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Introduction

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Introduction

  1. 1. I n t r o d u c t i o n Sound is s u c h a c o m m o n part of e v e r y d a y life that we rarely a p p r e c i a t e all its f u n c t i o n s . l t provides e x p e r i e n c e s s u c h as listening to music, it enables us spoken c o m m u n i c a t i o n , it a l e r t s and warns us. Sound permits us to m a k e q u a l i t y e v a l u a t i o n s and d i a g n o s e s - the rattling of a loose bolt, the squeaking of a bad bearing, the heart b e a t etc. Yet, too o f t e n in our m o d e r n society, sound annoys us. Many s o u n d s a r e u n p l e a s a n t or u n w a n t e d - these are ca 1 led noise. To a set of rock f a n s the t h u n d e r i n g sounds of a rock c o n c e r t a r e m u s i c but it a n n o y s p e o p l e living c l o s e by. S o u n d s do not n e c e s s a r i l y have to be loud to annoy. A c r e a k i n g floor or a d r i p p i n g tap is equally annoying as a jet taking off. What is m e d i c a l l y h a r m f u l to us is the c o n t i n u e d e x p o s u r e to h i g h levels of noise. T h i s results in a n n o y a n c e , f a t i g u e , t e m p o r a r y shift of the threshold of hearing , and p e r m a n e n t h e a r i n g loss .There is an increasing m e d i c a l e v i d e n c e that noise e x p o s u r e c a u s e s p h y s i o l o g i c a l d i s t u r b a n c e s , m e t a b o l i c d i s o r d e r s blood c i r c u l a t i o n c h a n g e s etc. S t a n d a r d s o n a l l o w a b l e n o i s e levels e x i s t worldwide. T h e r e are Indian S t a n d a r d s on this s u b j e c t also. The C e n t r a l P o l l u t i o n C o n t r o l B o a r d has d o n e an e x t e n s i v e study and d a t a c o l l e c t i o n on this s u b j e c t and have made a d e t a i l e d report. T h e C o m m i t t e e on N o i s e P o l l u t i o n Control has identified i n d u s t r i a l n o i s e , a u t o m o t i v e noise, d o m e s t i c a p p l i a n c e n o i s e and p u b l i c a d d r e s s s y s t e m noise as the major c a t e g o r i e s . L i m i t s have been set for all the c a t e g o r i e s and for s o m e c a t e g o r i e s these limits shail be lowered e v e r y few y e a r s to a l l o w m a n u f a c t u r e r s , R D in noise c o n t r o l to i m p r o v e q u a l i t y . P r o p a g a t i o n of S o u n d i n Air. Sound may be d e f i n e d as any p r e s s u r e v a r i a t i o n (in air , water or any m e d i u m ) that the ear can d e t e c t . P r e s s u r e v a r i a t i o n s of air that a r e c a u s e d by the c h a n g i n g w e a t h e r c o n d i t i o n s a r e far too s l o w for the h u m a n ear to detect But air p r e s s u r e c h a n g e s that occur m o r e rapidly - at least 20 times a s e c o n d - can be heard - thus ca! ied sound. A v a r i a t i o n of 20 t i m e s a s e c o n d is called 20 H e r t z ( H z . ) A f r e q u e n c y or v a r i a t i o n of 2 0 , 0 0 0 times a s e c c r, d is c a l l e d 2 0 , 0 0 0 Hz or 20 K Hz. T h e s e are sounds having a s i n g l e f r e q u e n c y and a r e c a l l e d a pure tone.
  2. 2. W Ikl D V E L Q C ^ r w; :ijPft! .! t H j! |: .fi'I' c./ / r c s / Soo"CE KS O VJ'v:^, AJ - V/jikjD VE t O C v c r M E L O c n X OP fZA"; (2 = ICLCCi'Y "S^CWt- 'H WOVK'O t XXJ*JEs P R O R & O M O U B E T W B E M L A Y E R S Of" C^FFECJEKTT V E U X l T t . r j I ij D tP-et u M T U P H F T E & E K I T V E L O C T T I C ^ .
