Document
Advisory
U.S. Department of Transportation
Circular
Federal Aviation Administration Date: 3 /22/ 89 Subject: COCKPIT NOISE AND SPEECH AC No: 20-133 lNTERFERENCE BEIWEEN CREWMEMBERS Initiated by: AIR-120 Change: 1. PURPffiE. This advisory circular (AC) provides information about the relationship between flight crew cockpit voice canmunicaticn and cockpit noise levels. Guidance, al speech interference levels, noise rreasurerrent and rreasurerrent systems, and rrethcx:l.s to improve cockpit camu:micatioo, is provided for those manufacturers, avners or operators who believe cockpit noise may be a proolem al their aircraft.. This guidance material is relevant to the operatial of all types of civil aircraft.
2. BACKGROlND.
a. Many modern aircraft provide ccmfort, convenience, and excellent perfonnance. At the sc111e titre that the manufacturers have developed m::>re pc,.,.erful engines, they have tried to give the cccupants better noise protection and cootrol, so that many of tcrlay's aircraft are more p~rful, yet quieter than ever. Still, the levels of sound associated with ~red flight are high enough in scree aircraft to raise coocern aba.lt the effect these noise levels may have en direct voice cannunication between flight crew rcembers.
b. The National Transportation Safety Board (N'ISB) investigation of an accident involving a twin-engine, small airplane, concluded that the cockpit noise levels of that particular airplane ~re loud enough to interfere with direct voice ccrrmunicaticn. In the opinion of the NTSB, this carmunicatioo interference could have affected crew coordination and ccntribJ.ted to the accident. The NTSB also believes that poor crew camunication, because of high cockpit noise levels, may have cootribJ.ted to other accidents.
3. DEFililrI'ICNS.
a. Noise - My sound which is undesirable because it interferes with speech and hearing.
b. Noise S~ctra - The descriptioo of noise sound waves by resolution of their coupcnents, each of different frequency and {usually) different arrplitude and phase.
c. FrequencyCHz) -- The number of oscillations per seccnd of a sine-wave of sound.
d. Decibel(dB) -- The unit in which the relative levels of int?....nsity of acoustical quantities, such as sound pressure levels, noise levels and pc-Mer levels, are expressed on a scale fran zero for the average least perceptible level to abcut 130 for the average pain level.
AC 20-133 3/22/89 e. A-Weighted Samd Level (dB(A)) -- A single event sound level which has been filtered or weighted to discriminate against the lOW' and high frequency extremes to approximate the atrlitory sensitivity of the human ear.
f. Octave Band -- All of the ccnponen ts, in a sound spectrum, whose frequencies are between two sine wave (pure tcne) coopcnents whose ratio of freq.iencies exactly two, ie. separate:'l by an octave.
4. DISCUSSICN. Tcrlay' s large, jet-powered, air-transport airplanes present few speech-interference prd:>lems for flight crew.;. HCMever, propeller or rotor driven aircraft, regardless of the poy;er plant used, have noisier cockpits for several reasoos. Much of the prc.peller or rotor noise energy lies in lower frequencies, which are much crore difficult to attenuate than high-frequency
sounds. In nonpressurized aircraft, ccnstructicn permits air leaks that are >
both sound transmitters and sound sources; propeller and rotor tips can travel at or near Mach 1, which rreans, in sane flight configurations, small scnic boans ccnstantly banbard the aircraft. In addition, techniques for minimizing sound pro:iuction or soond transmissicn require the addition of physical mass to the system, and where payload determines the value or utility of the aircraft, acHing encugh mass to reduce noise, can cost severely in payload. Streamlining can be very costly in new design costs (to remove air leaks) and it nay also require major changes in prcrluction methods. Scree of these methods require additiooal ~ight which reduces utility.
a. outside the aircraft, noise spectra vary greatly as a function of aircraft size and type and the variety of powerplaDt, b.lt the interactioos of those spectra with the sound-insulatioo properties of the various airfranes generally lead to strikingly similar spectra en the inside. Cockpit noise stu:iies have shown the spectral shapes of cockpit noises vary only slightly £ran one type of fixed-wing aircraft to another.
b. The primary energy in those noises lies in the low frequencies, ranging coostly from 100 to 300 Hz, with a rapid decrease as frequency increases.
