Page 67 - The Master Handbook Of Acoustics

42 CHAPTER THREE

the glass fiber wedges to pass the time. It is very eerie in here. The sea
of sound and noises of life and activity in which we are normally
immersed and of which we are ordinarily scarcely conscious is now
conspicuous by its absence.
The silence presses down on you in the tomblike silence, 10 min-
utes, then a half hour pass. New sounds are discovered, sounds that
come from within your own body. First, the loud pounding of your
heart, still recovering from the novelty of the situation. An hour goes
by. The blood coursing through the vessels becomes audible. At last, if
your ears are keen, your patience is rewarded by a strange hissing
sound between the “ker-bumps” of the heart and the slushing of blood.
What is it? It is the sound of air particles pounding against your
eardrums. The eardrum motion resulting from this hissing sound is
1 1
unbelievably small—only ⁄100 of a millionth of a centimeter—or ⁄10
the diameter of a hydrogen molecule!
The human ear cannot detect sounds softer than the rain of air par-
ticles on the eardrum. This is the threshold of hearing. There would be
no reason to have ears more sensitive, because any lower-level sound
would be drowned by the air-particle noise. This means that the ulti-
mate sensitivity of our hearing just matches the softest sounds possi-
ble in an air medium. Accident? Adaptation? Design?
At the other extreme, our ears can respond to the roar of a cannon, the
noise of a rocket blastoff, or a jet aircraft under full power. Special pro-
tective features of the ear protect the sensitive mechanism from damage
from all but the most intense noises.

A Primer of Ear Anatomy

The three principal parts of the human auditory system, shown in Fig.
3-1, are the outer ear, the middle ear, and the inner ear. The outer ear
is composed of the pinna and the auditory canal or auditory meatus.
The auditory canal is terminated by the tympanic membrane or the
eardrum. The middle ear is an air-filled cavity spanned by the three
tiny bones, the ossicles, called the malleus, the incus, and the stapes.
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 termi-
nated in the auditory nerve, which sends impulses to the brain.