Page 71 - Master Handbook of Acoustics

FIGURE 4-2 The auditory canal, closed at one end by the eardrum, acts as a quarter-wavelength

pipe. Resonance provides acoustical amplification for voice frequencies.

The acoustical similarity of this ear canal to an organ pipe is apparent. As with a pipe closed at
one end, the resonance effect of the ear canal increases sound pressure at the eardrum at certain
frequencies. The maximum increase occurs at the frequency at which the 2.5-cm pipe is one-quarter
wavelength—about 3,000 Hz.
Figure 4-3 shows the increase in sound pressure at the eardrum over that at the opening of the ear
canal. A primary peak is noted around 3,000 Hz caused by the quarter-wave pipe resonance effect.

The primary pipe resonance amplifies the sound pressure at the eardrum by approximately 12 dB at
the major resonance at about 4,000 Hz. There is a secondary resonance nearer 9,000 Hz of lower
peak pressure. In addition, a plane wave striking the front of the head will be diffracted; this
diffraction also increases sound pressure at the ears at midrange frequencies. These effects combine
to make the ear most sensitive to midrange frequencies, the same frequencies occupied by speech.
Unfortunately, these same resonances also make the ear relatively more susceptible to hearing loss at

these important midrange frequencies.

FIGURE 4-3 The transfer function (frequency response) of the ear canal shows the effect of
resonance. This is a fixed component that is combined with every directionally encoded sound
reaching the eardrum. See also Figs. 4-15 and 4-16. (Mehrgardt and Mellert.)

The Middle Ear

Transmitting sound energy from a tenuous medium such as air into a dense medium like water poses
challenges. Without some transfer mechanism, sound originating in air reflects on water like light on a

mirror. For efficient energy transfer, the two impedances must be matched; in this case the impedance
ratio is about 4,000:1. It would be very unsatisfactory to drive the 1-Ω voice coil of a loudspeaker
with an amplifier having an output impedance of 4,000 Ω; not much power would be transferred.