Page 145 - 9780077418427.pdf

/Users/user-f465/Desktop
tiL12214_ch05_115-138.indd Page 122 9/1/10 9:39 PM user-f465
tiL12214_ch05_115-138.indd Page 122 9/1/10 9:39 PM user-f465 /Users/user-f465/Desktop

Outer Ear Middle Inner Ear
5.4 SOUND WAVES External Ear
auditory canal
SOUND WAVES IN AIR AND HEARING
Horizontal
You cannot hear a vibrating door because the human ear normally
Posterior Semicircular
hears sounds originating from vibrating objects with a frequency Anterior canals
between 20 and 20,000 Hz. Longitudinal waves with frequen- Vestibule
cies less than 20 Hz are called infrasonic. You usually feel sounds Cochlea
below 20 Hz rather than hear them, particularly if you are listening
to a good sound system. Longitudinal waves above 20,000 Hz are Auditory
called ultrasonic. Although 20,000 Hz is usually considered the nerve
upper limit of hearing, the actual limit varies from person to per-
Round window
son and becomes lower and lower with increasing age. Humans
do not hear infrasonic or ultrasonic sounds, but various animals Eustachian tube
have different limits. Dogs, cats, rats, and bats can hear higher fre- Incus

Malleus
quencies than humans. Dogs can hear an ultrasonic whistle when Stapes Bone
a human hears nothing, for example. Some bats make and hear Tympanic Oval
sounds of frequencies up to 100,000 Hz as they navigate and search membrane window

for flying insects in total darkness. Scientists discovered recently
that elephants communicate with extremely low- frequency sounds
FIGURE 5.11 Anatomy of the ear. Sound enters the outer ear
over distances of several kilometers. Humans cannot detect such
and, upon reaching the middle ear, impinges upon the tympanic

low-frequency sounds. This raises the possibility of infrasonic membrane, which vibrates three bones (malleus, incus, and
waves that other animals can detect that we cannot. stapes). The vibrating stapes hits the oval window, and hair cells
A tuning fork that vibrates at 260 Hz makes longitudinal in the cochlea convert the vibrations into action potentials, which
waves much like the swinging door, but these longitudinal follow the auditory nerve to the brain. Hair cells in the semicircular
waves are called audible sound waves because they are within canals and in the vestibule sense balance. The eustachian tube
connects the middle ear to the throat, equalizing air pressure.
the frequency range of human hearing. The prongs of a struck

tuning fork vibrate, moving back and forth. This is more readily
observed if the prongs of the fork are struck, then held against
a sheet of paper or plunged into a beaker of water. In air, the tiny bones to a fluid in a coiled chamber (Figure 5.11). Here,

vibrating prongs first move toward you, pushing the air mol- tiny hairs respond to the frequency and size of the disturbance,
ecules into a condensation of increased density and pressure. activating nerves that transmit the information to the brain.
As the prongs then move back, a rarefaction of decreased den- The brain interprets a frequency as a sound with a certain pitch.

sity and pressure is produced. The alternation of increased and High-frequency sounds are interpreted as high-pitched musical
decreased pressure pulses moves from the vibrating tuning fork notes, for example, and low-frequency sounds are interpreted as
and spreads outward equally in all directions, much like the low-pitched musical notes. The brain then selects certain sounds

surface of a rapidly expanding balloon (Figure 5.10). When the from all you hear, and you “tune” to certain ones, enabling you
pulses reach your eardrum, the eardrum is forced in and out by to listen to whatever sounds you want while ignoring the back-
the pulses. It now vibrates with the same frequency as the tun- ground noise, which is made up of all the other sounds.

ing fork. The vibrations of the eardrum are transferred by three
MEDIUM REQUIRED

Condensations The transmission of a sound wave requires a medium, that is, a
solid, liquid, or gas to carry the disturbance. Th erefore, sound
does not travel through the vacuum of outer space, since there
is nothing to carry the vibrations from a source. The nature of

the molecules making up a solid, liquid, or gas determines how
well or how rapidly the substance will carry sound waves. Th e
two variables are (1) the inertia of the molecules and (2) the

strength of the interaction. Thus, hydrogen gas, with the least
Rarefactions massive molecules, will carry a sound wave at 1,284 m/s (4,213 ft /s)
when the temperature is 0°C. More-massive helium gas mole-
FIGURE 5.10 A vibrating tuning fork produces a series of cules have more inertia and carry a sound wave at only 965 m/s
condensations and rarefactions that move away from the tuning
(3,166 ft/s) at the same temperature. A solid, however, has mol-

fork. The pulses of increased and decreased pressure reach your
ear, vibrating the eardrum. The ear sends nerve signals to the ecules that are strongly attached, so vibrations are passed rapidly
brain about the vibrations, and the brain interprets the signals from molecule to molecule. Steel, for example, is highly elastic,
as sounds. and sound will move through a steel rail at 5,940 m/s (19,488 ft /s).
122 CHAPTER 5 Wave Motions and Sound 5-8