which remained at a constant level before and after the change in the muffler. The improved mufflers were supplied by Cessna Aircraft for cabin noise evaluation [79].

.

Vibration transmission from the engine through the engine mounts and into supporting bed mount structure was very high. Structure-borne vibration transmission via engine mount tunnel appears highly likely, however, time did not allow further evaluation of this potential noise source.

General Aviation Interior Noise: Page 4-2 Part III- Noise Control Measure Evaluation Cessna 206 Cruise @ 2,400 75% Power - Baseline Windshield Bight Side Center AC10=1.274 grms u_ o.1 o.Ol o.ool o 19o 29o _o 4o0 9oo 69o 79o 89o 9o0 lO9o Frequency-Hz Cessna 206 Cruise @ 2,400 75% Power - Baseline CoPilot's Side Window Center AC11 =1.554 grins u_ 0.1 o: O.Ol o.ool o lOO 2o0 3OO 4_ 5OO 6OO 700 8OO 9O0 IOO0 Frequency - Hz Figure 4.2 Model 206 Window Vibration Spectra.

General Aviation Interior Noise: Page 4-3 Part III - Noise Control Measure Evaluation Cessna 206 Cruise @ 2,400 75% Power - Baseline Firewall Mid Center EC2=3.552 grms o= 1(o 2(0 300 4(0 5(0 600 700 8(0 Frequency - Hz Cessna 206 Cruise @ 2,400 75% Power - Muffler Treatment Cessna 206 Cruise @ 2,400 75% Power - Firewall Treatment Firewall Mid Center Firewall Mid Center EC2=1.483 grms EC2=1.223 grms o.1 o.1 o: o: O.Ol O.Ol 1(0 2(0 300 4(0 5(0 600 700 8(0 1(0 2(0 3(0 4o0 5(0 600 700 800 900 1(0o Frequency-Hz Frequency-Hz Figure 4.3 Model 206 Firewall Vibration Spectra.

General Aviation Interior Noise: Page 4-4 Part III - Noise Control Measure Evaluation Cessna 206 Cruise @ 2,400 75% Power - Baseline Above Pilot's Control Column 1oo -_ 90 so

iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii

"_ 70 I[ hJ_ 1(;0 2(;0 300 4o0 5(;0 600 7(;0 800 900 1(;00 Frequency - Hz Cessna 206 Cruise @ 2,400 75% Power - Firewall Treatment Cessna 206 Cruise @ 2,400 75% Power - Muffler Treatment Above Pilot's Control Column Above Pilot's Control Column AC1=112.9 dB AC1=116.9 dB 100 lOO 95.6 dBA 95.7 dBA -_ 90 =, _ so == so == i 7o i 7o 50 50 0 100 2(;0 1(;0 2(;0 300 4o0 5(;0 600 7(;0 800 900 1(;00 _0 4o0 5_ 6_ 7_ _0 _0 1_0 Frequency - Hz Frequency-Hz Figure 4.4 Model 206 Cabin Noise Spectra In Forward Cabin.

General Aviation Interior Noise: Page 4-5 Part III - Noise Control Measure Evaluation Table 4.1 Model 206 Tone Phase Evaluation.

Tone Microphone Microphone Frequency AC4 AC5 AC4-AC5 Hz Mag - dB Phase - deg Mag - dB Phase - deg Phase - deg 118.75 80.2 -73 80.2 -68 5 120.00 106.2 -93 106.9 -91 2 121.25 91.4 89 90.1 73 16 178.75 83.6 -35 79.1 -175 140 180.00 95.5 -57 95.4 143 200 181.25 85.7 112 85.5 -33 145 238.75 84.4 -160 85.3 48 208 240.00 91.9 -168 97.6 63 231 241.25 80.2 18 87.9 -143 161 298.75 77.8 148 66.9 -17 165 300.00 87.5 38 85.1 -137 175 301.25 77.4 -168 80.3 59 227 4.2 Panel Tap Test An extensive panel tap test was conducted on the Model 206 aircraft to support the development of an Active Structural Acoustic Control (ASAC) investigation by NASA and VPI engineers. Frequency response functions were generated from hammer impact data recorded from seven accelerometers placed on various structural panels and cabin windows. A total of 22 data sets were generated during the study. In general, the panels were very rich in low frequency response [80].

4.3 Linear Array Measurements The purpose of this effort was to evaluate the acoustic environment of the aircraft interior and characterize the environment as either standing wave or free field traveling wave at select frequencies of interest. The acoustic array consisted of 16 microphones spaced 6 inches apart. The array was positioned near the centerline of the aircraft at mid-window height. Microphone A1 was located just aft of the instrument glare shield.

A photograph of the installed array is shown in Figure 4-5 and a typical noise spectrum, recorded at the first microphone in the array is shown in Figure 4-6. In general a slight decrease in SPL occurs from forward to aft along the fuselage with the total decrease being approximately 2.5 dBA over the 90-inch span of the array [78].

With the engine speed set at 2,400 rpm, the firing and the three-bladed propeller fundamental frequencies are at 120 Hz. The relative magnitude and phase variations along the aircraft for the 120 Hz tone and first harmonic at 240 Hz are given in Figures 4-7 and 4-8, respectively. The equivalent linear phase distribution for a traveling wave is given in the figures. The 240 Hz harmonic exhibits a strong traveling wave phase distribution along the entire length of the cabin.

General Aviation Interior Noise: Page 4-6 Part III- Noise Control Measure Evaluation

_ /iiiiiiiiiiiiiiii_ii!

iiiiiiiij ili ¸ _ ili!i!iiii!iiiiiiiiiiii :::::::::: Figure 4.5 Cessna 206 Microphone Array.

_A1 = 113.4 dB _A1 = 100.0 dBA == ._1 90 .= II I I I .= 13.

"o c 70 U3 0 100 200 300 400 500 600 700 800 900 1000 Frequency- Hz Figure 4.6 Narrow Band Spectrum at Microphone AI.

