Audio Engineering

8 Chapter 1

4 kHz. Sound vibrates the eardrum or tympanic membrane, which seals the outer ear from
the middle ear. The inner ear or cochlea works by sound traveling though a fl uid. Sound
enters the cochlea via a membrane called the oval window.

If airborne sound were to be incident on the oval window directly, the serious impedance
mismatch would cause most of the sound to be refl ected. The middle ear remedies that
mismatch by providing a mechanical advantage. The tympanic membrane is linked to
the oval window by three bones known as ossicles, which act as a lever system such that
a large displacement of the tympanic membrane results in a smaller displacement of the
oval window but with greater force. Figure 1.5 shows that the malleus applies a tension
to the tympanic membrane, rendering it conical in shape. The malleus and the incus are
fi rmly joined together to form a lever. The incus acts on the stapes through a spherical

Ossicles

Auditory

nerve

Outer ear

(pinna)

Inner ear

Eardrum

Ear canal Eustachian

tube

Figure 1.4 : The structure of the human ear. See text for details.

Malleus Incus

Footplate

Stapes

Tympanic membrane

Ear canal

Figure 1.5 : The malleus tensions the tympanic membrane into a conical shape.

The ossicles provide an impedance-transforming lever system between the

tympanic membrane and the oval window.