  3. 3. T ^ v a P ^ MOP^ual •. O O E T O I E K P P-e-axiuq w r m UE^OUT •z INX/e^T : T E M P TO A. M E ^ C ^ T AVtO T M E N N O R M A L E E D O C T I O M VXTH HEIGHT •^•COO&US iviV/EC-T'. " D U B "TO A P c O B L - E T t V A P
  4. 4. T h e r e are many factors w h i c h s i g n i f i c a n t l y affect the p r o p a g a t i o n of sound in the a t m o s p h e r e . Uind velocity and t e m p e r a t u r e c h a n g e s a l t e r w a v e d i r e c t i o n s , turbulence d i s t o r t s it and v i s c o s i t y c a u s e s a b s o r p t i o n . A b s o r p t i o n by air is far g r e a t e r for high frequency than for low f r e q u e n c i e s . T h e a t m o s p h e r e t h e r e f o r e attenuates high f r e q u e n c i e s and thus d i s t o r t s the spectrum of noise, it r e d u c e s its s t r e n g t h and c h a n g e s the p r o p a g a t i o n path. In a d d i t i o n most m e a s u r e m e n t s a r e m a d e at ground level where p e o p l e live and w o r k and w h e r e noise is produced and received. For these r e a s o n s the r e f l e c t i o n and a b s o r p t i o n of the ground under the path b e t w e e n s o u r c e and receiver is very important, and m u s t be taken into account as a matter of c o u r s e when s t u d y i n g the t r a n s m i s s i o n of sound out doors. W i rtd W h e n a. sound w a v e i m p i n g e s on a layer of air which has a d i f f e r e n t s p e e d , the w a v e ' s direction of travel c h a n g e s . This h a p p e n s b e c a u s e the speed of sound d e p e n d s only upon the m e d i u m in w h i c h it propagates, so any m o v e m e n t of that m e d i u m will i m p o s e a similar movement on the s o u n d . If it has a c o m p o n e n t in the same d i r e c t i o n as the wind, the sound w a v e is r e f r a c t e d towards the i n t e r f a c e b e t w e e n the two d i f f e r e n t v e l o c i t y regions when entering that w i t h the h i g h e r s p e e d , or away from the interface w h e n e n t e r i n g a r e g i o n of lower speed. The e f f e c t s a r e simply r e v e r s e d if the d i r e c t i o n of sound p r o p a g a t i o n is o p p o s i t e to that of the wind. T e m p e r a t u r e The v e l o c i t y of s o u n d in air increases with t e m p e r a t u r e , and in a normal a t m o s p h e r e the t e m p e r a t u r e itself d e c r e a s e s w i t h h e i g h t . A r i s i n g sound ray, on e n t e r i n g a layer w i t h lower t e m p e r a t u r e u n d e r g o e s a r e d u c t i o n in p r o p a g a t i o n v e l o c i t y and in refracted away from the i n t e r f a c e b e t w e e n the two layers. The result is that in the a b s e n c e of w i n d the r a y s a r e c o n t i n u o u s l y bent away from the ground s u r f a c e . H u m i d i t y and P r e c i p i t a t i o n s :- T h e a b s o r p t i o n of sound in air varies w i t h f r e q u e n c y , h u m i d i t y and t e m p e r a t u r e and in e x t r e m e l y c o m p l i c a t e d . T H e o n l y general trend is that a b s o r p t i o n is h i g h e r at h i g h f r e q u e n c i e s and shows a tendency to i n c r e a s e w i t h t e m p e r a t u r e and to d e c r e a s e w i t h higher r e l a t i v e h u m i d i t y . A b s o r p t i o n by natural f e a t u r e s if the ground s u r f a c e is p e r f e c t l y flat and r e f l e c t i n g the w a v e would p r o p a g a t e w i t h o u t any a b s o r p t i o n . In real life this does not h a p p e n as most natural ground has s i g n i f i c a n t a b s o r p t i o n . The a t t e n u a t i o n is h i g h e r at h i g h e r f r e q u e n c i e s and d e p e n d s on the r e l a t i v e r o u g h n e s s of the s u r f a c e . The values are low for o r d i n a r y grass lands b u t v a r y and rise upto 20 ti 8 per 100 m e t e r s for long g r a s s , p a d d y , s h r u b s and trees.
  5. 5. Soorclie & r r c - "' kJC — .! - ' / I WACvE. Ps^FUE.CTlC>M^ R O U A. PuAtMH. ^ U R F A ^ E . REFLEiCTIOM t=?XsM < 3 0 0 * = fc»lt=f= E(££ MT <^lU-PE.<~v KETO*-H fROU RAGUT •vT^MCit-iq wave.'