General Aviation Interior Noise: Page 4-7 Part III- Noise Control Measure Evaluation 122 180 120 150 --e--Amplitude 118 120 Phase _Travelin( 116 90 114 60 112 30 _ '10 "1o | i 0 _ .-I Iz.

or) -30 _.

106 -60 104 -90 102 -120 100 -150 98 - 180 0 10 20 30 40 50 60 70 80 90 Distance Aft of Instrument Panel - in Figure 4.7 Model 206 Microphone Array Measurement - 120 Hz Tone.

-e-- Phase --e--Amplitude _Traveling Wave "o i .-I o.

u) 0 10 20 30 40 50 60 70 80 Distance Aft of Instrument Panel - in Figure 4.8 Model 206 Microphone Array Measurement - 240 Hz Tone.

General Aviation Interior Noise: Page 4-8 Part III- Noise Control Measure Evaluation 5. CESSNA MODEL 182F Flight tests were conducted on a Cessna Model 182F during the two-week period from August 10 through August 23, 2001 with the purpose to evaluate passive and active noise control measures for cabin noise reduction. A photograph of the test aircraft is given as Figure 5-1. The aircraft was fitted with a three-bladed propeller supplied to the project by McCauley Propeller Systems. Flight test operations were carried out of Check Six Aviation, San Antonio, Texas. All recorded flight tests of the aircraft were conducted at 2,400 rpm, 75% power cruise at an altitude of 5,000 feet. The instrumentation schedule used during the flight tests included both microphones and accelerometers, according to the schedule given in Table 5-1. In the active control evaluation, several of the accelerometers were replaced by four microphones (AC31- AC34) to aid in global control as noted in the table. Detailed spectra for all measured response parameters for the various control configurations are contained in the References 89 through 95.

Figure 5.1 Cessna Model 182F Test Aircraft.

The cabin noise control challenge for the test aircraft is best visualized by the summary of bare cabin microphone spectra given in Figure 5-2. Here we see the propeller and engine firing harmonics at 120 Hz, 240 Hz, 360 Hz, 480 Hz, 600 Hz, 720 Hz clearly dominate the spectra, along with a cluster of tones from 460 Hz through 520 Hz at a 20 Hz frequency increment. The immediate noise control targets are the coincident fundamental propeller and exhaust firing tones at 120 Hz, the first harmonic at 240 Hz, and the cluster of tones around 500 Hz.

General Aviation Interior Noise: Page 5-1 Part III- Noise Control Measure Evaluation Table 5.1 Instrumentation Schedule During Passive Treatment Evaluation.

Channel Type - Nomenclature Description 1 Optical Pickup Prop Fundamental 3 per rev.

2 Accelerometer - EC12 Firewall normal acceleration - mid center 3 Microphone - EC14 Firewall sound pressure level - upper center 4 Microphone -AC1 Above pilot's control column 5 Microphone -AC2 Above copilot's control column 6 Microphone - AC3 Near right rear seat passenger's head 7 Microphone - AC4 Near left rear seat passenger's head 8 Microphone - AC20 Between Pilot and Co-pilot ear height 9 Microphone - AC21 Behind pilot's head 10 Microphone - AC22 Behind co-pilot's head 11 Accelerometer -SP1 Structural Panel Pilot Side Foot Well 11a Microphone - AC31 Forward Cabin Pilot Side 12 Accelerometer - SP3 Structural Panel Pilot Side Mid Cabin 12a Microphone- AC32 Forward Cabin Co-Pilot Side 13 Accelerometer - AC7 Windshield right side 13a Microphone - AC33 Far Aft Cabin Pilots Side 14 Accelerometer - AC9 Pilot's side window center 14a Microphone - AC34 Far Aft Cabin Co-Pilot Side 15 Accelerometer - AC11 Right rear passenger's window center 16 Accelerometer - SP2 Structural Panel Forward Center Roof Panel 182F 3 B Propeller 2,400 RPM - 75% PC - No Interior Interior Microphones: AC1-AC4, AC20-AC21 m 80 "O | "_ 75 --1 -_ 7o a. 65 e- --I _ 6o i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i 50 ' ' 0 100 200 300 400 500 600 700 800 900 Frequency-Hz Figure 5.2 Model 182F Bare Cabin Interior Noise Spectra.

General Aviation Interior Noise: Page 5-2 Part III- Noise Control Measure Evaluation 5.1 Passive Treatment Evaluation Table 5-2 lists the passive treatment evaluations and the nomenclature used to reference a particular treatment. A summary of the weights of the aircraft interior and passive treatments is given in Table 5-3 [89].

Table 5.2 Passive Treatment Test Configurations.

Test Configuration Nomenclature Est. Weight - Ibs Standard Interior Trim Std. Interior 43.08 Bare Fuselage - No Trim or Rear Seats No. Interior 0.0 Firewall Treated with WB10 ~ 90% Coverage Firewall Only 9.62 Distributed Vibration Absorbers - Standard 120 Hz and DVAsl + FW 16.85 240 Hz with Firewall Treatment - Run #1 Distributed Vibration Absorbers - Standard 120 Hz and DVAsl + FW + SW 27.23 240 Hz with Firewall Treatment with WB10 on all Side Windows Distributed Vibration Absorbers - Standard 120 Hz and DVAs2 + FW 16.85 240 Hz with Firewall Treatment - Run #2 Equivalent Masses Replacing Standard DVAs with DVA Masses 16.85 Firewall Treatment Distributed Vibration Absorbers- Special Design DVAs Spec + FW 14.70 Aimed at 240 Hz Broadband with Firewall Treatment Table 5.3 Passive Treatment Weights.

Treatment Weight (Ibs) Comments Standard Interior 43.08 Not including rear seat at 30.52 Ibs Firewall Treatment WB10 9.62 Approximately 90% coverage @ 1.0 Ibs/sq.ft.

Side Window Treatments 10.38 100% coverage @ 1.0 Ibs/sq.ft.