This spectral ccnfiguration may peak at different sound levels for different airplanes. The overall soond intensity varies from about 70 dB(A) to more than 100 dB(A). Generally, the quietest cockpits are found in jet aircraft: the noisiest are found in open cockpit airplanes such as those use:1 for aerial application in agriculture and in sorre small military jets that use afterburners.
c. Within a general class of aircraft (for exarrple, light, single-engine airplanes), the variations in cockpit noise level amcng airplanes of a single type 1nay be abo.J.t as large as the variations found arrong all the types within the class. Age and history seem to be irrportant determinants of the cockpit noise level as much as the original design. Therefore, little is to be gained by locking at a single sound spectrum from a single airplane as if it were typical of its type and v.0uld remain typical of its type.
d. The following sections present an overview of a ~ans to assess the level of cockpit speech interference due to noise and methods to rreasure and irrprove cockpit ccmnunications.
Par 3 AC 20-133 3/22/89 (1) Speech Interferenoe Level. This PC utilizes a noise interference metric known as the perferred-frequency speech interference level (PSIL). The PSIL is an average of the un...eighted noise sound pressure level of three octave bands at 500,1000 and 2000 Hz and relates to an "A" ...eighted decibel rreasureirent (dB(A)). Tl'E PSIL has b:!en accepted as a suitable predictor for a much rrore carplex rreasure of speech intelligibility known as the artiailation index (AI).
Th: AI ranges from 0. 0 to 1. 0 with an increasing value indicating a :nore perfect caranunicaticn. The Armed Forces maintain that for canmunications approximately 3 feet apart, an AI bet,ween 0. 2 and O. 3 represents an acceptable minimum intelligibility level. The maxi.rm.un PSIL for AI=0.2 is 83 and for AI=0.3 is 78.
The FM. :t:Blieves that in cockpits with noise levels aoove 88 dB(A) (PSIL=78), efforts should be made to aid comrunications by use of cne or more of the rrethods discussed in this PC. Th: evolution of speech intelligibility research and the develcpnent of criteria regarding speech interferenoe is covered in sane detail in appendix 1.
(2) Cockpit Noise Measurement. A tortable sound level rreter (SIM) which indicates the sound output in "A" weighted decibels (dB(A)) is recamnended for the rreasurerrent of cockpit noise.
(a) A quick noise survey of the cockpit can be wade by observing the sound level for approximately 20 secoods while the aircraft is in stabilized flight. One or two repeat readings are reccrnrrended to average the data.
Readings should be taken in the takeoff, approach, cruise and descent mcx:les of flight so that a ccnprehensi ve noise picture is obtained.
Cb) If the above tests indicate a noise problem or a borderline noise prd>lem exists, addi tiooal noise measurements should be taken and recorded, as discussed in appendices 2 and 3. Recording noise levels is desirable as this will allow a more ccrcplete noise analysis to be rnade. In addition, a sanple calculation of PSIL is shown in appendix 4.
(3) Methcx:ls to Improve/Aid Cockpit Ccmnunication. When the noise level in the cockpit, exceeds 88 dBCA) (PSIL=78), the noise will be of sufficient magnitude as to interfere with norrral cockpit corrmunications, i.e.
voice and radio. Therefore, efforts should be made to aid ccmnunicatioos. The following rrethods are suggested to irrprove the signal (voice )-to-noise ratio, which will enhance the intelligibility of cockpit ccmnunicatioos. Appropriate FAA approvals must be obtained for any type design changes resulting frcm any of the fallowing rrethods enployed : (a) Decrease the cockpit noise level.
( i > Use of door seals (ii) Acoustical insulation.
(b) Increase the voice signal levels or rrodify the signal-to-noise ratio.
(i) Increase the gain of intervening audio arrplifiers.
(reference TSO-CSOc, Aircraft Audio and Int.erphone Anplifiers) Par 4 AC 20-133 3/22/89 < ii) Use of electrcnic ooadsets, noise cancelling or boan microphcnes and intercom systerrs. ( reference TS.J-C57b, Aircraft Headsets and Speakers (for Air Carrier Aircraft) and TS0-C58a, Aircraft Microphones(for Air Carrier Aircraft)) (iii) A:wropriate use of rearing protectors.
( iv) Move the flight crewrreml:ers closer 1:.og'ether.
e. Appendix 5 discusses in detail the a:ivantages and disadvantages of the rrethcrls described above to improve cockpit carrnunications. The overall objective of the rrodification should be to irrprove the intelligibility of carrnunication'3. The minimum gool shculd be to achieve an articulaticn index CAI) of 0.3., identifiable by a PSIL of 78 or a rreasured noise level of 88 dB(A) or less.
f. Regardless of the rrethcrl used to aid cacmWlications care should be taken to assure that aural warnings (i.e. overspeed, stall, and landing gear) can be heard with or without the cannunications aid in place.
