http://www.ck12.org Chapter 5. Energy
FIGURE 5.73
- The eardrum is like the head of a drum. It is a thin membrane stretched tight across the end of the ear canal.
The eardrum vibrates when sound waves strike it, and it sends the vibrations on to the middle ear.
Q:How might cupping his hands behind his ears help the boy pictured in the opening image hear better?
A:His hands might help the pinna of his ears catch sound waves and direct them into the ear canal.
Middle Ear
The middle ear contains three tiny bones (ossicles) called the hammer, anvil, and stirrup. If you look at these bones in
theFigure5.73, you might notice that they resemble the objects for which they are named. The three bones transmit
vibrations from the eardrum to the inner ear. The arrangement of the three bones allows them to work together as a
lever that increases the amplitude of the waves as they pass to the inner ear.
Q:Wave amplitude is the maximum distance particles of matter move when a wave passes through them. Why
would amplifying the sound waves as they pass through the middle ear improve hearing?
A:Amplified sound waves have more energy. This increases the intensity and loudness of the sounds, so they are
easier to hear.
Inner Ear
The stirrup in the middle ear passes the amplified sound waves to the inner ear through the oval window. When the
oval window vibrates, it causes the cochlea to vibrate as well. The cochlea is a shell-like structure that is full of fluid
and lined with nerve cells called hair cells. Each hair cell has many tiny “hairs,” as you can see in the magnified
image 5.74. When the cochlea vibrates, it causes waves in the fluid inside. The waves bend the “hairs” on the hair
cells, and this triggers electrical impulses. The electrical impulses travel to the brain through nerves. Only after the
nerve impulses reach the brain do we hear the sound.
Summary
- Sound is a form of energy that travels in waves through matter. The ear gathers and amplifies sound waves and
changes them to electrical signals. The brain receives the signals and interprets them as the sounds we hear.