Sound is a mechanical disturbance that travels through an elastic medium at a
speed characteristic of that medium. Sound propagation is essentially a wave phenomenon,
as with the case of a light beam. But acoustical phenomena are mechanical
in nature, while light, X rays, and gamma rays occur as electromagnetic
phenomena. Acoustic signals require a mechanically elastic medium for propagation
and therefore cannot travel through a vacuum. On the other hand, the
propagation of an electromagnetic wave can occur in empty space. Other types of
wave phenomena include those of ocean movement, the oscillations of machinery,
and the quantum mechanical equivalence of momenta as propounded by de
Broglie.1
Consider sound as generated by the vibration of a plane surface at x = 0 as shown
in Figure 2.3. The displacement of the surface to the right, in the +x direction,
causes a compression of a layer of air immediately adjacent to the surface, thereby
causing an increase in the density of the air in that layer. Because the pressure of that
layer is greater than the pressure of the undisturbed atmosphere, the air molecules
in the layer tend to move in the +x direction and compress the second layer which,
in turn, transmits the pressure impulse to the third layer and so on. But as the plane
surface reverses its direction of vibration, an opposite effect occurs. A rarefaction
of the first layer now occurs, and this rarefaction decreases the pressure to a value
below that of the undisturbed atmosphere. The molecules from the second layer
now tend to move leftward, in the −x direction, and a rarefaction impulse now
follows the previously generated compression impulse.
This succession of outwardly moving rarefactions and compressions constitutes
a wave motion. At a given point in the space, an alternating increase and decrease
in pressure occur, with a corresponding decrease and increase in the density. The
spatial distance λ from one point on the cycle to the corresponding point on the next
cycle is the wavelength. The vibrating molecules that transmit the waves do not, on
the average, change their positions, but are merely moved back and forth under the
influence of the transmitted waves. The distances these particles move about their
respective equilibrium positions are referred to as displacement amplitudes. The
velocity at which the molecules move back and forth is termed particle velocity,
which is not to be confused with the speed of sound, the rate at which the waves
travel through the medium.
The speed of sound is a characteristic of the medium. Sound travels far more
rapidly in solids than it does in gases. At a temperature of 20◦C sound moves at
the rate of 344 m/s (1127 ft/s) through air at the normal atmospheric pressure of
1 The de Broglie theory assigns the nature of a wave to the momentum of a particle of matter in motion
in the following way:
mv = hν
c
where mv represents the moment of the particle, h Planck’s constant = 6.625 × 10–27 erg s, c the
velocity of light = 3 × 108 m/s, and ν the radial frequency of the wave attributable to the particle.
speed characteristic of that medium. Sound propagation is essentially a wave phenomenon,
as with the case of a light beam. But acoustical phenomena are mechanical
in nature, while light, X rays, and gamma rays occur as electromagnetic
phenomena. Acoustic signals require a mechanically elastic medium for propagation
and therefore cannot travel through a vacuum. On the other hand, the
propagation of an electromagnetic wave can occur in empty space. Other types of
wave phenomena include those of ocean movement, the oscillations of machinery,
and the quantum mechanical equivalence of momenta as propounded by de
Broglie.1
Consider sound as generated by the vibration of a plane surface at x = 0 as shown
in Figure 2.3. The displacement of the surface to the right, in the +x direction,
causes a compression of a layer of air immediately adjacent to the surface, thereby
causing an increase in the density of the air in that layer. Because the pressure of that
layer is greater than the pressure of the undisturbed atmosphere, the air molecules
in the layer tend to move in the +x direction and compress the second layer which,
in turn, transmits the pressure impulse to the third layer and so on. But as the plane
surface reverses its direction of vibration, an opposite effect occurs. A rarefaction
of the first layer now occurs, and this rarefaction decreases the pressure to a value
below that of the undisturbed atmosphere. The molecules from the second layer
now tend to move leftward, in the −x direction, and a rarefaction impulse now
follows the previously generated compression impulse.
This succession of outwardly moving rarefactions and compressions constitutes
a wave motion. At a given point in the space, an alternating increase and decrease
in pressure occur, with a corresponding decrease and increase in the density. The
spatial distance λ from one point on the cycle to the corresponding point on the next
cycle is the wavelength. The vibrating molecules that transmit the waves do not, on
the average, change their positions, but are merely moved back and forth under the
influence of the transmitted waves. The distances these particles move about their
respective equilibrium positions are referred to as displacement amplitudes. The
velocity at which the molecules move back and forth is termed particle velocity,
which is not to be confused with the speed of sound, the rate at which the waves
travel through the medium.
The speed of sound is a characteristic of the medium. Sound travels far more
rapidly in solids than it does in gases. At a temperature of 20◦C sound moves at
the rate of 344 m/s (1127 ft/s) through air at the normal atmospheric pressure of
1 The de Broglie theory assigns the nature of a wave to the momentum of a particle of matter in motion
in the following way:
mv = hν
c
where mv represents the moment of the particle, h Planck’s constant = 6.625 × 10–27 erg s, c the
velocity of light = 3 × 108 m/s, and ν the radial frequency of the wave attributable to the particle.
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