Audio Principles 9joint. As the area of the tympanic membrane is greater than that of the oval window, there
is further multiplication of the available force. Consequently, small pressures over the
large area of the tympanic membrane are converted to high pressures over the small area
of the oval window.
The middle ear is normally sealed, but ambient pressure changes will cause static
pressure on the tympanic membrane, which is painful. The pressure is relieved by the
Eustachian tube, which opens involuntarily while swallowing. The Eustachian tubes
open into the cavities of the head and must normally be closed to avoid one’s own speech
appearing deafeningly loud.
The ossicles are located by minute muscles, which are normally relaxed. However,
the middle ear refl ex is an involuntary tightening of the tensor tympani and stapedius
muscles, which heavily damp the ability of the tympanic membrane and the stapes to
transmit sound by about 12 dB at frequencies below 1 kHz. The main function of this
refl ex is to reduce the audibility of one’s own speech. However, loud sounds will also
trigger this refl ex, which takes some 60 to 120 ms to occur, too late to protect against
transients such as gunfi re.
1.5 The Cochlea
The cochlea, shown in Figure 1.6(a) , is a tapering spiral cavity within bony walls, which
is fi lled with fl uid. The widest part, near the oval window, is called the base and the
distant end is theapex. Figure 1.6(b) shows that the cochlea is divided lengthwise into
three volumes by Reissner’s membrane and the basilar membrane. The scala vestibuli
and thescala tympani are connected by a small aperture at the apex of the cochlea known
as thehelicotrema. Vibrations from the stapes are transferred to the oval window and
become fl uid pressure variations, which are relieved by the fl exing of the round window.
Essentially the basilar membrane is in series with the fl uid motion and is driven by it
except at very low frequencies where the fl uid fl ows through the helicotrema, bypassing
the basilar membrane.
The vibration of the basilar membrane is sensed by the organ of Corti, which runs along
the center of the cochlea. The organ of Corti is active in that it contains elements that can
generate vibration as well as sense it. These are connected in a regenerative fashion so
that theQ factor, or frequency selectivity of the ear, is higher than it would otherwise be.
The defl ection of hair cells in the organ of Corti triggers nerve fi rings and these signals