Rating helicopter noise

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RATING HELICOPTER NOISE John W. Leverton, B. J. Southwood, and A. C. Pike Westland Helicopters Limited SUMMARY I n t h e case of h e l i c o p t e r s t h e main problem from t h e community point of v i e w is t h e n o i s e heard on t h e approach. This i s p a r t i c u l a r l y t r u e i n t h e case of h e l i c o p t e r s with high levels of b l a d e s l a p and/or t a i l r o t o r noise. The EPNL concept does n o t appear p a r t i c u l a r l y w e l l s u i t e d on quantifying h e l i c o p t e r n o i s e s i n c e - i t is i n s e n s i t i v e t o t h e n o i s e Heard on approach some d i s t a n c e from t h e flyover position. Blade s l a p and t a i l r o t o r n o i s e b o t h need an a d d i t i o n a l correction. The former i s now r e a d i l y agreed and preliminary evidence i s pre- sented t o show t h a t a similar c o r r e c t i o n is required i n t h e case of t a i l r o t o r noise. The impact of t h e use of such c o r r e c t i o n s is examined and although they improve t h e c o r r e l a t i o n , w i t h t h e p r a c t i c a l s i t u a t i o n t h e r e is s t i l l considerable d i f f i c u l t y due t o t h e i n h e r e n t c h a r a c t e r i s t i c s of t h e EPNL procedure.

INTRODUCTION The use of h e l i c o p t e r s f o r general commercial purposes has continued t o i n c r e a s e and although i n many instances they operate from remote sites on spec- i a l i s t operations, t h e i r use w i t h i n b u i l t up areas has coctinued t o grow. A s a n a t u r a l consequence o v e r f l i g h t s over many areas f a r from h e l i p o r t sites have well-defined f l i g h t paths have showed a s i g n i f i c a n t increase. I n urban areas, been e s t a b l i s h e d f o r s a f e t y and ATC reasons and, as a r e s u l t , t h e r e has been a tendency t o concentrate f l i g h t s over p a r t i c u l a r l o c a t i o n s . This wider use of h e l i c o p t e r s has given rise t o concern over n o i s e and although t h e number of complaints are s t i l l s m a l l , they have tended t o i n c r e a s e over t h e last few years. The p o i s e levels of h e l i c o p t e r s have a l s o shown a tendency t o increase as modern technology has allowed high speed r o t o r s t o be employed and helicop- ters with h i g h e r forward f l i g h t speeds. Noise c e r t i f i c a t i o n f o r h e l i c o p t e r s has now been under consideration by t h e ICAO Working Group B f o r a number of There appears t o be a years and c e r t i f i c a t i o n 'proposals are expected s h o r t l y .

need, t h e r e f o r e , t o re-examine t h e whole t o p i c of e x t e r n a l h e l i c o p t e r n o i s e and (WHL) are t h e r e s u l t s of such a survey conducted by Westland Helicopters Ltd.

presented i n this paper. I n t h i s context i t is worth noting t h a t i n a d d i t i o n t o manufacturing h e l i c o p t e r s , WHL operates t h e London (Battersea) H e l i p o r t and h a s been involved i n helicopter/community i n t e r f a c e problems encountered by t h e B r i t i s h Army of t h e m i n e i n West Germany. The WHL a i r f i e l d is a l s o s i t u a t e d within t h e boundaries of a s m a l l town; thus, considerable experience h a s been gained from t h e operators, manufacturers, and community p o i n t of view. The author has a l s o been c l o s e l y involved, as an a d v i s e r t o t h e United Kingdom (UK) delegation, with t h e ICAO Working Group B.

HELICOPTER NOISE - THE REAL PROBLEM

I n t h e case of h e l i c o p t e r s t h e main n o i s e problem from t h e community point of view is t h e n o i s e heard on approach. This is p a r t i c u l a r l y t r u e i n t h e case of h e l i c o p t e r s with hegh levels of blade s l a p and/or t a i l r o t o r noise. It is t h e n o i s e heard at d i s t a n c e which is of main concern. I f t h i s n o i s e i s im- pulsive (blade s l a p ) o r contains a d i s t i n c t i v e whine ( t a i l r o t o r n o i s e ) , then it r e a d i l y attracts a t t e n t i o n , becomes d i s t u r b i n g , and, because it can b e heard f o r a r e l a t i v e l y long period as t h e h e l i c o p t e r approaches, gives rise t o complaints.

I n many cases, b u t n o t always, blade s l a p decreases r a p i d l y in l e v e l as t h e o v e r f l i g h t p o s i t i o n i s approached. S i m i l a r l y , on p r a c t i c a l l y a l l h e l i c o p t e r s with a high degree of t a i l r o t o r n o i s e , t h e n o i s e d i e s away w e l l b e f o r e t h e o v e r f l i g h t p o i n t is reached. This is due t o t h e d i r e c t i o n a l c h a r a c t e r i s t i c s of t h e s e two n o i s e sources.

The e f f e c t s discussed above are i l l u s t r a t e d i n f i g u r e s 1, 2 , and 3 which show t i m e h i s t o r i e s (amplitude - time traces). Figure 1 shows r e s u l t s f o r a two-bladed s i n g l e r o t o r h e l i c o p t e r with and without blade s l a p where t h e blade s l a p decreases j u s t p r i o r t o t h e overhead p o s i t i o n ; as can be seen, t h e r e is a marked d i f f e r e n c e i n t h e duration of t h e noise. A l s o i n d i c a t e d on t h i s f i g u r e is t h e blade s l a p region and t h e "peak levels" determined using a peak d e t e c t o r developed w i t h i n WHL. This, i n e f f e c t , gives a measure of t h e blade s l a p and a t r u e r e p r e s e n t a t i o n of t h e d u r a t i o n a s s o c i a t e d w i t h t h i s h e l i c o p t e r . The d a t a on t h i s f i g u r e , l i k e t h a t on f i g u r e s 2 and 3 , are unfortunately l i m i t e d i n t h e sense t h a t t h e recordings w e r e n o t taken over a s u f f i c i e n t t i m e period, ( t h e d a t a w e r e Collected as a p a r t of o t h e r s t u d i e s ) and, as a r e s u l t , t h e t r u e dura- The g e n e r a l implications are, t i o n above t h e background n o i s e cannot be shown.

however, clear. Figure 2 shows r e s u l t s again f o r a h e l i c o p t e r with blade s l a p b u t i n t h i s case t h e blade s l a p occurs during t h e complete f l i g h t . These d a t a w e r e obtained from a tandem r o t o r h e l i c o p t e r and are t y p i c a l of t h e n o i s e l e v e l s generated by t h i s type of h e l i c o p t e r . Indicated on t h e f i g u r e is t h e 'peak' l e v e l f o r t h e blade s l a p case and t h e dB(A) level f o r t h e same h e l i c o p t e r when flown (during s p e c i a l t e s t s ) with no blade s l a p . Figure 3 shows t h e corres- ponding r e s u l t s f o r a h e l i c o p t e r w i t h a high degree of t a i l r o t o r n o i s e compared with a h e l i c o p t e r which has a low l e v e l of t a i l r o t o r noise. A s can be seen, t h e duration is increased when t a i l r o t o r n o i s e is present.

