Audio Engineering

10 Chapter 1

are conducted to the brain by the auditory nerve. Some of these signals refl ect the time
domain, particularly during the transients with which most real sounds begin and also
at low frequencies. During continuous sounds, the basilar membrane is also capable of
performing frequency analysis.

Figure 1.6(c) shows that the basilar membrane is not uniform, but tapers in width and
varies in thickness in the opposite sense to the taper of the cochlea. The part of the basilar
membrane that resonates as a result of an applied sound is a function of the frequency.
High frequencies cause resonance near the oval window, whereas low frequencies cause
resonances further away. More precisely, the distance from the apex where the maximum
resonance occurs is a logarithmic function of the frequency. Consequently, tones spaced
apart in octave steps will excite evenly spaced resonances in the basilar membrane. The
prediction of resonance at a particular location on the membrane is calledplace theory.
Essentially the basilar membrane is a mechanical frequency analyzer.

Reissner’s

membrane Tectorialmembrane

Hair

cells

Scala

media

Basilar

Scala membrane

tympani

Auditory

nerve

(a) (b)

(c)

Round

window

Basal

end

20 kHz

10 kHz 1 kHz 100 kHz 20 kHz

Apical

end

Oval

window

Helicotrema

Scala

vestibuli

Figure 1.6 : (a) The cochlea is a tapering spiral cavity. (b) The cross section of the cavity is

divided by Reissner’s membrane and the basilar membrane. (c) The basilar

membrane tapers so that its resonant frequency changes along its length.