Periodic and Aperiodic Sounds
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P er i o d i c an d ap eri o d i c s ou n d s 1. So u n d i s creat ed b y a d ist u rban ce t r av el l in g i n an el ast i c m ed i u m l i k e ai r an d w ater. Fo r instance, w h en an excess pressure is prod u ced o n som e r eg i o n of t h e air , t h at region t en d s to exp an d t ow ards t h e nei gh bo u r i n g zon es. Tis, i n t u r n , com p resses t h ose zon es, creati n g a new excess pres sure w h i ch wi l l ten d t o exp an d n ext, an d , ag ai n , a n ew excess pres sure i s f u r t h er cr eat ed . Te p r essure d ist u rban ce w i l l t h u s prop ag ate t h r o u g h t he air , and ev en t u all y i t w i l l reach some recei ve r ( f o r inst an ce a m icr o p h o ne or an ear) . E xcess p ress u r e i s called sou n d p res sure( see L ad ef o g ed ,1996: 3 ) . 2. Ti s ki n d o f m ovem en t i n w h i ch i t is n ot t h e m edi u m i t s el f bu t s om e di s t u r ban ce w h at is trave l lin g , is cal l ed a w ave. Ter e are m an y ot h er t y p es of w av es, such as r ad io w ave s, l i gh t, h eat radiati o n , t he ri p p les on t h e su rface of a l ak e, eart h q u ak es, etc. Wh en t h e wave t ak es p l ace in a l i qu i d or gaseou s m edi u m ( excep t s u r f ace w aves) , t h e w ave is c al l ed an acou s t ic wave. Wh en a w av e i s au d ible, it is cal l ed a so u n d w ave. 3. M ost w ave s are t h e r esul t o f m an y successi ve d ist u r ba nces of t h e m ed i u m , i n s t ead o f on l y o n e. When t hose dist u r ba n ces are gen erat ed at r egu lar int erval s an d are al l t h e same s h ap e we are i n t h e p r esen ce o f a p er i o d i c wave, an d t h e n u m b er of d i st u r ba n ces per u n i t t i m e is c al l ed t h e f r eq u en cy o f t h e wave. I t i s exp r essed i n a u n i t called H ertz (Hz) , m ean i n g cy cles per s eco n d (a cy cl e i s all t h at h ap p en s i n b et w een a d ist u rban ce) . I n t h e case o f sou n d waves, f r eq u en cy is bet w ee n 20 H z and 20, 000 H z. 4. A cou stic w av es of f r eq u en cy sm al l er t h an 20 Hz ar e cal l ed i n f r asou n d s, an d t h ose o f f r eq u en cy gr eat er t h an 20,000 H z ar e cal l ed u l tr asou n d s. N ei t h er of t h emcan o r di nari l y b een h eard b y h u m ans. Several an i m als ( such as t h e do g , for i n stan ce) can h ear very l ow f r eq u en cy sou n d s, such as t h ose creat ed b y groun d w aves du r i n g an eart h qu ake. 5. E ven i f t h er e ar e m an y s ou n ds w h i ch are nearl y p eriodic, such as t h ose s ou n ds p rod u ced b y pi t ch ed m u si cal i n st rum en ts, t h e v ast m aj o ri ty o f sou n d s in N at u re ar e a p eri o d i c, t h at is, successi ve dist u r ba n ces ar e n ot equal l y spaced in t i m e, an d ar e n ot o f con st an t sha p e ei t her. Tis i s w h at i n a t ech n i cal sense is called n oise. A p eri o d i c w av es u sual l y can n ot con vey t h e sensat i o n of p it ch . So m e exam p l es are t h e con sonan t s of speech , u r ban n oi se , t h e noise o f t h e w i n d an d t h e sea, and t h e sou nd o f m an y p erc u ss i ve i n str u m ents such as dr u m s, char l est on s, etc. 6. P E R IO D ( n .) A ter m deri ved f r om t h e st u d y of t h e p h ysi cs of s ou n d, and u sed in acou stic p h o n eti cs, ref erri n g to t he t i m e i t t ak es f o r a cy cl e o f p r essure v ariation i n a sou n d wave to r ep eat it sel f regu l arl y o ve r an d o ve r. Te sh o rt er t h e p erio d , t h e m o r e cy cl es t h ere w i l l b e i n a g i ve n u n it of t i m e, and t h u s t h e h i gher t h e fr equ en cy. W avef orm s w h i ch sh ow a repeating p a er n of vibrat i on ar e p er i od i c waves; t h ose w h i ch d o n ot ar e ap er i od ic. Sp eech m ak es use of bot h t y p es of w av ef orm : vo w el sou n d s ha ve p er i od i c w av ef orm s; f r icat i ves, f o r exam ple, i n vo l ve aperi od i c w av ef orm s. C ryst al ( 2008:357 ) 7. Sp eech com m u n icat i on i n vo l ves bo t h p eri od i c an d ap er i od i c sou nds wh i ch ar e characterized, r esp ectivel y, b y t h e p resence an d t h e ab sen ce o f t h e p erio d i c aco u stical excitation p rod u ced b y t h e vi brat i on o f t h e v o cal f ol d s i n t h e h u man l aryn x. D ep en di n g on t h e v o cal f ol d f u n ct i on , t h e d eg r ee of peri od ici t y m ay vary even w it h i n s peech sounds belon gi n g to the same ph on em i c ca tego r y. 8. M any speech sou n ds, such as vo w el s, t h at t y p i cal l y o ccu r i n p eri od i c f orm s c an al so b e p rod u ced w i t h o u t p erio d ic vi brat ion o f t h e vo cal f o l d s ( e. g ., in w h i spe red sp eech ) . Te d eg ree of p eri o d icit y al so varies in vo w el s o f n at u ral spe ech d u e to a spi rati o n n o i se an d t h e irr eg u l ar o scillat i o n of t h e vocal f o l d s t h at t ak es p lace, f o r ex am p l e, i n t h e p rod u ct i o n of creaky v o ices. I n ad d i t i o n , t h e d eg ree o f sou n d p eri o dici t y a l lows l i n g u i stic di ff eren tiatio n b et w ee n cert ai n