Director, Aircraft Certification Service, AIR-1 Par 4 4 AC 20-133 3/22/89 Appendix 1 APPENDIX 1 QUANTIFYING SPEEOI INTERFERENCE Several researchers have contrituted landmark stuHes of the ways in which noise can interfere with the understandability or intelligibility of speech.
It has been derrai.strated that the frequencies necessary for 100 percent intelligibility of a speech signal cover the range frcm about 300 Hz to about 7000 Hz.
A rreasure of that portion of the speech intelligibility range that is available in a specific carmunication situation is knCMn as the articulation index (AI).
The AI was developed by French and Steinberg and is a number falling between 0 and 1.0.* AI accounts for the level and spectra of ambient noise, and describes the relative ease or difficulty of a particular cannunication situation. An AI of 1.0 is considered perfect, with lcwer values indicating can:nunications of lesser quality.
* French, N.R. and Steinberg J., "Factors governing the intelligibility of speech sounds," Journal of Acoustical Society of America, 19,90-119, 1947.
Researchers have devised a set of relationships between AI and speech intelligibility for several sorts of speech test materials ranging from nonsense syllables, in which the content is quite unpredictable, to sentences, which are, catparatively, perceptually re:lundant-if you hear part of a sentence, yoo have a rea.sonably good chance to guess correctly what the rest of it is.
In 1947, Beranek published a report that serves as a further basis for determining how noise interferes with speech.* The speech interference level(SIL) is an average of the octave-band noise levels at sooe preselected set of center frequencies. In his original proposal, Beranek used the three octaves running fran 600-4800 Hz. Later work, prirmrily by Webster and by Klurrpp and Webster, shCMed that the inclusion of different frequency bands in the averages leads to AI predictions that are accurate for different ccmnunication ccriditions.** Thus, an average of the octave band levels at 500, 1000, and 2000 Hz seems well suited for predicting an AI of 0.2; i.e. a minirral carmunication emrironrrent. An average of 500, 1000, 2000, and 4000 Hz corresponds fairly well with an AI of 0.5 and an average of 1000, 2000, and 4000 Hz seems to go with an AI of 0.8. The 500, 1000, and 2000 Hz SIL has cane to l:e known as the preferred-frequency SIL (PSIL), and it is often closely related to a dB(A) neasurerrent of the sane noise, though the relationship is not perfect.
*L. L. Beranek, "The design of speech carmunication systems," Proceedings of the Institute of Radio Engineers, 35, 880, 1947.
**J. C. Webster, ''Relations cetween speech-interference contours and idealized articulation-in<Ex c01toors," Journal of the Acoustical Society of America, 36, 1662, 1964; J. C. Wel:.:>ster, ''Noise and Ccmnunication," in D. Jones and T.
Chapman (editors), Noise and Society, Lendon: Wiley in preparation: R. G.
Klunpp and J.C. Webster, "Physical rreasurerrents of equal speech-interfering navy noises," Journal of the Acoustical Society of Arrerica, 35, 1328, 1963.
3/22/89 AC 20-133 Appendix 1 TM maximum PSIL for cannunications approximately 3 feet apart for an AI of 0.2 is 83. The maxirrum for an AI of 0. 3 is 78. As will be shc:Mn belcw, these two AI' s represent the range of acceptable minimum intelligibility levels.
Therefore, when a cockpit has a noise level above a PSIL level of 78, talkers and listeners can be expected to have sooe voice-ccmnunication problems. This predictioo can be m::x:lified slightly by the fact that, in many cockpit5, the pilot and copilot can be rrore or less than 3 feet apart. H~ver, in the cockpits of aircraft likely to be relatively noisy, i.e. small aircraft, crev.nembers would probably be seated at distances be~en 2 and 3 feet apart.
The rressages that are expected to be transmitted in aviation conmunications care f ran a prescribed vocab.ilary. HC111ever, even when that vocab.ilary is ignored, the messages are spoken in context, which usually neans that they are more intelligible. Too Armed Forces have set acceptable levels of performance for cannunications equipcrent, and those perforrrance levels can be transforrred into AI values: they range from 0. 25 to 0. 3. The Air Force, for exanple, defines an 80 percent score oo a rhyrre test as passing and a 70 percent score a'3 ccnditicnally passing. In figure 1, it can be seen that tre 80 percent criterion is almost exactly 0.3 and that the 70 percent criterion is very close to 0.25.