  6. 6. original A^LD ^erco t>*eY (MTeeteec A.T PoiKJT 6- „ P 1 F F P A C T I 0 N £k~T U 3 W P F ^ S i . fJEW WAVE FROtJT^ S f R E A D SPHtRlOU-Y R4,.<bT T U t O B ^ T A C L E - m i l l ] 1 1 ) ' / / I I I THCOOCtH AH OPEUIUC, IT F t e w ^ a P i f f f w t t i o k j A T UICW F K E ^ .
  7. 7. - J
  8. 8. Reflection :- W h e n sound waves c o m e into c o n t a c t with a s u r f a c e , p a r t of the energy is reflected and part of it is absorbed and part transmitted through it d e p e n d i n g on the absorption and t r a n s m i s s i o n c o e f f i c i e n t s . T h e instantaneous sound p r e s s u r e is therefore d e p e n d e n t on the d i r e c t and ref1ected fields. The effect of curved and fiat r e f l e c t i n g s u r f a c e s on sound is s i m i l a r to that as on 1 i g h t . C o n c a v e s u r f a c e s reflect sound and focus it, w h i l e c o n v e x s u r f a c e s d i s p e r s e sound . R e f l e c t i o n from a right a n g l e s u r f a c e has the effect of r e t u r n i n g the wave at the s a m e angle but d i sp1aced . Two parallel r e f l e c t i n g s u r f a c e s c a u s e two important effects. F i r s t l y the f o r m a t i o n of s t a n d i n g waves, jeading to large v a r i a t i o n in p r e s s u r e from n o d e to a n t i n o d e . The second e f f e c t is a flutter echo, is c a u s e d by the continuous and regular r e f l e c t i o n s of a pulse from a parallel s u r f a c e w i t h low a b s o r p t i o n . - / i D i f f r a c t i o n ;- W h e n a sound w a v e e n c o u n t e r s an o b s t a c l e which is small in relation to its w a v e l e n g t h , the w a v e passes round it a l m o s t as if it did not e x i s t f o r m i n g very little s h a d o w . But if the f r e q u e n c y of the sound is sufficiently h i g h and the w a v e l e n g t h is t h e r e f o r e short, a n o t i c e a b l e s h a d o w if formed. A large ratio of w a v e l e n g t h to o b s t a c l e s i z e c a u s e s the d i f f r a c t i o n p a t t e r n as shown. A smal1 v a l u e leads to the formation of a m o r e d i s t i n c t s h a d o w b e h i n d the barrier or a beam of sound through an o p e n i n g . The g r e a t e s t a t t e n u a t i o n behind a b a r r i e r is w h e n the barrier is as near as p o s s i b l e to e i t h e r the s o u r c e or the recei ver. When taking m e a s u r e m e n t s of n o i s e s o u r c e s in unobstructed s i t u a t i o n s a r e to be c h o s e n barrier e f f e c t is so d e s i r e d . the field, u n l e s s the A b s o r p t i o n :- certain a m o u n t absor pt ion roughness absorption abso r pt i on represents absor ption sound w a v e m e e t s a s u r f a c e a is lost or a b s o r b e d . T h e d e p e n d s o n the m a t e r i a l ' s of 1 ed W h e n ever a of energy of a s u r f a c e p o r o s i t y , f l e x i b i l i t y etc. The e f f i c i e n c y is e x p r e s s e d in a n u m b e r f r o m 0 to 1 ca c o e f f i c i e n t . 0 r e p r e s e n t s no a b s o r p t i o n and 1 p e r f e c t a b s o r p t i o n . M o s t m e c h a n i s m s of are f r e q u e n c y d e p e n d e n t .
  9. 9. T H E D E C I B E L ( d B ) T H E D E C I B E L IS A L O G RfiTIO B E T W E E N T H E M E A S U R E D Q U A N T I T Y A N D f t R E F E R E N C E M E A S U R E D Q U A N T I T Y = 2 0 0 0 - F a R E F E R E N C E = 20 - Pa D E C I B E L S = 2 0 L o g 2 0 0 0 ^ P a 1 0 2 0 nP a = 2 0 L o g < 1 0 0 ) 1 0 = 2 0 X 2 = 4 0 dB- S I M I L A R L Y A U A R I A T I O N O F 1 , 0 0 0 , 0 0 0 T I M E S I S O N L Y 1 2 0 d B . Fig.l

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