Page 68 - Master Handbook of Acoustics

our hearing just matches the softest sounds possible in an air medium.
Leaving the anechoic chamber, imagine being thrust into the loudest imaginable acoustical
environments. At this extreme, our ears can respond to the roar of a cannon, the noise of a rocket

blastoff, or a jet aircraft under full power. Physiological features of the ear help protect the sensitive
mechanism from damage at the threshold of feeling (where a tingling sensation is felt), but sudden or
intense noises can easily cause temporary shifts in the hearing response. At the extreme threshold of
pain, some permanent damage is probably inevitable.

Ear Anatomy

The three principal parts of the human auditory system are the outer ear, middle ear, and inner ear, as
shown in Fig. 4-1. The outer ear is composed of the pinna and the auditory canal. The auditory canal
is terminated by the tympanic membrane or the eardrum. The middle ear is an air-filled cavity
spanned by three tiny bones collectively called the ossicles, and comprising the malleus, incus, and

stapes. These three bones are also sometimes called the hammer, anvil, and stirrup, respectively,
because of their shapes. About the same size of a grain of rice, the stapes is the smallest bone in the
body. The malleus is attached to the eardrum and the stapes is attached to the oval window of the
inner ear. Together these three bones form a mechanical, lever-action connection between the air-
actuated eardrum and the fluid-filled cochlea of the inner ear. The inner ear is terminated in the
auditory nerve, which sends electrical impulses to the brain.