Navy and ~y li.mits of acceptability are approximately the sarce as the Air Force's. Webster and Allen specified an 80 percent rhyme test score as (the Navy fence) the lowest acceptable value*. They reasoned that "95 percent of standard test sentences will be understood over a system that will pass 80 percent" of rhyme test v;ords. Following identical reasoning, t.h.e FAA believes that, short of rreasudng human perfonnance on rhyme tests in cockpit-noise environrrents, t.h.e choice of AI=O. 3 is both reasonable and acceptable. This AI equates to a PSIL of approxirrately 78 at a distance of 3 feet.
*J. C. Webster and C. R. Allen, "Speech intelligibility in naval aircraft radios," Naval Electronics laboratory Center Report, 'IR 1830, 1972.
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0.1 11.2 0.1 11.4 o.s 0.1 0.1 o.a o.e 1.0 ARTICUlATION INDEX FIGURE 1 AC 20-133 3/22/89 Appendix 1
Tre following table also corroborates the relationship between the various test
results and Articulation Index: Table 1. Expected Ward or Sentence Soores far Various Articulation Indices (AI) PERCENT lliTELLIGIBILI'IY Mcx:lif ied** Sentence* Articulation Phonetically* Index Balanced Test Rhyme Tests Test 54 77 0.2 22 72 92 0.3 41 78 95 0.35 50 o. 40 62 86 96 77 91
o.so
85 94 0.60
o.~ 92 98 99
(1963) *Fran Kryter and Whitman (1972) **Fran Webster and .;n.llen Assuming that pilots can camtunicate visually with each other, an AI of 0.3 actually can be elevated to 0. 47 as indicated by the folla\fing chart (figure 2).
u, 1.0 , w
5 0.9
,.I/
,., :-i! 0.8
:::, .J u, 0.7 V s: ~ :I: 0.6 t
/
i 0.5
i'
cC 0.4 w I/ > 0.3
/
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W 0.1 ~ 0 0.1 0.2 0.3 0.4 o.s 0.6 0.7 0.8 0.9 1.0 CALCULATED ARTICULATION INDEX Relation between calculated Al and effective Al for a communication system wherein the listener can see the lips and face of the talker (Sumby and Pollack, 1954).
FIGURE 2 3/29/89 AC 20-133 Appendix 1 Thus, an AI of O. 3, if aided by visual cues, can rai5e the intelligibility level to approximately 98 percent (as shown figures 1 & 2). HCJNever, visual ccmnunication, while it can inprove intelligibility, requires the persons to look directly at each other. This full-face orientation in the cockpit bet~en the pilot and ccpilot is an unusual occurrence. Cockpit noise levels in many rotorcraft and propeller-driven airplanes, especially the piston-engine types, can possibly exceed the maximum practical PSIL values noted above.
If one coosiders the distance between the heads of a pilot and copilot to be 3 feet, then in a noise field whose intensity exceeds a PSIL of about 90 ( about 97 d.B(A)), vocal effort cannot overcare the intelligibility problem created by the noise. First, shoutoo speech is not as intelligible as speech prcrluced with less effort (~e figure 3). Second, in that much noise, human vocal systems are, on the average, just about at the limit of their loudness. (Reflexively, talkers raise their voices in order to be heard above the b~kground noise. In this instance,though, where noise levels are quite high, the reflex cannot lead to rrore in tense speaking levels: the vocal system has already reached its physiolQ:Jical end point.) When PSIL = 90, AI approaches zero as dces intelligibility--that PSIL condition is unacceptable at a 3-foot distance.
FR.CM: Pickett, J.M.: Limits of Direct Sp;!ech Cannunication in Noise. J.
Acoust. Soc. Arrerica, vol. 30, no. 4, Apr. 1958, pp. 278-281.
IO ~ .... SIN iD 70 +I ::; 80 .., w ...
!
..
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f
MAXIMUM VERY
f SHOUT
~
0 1 LOUD
NORMAL LOW FORCE FORCE 30 40 liO IO 70 ID IO SPEECH INTENSITY, DB Relations be1ween speech lntelllglblllty In· nol·se and wocal force. Vocal force measured as speech Intensity one m from lips in a free field. Parameter, over-all algnal-to-nolae ratio, db.
Noise, 70 db, flat spectrum.