From a review of test d a t a and a b r i e f review of complaints and observa- t i o n of h e l i c o p t e r n o i s e , it h a s been concluded t h a t although i n many cases t h e absolute l e v e l Qf h e l i c o p t e r n o i s e is h i g h e s t a t t h e o v e r f l i g h t p o s i t i o n , t h e annoying c h a r a c t e r i s t i c s have decreased and t h e r e i s l i t t l e n o t i c e taken of t h e maximum o v e r f l i g h t noise. This is n o t t o imply t h a t t h e complaints a g a i n s t h e l i c o p t e r n o i s e are completely independent of t h e l e v e l s i n c e obviously a ldw a l t i t u d e f l y o v e r which generates a very high level w i l l prompt an adverse re- action. I n t h e real environment h e l i c o p t e r s are t y p i c a l l y 500 f t (150 m) o r mor€ from t h e n e a r e s t residence and then it would appear t h a t t h e c h a r a c t e r of t h e n o i s e is e q u a l l y , o r more, important than t h e absolute l e v e l . There h a s , how- ever, been l i t t l e o r no t e c h n i c a l s t u d i e s i n t o t h e s e a s p e c t s , although experiencc gained from h e l i c o p t e r f l i g h t s over London and generally w i t h i n t h e UK supports these g e n e r a l observations.

According t o Greater London Council t h e r e is i n London "a small b u t steady flow of complaints about noise" ( r e f , 1). Y e t , when 230 occupiers of p r o p e r t i e s within t h e v f c i n i t y of the London Battersea Heliport w e r e contacted, only 3 ob- j e c t i o n s on n o i s e were received. I n f a c t , t h e r e appears t o b e more o b j e c t i o n s from l o c a t i o n s w e l l away from t h e h e l i p o r t ; t h i s r e s u l t agrees with t h e conclusion from t h e W H L review t h a t t h e main problems arise from the n o i s e generated on approach. I n t h i s context it is a l s o worth notlng t h a t i n a d d i t i o n t o t h e ab- s o l u t e level of h e l f c o p t e r n o i s e decaying r a p i d l y a f t e r t h e o v e r f l i g h t p o i n t is reached, n e i t h e r b l a d e s l a p n o r t a i l r o t o r n o i s e is t h e r e a f t e r s u b j e c t i v e l y d e t e c t ab le.

I n t h e p r a c t i c a l s i t u a t i o n , high n o i s e levels are generated during "bank turns", manoeuvres etc. p r l o r t o landing and take-off at h e l i p o r t s . These are obviously a f u n c t i o n of t h e s p e c i f i c f l i g h t procedures used o r t h e ATC con- s t r a i n t s and t h u s should n o t b e b c l u d e d i n any c e r t i f i c a t i o n scheme. They can, and do, however, have a major influence on t h e s u b j e c t i v e r e a c t i o n t o helicop- ter n o i s e and it would appear from t h e a v a i l a b l e evidence t h a t it is such aspects wlilich d e f i n e tlie a c c e p t a b i l i t y t o t h e general p u b l i c of a p a r t i c u l a r h e l i c o p t e r near a h e l i p o r t . The d e t a i l s of t h e f l i g h t path are important i n t h i s context and if they are chosen such t h a t a h e l i c o p t e r has t o t u r n sharply t o avoid over- f l y i n g a p a r t i c u l a r l o c a t i o n , t h i s can o f t e n generate higher n o i s e l e v e l s than w a s allowed t o f l y overhead.

would occur i f t h e h e l i c o p t e r RATING HELICOPTER NOISE It follows from t h e p o i n t s o u t l i n e d previously t h a t s i n c e t h e annoyance of a h e l i c o p t e r is l a r g e l y dependent on t h e n o i s e heard on approach t h a t any r a t i n g o r c e r t i f i c a t i o n scheme f o r h e l i c o p t e r s should b e completely d i f f e r e n t from t h a t derived f o r f i x e d wing (CTOL) a i r c r a f t . This should t a k e i n t o account t h e s u b j e c t i v e c h a r a c t e r of t h e h e l i c o p t e r n o i s e on approach some d i s t a n c e from t h e f l y o v e r p o i n t a d t h e t i m e a s s o c i a t e d with t h e n o i s e , as w e l l as t h e maxi- m u m l e v e l measured during t h e o v e r f l i g h t . It is o f t e n argued t h a t it is not t h e r o l e of c e r t i f i c a t i o n t o c o n t r o l t h e "operational n o i s e s i t u a t i o n " , b u t blade s l a p n o i s e and t a i l r o t o r n o i s e are of fundamental importance i n t h e case of t h e h e l i c o p t e r , p a r t i c u l a r l y s i n c e t h e s e n o i s e sources can occur during is a very d i f f e r e n t s i t u a t i o n from " s t r a i g h t and l e v e l " c r u i s e f l i g h t . This t h e tra- t h a t a s s o c i a t e d with f i x e d wing a i r c r a f t and t h e r e f o r e , by i m p l i c a t i o n , It is d i t i o n a l m e t h o d of r a t i n g a i r c r a f t n o i s e is b a s i c a l l y inappropriate.

d i f f i c u l t , however, t o kmagine how a "new scheme" f o r h e l i c o p t e r s could be formulated p a r t i c u l a r l y s i n c e t h e v a r i o u s c e r t i f i c e t i o n a u t h o r i t i e s p l a c e signi- f i c a n t emphasis on developing a scheme which is, as f a r as p o s s i b l e , compatible with ' f i x e d wing' procedures. A s a consequence, t h e h e l i c o p t e r n o i s e certifica- t i o n concept c u r r e n t l y being considered w i t h t h e I n t e r n a t i o n a l C i v i l Aviation Organization (ICAO) and by t h e Federal Aviation Administration (FAA) are based on t h e EPNL method which is dependent mainly on t h e a b s o l u t e l e v e l of t h e n o i s e and t h e d u r a t i o n between t h e '10 dB down' p o i n t s .

