Longitudinal Waves and
Guitar Strings
A GIF Animation
A sound wave is produced by a vibrating object. As a guitar
string vibrates, it sets surrounding air molecules into vibrational
motion. The frequency at which these air molecules vibrate is equal
to the frequency of vibration of the guitar string. The back and
forth vibrations of the surrounding air molecules creates a pressure
wave which travels outward from its source. This pressure wave
consists of compressions and rarefactions. The compressions are
regions of high pressure, where the air molecules are compressed into
a small region of space. The rarefactions are regions of low
pressure, where the air molecules are spread apart. This alternating
pattern of compressions and rarefactions is known as a sound wave.
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 guitar string - from left to right. A careful inspection of the
particles of the medium (represented by lines) in the animation above
reveal that the particles of the medium are displaced rightward and
then move back leftward to 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.
A guitar string vibrating by itself does not produce a very
loud sound. The string itself disturbs very little air since its
small surface area makes very little contact with surrounding air
molecules. However, if the guitar string is attached to a larger
object, such as a wooden sound box, then more air is disturbed. The
guitar string forces the sound box to begin vibrating at the same
frequency as the string. The sound box in turn forces surrounding air
molecules into vibrational motion. Because of the large surface area
of the sound box, more air molecules are set into vibrational motion.
This produces a more audible sound.
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.