FIGURE 3 AC 20-133 3/22/89 Appendix 2 APPENDIX 2 CCO<PIT NOISE MFASUREMENT TEST SEIUP Measurenent in the cockpit should be made at the typical head location of each flight crewrrember. The microphone shOU.ld be placed at the representative ear position oo the side where speech carmunication is normally received and rroved around slightly to cbtain a spatial average of noise at the head pcsiticn.
Wh:never possible, the nea«:Jurerrent shall be made with the crewrmmber al:sent fran his locatioo so as to minimize interference and shielding effects. During the rreasurerrents care should be taken not to hold the microphone close to a sound-reflecting or sound-refracting surface. A ccrnncn recarcrendatim is to stay at least cne foot at1ay; in practical use, a 6-inch distance is probably adequate.
TEST CCNDITICNS The aircraft interior should be in a fully furnished configuration for its intended use (passenger, cargo, other) with tie downs, carpets, seats, curtains, interior trim panels, etc., installed. Systems used for providing conditioned air (i.e., pressurizaticn, cooling, !Eating,) should be operaticnal. Cabin pressure should be noted so that adjustrrents for differences in air pressure may be made, if necessary. Cabin pressure can af feet noise measurerrents taken on the ground or inflight. Tl'E difference between these rreasurerrents is aboot 0.25 dBCA). 01 sane aircraft, windONS can be cpen during flight and could adversely affect the noise level in the cockpit. If this case exists this coodition should also be tested.
If a tape recorder is used, the acoustic sensitivity calibration can be recorded during flight to establish the reference acoostic level for subsequent data processing and for catparison with the preflight recording of acoustic-sensitivity signals. Recorded noise levels should be neasured oo the ground and inflight to establish the proper gain to be used for recording above the backgrCllJld noise levels. At least aie reel of tape used during the test should have a recording of acoustic-sensitivity calibration signals.
Wh:!re possible neasurenents should be made woon all aircraft cperating caiditioos (such as altitude, airspeed and engine paver settings) are stabilized.
The aircraft cockpit noise should be tested in take-off, approach, landing, cruise, and descent at high speErl.
On multi~gine aircraft the use of engine synchrooizatioo is optional depending oo the test d:>jectives. Installaticn and operation of engine synchrcnizers or propeller synchrophasers is frequently desirable for increased passenger canfort.
~ratioo of such devices during acoustical testing is advisable if the t.est objective is to ~ure the optimum cabin environirent. Ho-ever, irrperfect synchrooizer operation may intrcxluce very low frequency beats which ccxrpranise AC 20-133 3/22/89 Appendix 2 the data, so that intentional operation out of sync may be necessary. In such cases, the engines should be set to prcrluce a knCMn beat frequency high enough to . allow reasonable length data records and minimize arrpli tu:'le effects.
The following flight data should tie ohserved and noted while the acoustic data is being obtained: a. Flight Regirre - takeoff, cruise, approach, landing, descent etc.
b. Airplane pressure al ti tu::le .
c. Airplane indicated airspeed and/or Mach numoor.
d. Propeller RPM ( if applicable) .
e. Engine ~r settings.
f. Synchronizer or synchrophaser operation.
g. External ambient air tercperature.
h. cabin pressure and terrperature.
i. Cabin system operation modes.
DATA ACQJISrrICN
If tape recording is used, the record length at each location should oo at least
2 1/2 tines the data reduction integratioo pericd, rut in no case less than 20
seconds. If auiible beats are present the record shall incltrle at least 3 corrplete beats. Sufficient precauticns should be taken to ensure the data signals are not carpranised by inappropriate tape recorded gain settings. Data should be recorded with the sound level rreter in the flat m::x:le (unw:iighted) • WlEn portable sound level rret.ers are used for direct rreasurerrent of sound pressure levels, (use the A-weighting network with SI..09' response setting) the data to be reported shall be the maximum reading noted on the ~ter. Wren audible beats are present the meter should be ol:Eerved for a pericd of time long enough to incltrle at least three beats, and the maximum rreter reading noted shall be reported. If the sound level rreter has integrating capability where the ti!l'E pericrl is q:ierator-controlled, the tirre pericrl used shall be at least 10-20 seccnds. If audible beats are present, the tirre pericxl shall be sufficient to incltrle at least 3 ccuplete beats, but not less than 20 secoods.
DATA REDUCTICN Data reduction, £ran the recording, when errployed, should be perforimd by tin-e averaging data sarrples of at least 8 secoods duratioo. When audible beats are present, the integration period should be extended to inclu:'ie at least a three-beat period.