The EPNL method a l s o t a k e s i n t o consideration t h e t o n a l content of t h e t h e s u b j e c t i v e noise and thus it would b e expected t h a t it would t a k e account of impact of t a i l r o t o r n o i s e as w e l l as t h e high frequency engine "whine.' Pre- liminary s t u d i e s witf-,in W H L suggested t h a t t h i s is not t h e case and t h a t t h e 'tone c o r r e c t i o n s ' are o f t e n , t o a f i r s t o r d e r , independent of t h e l e v e l of t a i l r o t o r n o i s e c ThAs is a complex s u b j e c t and t h e r e has been very l i t t l e work on t h i s t o p i c t o d a t e , b u t ft would appear from a n a l y s i s made by t h e 'author that problems arise from the f act t h a t h e l i c o p t e r n o i s e , p a r t f c u l a r l y below 500 Hz, c m t a f n s many d i s c r e t e frequencies from t h e main and t a i l r o t o r and t h e one-thlrd octave band s p e c t r a is n o t a t r u e r e f l e c t i o n of t h e annoying charac- teristics of t h e t a i l r o t o r noise. To remove t h e u n c e r t a i n t i e s of applying t h e tone c o r r e c t i o n procedure, ICAO Working Group B suggested at one s t a g e t h a t tone c o r r e c t i o n s should be applied only above 500 H z , b u t r e c e n t l y t h i s approach has been dropped and t h e latest view appears t o b e t h a t tone c o r r e c t i o n s should b e applied over t h e complete frequency range as i n t h e standard EPNL procedure (Annex 16).

A s mentioned previously;, b l a d e s l a p i s t h e most annoying source associated with t h e h e l i c o p t e r . There is ample evidence t o show t h e inadequacies of t h e standard EPNL procedure and t h a t an a d d i t i o n a l c o r r e c t i o n factor is required t o account f o r t h i s source. There is, however, some opposition t o t h i s procedure and, although t h e I n t e r n a t i o n a l Organization f o r Standardization (ISO) has pro- posed a procedure f o r accounting f o r , b l a d e s l a p , t h i s has n o t received wide acceptance i n t h e U . S . A . I n f a c t , it would appear t h a t even some groups, which have accepted i n p r i n c i p l e t h a t a b l a d e s l a p c o r r e c t i o n i s necessary, are more concerned w i t h developing a l t e r n a t i v e schemes r a t h e r than assessing t h e r e l a t i v e merits of those already proposed.

t h e use of t h e EPNL concept f o r r a t i n g The concern over t h e s u i t a b i l i t y of h e l i c o p t e r n o i s e w i l l still apply even i f a blade s l a p c o r r e c t i o n i s f i n a l l y accepted. The adoption of such a c o r r e c t i o n w i l l , however, s i g n i f i c a n t l y improve t h e r a t i n g of h e l i c o p t e r n o i s e r e l a t i v e t o t h e use of an unmodified EPNL .

RATING UNITS It is n o t proposed i n t h i s paper t o review t h e various u n i t s f o r r a t i n g h e l i c o p t e r n o i s e s i n c e it is clear t h a t f o r a number of reasons t h e perceived n o i s e l e v e l (PNL) w i l l be used as t h e b a s i c u n i t with possibly t h e dB(A) i n some s i t u a t i o n s . I n t u i t i v e l y , it would be expected t h a t t h e PNL method should adequately account f o r h e l i c o p t e r n o i s e providing t h e s j g n a l is r e l a t i v e l y broadband i n n a t u r e and without any pronounced d i s c r e t e n o i s e sources. Also, s i n c e it is used f o r r a t i n g CTOL a i r c r a f t , it should a l s o be equally a p p l i c a b l e f o r r a t i n g h e l i c o p t e r engine noise. It follows, t h e r e f o r e , t h a t with t h e ex- ception of t h e cases when t h e s i g n a l is dominated by blade s l a p and/or t a i l r o t o r n o i s e , t h e PNL u n i t , and by implication t h e dB(A), should b e s u i t a b l e f o r r a t i n g h e l i c o p t e r noise. For b l a d e s l a p and t a i l r o t o r n o i s e , some a d d i t i o n a l c o r r e c t i o n terms are required and t h i s is discussed i n t h e following s e c t i o n s .

BLADE SLAP Correcting f o r Blade Slap It is clear from t h e s t u d i e s conducted w i t h i n t h e UK by t h e National Phy- sical Laboratory (NPL) and W H L ( r e f s . 2 and 3 ) , by Aerospatiale i n France ( r e f . 4 ) and from some of t h e work conducted i n t h e States ( r e f s . 5 and 6 ) t h a t a s u b j e c t i v e c o r r e c t f o n is required t o account f o r b l a d e s l a p . There i s very l i t t l e disagreement between t h e r e s u l t s and t h e r e is a g e n e r a l consensus t h a t t h e penalty a s s o c i a t e d with severe blade s l a p is 6 dB. This is i l l u s t r a t e d i n f i g u r e 4 which shows t h e r e s u l t s of psychoacoustic tests conducted w i t h i n W H L ( r e f . 7 ) . The f i g u r e shows t h e Subjective c o r r e c t i o n i n terms of dB(A) a g a i n s t a measure of t h e impulsive n a t u r e of t h e s i g n a l based on t h e crest f a c t o r developed by WHL some y e a r s ago ( r e f s . 3 and 7). The s u b j e c t i v e r a t i n g of blade s l a p , from none t o severe, is i n d i c a t e d on t h e f i g u r e , t o g e t h e r w i t h a proposed correcti'on c u w e . For a l l p r a c t i c a l purposes, t h e r e s u l t s would b e i d e n t i c a l t h e c o r r e c t i o n s had been determined i n terms of PNL values.

if, i n s t e a d of dB(A), Since t h e analogue crest f a c t o r method developed by W H L was proposed, NPL and Aerospatiale have developed impulsive n o i s e d e s c r i p t o r s which are based on Recently, t h e r a t i n g of blade s l a p has been d i g i t a l a n a l y s i s of t h e s i g n a l s .

reviewed by t h e I n t e r n a t i o n a l Standards Organization (ISO) and they have recommended t h e u s e of a method based on t h a t o r i g i n a l l y devised by NPL (ref. 8 ) .

