Page 104 - The Master Handbook Of Acoustics

79
THE EAR AND THE PERCEPTION OF SOUND

constellation, is that of a 50-ish sound mixer in a recording studio. The
indications are that this loss, centered on 4 kHz, is the accumulation of
many years of listening to high-level sounds in the control room.

Summary

The ear is sensitive enough to hear the tattoo of air particles on
the eardrums in the quietude of an anechoic chamber.

The auditory canal, acting as a quarter-wave pipe closed at one
end by the eardrum, contributes an acoustical amplification of
about 10 dB, and the head diffraction effect produces another 10
dB near 3 kHz. These are vital speech frequencies.

The leverage of the ossicle bones of the middle ear and the ratio
of areas of the eardrum and oval window successfully match the
impedance of air to the fluid of the inner ear.
The Eustachian tube and round window provide pressure
release and equalization with atmospheric pressure.

Waves set up in the inner ear by vibration of the oval window
excite the sensory hair cells, which are connected to the brain.
There is a “place effect,” the peak of hair cell agitation for higher
frequencies being nearer the oval window, and low frequencies
at the distal end.

The area of audibility is bounded by two threshold curves, the
threshold of audibility at the lower extreme and the threshold
of feeling or pain at the loud extreme. Our entire auditory
experience occurs within these two extremes.

The loudness of tone bursts decreases as the length of the burst
is decreased. Bursts greater than 200 msec have full loudness,
indicating a time constant of the ear at about 100 msec.

Our ears are capable of accurately locating the direction of a
source in the horizontal plane. In a vertical median plane, how-
ever, localization ability is less accurate.