AC 20-133 3/22/89 Appendix 2 Sound pressure levels should be obtained for the eight-octave bands center frequencies fran 63 Hz to 8 KHz. overall soond pressures shoold be cbtained by sumning antilogari thmically the octave band data. Preferred speech interference level (PSIL) should be calculated by algebraically averaging the un~ighted levels in the 500, 1,000, and 2, 000 Hz octave bands.
Frequency W:ighting may be added to octave band sound pressure level data. The ~ighting functim should correspmd to that referenced in Internatimal Electrarechanical C(J'llllissim (IEC) 651. Frequency ~ighted overall sound pressure levels are cbtained by antilogaritlmically summing the octave-band data after ~ighting is applied.
Presentation of the acoustical data should incllrle at least the following information: 1. Overall A-weighted sound pressure levels at each rreasurercent location.
2. Preferred speech interference levels at each rreasurenent location.
3/22/89 AC 20-133 Appendix 3 APPENDIX 3 MEASUREMENT SYSTEM A ~rtable sound level rreter (SIM > and a portable batte.ry-powered FM recorder are recatm:mded to rceasure cockpit noise. The SIM includes the mic:rq,hone, amplifier, rectifier and a ~ter which gives a somd output directly in deci~ls.
A coonecting jack is provided so the arcplifier output can also be recorded oo a magnetic recorder for further study.
Mcst sound level rreters also inclu:le weighting networks selected by a panel switch. The "flat" positicn sums all frequencies evenly. The "C" positicn is almost the sane as "flat" and one or the other may be anitted on cheaper instruments. The "A" and "B" weightings are designed to approximate the ear's response and to give a truer approach to lomness of carplex somds. ('!be "B" scale is little used today, while tre "A" weighting is used extensively. The designation "d.B(A)" or, less prq;,erly, "dBA", indicates the reading with the "A" ~ighting. ) More expensive rreters inclu:le, either as an attacment or int.ernally, a series of band pass filters, usually of one octave width. Eight such bands will cover the usual ~asurerrent range of 50 to 10,000 Hz. Su:h ;filters provide a convenient rreans for a quick evaluation of tre frequency structure of a corrplex sound.
In order that sound level rreters made by different manufacturers will agree adequately when measuring various sources, their charact.eristics are specified by the International Standards Organization (ISO) and 11nerican National Standards Institute (ANSI). This includes the characteristics of the weighting networks and the meter danping, as Tftlell as the overall accuracy. Somd level rceters are divided by ANSI standards into several groups: 'l'Ype 1 or "Precisicn" rreters; Type 2, or "General Purpose," ~ 3, or "Survey," and Type s er "S~cial Purpose." Type 1 neters meet the rigid tolerances for Precision rreters and provide filtering and inpulse measuring optia,s. A Type 1 rreter is recamended for evaluating cockpit noise.
A high quality rM tape recorder should be used to record the noise in the cockpit. Good results can be obtained frcm a portable batt.ery-p~red system. Several manufacturers now advertise high quality cassette recorders for instrurrentation use.
Tre sound level rreter or the recording system, if recordings are ma.de should be calibrated using a PISTCN-PHCNE, or other calibration instruments, before and after the test data is recorded. These calibration devices are available fran manufacturers of sound level meters and measurert'elt microphones. It is designed to fit tightly on the m.icrq:,hone, with adapters for various micrephone sizes, and it prcxluces a tone of accurately kn°"1111 sound pressure at the micrq:,hcne diaphragm at one or more standard frequencies. A set-screw is usually provided in the sound level rreter to standardize its output.
3/22/89 AC 20-133 Appendix 3 A calibration signal is particularly necessary when the microphone is used with anplifiers other than a standard sound level rreter or when a recorder is used.
This "end-to-end" calibration should be made both at the beginning and end of a test run, and at any other t:i.rre where there is a possibility that the system gain may have been changed.
It is irrportant in all test cperations to maintain an accurate log of all conditioos: microphooe placement, weather conditions if outdoors, system channel connections (if rrore than one channel), all attenuator and calibrated anplifier gain settings, time of day and date, source and distance fran source to microphone, etc. Wl'En a tape recorder is used, the log information should be recorded vocally on the tape.