WB10 Standard DVAs 7.23 22 ea. 120 Hz @ 5.68 Ibs and 20 ea. 240 Hz @ 1.55 Ibs Special DVAs 5.08 20 ea. 240 Hz Broadband @ 5.08 Ibs 5.1.1 Firewall Treatment The WB 10 treatment, a 1.0 lbs/sq, ft. self-adhesive backed loaded vinyl, was used as mass loading over approximately 90 percent of the firewall area. The mass loading treatment reduced the firewall vibration levels in most all the spectra, except at the propeller and engine firing fundamental at 120 Hz, as is shown in Figure 5-3. It appears that a firewall resonance may have been shifted down to near the 120 Hz tone, thereby, producing an amplified vibration response. The source of the 460 Hz to 520 Hz cluster of energy is not as apparent as the propeller and engine firing harmonics; however, it was believed to be associated with engine valve noise from the CRA results presented in Section 2. This is supported by the firewall vibration reduction given in Figure 5-3 and the corresponding under cowling noise spectra given in Figure 5-4, both having rich response in the mid frequency region. Unfortunately, the cabin noise reduction in the 460 Hz to 520 Hz range does not directly track the firewall vibration reduction at the particular point of measurement and, therefore, the source must be more widely distributed.

General Aviation Interior Noise: Page 5-3 Part III- Noise Control Measure Evaluation 182F 3 B Propeller 2,400 RPM - 75% PC - No Interior EC12=5.365 grms (/) t- o 0.1 G) o o ,¢ 0.01 0.001 100 200 300 400 500 600 700 800 900 1000 Frequency- Hz a) No Treatment.

182F 3 B Propeller 2,400 RPM - 75% PC - Firewall Only EC12=2.670 grms u) -m _ 0.1 0.01 0.001 0 100 200 300 400 500 600 700 800 900 1000 Frequency- Hz b) Firewall Treatment Applied.

Figure 5.3 Effect of Firewall Treatment on Firewall Vibration.

General Aviation Interior Noise: Page 5-4 Part III- Noise Control Measure Evaluation 182F 3 B Propeller 2,400 RPM - 75% PC - No Interior 11o _ Ec14:!23 2aP i -_ 100 so 60 , t 0 100 200 300 400 500 600 700 800 900 1000 Frequency- Hz Figure 5.4 Typical Under Cowling Noise Spectra.

5.1.2 Distributed Vibration Absorbers The Distributed Vibration Absorbers (DVAs) were developed by VPI engineers and consist of a distributed mass plate supported on a distributed stiffness and damping foam material. The DVAs are designed to have a resonant tuned response at prescribed frequencies corresponding to the driven excitation of the structural panel to which they are attached. Thus, tuning the DVAs to the 120 Hz and 240 Hz tones and applying them to the various cabin panels should produce reduced panel vibration and, thus, reduce noise radiation into the cabin. A photograph of the DVAs used during the evaluation is given in Figure 5-5 and photographs of typical installations are given in Figures 5-6 and 5-7. The standard installation consisted of 22 of the larger 120 Hz DVAs and 20 of the smaller 240 Hz DVAs. The DVAs were placed on nearly all exposed structural panels of the aircraft. The larger panels, such as sidewall panels, were fitted with both 120 and 240 Hz DVAs. Two flight tests were conducted with the standard DVA set to provide a check on repeatability. Limp masses, cut from WB10, of equivalent weights to the standard DVAs replaced the DVAs to provide a check on the blocking mass effects of the DVAs versus their absorptive characteristics. A special set of DVAs aimed at the 240 Hz tone (see Figure 5-5) was also flight-tested.

Sound pressure levels at the target 120 Hz and 240 Hz tones for the seven cabin microphones were extracted from the various in-flight spectra and listed in Tables 5-4 and 5-5, respectively. The under cowling microphone, EC14, levels are also given to indicate the steadiness of the source. Several observations can be drawn from the data presented in Tables 5-4 and 5-5, and reference is made to the detailed evaluations contained in Reference 91.

.

The firewall treatment provided a measurable level of noise source isolation in the forward cabin for the 120 Hz tone and in the aft cabin for the 240 Hz tone.

General Aviation Interior Noise: Page 5-5 Part III- Noise Control Measure Evaluation

Figure 5.5 Distributed Vibration Absorbers.

iiii_%iiii!!!iiiiiiiii_ Figure 5.6 Typical DVA Installation: Cabin Roof.

General Aviation Interior Noise: Page 5-6 Part III- Noise Control Measure Evaluation Figure 5.7 Typical DVA Installation: Cabin Sidewall.

Table 5.4 Effect of Treatment Configuration on the 120 Hz Tone.

Microphone - dBA AC3 AC4 Configuration EC14 AC1 AC2 AC20 AC21 AC22 No Interior 101.7 92.7 91.4 78.8 81.4 89.3 87.7 76.6 Std. Interior 68i;8 102.1 87.8 87.7 75.6 84.2 84.5 86.5 DVAs1 + FW 101.7 88.6 85.9 79.1 79.7 83.8 74.8 DVAsl + FW + SW 102.1 88.7 86.7 78.1 78.3 82.2 69.4 DVA Masses 101.9 88.5 87.6 77.2 79.7 82.4 83.6 74.5 DVAs2 + FW 102.8 87.7 85.2 _6 80.5 86.1 85.9 74.9 102.1 83.9 83.6 84.3 87.5 85.7 DVAs Spec + FW 102.0 90.0 87.4 71.7 Z6i_B 89.7 87.9 77.6 Firewall Only Minimum 101.7 84.4 81.6 70.6 76.0 80.9 83.0 68.5 83.9 84.2 Maximum 102.8 92.7 91.4 89.7 87.9 85.7 Table 5.5 Effect of Treatment Configuration on the 240 Hz Tone.