The NPL and Aerospatiale C I d e s c r i p t o r s are compared i n general terms on f i g u r e 5 which shows t h e r e s u l t s f o r a 250-Hz s i n e wave pulse as a f u n c t i o n of r e p e t i t i o n rate. Also i n d i c a t e d on t h i s f i g u r e is t h e t r u e crest f a c t o r and, as can b e seen, providing t h e i n t e g r a t i o n t i m e employed i n t h e NPL method is small, then f o r a l l p r a c t i c a l purposes t h e r e s u l t s of t h e NPL and t h e C I desc- r i p t o r follows c l o s e l y those given by t h e crest f a c t o r . Thus, from a fundamental point of view, t h e r e is l i t t l e t o choose between t h e methods (assuming t h e i n t e - t h e s i g n a l as orig- g r a t i o n i s 0.2 ms) and determination of t h e crest f a c t o r of chose an i n a l l y proposed by wHLi The ISO, when it adopted t h e NPL d e s c r i p t o r , i n t e g r a t i o n t i m e of 0.2 m s which is i d e n t i c a l t o t h e value used i n t h e W H L ana- logue peak d e t e c t o r ( r e f . 9 ) .

merits of t h e s e It is n o t proposed i n t h i s paper t o discuss t h e r e l a t i v e methods b u t , s i n c e i n p r a c t i c a l terms they r e s u l t i n t h e same o r d e r of correc- t i o n , t o review implications which r e s u l t from t h e i r use. There is, however, one exception t o t h i s g e n e r a l t r e n d i n t h a t t h e method proposed by Galloway contains a crest f a c t o r and r e p e t i t i o n rate term and can give very ( r e f . 6) d i f f e r e n t r e s u l t s . F i r s t l y , t h e crest f a c t o r , by d e f i n i t i o n , already contains a r e p e t i t i o n rate term and seconjily, i f t h i s procedure is adopted, then h e l i - w i l l have a high copters w i t h a high b l a d e passing frequency ( r e p e t i t i o n r a t e ) s u b j e c t i v e penalty o r c o r r e c t i o n even i f t h e impulsive content of t h e s i g n a l is s m a l l ( r e f . 10). This r e s u l t i s opposite t o what occurs i n practice, where t h e r e is a marked tendency f o r t h e s e v e r i t y of blade s l a p t o decrease with an increased.

i n c r e a s e i n t h e number of blades; hence, t h e blade passing frequency is There is a number of o t h e r d i f f i c u l t i e s a s s o c i a t e d w i t h t h e u s e of t h i s method and thus it would n o t seem t o b e a s u i t a b l e d e s c r i p t o r for blade s l a p .

The blade s l a p c o r r e c t i o n w a s o r i g i n a l l y based on steady state (hover) recordings, although r e c e n t l y Aerospatiale ( r e f . 11) and Galloway ( r e f . 6 ) have conducted physcoacoustic tests u s h g f l y o v e r recordings. These have shown t o a first order t h a t t h e steady state and flyover tests g i v e f o r a l l p r a c t i c a l pur- poses i d e n t i c a l r e s u l t s . The f l y o v e r s used appear, however, i n general t o be of s h o r t e r d u r a t i o n than s i g n a l s commonly encountered i n t h e real environment.

Even so, P t h a s been argued t h a t t h e r e s u l t s are equally applicable t o hover and flyover signals.

The b l a d e s l a p c o r r e c t i o n f a c t o r , as c u r r e n t l y proposed by I S O , i s added t o t h e PNLT t i m e h i s t o r y , i n 8 s i m i l a r manner t o t h e tone c o r r e c t i o n , every h a l f second t o give t h e PNLT(1) t i m e h i s t o r y from which t h e EPNL(1) is c a l c u l a t e d The d i f f e r e n c e between t h e EPNL(1) and t h e standard using t h e normal approach.

t h e h e l i c o p t e r n o i s e o r i n EPNL is a measure of t h e o v e r a l l impulsiveness of o t h e r words t h e magnitude of t h e annoyance associated w i t h t h e blade s l a p .

'Use of t h e Blade Slap Correction A number of s t u d i e s have been conducted w i t h i n W H L on t h e effect of using t h e various proposed blade s l a p c o r r e c t i o n procedures. These have been based on t h e NPL, t h e Aerospatiale C I and/or t h e WHL analogue crest f a c t o r d e t e c t o r methods s i n c e , as mentioned previously, t h e r e s u l t s are e s s e n t i a l l y independent of t h e method used. R e a l h e l i c o p t e r and simulated h e l i c o p t e r s i g n a l s have been t h e time a t which t h e blade s l a p d i e s away, r e l a t i v e t o used and t h e impact of t h e t i m e of t h e m a x i m u m n o i s e level, e s t a b l i s h e d . The g e n e r a l trends are ill- u s t r a t e d i n f i g u r e s 6 and 7. Two cases are shown on f i g u r e 6 which gives re- s u l t s of a t h e o r e t i c a l a n a l y s i s . I n t h e first t h e maximum c o r r e c t i o n of 6 dB, which corresponds t o severe b l a d e s l a p , is assumed t o apply over t h e b l a d e s l a p range w h i l s t i n t h e o t h e r a lower i n t e n s i t y b l a d e s l a p with a c o r r e c t i o n of 4 dB w a s considered. A s w i l l be observed, t h e impact, as expected, of applying t h e t h e t i m e from overhead p o s i t i o n is increased. Figure c o r r e c t i o n is decreased as 7 shows sfmilar r e s u l t s f o r a simulated f l y o v e r w i t h blade s l a p and r e s u l t s of real h e l i c o p t e r a n a l y s i s .

It is d i f f i c u l t from t h i s a n a l y s i s t o draw s p e c i f i c conclusions, although, if blade s l a p occurs during t h e complete f l i g h t , t h e c o r r e c t i o n is more l i k e l y t o be i n t h e o r d e r of 5.5 PNdB r a t h e r than t h e t h e o r e t i c a l 6 dB. From a review of a wide range of h e l i c o p t e r s , it would appear t h a t f o r t h e h e l i c o p t e r with severe blade s l a p on approach which "dies away" 2 o r 3 seconds p r i o r t o t h e overhead p o s i t i o n t h a t t h e c o r r e c t i o n w i l l b e i n t h e o r d e r of 4.5 EPNdB. Another d i f f i c u l t y a s s o c i a t e d with t h e b l a d e s l a p d e s c r i p t o r s c u r r e n t l y being proposed by IS0 is t h a t nonimpulsive h e l i c o p t e r s g i v e rise t o a c o r r e c t i o n i n terms of EPNL of 1 t o 2 EPNdB. This r e s u l t s from t h e f a c t t h a t although s u b j e c t i v e l y they are more impulsive than t h e broadband white n o i s e they are n o t impulsive, a reference i n determination of t h e impulsive d e s c r i p t o r . The s i g n a l used as p r a c t i c a l e f f e c t of t h i s is t h a t t h e d i f f e r e n c e between t h e impulsive and non- impulsive h e l i c o p t e r is f u r t h e r decreased.

