Sound Waves and Music
The Nature of a Sound Wave
Sound is a Mechanical Wave
Sound
and music are parts of our everyday sensory experience. Just as humans have
eyes for the detection of light and color, so we
are equipped with ears for the detection of sound. We seldom take the time to
ponder the characteristics and behaviors of
sound and the mechanisms by which sounds are produced, propagated, and
detected. The basis for an understanding of sound, music and hearing is the
physics of waves. Sound is a wave that is created by vibrating objects and
propagated through a medium from one location to another. In this unit, we will
investigate the nature, properties and behaviors of
sound waves and apply basic wave principles towards an understanding of music.
As discussed in the previous unit of The Physics Classroom
Tutorial, a wave can be described as a disturbance that travels
through a medium, transporting
energy from one
location to another location. The medium is simply the material through which the
disturbance is moving; it can be thought of as a series of interacting
particles. The example of a slinky wave is often used to illustrate the nature
of a wave. A disturbance is typically created within the slinky by the back and
forth movement of the first coil of the slinky. The first coil becomes
disturbed and begins to push or pull on the second coil. This push or pull on
the second coil will displace the second coil from its equilibrium position. As the
second coil becomes displaced, it begins to push or pull on the third coil; the
push or pull on the third coil displaces it from its equilibrium position. As
the third coil becomes displaced, it begins to push or pull on the fourth coil.
This process continues in consecutive fashion, with each individual particle acting to displace the adjacent particle.
Subsequently the disturbance travels through the slinky. As the disturbance
moves from coil to coil, the energy that was originally introduced into the
first coil is transported along the medium from one location to another.
A sound wave is similar in nature to a slinky wave for a variety
of reasons. First, there is a medium that carries the disturbance from one
location to another. Typically, this medium is air, though it could be anymaterial such as water or steel. The medium is
simply a series of interconnected and interacting particles. Second, there is
an original source of the wave, some vibrating object capable of disturbing the
first particle of the medium. The disturbance could be created by the vibrating
vocal cords of a person, the vibrating string and soundboard of a guitar or
violin, the vibrating tines of a tuning fork, or the vibrating diaphragm of a
radio speaker. Third, the sound wave is transported from one location to
another by means of particle-to-particle interaction. If the sound wave is
moving through air, then as one air particle is displaced from its equilibrium
position, it exerts a push or pull on its nearest neighbors,
causing them to be displaced from their equilibrium position. This particle
interaction continues throughout the entire medium, with each particle
interacting and causing a disturbance of its nearest neighbors.
Since a sound wave is a disturbance that is transported through a medium via
the mechanism of particle-to-particle interaction, a sound wave is
characterized as a mechanical
wave.
Production
and Propagation of Sound Waves
The
creation and propagation of sound waves are often demonstrated in class through
the use of a tuning fork. A tuning fork is a metal object consisting of two
tines capable of vibrating if struck by a rubber hammer or mallet. As the tines
of the tuning forks vibrate back and forth, they begin to disturb surrounding
air molecules. These disturbances are passed on to adjacent air molecules by
the mechanism of particle interaction. The motion of the disturbance,
originating at the tines of the tuning fork and traveling through the medium
(in this case, air) is what is referred to as a sound wave. The generation and
propagation of a sound wave is demonstrated in the animation below.
Many
Physics demonstration tuning forks are mounted on a sound box. In such
instances, the vibrating tuning fork, being connected to the sound box, sets the sound box into
vibrational motion. In turn, the sound box, being connected to the air inside of it, sets the air inside
of the sound box into vibrational motion. As the tines of the tuning fork, the
structure of the sound box, and the air inside of the sound box begin vibrating
at the same frequency, a louder sound is produced. In fact, the more particles
that can be made to vibrate, the louder or more amplified the sound. This
concept is often demonstrated by the placement of a vibrating tuning fork
against the glass panel of an overhead projector or on the wooden door of a
cabinet. The vibrating tuning fork sets the glass panel or wood door into
vibrational motion and results in an amplified sound.
We know that a tuning fork is vibrating because we hear the
sound that is produced by its vibration. Nonetheless, we do not actually
visibly detect any vibrations of the tines. This is because the tines 
are
vibrating at a very high frequency. If the
tuning fork that is being used corresponds to middle C on the piano keyboard,
then the tines are vibrating at a frequency of 256 Hertz; that is, 256
vibrations per second. We are unable to visibly detect vibrations of such high
frequency. A common physics demonstration involves slowing down the vibrations by through the use of a strobe
light. If the strobe light puts out a flash of light at a frequency of 512 Hz
(two times the frequency of the tuning fork), then the tuning fork can be
observed to be moving in a back and forth motion. With the room darkened, the
strobe would allow us to view the position of the tines two times during their
vibrational cycle. Thus we would see the tines when they are displaced far to
the left and again when they are displaced far to the right. This would be
convincing proof that the tines of the tuning fork are indeed vibrating to
produce sound.
In a previous unit of The Physics
Classroom Tutorial, a distinction was made between two categories of waves: mechanical waves and electromagnetic waves.
Electromagnetic waves are waves that have an electric 
and
magnetic nature and are capable of traveling through a vacuum. Electromagnetic
waves do not require a medium in order to transport their energy. Mechanical
waves are waves that require a medium in order to transport their energy from
one location to another. Because mechanical waves rely on particle interaction
in order to transport their energy, they cannot travel through regions of space
that are void of particles. That is, mechanical waves cannot travel through a
vacuum. This feature of mechanical waves is often demonstrated in a Physics
class. A ringing bell is placed in a jar and air inside the jar is evacuated.
Once air is removed from the jar, the sound of the ringing bell can no longer
be heard. The clapper is seen striking the bell; but the sound that it produces
cannot be heard because there are no particles inside of the jar to transport
the disturbance through the vacuum. Sound is a mechanical wave and cannot
travel through a vacuum.
Check Your Understanding
1.
A sound wave is different than a light wave in that a sound wave is
a. produced by an oscillating object and a light wave is not.
b. not capable of traveling through a vacuum.
c. not capable of diffracting and a light wave is.
d. capable of existing with a variety of frequencies and a light
wave has a single frequency.
Answer: B
Sound is a
mechanical wave and cannot travel through a vacuum. Light is an electromagnetic
wave and can travel through the vacuum of outer space.