Microphone - dBA AC3 AC4 Configuration EC14 AC1 AC2 AC20 AC21 AC22 No Interior 97.9 79.2 78.8 80.5 84.8 78.4 78.4 80.9 Std. Interior 99.0 75.3 79.7 75.4 DVAsl + FW 99.0 76.5 80.9 76.5 81.5 82.3 83.8 DVAsl + FW + SW 99.3 80.3 79.6 84.4 82.4 86.0 76.4 DVA Masses 99.9 75.9 80.4 76.3 76.6 83.5 79.0 DVAs2 + FW 99.3 81.6 79.8 77.2 81.4 80.1 83.1 79.0 80.4 74.4 DVAs Spec + FW 100.2 79.3 82.7 80.1 79.4 81.5 76.5 76.7 Firewall Only 99.6 79.0 77.5 78.1 79.1 77.2 72.2 72.7 Minimum 97.9 74.3 75.8 77.0 78.9 74.5 80.4 84.4 Maximum 100.2 81.6 82.7 83.5 86.0 81.5 General Aviation Interior Noise: Page 5-7 Part III- Noise Control Measure Evaluation .

The standard cabin interior performed as well as any of the treatments at the 240 Hz tone.

.

The DVAs masses performed nearly as well as the standard DVAs for either of the tones analyzed.

.

The special DVAs, aimed at the 240 Hz tone, performed better in the forward cabin at 120 Hz than any of the other control measures and less effective in the forward cabin at 240 Hz than the standard DVAs.

5.2 Active Structural Acoustic Control An Active Structural Acoustic Control (ASAC) system, developed by VPI and NASA engineers, was flown on the Model 182F aircraft aimed at control of the 120 Hz and 240 Hz tones. The ASAC system consisted of placing small Motran inertial exciters on low impedance, high mobility, locations within the aircraft along with collocated accelerometers.

Six to eight actuators were used in the control scheme; photographs of typical actuator installations are given in Figures 5-8 and 5- 9. Various combinations of actuators and error microphones were investi- gated to obtain an optimum control set [92, Figure 5.8 Typical Motran Installation: Co-Pilot's Door.

93]. The error microphones were those listed under channels 4 through 11 in Table 5-1, which covered both the forward and aft cabin areas. During flight, the control algorithm was turned on and then off to record sample averaged responses at each of 12 sensors.

The best performing ASAC system consisted of 8 actuators and 8 error microphones, and the control achieved for selected tones in the spectrum are given in Table 5-6. The difference in tone levels when the ASAC system was activated is listed in the last set of data in Table 5-6. The maximum control level achieved at 120 Hz was 12.3 dB at AC4, and the minimum was a slight increase at the far aft cabin at AC33. AC33 was not included in the error microphone set. The minimum control at the error microphones was 2.2 dB at AC22. The maximum control achieved at 240 Hz among the error microphones was also at AC4 at a level of 4.8 dB, and the minimum control was actually a gain of 5.5 dB on the opposite side of the aircraft at AC3. The far aft cabin microphone AC34, not included in the error microphone set, exhibited a rather large General Aviation Interior Noise: Page 5-8 Part III- Noise Control Measure Evaluation increase in noise level. Thus, the ASAC system did not achieve global control in the aircraft cabin.

The ASAC system was flown with the standard set of DVAs and the firewall treatment.

By comparing noise levels to the baseline runs with no interior and firewall, only the combined effects of passive and active Figure 5.9 Typical Motran Installation: Windshield.

controls can be sorted out as given in Table 5-7. The level of control for the combined ASAC, standard DVAs, and firewall treatment at the 120 Hz tone was quite high for all but one of the error microphones, microphones AC31 through AC34 were not included during the baseline flights. Summary spectra out to 500 Hz for the aircraft with no interior, standard interior, and full treatment consisting of the best ASAC system with DVAs and firewall treatment are given in Figures 5-10 through 5-13, respectively, for microphones AC1 through AC4. Here we see a good level of control for the 120 Hz tone with little control of the 240 Hz tone with degraded performance in the higher frequencies over that of the standard interior [94, 95].

Overall noise control performance in the frequency range out to 1,000 Hz is summarized in Table 5-8. In general, the standard aircraft interior provided nearly 4 dB noise reduction while the full treatment, consisting of ASAC plus standard DVAs and firewall treatment, provided only 3 dB overall noise reduction. It appears that a combination of passive and active treatment for low frequency control and standard interior for high frequency control may provide the optimum control measure for the aircraft.

General Aviation Interior Noise: Page 5-9 Part III- Noise Control Measure Evaluation Table 5.6 Best ASAC Results: 8 Actuators and 8 Error Microphones.