Based on experience obtained within W H L from evaluating p u b l i c r e a c t i o n t o h e l i c o p t e r n o i s e , it seems f a i r t o conclude, t h e r e f o r e , t h a t t h e EPNL(1) 4 2 4 concept as c u r r e n t l y being proposed g t g l l underest-tes, i n r e l a t i o n t o a non- slapping h e l i c o p t e r , the impact o f h e l i c o p t e r b l a d e slap. This i s not meant t o imply t h a t the c o r r e c t i o n procedure is h a p p r o p r i a t e b u t r a t h e r t h a t b a s i c EPNL concept is inadequate, TAIL ROTOR NOISE C h a r a c t e r i s t i c s T a i l r o t o r n o i s e , l2ke blade s l a p , shows up on a n a l y s i s as a serees of p u l s e s spaced a t t h e b l a d e passing i n t e r v a l . Thus, except f o r t h e d i f f e r e n c e s i n p u l s e frequency and b l a d e passing frequency, t h e s i g n a l s , which s u b j e c t i v e l y are c l a s s e d "whine", are very s i m i l a r t o those associated with b l a d e s l a p . This i s i l l u s t r a t e d diagrammatically i n f i g u r e 8 which shows r e p r e s e n t a t i v e blade s l a p and t a i l r o t o r n o i s e p u l s e chains. Thus, from a r a t i n g p o i n t of v i e w , t a i l r o t o r n o i s e is impulsive i n c h a r a c t e r and hence "rated" by t h e v a r i o u s blade s l a p d e s c r i p t o r s discussed prevlously. This is a very important p o i n t which is over- looked by many i n v e s t i g a t o r s who simply associated t a i l r o t o r n o i s e with a dis- crete frequency spectrum.

The Tone Correction Procedure T a i l r o t o r n o i s e , as discussed previously, is very dominant on approach, but unlike blade s l a p , it d i e s away w e l l before t h e overhead p o s i t i o n , as i l l u s - When t a i l r o t o r n o i s e is pronounced, then it can be detec- t r a t e d i n f i g u r e 3.

t e d on a one-third octave band p l o t ; a t y p i c a l r e s u l t is shown on f i g u r e 9. The E1NL procedure is, however, r e l a t i v e l y i n s e n s i t i v e t o such tones as can be seen I f t h e f l y o v e r s shown i n f i g u r e 3 are exami- from r e s u l t s i n d i c a t e d on t h e f i g u r e .

ned, then it can b e shawn.that t h e EPNL f o r t h e Scout with t h e high l e v e l of t a i l r o t o r n o i s e is only 2 EPNdB higher than t h a t f o r t h e Wessex. P a r t of t h i s is due t o t h e d i f f e r e n c e i n t h e duration c o r r e c t i o n a r i s i n g from t h e s l i g h t l y d i f f e r e n t f l i g h t speeds between t h e two h e l i c o p t e r s . I f t h i s is taken i n t o consideration, then t h e c a l c u l a t e d d i f f e r e n c e is less than 1 EPNdB. I n an t h e equivalent continuous n o i s e l e v e l attempt t o h i g h l i g h t t h i s problem f u r t h e r , (Leq) has been c a l c u l a t e d f o r a Scout f l y o v e r when t h e t a i l r o t o r n o i s e is very pronounced and compared w i t h t h e p r e d i c t i o n of t h e Leq f o r an equivalent f l i g h t with no t a i l r o t o r noise. This is i l l u s t r a t e d i n f i g u r e 10 and it w i l l be noted t h a t t h e t a i l r o t o r 'hump' is w i t h i n 3 dB(A) of t h e maximum dB(A) level.

The d i f f e r e n c e between' t h e two Leq v a l u e s , based on t h e levels w i t h i n t h e

region covered by t h e 'maximum - 25 dB(A), is 1.9 dB(A) - y e t obviously t h e two

conditions sound very d i f f e r e n t , When analysing f l y o v e r s i g n a l s , it has a l s o been observed t h a t tone c o r r e c t i o n s r e s u l t i n a constant d i f f e r e n c e between t h e PNLT and PNL values, being t y p i c a l l y 1.5 dB. Owing t o t h e v a r i a b i l i t y of t h e one-third octave band s p e c t r a , t h e band which is responsible f o r t h e c o r r e c t i o n does not appear t a be represen- is considered t o b e due t o t h e complex n a t u r e tative of t h e real s i t u a t i o n . This of h e l i c o p t e r noise.

It is concluded t h e r e f o r e from the a n a l y s i s o u t l i n e d above and d e t a i l e d reviews of a wide range of f l i g h t conditions t h a t tone correctfons i n t h e EPNL procedure b e a r l i t t l e r e l a t i o n t o t h e t r u e annoyance of h e l i c o p t e r t a i l r o t o r noise.

Rating by Impulsive Noise Descriptors Since t a i l r o t o r n s i s e takes t h e same form as blade s l a p , then any impulsive d e s c r i p t o r w i l l be e q u a l l y s e n s i t i v e t o both of these sources of n o i s e and, of course, t o any o t h e r impulsive sources of noise. This can be seen on f i g u r e 5 which, i n a d d i t i o n t o v a l u e s f o r i d e a l i s e d blade s l a p pulses (low r e p e t i t i o n tail r o t o r n o i s e . A s discussed previously, r a t e s ) shows r e s u l t s f o r i d e a l i s e d values are shown f o r t h e NPL (ISO) and Aerospatiale C I d e s c r i p t o r s and t h e crest f a c t o r . The main d i f f e r e n c e between t h e two sets of r e s u l t s is t h a t whereas t h e NPL (ISO) method gives a measure of blade s l a p when an i n t e g r a t i o n This e f f e c t time of 10 ms is used, it e f f e c t i v e l y rejects t a i l r o t o r noise.

can be b e t t e r appreciated from t h e p l o t f o r i d e a l i s e d s i g n a l s shown i n f i g u r e 11.

