PHY238YLecture 11

The human ear: amplification of the outer/inner ear Acoustic impedance. Physics of hearing (III) Protection mechanisms of the human ear

References: Hallett et al.: Physics for the life sciences, 4th ed., Ch.2 (2.6)K. Bogdanov: Biology in Physics: is Life Matter, Acad. Press 2000, Ch. 6Hyper Physics: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/ear.html#c1Thanks to dr. Rod Nave for the permission to use the above resource

PHY238YLecture 11

The middle ear The tympanic

membrane or eardrum receives vibrations traveling up the auditory canal and transfers them through the tiny ossicles to the oval window, the port into the inner ear. The eardrum is ~ fifteen times larger than the oval window, giving an amplification of about fifteen compared to the oval window alone.

PHY238YLecture 11

The middle ear: ossicles

The three tiniest bones in the body couple the vibration of the eardrum and the forces exerted on the oval window of the inner ear

The ossicles form a compound lever which achieves a multiplication of force.

PHY238YLecture 11

The middle ear: Ossicle vibration

Ossicles achieve an amplification by lever action. The lever is adjustable under muscle action and may actually attenuate loud sounds for protection of the ear.

PHY238YLecture 11

The outer- middle ear amplification

Funneleffect and resonancesof the auditory canal

Tympanic

membrane (eardrum) area vs. oval window

area

Ossicles

(hammer, anvil and stirrup) contribute by a lever-type amplification when listening to soft sounds.

Outer ear~2x

Tympanic membrane

~15x

Ossicles~2x

(soft sounds only)

PHY238YLecture 11

Why is the amplification needed? Outer and middle ear are filled with air at normal pressure; inner ear is filled with a

liquid. The oscillation of ossicles causes a vibration of the oval window, but part of the

acoustic wave is reflected from the interface due to the difference in acoustic impedance between the middle and inner ear.

We define the acoustic impedance Z and we use the definition of the wave velocity v:

BZ

waveallongitudinaforB

vbut

v

BZ

:

PHY238YLecture 11

Ear protection to loud sounds

PHY238YLecture 11

Ear protection mechanisms

In response to loud sounds, the tensor tympani muscle tightens the eardrum and through the tendon between the hammer and anvil and shifts the stirrup backward from the oval window of the inner ear. This shifting of the ossicles reduces the transmitted force to the inner ear, protecting it. However, it is a relatively slow action and cannot protect the ear from sudden loud sounds like a gunshot. The process is less effective in older ears