ASAC On AC4 AC20 AC21 AC22 AC31 AC32 AC33 AC34 Frequency EC14 AC1 AC2 AC3 SPL dBA SPL dBA 3PL dBA SPL dB._ SPL dBA SPL dBA SPL dBA 3PL dBA (Hz) SPL dBA SPL dB._ SPL dBA 3PL dBA 120 100.7 80.5 76.6 71.0 69.4 75.0 72.7 77.4 78.3 81.4 78.2 77.1 240 97.6 74.0 76.7 79.9 77.4 72.5 75.2 73.4 71.9 75.6 81.3 83.9 360 96.2 75.7 71.8 71.3 72.9 80.0 76.1 75.5 75.7 79.9 74.0 75.6 480 87.6 78.9 80.6 75.6 72.6 77.4 73.9 80.2 74.7 79.1 72.7 78.5 520 95.9 83.4 80.7 76.5 77.5 76.6 77.0 75.0 82.3 83.0 72.6 67.8 560 99.0 74.5 69.8 68.9 70.7 68.2 69.0 68.3 70.5 72.7 67.7 68.6 600 93.9 76.7 79.0 72.0 72.5 72.6 74.0 73.6 76.3 79.3 71.8 72.8 Overall 113.4 93.8 93.2 90.9 91.5 91.2 92.0 91.8 94.8 94.3 92.5 92.2 ASAC Off AC4 AC20 AC21 AC22 AC31 AC32 AC33 AC34 Frequency EC14 AC1 AC2 AC3 SPL dBA SPL dBA 3PL dBA SPL dB._ SPL dBA SPL dBA SPL dBA 3PL dBA (Hz) SPL dBA SPL dB.a SPL dBA 3PL dBA 120 98.9 86.9 86.3 78.6 81.7 80.2 83.0 79.6 87.9 87.7 77.0 77.9 240 95.7 75.3 76.9 74.4 82.2 75.9 77.8 76.2 74.9 79.9 83.6 73.4 360 95.7 72.8 73.0 70.7 71.0 74.8 73.4 72.9 74.4 78.1 72.5 69.7 480 94.1 74.3 77.5 73.0 70.1 75.8 71.9 75.6 73.3 76.0 68.2 73.4 520 100.7 79.1 74.7 71.7 73.7 71.2 72.4 70.2 74.7 78.5 69.0 67.7 560 93.1 70.1 69.0 66.2 67.6 68.5 68.5 68.8 70.4 70.9 68.8 67.2 600 89.6 70.7 72.0 70.1 67.1 70.2 71.4 71.0 73.3 73.0 68.7 66.5 Overall 113.3 94.2 94.7 91.4 92.2 91.8 92.5 92.4 95.2 95.1 91.7 91.6 Effect of ASAC AC4 AC20 AC21 AC22 AC31 AC32 AC33 AC34 Frequency EC14 AC1 AC2 AC3 SPL dBA SPL dBA 3PLdBASPLdB_SPLdBASPLdBASPLdBA3PLdBA (Hz) SPL dBA SPL dB._ SPL dBA 3PL dBA 120 1.8 -6.4 -9.7 -7.5 -12.3 -5.2 -10,4 -2.2 -9.6 -6.4 1.2 -0.8 240 1.9 -1.3 -0.2 5.5 -4.8 -3.4 -2.7 -2.8 -3.0 -4.4 -2.2 10.5 360 0.5 2.9 -1.3 0.6 1.9 5.2 2.7 2.7 1.3 1.8 1.4 5.9 480 -6.5 4.6 3.1 2.6 2.5 1.6 2.0 4.7 1.5 3.1 4.6 5.2 520 -4.8 4.3 6.0 4.7 3.8 5.3 4.6 4.8 7.6 4.5 3.6 0.1 560 5.9 4.4 0.8 2.7 3.2 -0.4 0.6 -0.5 0.0 1.8 -1.1 1.5 600 4.3 6.0 7.0 2.0 5.4 2.4 2.6 2.6 3.0 6.4 3.1 6.3 Overall 0.1 -0.4 -1.5 -0.5 -0.8 -0.6 -0.5 -0.6 -0.4 -0.8 0.8 0.6 General Aviation Interior Noise: Page 5-10 Part III - Noise Control Measure Evaluation Table 5.7 Best ASAC Plus Passive Treatment.

Effect of ASAC + Standard DVAs Frequency EC14 AC1 AC2 AC3 AC4 AC20 AC21 AC22 AC31 AC32 AC33 AC34 (Hz) SPL dBA SPL dBA SPL dBA SPL dBA SPL dBA SPL dBA SPL dBA SPL dBA SPL dBA SPL dBA SPL dBA SPL dBA 120 -1.3 -9.4 -10.8 -0.7 -6.6 -14.7 -15.2 -0.2 -13.6 -8.2 0.0 -8.4 240 -2.0 -4.9 -0.8 3.3 0.7 -5.6 -3.9 -3.9 -4.3 0.0 1.6 2.5 360 -3.1 5.5 -4.9 -4.9 -4.6 8.4 -1.0 1.6 0.5 1.0 0.4 4.1 480 -7.0 2.9 -0.1 -5.6 -3.7 -3.1 3.5 5.0 -3.0 1.7 -2.1 2.2 520 1.6 3.4 -1.1 -6.1 -2.6 -4.9 -3.1 -4.2 5.4 4.4 1.3 -9.6 560 -0.7 1.9 -1.4 -0.4 1.4 -1.5 -1.2 -2.0 -1.8 0.6 -3.4 -3.4 600 -2.4 -3.0 1.9 -6.9 -5.1 -2.1 -5.0 -2.3 0.4 3.9 0.2 0.9 Overall 0.3 -0.8 -0.8 -2.6 -1.4 -3.3 -2.4 -1.5 -0.9 -0.9 -0.1 -2.4 Effect of ASAC + Standard DVAs + Firewall Treatment Frequency EC14 AC1 AC2 AC3 AC4 AC20 AC21 AC22 (Hz) SPL dBA SPL dBA SPL dBA SPL dBA SPL dBA SPL dBA SPL dBA SPL dBA 120 -1.0 -12.2 -14.8 -7.8 -12.1 -14.2 -15.0 0.8 240 -0.3 -5.1 -2.1 1.5 -3.5 -8.0 -9.6 -5.0 360 -2.4 0.6 -3.4 -4.0 -3.3 3.8 -2.5 1.0 480 -5.3 3.9 2.9 -0.8 -1.7 1.0 1.0 2.6 520 4.4 0.2 -0.4 -0.4 2.9 -2.0 -4.9 -8.0 560 0.7 4.4 -1.1 0.5 2.5 1.0 -2.2 -3.4 600 -3.3 -1.8 4.5 -4.4 0.3 -0.7 -0.2 -4.3 Overall 0.2 -2.3 -2.2 -1.8 -1.6 -3.0 -3.0 -2.2 General Aviation Interior Noise: Page 5-11 Par_ III - Noise Control Measure Evaluation Table 5.8 Summary of Overall Noise Control.

