Tutorials and Topics - Peabody Computer Music

Tutorial 29Processing:FlangeWhile this avoids the click that could be caused by a sudden discontinuity, it does meanthat the pitch of the output signal will change while the delay time is being changed,emulating the Doppler effectTechnical detail: The Doppler effect occurs when a sound source ismoving toward or away from the listener. The moving sound source is, tosome extent, outrunning the wavefronts of the sound it is producing. Thatchanges the frequency at which the listener receives the wavefronts, thuschanging the perceived pitch. If the sound source is moving toward thelistener, wavefronts arrive at the listener with a slightly greater frequencythan they are actually being produced by the source. Conversely, if thesound source is moving away from the listener, the wavefronts arrive atthe listener slightly less frequently than they are actually being produced.The classic case of Doppler effect is the sound of an ambulance siren. Asthe ambulance passes you, it changes from moving toward you (producingan increase in received frequency) to moving away from you (producing adecrease in received frequency). You perceive this as a swift drop in theperceived pitch of the siren.A delayed signal emulates a reflection of the sound wave. As the delay timedecreases, it is as if the (virtual) reflecting wall were moving toward you.The source of the delayed sound (the reflecting wall) is “moving towardyou”, causing an increase in the received frequency of the sound. As thedelay time increases, the reverse is true; the source of the delayed sound iseffectively moving away from you. That is why, during the time when thedelay time is actually changing, the perceived pitch of the output soundchanges.A delayed signal emulates a reflection of the sound wave. As the delay time decreases, it isas if the (virtual) reflecting wall were moving toward you. The source of the delayedsound (the reflecting wall) is “moving toward you”, causing an increase in the receivedfrequency of the sound. As the delay time increases, the reverse is true; the source of thedelayed sound is effectively moving away from you. That is why, during the time whenthe delay time is actually changing, the perceived pitch of the output sound changes.A pitch shift due to Doppler effect is usually less disruptive than a click that’s caused bydiscontinuity of amplitude. More importantly, the pitch variance that results fromcontinuously varying the delay time can be used to create some interesting effects.228