It may appear from t h e s e p l o t s t h a t i f a 10 m s i n t e g r a t i o n t i m e w a s used, then t a i l r o t o r n o i s e would be r e j e c t e d and t h e measured value i n p r a c t f c e would depend s o l e l y on t h e l e v e l of t h e impulsive (blade s l a p ) noise. There are, however, a number of major objections t o t h i s . F i r s t l y , t h e r e l a t i o n s h i p between t h e NPL d e s c r i p t o r (10 log I) and t h e crest f a c t o r v a r i e s with r e p e t i - t i o n rate .- i n o t h e r words, on f i g u r e 5, t h e values are not p a r a l l e l t o those is t h e f a c t t h a t use of such of t h e crest f a c t o r . More importantly, however, a method would give a r e s u l t independent of t h e pulse frequency, and hence c r e s t f a c t o r , as i n d i c a t e d i n f i g u r e 12. Thus, such a s o l u t i o n is n o t p r a c t i - c a l .

I n t h e method o r i g i n a l l y proposed by WHL f o r r a t i n g blade s l a p , which w a s based on t h e use of t h e crest f a c t o r , t h i s problem w a s overcome by passing t h e s i g n a l through a band p a s s f i l t e r centered on 250 Hz i n order t h a t a l l impul- s i v e s i g n a l s except those a s s o c i a t e d with blade s l a p were r e j e c t e d ( r e f . 3).

Such a method is used w i t h i n W H L f o r assessing t h e magnitude of blade s l a p , b u t o b j e c t i o n s w e r e r a i s e d a g a i n s t t h i s method on t h e grounds t h a t s i n c e the widest standard f i l t e r which could be used w a s an octave band (177 t o 354 Hz), some h e l i c o p t e r s could generate blade s l a p with t h e main energy above t h e upper A n example o f t e n quoted is t h e Bolkow BO 105 which appears t o frequency l i m i t .

have t h e blade s l a p energy m a x i m u m centered on 600 Hz ( r e f . 12), while on most it is around 250 t o 300 Hz ( r e f . 13). It is considered t h a t o t h e r h e l i c o p t e r s t h e W H L proposal could possibly be f u r t h e r developed t o take account of such cases and it i s 'questionable whether a s i g n a l w i t h a r e p e t i t i o n rate, as is t h e case of t h e BO 105, of 28 Hz and "pulse frequency" of 600 Hz w i l l subjective11 sound t h e same as t h e blade s l a p generated by o t h e r h e l i c o p t e r s . It h a s a l s o n o t y e t been shown whether t h e standard PNL/dB(A) method f a i l s t o penalize such blade s l a p i n a similar manner t o t h a t found f o r Glade s l a p of t h e type used i n W H L h a s , however, not pursued t h i s method t h e s t u d i e s summarezed f n f i g u r e 4.

r e c e n t l y s i n c e c u r r e n t proposals by IS0 provide an adequate d e s c r i p t o r f o r b l a d e s l a p and t h e r e is an h t u i t i v e f e e l i n g - r e c e n t l y confirmed by preliminary

tests - t h a t t h e IS0 method could be used t o account for both t a i l

s u b j e c t i v e r o t o r n o i s e and b l a d e s l a p .

I f f i g u r e s 5 and I1 are examined i n d e t a i l , f t w k l l be observed t h a t t h e crest f a c t o r assocfated with t a i l r o t o r n o i s e is less than t h a t f o r blade s l a p .

This is a genuine e f f e c t and agrees w e l l with t h e real p r a c t i c a l s i t u a t i o n i n t h a t severe blade s l a p is always more pronounced and a n n o y h g than t h e c o k e s - ponding high l e v e l of t a i l r o t o r noise.

Subjective Evaluatfm Preliminary p sckoacoustlc tests have been conducted w i t h i n WHL using simulated and real h e l i c o p t e r steady state (hover) recordfngs ( r e f . 14). The This r e s u l t s obtained t o d a t e are shown i n terms of dB(A) values i n f f g u r e 13.

f i g u r e is d i r e c t l y comparable t o t h e b l a d e s l a p r e s u l t s shown i n f i g u r e 4 an?, as w i l l be noted, it suggests t h a t pronounced tail r o t o r n o i s e r e q u i r e s an addi- t i o n a l c o r r e c t i o n of 4 dB(A). It follows t h a t , t o a f i r s t o r d e r , t h i s is s i m - i l a r t o blade s l a p and t h e r e f o r e , by taking i n t o account t h a t t a i l r o t o r n o i s e g i v e s sli-ghtly lower crest f a c t o r than a s s o c i a t e d with blade s l a p , it can be argued t h a t an kmpulsive d e s c r l p t o r of t h e type proposed by IS0 can adequately I f t h i s approach w a s adopted, then account f o r both sources of impulsive noise.

obviously t h e conventfonal tone c o r r e c t i o n s would n o t b e required. The impul- s i v e r a t i n g procedures, as c u r r e n t l y envisaged, have a ' c u t o f f ' a t around 2 kHz and t h e l e v e l of t a i l r o t o r n o i s e is very low above 1 kHz. It would seemsappro- p r i a t e , t h e r e f o r e , t o l i m i t t h e tone c o r r e c t i o n procedure i n t h e present EPNL procedure t o , say, 1 kHz and above. This would provide, assuming an. impulsive impulsive n o i s e , n o i s e d e s c r i p t o r w a s used, a good measure of both sources of while ensuring t h a t high frequency d i s e r e t e s tones from t h e engine etc. were adequately covered and t h a t t a i l r o t o r n o i s e was n o t penalized twice.

THE REQUIREMENTS FOR NOISE STANDARDS t h e UK it would seem reasonable t o assume t h a t From t h e experience within t h e a i m of ' n o i s e c e r t i f i c a t i o n ' should b e , i n general, t o contain t h e c u r r e n t s i t u a t i o n s i n c e , u n l i k e CTOL a i r c r a f t , h e l i c o p t e r s do not cause any major n o i s e disturbance. There are, of course, a number of noisy h e l i c o p t e r s which are ' c e r t i f i c a t i o n ' lirrtits should be such exceptions t o t h i s g e n e r a l r u l e and hence as t o p r o h i b i t t h e i r development and ensure t h a t they are phased out of c i v i l use. Even so, t h e r e does n o t appear t o be any case f o r s e t t i n g standards which would r e q u i r e a dramatic reduction i n t h e n o i s e generated by t h e majority of h e l i c o p t e r s .