While the PISfON-PHONE calibrator is an essential part of any acoustic rreasurerrent program, it d~s not give an adequate check of microphone, amplifier
and recorder frequency characteristics. Tre instrurren tation and procedures
required for full calibration are beycnd the sccpe of this discussion, but sare provision should be made for periodic recalibration of system canponents by the manufacturer or by a reliable and well-equipped standardization laboratory.
CALIBRATICliJ A preflight sensitivity check should be used to adjust the gain of the sound level rooter to 11\3.tch the output of the acoustic calibrator as aijusted for atmospheric pressure. A ''warm-up" tirre of at least 1 minute should be all~ before checlcing the sensitivity of the sound level meter. If a tape recorder is used, the sensitivity checks shall also be recorded.
If an in-flight acoustic sensitivity check is used, it should be taken when the aircraft has reached the desired cruise altitude and the aircraft's internal pressure is at the desired value. The indicated somd pressure level of the output of the acoustic calibrator should be noted: t0= gain of the sound level ~ter should not be adjusted in flight if the indicated level is not the sa.rre as the acoustic calibration level obtained before takeoff. If necessary, cabin pressure should be noted so that adjustaents for differences in air pressure may be ma&.
3/22/89 AC 20-133 Appendix 4 APPENDIX 4 EXAMPLE CALCULATICN OF PSIL
PSIL = Lsaa + L1aoa + L2000 *
T.O. P~r N onnal Cruise Parer Approach P<:Mer Octave Bmd Avg. Meas. Avg. Meas. Avg. Meas.
Cntr. Freq. Data Data Data 106.2 63 103.0 102.8 125 114.5 111.6 110.0 250 110.0 109.2 100.S 500 99.1 95.8 86.5 1000 84.6 00.1 73.9 2000 81.2 78. 4 73.2 4000 76.9 73.8 74.8 8000 76.1 74.l 73.7 db(C) 116.2 113. 8 110.9 db(A) 104.3 102.7 96.6 PSJL 88.3 84.7 77. 9 Fran the above it can be seen that the takeoff and normal cruise power noise
levels exceed a PSIL of 78 and speech interference can oo expected int~
cockpit in those flight regiires. The db(A} in all three flight regirres also exceed the reccmnended level of 88.
* L is the noise level C flat} at the specified octave band center frequency.
3/22/89 AC 20-133 Appendix 5 APPENDIX 5 MCDIFICATICNS CF SI~AL-TO-NOISE RATIOO An easy speech intelligibility ccncept to grasp is that the louier the speech is in canparison to the background noise, the easier it is to understand.
01:Niously, there are practical limits to the concept, rut through rrost of the
range of audible sound pressures, this statement about the speech-to-noise or the signal-to-noise ratio (S/N) is true. (~re both speech and noise are extrerrely quiet or extrerrely intense, nonlinearities arise. For the cockpit-noise situation, Clle may confront a degree of high-intensity nonlinearity. ) An improvenent in S/N, then, will serve to :improve the intelligibility of speech.
Tha most direct approaches call for an increase in alEolute signal level or a decrease in absolute noise level. One may also try to create relative differences between the signal and the noise levels.
The difficulty with trying to decrease cockpit noise levels at the source has already been discussed. Hc:Mever, it should be noted that noise attentuaticn rraterials are available for light aircraft. The use of inflatable door seals and acoustic blankets can reduce interior noise levels. Nevertheless, the most effective option may be to increase signal levels or modify the relationship between signal and noise.
Signal levels can be increased by increasing the gain of an intervening arcplifier (for electronically transmitted ccmnunications), or by moving tha talker and listener closer together. Research has shown a deterioration of intelligibility with an extrerrely weak or strong vocal force.
Hearing protectors for aviators can provide protection against rearing loss that results £ran noise exposure and irrproves speech intelligibility. Thay perform the intelligibility i.rrproverrent task in two ways. The lesser of these is that tooy 1~ tha overall intensity of the sound that enters the hwnan auiitory system into a middle range of sound pressures where the system q;:ierates cptirrally. (Note that hearing protectors do not reroove sound; they only decrease its intensity). Toe other way is selective filtering which can be effective in scrre noise envirorurents.
