Audio Engineering

Audio Principles 17

The term used in psychoacoustics to describe the fi nite width of the vibration envelope
iscritical bandwidth. Critical bands were fi rst described by Fletcher.^5 The envelope of
basilar vibration is a complicated function. It is clear from the mechanism that the area
of the membrane involved will increase as the sound level rises. Figure 1.11 shows the
bandwidth as a function of level.

As seen elsewhere, transform theory teaches that the higher the frequency resolution of
a transform, the worse the time accuracy. As the basilar membrane has fi nite frequency

Figure 1.10 : The basilar membrane symbolically uncoiled. (a) Single frequency

causes the vibration envelope shown. (b) Changing the frequency moves the

peak of the envelope.

Figure 1.11 : The critical bandwidth changes with SPL.