The h e l i c o p t e r s i n t h e s u b j e c t i v e l y 'noisy' category are u s u a l l y those which generate high levels of 'blade s l a p ' (impulsive main r o t o r n o i s e ) o r t o of t a i l r o t o r noise. It has been estab- a lesser e x t e n t t h o s e w i t h a high l e v e l l i s h e d t h a t t h e standard dBA and PNL (PNdB) u n i t s do n o t adequately account The p o s i t i o n r e l a t i n g t o t h e f o r blade s l a p and ' c o r r e c t i o n s ' are required.

s u b j e c t i v e impact of t a i l r o t o r n o i s e has n o t been s t u d i e d i n such depth, b u t it appears from t h e a v a i l a b l e evidence t h a t t h e standard procedures do n o t f u l l y quantify t h i s source even i f t h e 'tone c o r r e c t i o n procedure' i n t h e EPNL procedure is applied. It follows, t h e r e f o r e , t h a t it is completely f a l s e t o use a u n i t which does n o t t a k e t h e s e a s p e c t s f n t o account when s e t t i n g t h e a p p r o p r i a t e l i m i t .

Noise c e r t i f i c a t i o n is an important i s s u e and it is v i t a l t h a t care should be taken i n b o t h t h e s e l e c t i o n of t h e f l i g h t test conditions and t h e r a t h g u n i t s i n c e c e r t i f i c a t f o n w i l l Fiave a long term e f f e c t on t h e h e l i c o p t e r indus- It seems e s s e n t i a l t h a t a method and a r a t i n g u n i t t r y and t h e community.

which takes i n t o account t h e s u b j e c t i v e impact are derfved o r otherwise p u b l i c inadequate and then r e a c t i o n w i l l be based on t h e f e e l i n g t h a t c e r t i f i c a t i o n is numerous l o c a l rules w i l l be applfed. mese could be more severe than t h e cer- t i f i c a t i o n requirements and t h e i n d u s t r y would b e burdened by t h e need t o m e e t c o n f l i c t i n g requirements, It is, t h e r e f o r e , considered t h a t t h e c e r t i f i c a t i o n scheme should be based on a f u l l t e c h n i c a l evaluation of a l l t h e i s s u e s invol- ved including those o u t l f n e d i n t h i s paper.

CONCLUDING REMARIG Helicopter n o i s e is n o t t h e major problem o f t e n suggested, although ob- viously t h e r e are a number of nofsy h e l i c o p t e r s which give rise t o complaints.

and possibly lower Thus, it would appear t h a t although t h e r e is a need t o l i m i t , s l i g h t l y , t h e n o i s e l e v e l s a s s o c i a t e d with c u r r e n t h e l i c o p t e r s , t h e r e is l i t t l e j u s t i f i c a t i o n i n attempting t o o b t a i n a dramatic reduction. This is p a r t i c u - l a r l y t r u e i f t h e economic p e n a l t i e s involved i n o b t a i n n o i s e reductions on r e a l environment are h e l i c o p t e r s and t h e l i k e l y gains i n r e a l n o i s e terms i n taken i n t o account. Furthermore, it would a p p e a r from t h e experience of W H L t h a t many of t h e problems t h a t occur i n p r a c t i c e are a s s o c i a t e d almost e n t i r e l y with blade s l a p and/or high l e v e l s of t a i l r o t o r n o i s e which occur on approach during c r u i s e f l i g h t . I n simple terns t h e r e is ample evidence t o suggest t h a t a blade s l a p c o r r e c t i o n is required and t h a t when t h e blade s l a p is severe, t h i s should be 6 dB. A s i m i l a r s i t u a t i o n occurs i n t h e case of t h e t a i l r o t o r n o i s e and although preliminary r e s u l t s i n d i c a t e t h i s can be tackled i n a similar manner t o blade s l a p , it i s recognized t h a t it w i l l b e some t i m e b e f o r e such a correc- t i o n i s accepted. The main problem appears t o be, however, t h a t i f t h e s e cor- then s i n c e t h e r a t i n g r e c t i o n s a r e taken i n t o account on a "half second" b a s i s , methods are being based on t h e EPNL concept, they w i l l n o t account f o r t h e There is a l s o t h e p o s s i b i l i t y t h a t annoyance caused by h e l i c o p t e r s i n p r a c t i c e .

even i f c e r t i f i c a t i o n r e s u l t s i n a reduction of t h e m a x i m u m n o i s e emitted by a h e l i c o p t e r during f l y o v e r , t h e c h a r a c t e r ( c r e s t f a c t o r ) of t h e n o i s e generated on approach due t o blade s l a p and/or a high l e v e l of t a i l r o t o r n o i s e w i l l re- main t h e same. Thus, it is p o s s i b l e t h a t t h e annoyance t o t h e p u b l i c w i l l , f o r a l l p r a c t i c a l purposes, b e t h e same even though t h e a b s o l u t e l e v e l is reduced.

Added t o t h i s is t h e f a c t t h a t n e a r a h e l i p o r t t r a n s i e n t manoeuvre n o i s e (bank t u r n s , e t c . ) may still occur and s i n c e t o some e x t e n t t h e s e are independent of t h e maximum n o i s e generated by t h e h e l i c o p t e r , t h e impact of n o i s e c e r t i f i c a t i o n i n . t h i s case may again be s m a l l . This p a r t i c u l a r aspect can, however, b e con- t r o l l e d i n p r a c t i c e by l o c a l h e l i p o r t o r ATC r u l e s .

A review has been made of p o s s i b l e a l t e r n a t i v e methods of r a t i n g h e l i - copter nokse and t o d a t e W H L f r e e l y admits t h a t it has n o t y e t been a b l e t o devise a completely acceptable scheme. Aspects considered have included, i n t h e EPNL procedure, changing t h e c a l c u l a t i n g t h e t o t h e !maximum l e v e l - 20 dB' a correc- b u t this does n o t give any marked improvement and a method i n t i o n based on t h e crest f a c t o r of t h e signal 5n the f a r f i e l d d i t i o n ) is added t o t h e computed PNLM o r EPNL value. U s e of a l s o been evaluated, t o g e t h e r w i t h a number of 'ad hoc' allowances have been taken i n t o account f o r blade s l Since t h e r e is obviously d i f f i c u l t y at t h e p r e s e n t t l m e w5th t h e o v e r a l l r a t i n g procedure t o be used, t h i s again gives support t o t h e view t h a t c e r t i f i - c a t i o n should e s s e n t i a l l y p r w i d e a scheme which l i m i t s t h e use and develop- ment of very n o j s y "slapping" type h e l i c o p t e r s o r those with high l e v e l s of t a i l r o t o r n o i s e , r a t h e r than attempt an o v e r a l l reduction of h e l i c o p t e r noise.