Overall Sound Pressure Level - dBA Treatment Energy AC1 AC2 AC3 AC4 AC20 AC21 AC22 Average Bare Cabin 96.0 95.4 92.7 93.1 94.2 95.0 93.9 94.5 Standard Interior 93.2 93.1 87.1 88.7 90.5 90.7 87.4 90.7 Full Treatment 92.6 92.2 90.6 90.3 90.9 91.1 91.2 91.3 _No Interior 96.0 dBA _Std. Interior 93.2 dBA 9O _Full Treatment 92.6 dBA < "o > 8O ,-I u) u) 7O Q.

"o t- o or) 6O 5O 0 100 200 300 400 500 Frequency - Hz Figure 5.10 Summary of Control at AC1.

_No Interior 95.4 dBA --Std. Interior 93.1 dBA 9O Full Treatment 92.2 dBA 8O 7O 6O 50 '' '_ 0 100 200 300 400 500 Frequency - Hz Figure 5.11 Summary of Control at AC2.

General Aviation Interior Noise: Page 5-12 Part III- Noise Control Measure Evaluation IO0 _No Interior 92.7 dBA _Std. Interior 87.1 dBA _Full Treatment 90.6 dBA "o | _I n "o C o (/) 0 100 200 300 400 500 Frequency - Hz Figure 5.12 Summary of Control at AC3.

_No Interior 93.1 dBA _Std. Interior 88.7 dBA _Full Treatment 90.3 dBA "1o i == ._1 .= "t U_ U_ .= o.

"1o p.

"t o or) 0 100 200 300 400 500 Frequency - Hz Figure 5.13 Summary of Control at AC4.

General Aviation Interior Noise: Page 5-13 Part III- Noise Control Measure Evaluation

6. OBSERVATIONS AND CONCLUSIONS

Several observations and conclusions may be drawn from the results obtained during ground and flight tests of three single engine propeller driven General Aviation aircraft: .

The Conditioned Response Analysis (CRA) conducted on the Model 182E aircraft clearly demonstrated that a fundamental source associated with the propeller was not included in the set of simulation vectors used in the CRA evaluation. The missing source is believed to be the propeller wake vortex impingement on the fuselage.

.

In-flight linear array measurements recorded on two of the test aircraft clearly showed the cabin acoustic environment to be comprised of traveling waves for several of the fundamental tones in the spectra. This being the case, local control of the source(s) in the forward cabin should potentially provide global control within the cabin; however, this was not the case. It is believed that there must be a noise source being convected along the cabin, reinforcing the cabin sound field.

Propeller wake vortex impingement may be the convected source.

.

In addition to exhaust impingement as a source of cabin noise, under cowling engine case radiation appears to be a contributing source, particularly in the mid frequency region (500 Hz).

.

Structure-borne noise transmission from engine vibration does not appear to be a major source of cabin noise for the truss type engine mount configuration found on the Model 182 aircraft tested. Some indication of potential structure-borne noise transmission in the higher frequency region was found on the Model 206 aircraft, which uses a light weight frame type bed mount structure.

.

In general, only small noise control gains can be accomplished by treating selected areas of the cabin fuselage independently. However, from the surface treatments evaluated, it was determined that treating the windshield and forward cabin windows with increased mass loading or damping treatments appeared to have good potential for cabin noise reduction. Time and resources did not allow further evaluation of this potential passive control measure.

.

Under cowling treatment consisting of firewall mass loading, cowling absorption, and muffler isolation was a viable low frequency noise control treatment for the forward cabin of the aircraft.

.

Treatment of the fuselage tail cone volume and the rather flexible bulkhead separating the aft cabin from the tail cone provided little or no influence on cabin noise levels during flight. These results support the traveling wave environment found with the linear array measurements.

General Aviation Interior Noise: Page 6-1 Part III- Noise Control Measure Evaluation .

Measurable overall noise reduction was achieved at several engine power and speed points when replacing the standard two-bladed propeller with a three-bladed propeller on the Model 182 aircraft. The noise reductions are believed to be due to the reduced per blade loading. The coincidence of the fundamental propeller and exhaust tones when using the three-bladed propeller significantly reduced the number of offending tones in the cabin spectra, which was advantageous when implementing an active noise control system for the aircraft.

.

The standard Model 182 interior provided nearly 4 dB overall cabin noise reduction. Surprisingly, benefits were found at the 120 Hz tone and, as expected, in the higher frequencies of the spectrum.

10.

Distributed Vibration Absorbers (DVAs) were attached to nearly all exposed structural panels within the cabin (42 DVAs in all) targeting control of the 120 Hz and 240 Hz tones. Through the use of equivalent masses to those of the DVAs, it was found that the DVAs provided more of a blocking mass effect than that of energy absorption.

11.

Active Structural Acoustic Control (ASAC) using 8 Motran exciters on the cabin structure provided 6 to 12 dBA noise control at the 120 Hz tone in the forward cabin, and when combined with the firewall and DVA treatments, the control increased from 8 to 15 dBA. However, due to high frequency spillover, only 3 dBA overall control was achieved in the frequency range out to 1,000 Hz.

12.

It appears the combination of passive and active treatments for low frequency noise control and standard interior for high frequency control may provide the optimum control measure for the single engine General Aviation aircraft.

General Aviation Interior Noise: Page 6-2 Part III- Noise Control Measure Evaluation

7. REFERENCES

NASA General Aviation Database

Date Author Document Enclosures Description 1. Dec 1999 Source Path Jim Unruh General Aviation Identification Paul Till Interior Noise: Part I - Source Path Technology Identification Technology 2. June 2000 Jim Unruh General Aviation In-Flight Source Verification Paul Till Interior Noise: Part II - In-Flight Source Verification 3. Telecon Notes Rich Silcox Telecon Notes Aug 3,2000 4. Telecon Rich Silcox Telecon Notes Window AcceHerometer Aug 18, 2000 Cabin Phase TestpNan, V2 5. Telecon Notes Dan Palumbo Telecon Notes Aug 30,2000 6.