Sare. precautions are necessary, though, before cne elects to use hearing protectors for the purpose of .i.rcproving voice ccmnunication. First, a well-sealed, well-fitted protector is necessary. Second, sare atrli tory functions are changed by the introduction of hearing protectors into the transmission system. For example, sare pecple report a decrease in the ability to make fine pitch discriminations, many people report a decrease in the ability to judge tha azimuth of a sound source. However, the human atrl.itory system rapidly accanrocrlates itself to environnental change of all sorts, so one can assune that with a bit of practice these functions can be brought back into the normal range. Third, because one adjusts cne' s vocal effort to overcare the AC 20-133 3/22/89 Appendix 5 noise one hears, :tEaring-protector wearers (since tJ1ey hear less noise) usually don't speak loudly enough. Persoos who .....iear hearing-protectors must train themselves to spea.1( more louUy.
In ioost cockpits where noise is a problem, t.11e noise spectrum tends to have the saroo shape as the average speech spectrum. As a result, one cannot count on selective filtering to inprove speech intelligibility. Whatever changes are made in one spectrum will be made similarly in the other. The S/N stays about the sane. Thus, in cockpits with similar noise and speech spectrums, the ircproveroont in speech intelligibility for pilots and copilots who wear hearing protectors is probably limited to the srmll anomt that arises from bringing signal intensities into the linear, middle frequency range where the alrli tory system works better.
~ A microphone may help sare, because if it is held close to the rrouth, it is ,I sarewhat like reducing the distance to the ear. Coosiderably more improverrent in S/N can be obtained by using noise cancelling microphones in carmunication systems. The noise-canCElling micrcphone is ruilt to accept s0t.D1d fran the front, the back, or the top. In a fairly harogeneous sound field, appraxirra..tely the same ambient-noise wave form enters fran both sides, serving to cancel much of the effect of the noise on the micrcphone diaphragm. A talker, though, directs his or her speech to a1e side only, so the cancellatia1 effect for speech is far less than for noise--if the user understands the proper way to use the microphone. Covering the rear vents with the hand diminishes the cancellation effect. Holding the front of the microphone roc>re than a few inches fran the lips of the talker permits the speech to enter the back with nearly as much intensity as enters the front, thus cancelling speech as ~11 as noise. Another potential loss of S/N irrprovern.ent results £ran the reflex that leads a talker to speak with enough effort to be heard above the noise: if the talker expects to be heard (by the microphone) at a distance of 3 inches rather than 3 feet, l'E or she is likely to reduce vocal effort accordingly.
Miniature headsets have care into use arrong pilots in recent years.
The headsets, which are worn over the ear, conduct sound to the micrcphone diaphragm via a hard, plastic tube that is hinged so that it can be rroved about at will. Although these headsets are not noise-cancelling devices in the usual sense, the tip of the plastic tube can re rroved so close to the talker's lips as to make a significant irrproverrent in S/N over face-to-face ccrrmunications in the sa.rre noise environnB~t. Again, the likelihCXJd of improverrent is a direct function of how rruch vocal effort is exerted and of how close the tube is to the rcouth; if the tube has been moved out of tha way (as it needs to be for eating or drinking), any S/N ~rovercent will be markedly diminished.
Sare headsets are equipped with circunaural muffs which attenuate the cockpit
noise and enhance tre S/N for electrooic ccmnunicatioos. This type of ear muff
furnishes sorre hearing protection and acts sarewhat like an ear plug in normal cockpit voice carmunications. Headsets equipped with the bet.ter designai circumaural muffs may attenuate cockpit noise rcore that 20 dB. These headsets used with noise can<:Elling or txxm microphmes and an intercan system can substantially enhance the S/N and markedly inprove crew cam,.unications.
3/22/89 AC 20-133 Appendix 5 Proper use requires holding the noise-cancelling microphone so that the vents are not blocked, holding it close to the mouth, and speaking as loudly as if the listener wrere a few feet cMaY. Wh:!11 the microphone is used properly, it can make a significant difference in S/N.
It should be noted that increasing the gain of an anplifier or trying to do selective electronic filtering will make no useful change in the S/N; it will stay the San'e as it was at the face of the microphone whose sounds are being amplified or filtered. If the S/N is poor to begin with, c111plifying both the
speech and tre noise cannot make the situatirn any better. Also, electronic
filtering is no different in its effect than the acoustic filtering that a hearing protector does: if the spectrum of the noise and the spectrum of the speech are similar, selective filtering will not help.
Acliitional information en aircraft auiio system characteristics and standards can be found in Radio Technical Ccmnissioo for Aerooautiocs (R'ICA), Document No. Do-170, "Au:lio Systems Characteristics and Minimum Performance Standards, Aircraft Microphcnes (Except Carbal), Aircraft Headsets and Speakers, Aircraft Auiio Selector Panels and AIIplif iers," January, 1980.