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Transcript of Periodic and Aperiodic Sounds
8/15/2019 Periodic and Aperiodic Sounds
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Periodic an d ap eriodic s ounds
1. Sound is cr eated by a d isturban ce t ravelling in an elastic m edium like ai r an d water. Forinstance, when an excess pres sure i s prod uced on some r egion of t he ai r, that region tends t oexpand towards t he n eighbouring zones. Tis, in turn, compresses t hose zon es, creating a newexcess pres sure w hich will tend to expand next, and, again, a n ew excess pres sure i s f urther
created. Te p ressure d isturban ce w ill thus prop agate t hrough the ai r, and eventually it will reachsome r eceiver (for instance a m icrophone o r an ear). Excess pressure i s called sound p ressure( seeLadefoged,1996:3).
2. Tis kind o f movement in which i t is not the m edium itself but some di sturbance w hat istravelling, is cal led a w ave. Tere are m any other t ypes of w aves, such as r adio waves, light, heatradi ation, the ri pples on the su rface o f a l ake, eart hquakes, etc. When the w ave t akes p lace i n aliquid or gaseou s m edium (except surface w aves), the w ave i s c alled an acoustic w ave. When awave i s au dible, it is cal led a so und wave.
3. Most waves ar e t he r esult of many success ive d isturbances of the m edium, instead of only o ne.
When those di sturbances are generated at regular interval s and are al l the s ame s hape w e ar e i nthe p resence o f a p eriodic w ave, and the n umber of disturbances per u nit time i s c alled thefrequency of the w ave. It is exp ressed i n a u nit called Hertz ( Hz), meaning cy cles p er s econd (acycle i s al l that happens i n between a d isturban ce). In the case o f sound waves, frequency isbetween 20 H z and 20, 000 Hz.
4. Acoustic w aves of frequency sm aller than 20 H z ar e cal led infrasounds, and t hose o f frequencygreater t han 20,000 Hz ar e cal led ultrasounds. Neither of them can ordinarily been heard b yhumans. Several animals (such as t he do g, for i nstance) can hear very l ow frequency sou nds, suchas those c reated b y ground w aves du ring an earthquake.
5. Even if there ar e m any sounds w hich are nearl y periodic, such as t hose s ounds p roduced b ypitched musical instruments, the v ast majority of sounds i n Nature a re a periodic, that is,success ive disturbances ar e n ot equ ally spaced in time, and are n ot of constant shape ei ther. Tisis w hat in a t echnical sense i s cal led noise. Aperiodic w aves u sually cannot con vey the sen sationof pitch. Some exam ples ar e t he con sonants of speech , urban noise, the n oise o f the w ind and thesea, and t he sou nd o f many p ercussive i nstruments such as dr ums, charlestons, etc.
6. PERIOD (n.) A term derived from the st udy of the p hysics of sound, and used in acousticphonetics, referring to the t ime i t takes f or a cy cle o f pressure v ariation in a so und wave t o repeatitself regu larly over an d over. Te sh orter t he p eriod, the m ore cy cles t here w ill be i n a g iven unitof time, and thus t he h igher the f requency. Waveforms w hich sh ow a r epeating p aern ofvibration are p eriodic w aves; those w hich do not ar e ap eriodic. Speech makes use of bot h types ofwaveform: vowel sounds ha ve p eriodic w aveforms; fricatives, for exam ple, involve aperiodicwaveforms. Cryst al(2008:357)
7. Speech communication involves bo th periodic an d aperiodic sou nds w hich are ch aracterized,respectively, by the p resence an d the ab sence o f the p eriodic aco ustical excitation produced by thevibrat ion of the v ocal folds i n the h uman laryn x. Depending on the v ocal fold function, the d egreeof periodicity may vary even within speech sounds bel onging t o t he s ame ph onemic ca tegory.
8. Many speech sounds, such as vo wels, that typically occur in periodic f orms c an also b eproduced without periodic vi brat ion of the vo cal folds ( e.g., in whispered sp eech ). Te d egree ofperiodicity also varies i n vowels o f natural speech due to a spiration noise an d the i rregularoscillation of the v ocal folds t hat t akes p lace, for ex ample, in the p roduction of creak y voices. Inaddition, the d egree o f sound p eriodicity a llows linguistic di fferen tiation between cert ain
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