Guy's Comments Candidate Speaker Aug 30,2000 Speaker Locations Guy Billoud Locations.pdf 7. Acoustic Modes Steve Booth Acoustic Modes Aug 30,2000 8. Test Plan Jim Unruh Test Plan Aug 30,2000 TestPlan3.pdf 9. Discussion Aug 30,2000 Speaker Locations Guy Billoud 10. Comment Chris Fuller Chris's Comments Sept. 6,2000 11. Comment Sept. 7,2000 Guy Billoud Guy's Comments 12. Telecon Notes Dan Palumbo Telecon Notes Sept. 9,2000 13. Comments Sept9,2000 Guy Billoud Guy's Comments 14. Comments Ted Farwell Ted's Comments Sept. 11,2000 15. G/A Noise Schedule Jim Unruh Test Schedule.htm Test Schedule.PDF Sept12,2000 16.

Sept. 13,2000 17. Ted Farwell Sept. 13,2000 Tap Test Tap Test.htm 18. Reference Ted Farwell Sept. 13,2000 Enqine Installation Ih qenerator.ipq Ih Spinner.ip.q rh sp!nner:jpg rh vacuum pu mp_ip_ 19. Reference Ted Farwell Sept. 14,2000 182E En.qine ms3325.pdf HnstaHation 2 an4045_pdf as972pdf 20. Jim Unruh Sept. 19,2000 Tap Test Data Tap Test Analysis taptest.pdf 21.

Speaker Spec Sept. 20,2000 Speaker Spec Gary Gibbs 22. Telecon Notes Dan Palumbo Telecon Notes Sept. 25,2000 23. Junker 182 ZiP File of Jpe_qs Sept. 28,2000 Rick Wright 24. Telecon Notes Dan Palumbo Telecon Notes Sept. 28,2000 25. Ted Farwell Vacuum Pump PicOOOOl,jpg, PicOOOO2_jp_q Sept. 29,2000 Vacuum pump details PicOOOO3.ipq, PICOOOO4.JPG PicOOOO5.ip,q,PicOOOO6.jp,q 26.

Oct. 2, 2000 Muffler Benefit Ted Farwell Muffler Benefit 27. Oct. 4, 2000 Test Plan Jim Unruh Test Plan 28. Picture 1 Oct 5, 2000 Tach Drive Steve Booth Picture 2 29.

Oct. 5, 2000 Chris Fuller Cessna test VT test goals 30. Exhaust Extension & Ted Farwell Exhaust Extension Oct. 10,2000 Muffler 31. Test Plan Jim Unruh Oct. 11,2000 GA Noise Test Final Test PNan_pdf Matrix 32.

Oct 13, 2000 Borla Exhaust Data Ted Farwell BORLAREP.pdf 33. Ted Farwell Muffler O_.26,2000 Muffler Aqain General Aviation Interior Noise: Page 7-1 Part III- Noise Control Measure Evaluation Document Enclosures Date Description Author

34. Oct. Test Results Jim Unruh

Oct. 26,2000 GA Noise Overall Overa/! SPLPdf

35. Oct. Test Results Jim Unruh TailCone

Oct. 31,2000

36. Telecon Notes Dan Palumbo Telecon Notes

Nov. 3,2000

37. ASAC Mike Kidner

Nov. 7,2000 ASAC performance ASAC performance,pdf

38. Ran Cabell encoder files Encoder Data

Nov. 8,2000 Optical Encoder

39. Jim Unruh GA Noise - ANC

Nov. 9,2000 Speaker FRF's Speaker FRF.pdf

40. Encoder Failure Ted Farwell

Nov. 9,2000 Rotary Encoder disassembled.ip,q

41. Dan Palumbo

Nov. 13,2000 Project Status Project Status

42. Nov. 15,2000 SRI

Flight Matrix Fliqht Data

43. ANC FRFs SRI FRFs

Nov. 15,2000

44. Jim Unruh

Nov. 27,2000 Linear Array Linear Array.pdf

45. Jim Unruh

Nov. 28,2000 HiqhTone.pdf High Frequency Tone

46. GA ANC Results Mike Kidner

Nov. 28,2000 GA Optim!zat!ons_htm GA Opt,pdf

47. FRF Cal Factors Jim Unruh FRF Cal

Dec. 5,2000 Facto rs. htm

48. Jim Unruh

Dec. 6,2000 Travelinq Wave.pdf Traveling Wave

49. ANC Constrained Mike Kidner

Dec. 12,2000 ANC Constrained.pdf

50. Dan Palumbo

Dec. 13,2000 Maq PickUp MaqPickupRef.pdf Magnetic Pickup Reference.htm

51. Dan Palumbo telecon

Dec. 19,2000 Telecon Summary summarv.htm

52. Jim Unruh GA - Noise.htm

Jan. 8,2001 Happy New Year

53. ANC resuNts.htm Mike Kidner

Jan. 9,2001 Improved ANC results ANC Simulation.pdf

54. Call for Telecon Dan Palumbo

Jan. 10,2001 Te!econ Thursday Jan 11.htm

55. Jan 11TeHecon Dan Palumbo

Jan. 11,2001 Telecon Summary Summary.htm

56. ANC Coherence Mike Kidner ANC Coherence Limited.PDF

Jan. 15,2001

57. Dan Palumbo Jan 18 TeHecon

Jan. 19,2001 Telecon Summary Summary _htm 58.

Jan. 24, 2001 Firewall Treatment & Jim Unruh FirewaHTreatmentv2.pdf Test Schedule MarchTestScheduleRev0pdf

59. Coherence Limt Narrow

Jan. 24, 2001 ANC Coherence Narrow Mike Kidner Band Band.E_df

60. Grant Nash

Jan. 24, 2001 Windshield Treatment Windshield,pdf (VT)

61. VT Test Schedule

Jan. 25,2001 Rick Wright VT Test Schedule.pdf 62.

Jan. 29,2001 ATVA predictions ATVA Update.txt Rick Wright fb cont dsqn.pdf ATVASamp!eResu!ts.pdf

63. Steve Booth Pa 24pdf

Feb. l,2001 PA-24 testing

64. Latest Test Schedule

Feb. 14, 2001 Jim Unruh TestSchedule.pdf 65.

