Sound Waves and the
Eardrum
A GIF Animation
A sound wave sound wave traveling through a fluid medium (such as
a liquid or a gaseous material) has a longitudinal nature. This
means that the particles of the medium vibrate in direction which is
parallel (an anti-parallel) to the direction which the sound wave
travels. If the sound wave travels from west to east, then the
particles of the medium vibrate from west to east (and from east to
west). As a sound wave impinges upon a particle of air, that
particle is temporarily disturbed from its rest position. This
particle in turn impinges upon its nearest neighbor, causing it to be
displaced from its rest position. The displacement of several nearby
particles produces a region of space in which several particles are
compressed together. Such a region is known as a compression or high
pressure region. A restoring force typically pulls each particle
back towards its original rest position. As the particles are pulled
away from each other, a region is created in which the particles are
spread apart. Such a region is known as a rarefaction or low
pressure region. Because a sound wave consists of an alternating
pattern of high pressure (compressions) and low pressure
(rarefactions) regions traveling through the medium, it is known as a
pressure wave.
When a pressure wave reaches the ear, a series of high and low
pressure regions impinge upon the eardrum. The arrival of a
compression or high pressure region pushes the eardrum inward; the
arrival of a low pressure regions serves to pull the eardrum outward.
The continuous arrival of high and low pressure regions sets the
eardrum into vibrational motion. This is depicted in the animation
below.
The eardrum is attached to the bones of the middle ear - the
hammer, anvil, and stirrup. As these bones begin vibrating, the
sound signal is transformed from a pressure wave traveling through
air to the mechanical vibrations of the bone structure of the middle
ear. These vibrations are then transmitted to the fluid of the inner
ear where they are converted to electrical nerve impulses which are
sent to the brain.
Since the eardrum is set into vibration by the incoming
pressure wave, the vibrations occur at the same frequency as the
pressure wave. If the incoming compressions and rarefactions arrive
more frequently, then the eardrum vibrates more frequently. This
frequency is transmitted through the middle and inner ear and
provides the perception of pitch. Higher frequency vibrations are
perceived as higher pitch sounds and lower frequency vibrations are
perceived as lower pitch sounds.
The intensity of the incoming sound wave can also be
transmitted through the middle and inner ear and interpreted by the
brain. A high intensity sound wave is characterized by vibrations of
air particles with a high amplitude. When these high amplitude
vibrations impinge upon the eardrum, they produce a very forceful
displacement of the eardrum from its rest position. This high
intensity sound wave causes a large vibration of the eardrum and
subsequently a large and forceful vibration of the bones of the
middle ear. This high amplitude vibration is transmitted to the
fluid of the inner ear and encoded in the nerve signal which is sent
to the brain. A high intensity sound is perceived as a loud sound by
the brain.
For more information on physical descriptions of waves,
visit
The
Physics Classroom. Specific information is available there on the
following topics:
Other animations can be seen at the
Multimedia
Physics Studios. Other useful resources regarding the physics of
motion and waves is available through the
Glenbrook
South Physics Home Page.
This page was created by
Tom
Henderson of
Glenbrook South
High School.
Comments and suggestions can be sent by e-mail to
Tom
Henderson.
This page last updated on 8/17/98.