Psychoacoustics 47consume a lot of energy and nutrition, they are usually
damaged first due to loud sound or ototoxic medicines
(i.e., medicine that is harmful to the auditory system).
Not only does this kind of hearing loss make our hearing
less sensitive, it also makes our hearing less sharp. Thus,
as is easily confirmed with hearing-loss patients, simply
adding an extra gain with hearing aids would not totally
solve the problem.
3.3 Frequency Selectivity
3.3.1 Frequency Tuning
As discussed in Section 3.2.5, the inner hair cells are
sharply tuned to the characteristic frequencies with help
from the outer hair cells. This tuning character is also
conserved by the auditory neurons connecting to the
inner hair cells. However, this tuning feature varies with
level. Fig. 3-10 shows a characteristic diagram of tuning
curves from a particular location on the basilar mem-
brane at various levels. As can be seen in this graph, as
level increases, the tuning curve becomes broader, indi-
cating less frequency selectivity. Thus, in order to hear
music more sharply, one should play back at a relatively
low level. Moreover, above 60 dB, as level increases,
the characteristic frequency decreases. Therefore when
one hears a tone at a high level, a neuron that is nor-
mally tuned at a higher characteristic frequency is now
best tuned to the tone. Because eventually the brain per-
ceives pitch based on neuron input, at high levels, with-
out knowing that the characteristic frequency has
decreased, the brain hears the pitch to be sharp.
Armed with this knowledge, one would think that
someone who was engaged in critical listening—a
recording engineer, for example—would choose to
listen at moderate to low levels. Why then do so manyFigure 3-7. A traveling wave on the basilar membrane of
the inner ear. (After von Békésy, Reference 13.) The exag-
gerated amplitude of the basilar membrane for a 200 Hz
wave traveling from left to right is shown at A. The same
wave 1.25 ms later is shown at B. These traveling 200 Hz
waves all fall within the envelope at C.
AmplitudeA.B.C.20 22 24 26 28 30 321.25 msDistance from oval window—mmFigure 3-8. An illustration of vibration patterns of the hair
cells on the basilar membrane for various incident pure
tones. There is a localized peak response for each
audible frequency. (After von Békésy, Reference 13.)AmplitudeAmplitudeAmplitudeAmplitude25 Hz100 Hz400 Hz1600 HzDistance from oval window}mmDistance from oval window}mmDistance from oval window}mmDistance from oval window}mm0 10 20 300 10 20 300 10 20 300 10 20 30