I n t h i s context it is a l s o worth n o t i n g t h a t t h e p r e d i c t i o n of ' t o t a l h e l i - c o p t e r ' n o i s e is r e l a t 3 v e l y i n a c c u r a t e and less p r e c i s e than commonly a s s o c i a t e d w i t h f i x e d wing a i r c r a f t . This is understandable since( t h e research e f f o r t both i n terms of manpower and f i n a n c i a l resources has been s l g n i f i c a n t l y less than i n t h e case of CTOL a i r c r a f t . The main sources of r o t o r n o i s e are, how- e v e r , r e l a t f v e l y w e l l understood and it is t h e i n t e r a c t i o n e f f e c t s which cause problems during p r e d i c t i o n s . It a l s o implies t h a t t h e configuration (layout) of t h e h e l i c o p t e r has a s i g n i f i c a n t impact i n t h e r e s u l t i n g o v e r a l l noise. It follows, t h e r e f o r e , t h a t it is n o t p o s s i b l e t o design, w i t h i n t h e required accuracy, t o a s p e c i f i c l e v e l .

1, Greater London Council: Report (11.6.75) by t h e C o n t r o l l e r of Planning and Transportation. I t e m 11 P847.

F u l l e r H.C.: Rating Helicopter Noise - A Study of Subjective Reaction t o

2.

National P h y s i c a l Laboratory Report, November 1976.

Impulsive Sound.

3. Southwood B . J . , P i k e , A.C.: The Rating and Subjective Assessment of H e l i - copter Blade Slap. Westland Helicopters Ltd. (WHL) Applied Acoustics Department Note 1147, May 1976.

4. Damongest A: Gene Produit p a r les B r u i t s de Hature Impulsive. Aerospatiale November 1976.

Report H/DE - ER 351-67, Noise C e r t i f i c a t i o n Cor.siderations f o r Helicopters Based on Laboratory 5.

J u l y 1976.

I n v e s t i g a t i o n s - Man Acoustics Report.FAA-RD-76-116, Subjective Response t o Simulated and Actual Helicopter Noise.

6. Galloway W . J . : BBN Report 3573, December 1977.

7. Leverton J . W . , Southwood B . J . - A Correction f o r Helicopter Blade Slap.

W H L Applied Acoustics Department Note 1163, November 1976, 8. F i r s t D r a f t Proposal f o r Amendment t o IS0 3891 "Acoustics F Procedure f o r Describing Aircraft Noise Heard on t h e Ground!' --Measurement of Noise from Helicopters, ISO/TC 43/SC 1 ( S e c r e t a r i a t - 254) 356.

An Analogue Method f o r Quantifying Impulsive Pike A , C . , Leverton J . W . : 9 , Conference i n 'Zurich, Swit- Noise. Paper preeented at I n t e r n e i s e - 77.

. zerland, Marcti. 1977.

C-ents on BBN Report 3573 - Subjective Response t o

10. Southw,ood B. J. : Skmulated and Actual Helfcopter Noise, W€€L Applied Acoustics Department Note 1216, March 1978.

Psychoacoustic Studies of Impulsive Noise.

11. Wright D r . S.E., Damongeot A: ' Paper No.55 presented at t h e Third European Rotorcraft and Powered L i f t September 1977.

A i r c r a f t 'Forum, A i x e n Provence, France, 12. Laudien E, Huljer H.: Impulsive Helicopter Rotor Noise. Paper No. 2.4 presented a t t h e GARTEur - 5 S p e c i a l i s t Meeting on P r o p e l l e r and H e l i - copter Noise, Parfs, 1-2nd June 1977,

13. Leverton J . W . : Helicopter Noise - Blade Slap P a r t 2: Experimental Results

March 1972.

NASA Report CR-1983 " (Prepared by ISVR,' Southampton), 14, Southwood B . J . , Leverton J . W . , Pike A , C , : Rating T a i l Rotor Noise, Paper presented at the I O A Spring Conference, Cambridge, A p r i l 1978.

Figure 1.- Flyover t i m e h i s t o r y - 2 bladed main r o t o r h e l i c o p t e r

(UH-1B).

I m .

-10 -5 0 5

T I M € - 1 . (run ov:Rn#no W % W I ~ N )

(V107).

Figure 2.- Flyover time h i s t o r y - tandem r o t o r h e l i c o p t e r

-10 0 5 -2 5 -20 -IS OVCR CI En 0 T t M E AS. vomrront Figure 3.- Flyover t i m e history - Scout and Wessex.

Figure 4 . - Blade slap: subjective correction.

Figure 5.- C h a r a c t e r i s t i c s of impulse d e s c r i p t o r s - v a r i a t i o n with r e p e t i t i o n rate.

Figure 6.- Impact of blade s l a p d e s c r i p t o r s on EPNL ( i d e a l i s e d s i g n a l s ) .

s

E

..

\

P

r y 4 I n Y L

Q s 3

u

I

P

I

Figure 7 . - Impact of blade slap descriptors on EPNL - simulated

and real helicopter results.

BLADE S I R P I U L S E 2 S O M t ~ 4 n r &no€ ?Asshue INr€RVaI KAz 66 ~a TbiL ROTOR NOISE -4- 0.2- /UTEGRA nor4 PERtOO

I H - I n = 0.2ns

Figure 8.- Diagrammatic representation of blade slap and tail rotor noise.

HLC IcoQr&R w i r n TAIL Roroa NOISE (?AIL 90.3 P N d a PtvLr 9 2 P W B ) F E P 6 U E I N C Y-Uz Figure 9 . - octave band of helicopter noise.

Figure 10.- Flyover t i m e history. Scout helicopter wXth high level of t a i l rotor noise.

Figure 11.- V a r i a t i o n of main and t a i l r o t o r n o i s e w i t h i n t e g r a t i o n t i m e and d e s c r i p t o r .

. - -e I I Figure 12.- C h a r a c t e r i s t i c s of impulse d e s c r i p t o r s - v a r i a t i o n with p u l s e frequency.

Figure 1 3 . - Tail rotor noise. Subjective correction - preliminary

results.