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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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