The Special Senses
Fibers of basilar membrane
High-frequency sounds displace the basilar
membrane near the base.
(a) Route of sound waves through the ear
(b) Different sound frequencies cross the basilar membrane at
Medium-frequency sounds displace the basilar
membrane near the middle.
Low-frequency sounds displace the
basilar membrane near the apex.
Sound waves vibrate the
Auditory ossicles vibrate.
Pressure is ampliﬁed.
Pressure waves created by the
stapes pushing on the oval window
move through ﬂuid in the scala
Sounds with frequencies below
hearing travel through the
helicotrema and do not excite hair
Sounds in the hearing range go
through the cochlear duct, vibrating
the basilar membrane and
deﬂecting hairs on inner hair cells.
Pathway of sound waves and resonance of
the basilar membrane.
The cochlea is depicted as if uncoiled. The
graph in the bottom panel of (b) represents ﬁbers that span the
width of the basilar membrane. The stiffness of these ﬁbers “tunes”
speciﬁc regions of the basilar membrane to vibrate at speciﬁc
Transmission of Sound to the Internal Ear
Hearing occurs when the auditory area of the temporal lobe
cortex is stimulated. However, before this can happen, sound
waves must be propagated through air, membranes, bones,
and ﬂuids to stimulate receptor cells in the spiral organ
Airborne sound entering the external acoustic meatus strikes
the tympanic membrane and sets it vibrating at the same fre-
quency. Te greater the intensity, the farther the membrane is
displaced in its vibratory motion. Te motion of the tympanic
membrane is ampliﬁed and transferred to the oval window by
the ossicle lever system, which acts much like a hydraulic press
or piston to transfer the same total force hitting the eardrum to
the oval window.
Because the tympanic membrane is 17–20 times larger than
the oval window, the pressure (force per unit area) actually ex-
erted on the oval window is about 20 times that on the tympanic
membrane. Tis increased pressure overcomes the stiﬀness and
inertia of cochlear ﬂuid and sets it into wave motion. Tis situ-
ation can be roughly compared to the diﬀerence in pressure
relayed to the ﬂoor by the broad rubber heels of a man’s shoes
versus a woman’s tiny spike heels. Te man’s weight—say, 70 kg
(about 150 pounds)—is spread over several square inches, and
his heels will not make dents in a pliable vinyl ﬂoor. But spike
heels concentrate the same 70-kg force in an area of about 2.5
(1 square inch) and
dent the ﬂoor.