Longitudinal Waves and
Tuning Forks
A GIF Animation
A sound waves are produced by vibrating objects. Whether it be
the sound of a person's voice, the sound of a piano, the sound of a
trombone or the sound of a physics book slamming to the floor, the
source of the sound is always a vibrating object.
A tuning fork serves as a useful illustration of how a vibrating
object can produce sound. The fork consists of a handle and two
tines. When the tuning fork is hit with a rubber hammer, the tines
begin to vibrate. The back and forth vibration of the tines produce
disturbances of surrounding air molecules. As a tine stretches
outward from its usual position, it compresses surrounding air
molecules into a small region of space; this creates a high pressure
region next to the tine. As the tine then moves inward from its
usual position, air surrounding the tine expands; this produces a
low pressure region next to the tine. The high pressure regions are
known as compressions and the low pressure regions are known as
rarefactions. As the tines continue to vibrate, an alternating
pattern of high and low pressure regions are created. These regions
are transported through the surrounding air, carrying the sound
signal from one location to another.
In solids, sound can exist as either a longitudinal or a
transverse wave. But in mediums which are fluid (e.g., gases and
liquids), sound waves can only be longitudinal. The animation above
depicts a sound wave as a longitudinal wave. In a longitudinal wave,
particles of the medium vibrate back and forth in a direction which
is parallel (and anti-parallel) to the direction of energy transport.
In the animation above, the energy is shown traveling outward from
the tuning fork - from left to right. The air molecules are also
moving from left to right (as well as from right to left). This is
what makes a sound wave a longitudinal wave.
There is another important characteristic of waves depicted in
the animation above. A careful inspection of the particles of the
air (represented by dots) reveal that the air molecules are displaced
rightward and then move back leftward to their original position. Air
moleules are continously vibrating back and forth about their
original position. There is no net displacement of the air
molecules. The molecules of air are only temporarily disturbed from
their rest position; they always return to their original position.
In this sense, a sound wave (like any wave) is a phenomenon which
transports energy from one location to another without transporting
matter.
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 8/11/98.