TestScheduJev4,pdf Feb. 21,2001 Flight Test Topics Jim Unruh 206EnqineMount.pdf Opticai PickupCoherence.pdf

66. TestSchedu/eRev3pdf

Feb. 28, 2001 New Test Schedule Jim Unruh 67.

Feb. 28, 2001 DVA Characteristics Mike Kidner UVA.A_df 68.

March 7, 2001 Ralph Buehrle Constrained Layer cld._/.L&_ Damped Plexiglass Mike Kidner Foam Evaluation for 69. March 8, 2001 Fo am Eva!u at!on _pdf DVA 70.

March 13, 2001 March Test Schedule Jim Unruh SwRIScheduleRev5.pdf 71. VTschedu!ej3df March 14, 2001 VT 152 Test Schedule Mike Kidner 72.

March 14, 2001 ATVA 152 Test RickWright ATVAschedule.pdf Schedule Window Panel Shaker 73. March 15, 2001 ThinPanel.pdf Ralph Buehrle General Aviation Interior Noise: Page 7-2 Part III- Noise Control Measure Evaluation Enclosures Date Description Author Document Tests Thin Pane!4_pdf Th!ckPaneH:pdf ThickPanel4.pdf 74.

April 5, 2001 Baseline 152 Mike Kidner Base!ine152_pdf 75.

125 3rd octpdf April 5, 2001 CDL Radiation Results Gary Gibbs 125 radiation.pdf 125to250 rad!at!on_pdf 250 3rd oct:pdf 250 rad vs veipdf 250 radiation.pdf 76.

April 11,2001 Cessna 152 Excitation 152Excitation.pdf

77. Jim Unruh tt206apdf

April 25, 2001 Tap Test Data

78. Jim Unruh

April 25, 2001 Linear Array Results LinearArraypdf

79. Photos of 206 Jim Unruh 206 Photos

May 1,2001

80. Jim Unruh

May 1,2001 206 Tap Test Data 206TapTest

81. Jim Unruh

Observations.pdf May 6, 2001 206 Flight Test Data 206marchj3df 82.

tonephase.pdf May 23, 2001 Tone Phase Evaluation Jim Unruh 83.

May 24, 2001 Test Plan Mike Kidner Tech test plan requests.txt

84. Noise Contro! Flight Test

May 24, 2001 Flight Test Status Dan Palumbo Status.txt

85. Jim Unruh

June 4,2001 Time Data, 206 T!me H!story optical.x!s, time data.z!p Data.txt Microphones and Optical Pickup

86. Test Schedule

June 21,2001 1st week of 206 proj.txt GA206 Flight Test 4_txt July Strawman Schedule 87.

June 21,2001 152 ASAC Tests Kidner PreH!minary ASAC tests on 152 fuselage at VTech 88.

June 28, 2001 Coherence, Noise Palumbo nr2snr.pdf Reduction & Signal to Noise Ratio 89.

Sept. 20, 2001 Weights of Standard Unruh tr!m-weights:x!s Trim and DVAs

90. Passive

Sept. 20, 2001 Passive Treatment Unruh Evaluation

91. Individual Treatment Standard Trim

Sept. 20, 2001 Unruh No Trim Analyses Firewall Only DVA Run #1 DVA Run #2 DVA + Side Window 'Special Divas' DVA Equiv Mass

92. 6ac+7er

Sept. 20, 2001 ASAC Analyses Unruh 6ac+8er 8ac+8er

93. ASAC-6ac-7er.xls

Sept. 20, 2001 ASAC Data Unruh These are large Files!!! ASAC-6ac-8er.xls ASAC-8ac-8er.xls

94. Full Treatment

Sept, 20, 2001 Cabin Noise Spectra Unruh

95. asac-best.xls

Sept, 20, 2001 ASAC Data Unruh Comparison of Best Runs General Aviation Interior Noise: Page 7-3 Part III- Noise Control Measure Evaluation REPORT DOCU M ENTATION PAG E ror_ Approved OMB No. 0704-0188 Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington r DC 20503.

1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED May 2002 Contractor Report 4. TITLE AND SUBTITLE 5. FUNDING NUMBERS General Aviation Interior Noise: Part III- Noise Control Measure Evaluation NAG1-2288 6. AUTHOR(S) 781-10-13-01 James F. Unruh, and Paul D. Till 8. PERFORMING ORGANIZATION 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(kS) REPORT NUMBER Southwest Research Institute San Antonio, Texas 10. SPONSORING/MONITORING 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(kS) AGENCY REPORT NUMBER National Aeronautics and Space Administration NASA/CR-2002-211667 Langley Research Center Hampton, VA 23681-2199 11. SUPPLEMENTARY NOTES Langley Technical Monitor: Daniel L. Palumbo 12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE Unclassified-Unlimited Subject Category 03 Distribution: Nonstandard Availability: NASA CASI (301) 621-0390 13. ABSTRACT (Maximum 200 words) The work reported herein is an extension to the work accomplished under NASA Grant NAG1-2091 on the development of noise/source/path identification teclmiques for single engine propeller driven General Aviation aircraft. The previous work developed a Conditioned Response Analysis (CRA) teclmique to identify potential noise sources that contributed to the dominating tonal responses within the aircraft cabin. The objective of the present effort was to improve and verify the findings of the CRA and develop and demonstrate noise control measures for single engine propeller driven General Aviation aircraft.

14. SUBJECT TERMS 15. NUMBER OF PAGES 6O Noise Source/Path Identification, General Aviation Aircraft, Active Structural 16. PRICE CODE Acoustic Control, Distributed Vibration Absorbers 17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT OF REPORT OF THIS PAGE OF ABSTRACT LTL Unclassified Unclassified Unclassified N_SN/54U-U1-28U-55UO _Stanaara Porm 29_ (HEY. 2-_9) Prescribed by ANSI Std. Z-39-18 298-102