ELASTICITY AND DENSITY AND VELOCITY OF TRANSMISSION

Sound waves travel through any medium to a velocity that is controlled by the medium. Varying the frequency and intensity of the sound waves will not affect the speed of propagation. The ELASTICITY and DENSITY of a medium are the two basic physical properties that govern the velocity of sound through the medium.

Elasticity is the ability of a strained body to recover its shape after deformation, as from a vibration or compression. The measure of elasticity of a body is the force it exerts to return to its original shape.

The density of a medium or substance is the mass per unit volume of the medium or substance. Raising the temperature of the medium (which decreases its density) has the effect of increasing the velocity of sound through the medium.

The velocity of sound in an elastic medium is expressed by the formula:

Even though solids such as steel and glass are far more dense than air, their elasticities are so much greater that the velocities of sound in them are 15 times greater than the velocity of sound in air. Using elasticity as a rough indication of the speed of sound in a given medium, we can state as a general rule that sound travels faster in harder materials (such as steel), slower in liquids, and slowest in gases. Density has the opposite effect on the velocity of sound, that is, with other factors constant, a denser material (such as lead) passes sound slower.

At a given temperature and atmospheric pressure, all sound waves travel in air at the same speed. Thus the velocity that sound will travel through air at 32F (0C) is 1,087 feet per second. But for practical purposes, the speed of sound in air may be considered as 1,100 feet per second. Table 1-1 gives a comparison of the velocity of sound in various mediums.

Table 1-1. - Comparison of Velocity of Sound in Various Mediums

Q.25 How does density and temperature affect the velocity of sound?

ACOUSTICS

The science of sound is called ACOUSTICS. This subject could fill volumes of technical books, but we will only scratch the surface in this chapter. We will present important points that you will need for a better understanding of sound waves.

Acoustics, like sound, relates to the sense of hearing. It also deals with the production, control, transmission, reception, and the effects of sound. For the present, we are concerned only with the last relationship - the effects of sound. These same effects will be used throughout your study of wave propagation.

Echo

An ECHO is the reflection of the original sound wave as it bounces off a distant surface. Just as a rubber ball bounces back when it is thrown against a hard surface, sound waves also bounce off most surfaces. As you have learned from the study of the law of conservation of energy, a rubber ball never bounces back with as much energy as the initial bounce. Similarly, a reflected sound wave is not as loud as the original sound wave. In both cases, some of the energy is absorbed by the reflecting surface. Only a portion of the original sound is reflected, and only a portion of the reflected sound returns to the listener. For this reason, an echo is never as loud as the original sound.

Sound reflections (echoes) have many applications in the Navy. The most important of these applications can be found in the use of depth finding equipment (the fathometer) and sonar. The fathometer sends sound-wave pulses from the bottom of the ship and receives echoes from the ocean floor to indicate the depth of the ocean beneath the ship. The sonar transmits a pulse of sound energy and receives the echo to indicate range and bearing of objects or targets in the ocean depths.

Refraction

When sound waves traveling at different velocities pass obliquely (at an angle) from one medium into another, the waves are refracted; that is, their line of travel is bent. Refraction occurs gradually when one part of a sound wave is traveling faster than the other parts. For example, the wind a few feet above the surface of the earth has a greater velocity than that near the surface because friction retards the lower layers (see figure 1-16). The velocity of the wind is added to the velocity of the sound through the air. The result is that the upper portion of the sound wave moves faster than the lower portion and causes a gradual change in the direction of travel of the wave. Refraction causes sound to travel farther with the wind than against it.

Figure 1-16. - Refraction of sound.

Reverberation

In empty rooms or other confined spaces, sound may be reflected several times to cause what is known as reverberation. REVERBERATION is the multiple reflections of sound waves. Reverberations seem to prolong the time during which a sound is heard. Examples of this often occur in nature. For instance, the discharge of lightning causes a sharp, quick sound. By the time this sound has reached the ears of a distant observer, it is usually drawn out into a prolonged roar by reverberations that we call thunder. A similar case often arises with underwater sound equipment. Reverberations from nearby points may continue for such a long time that they interfere with the returning echoes from targets.

Interference

Any disturbance, man-made or natural, that causes an undesirable response or the degradation of a wave is referred to as INTERFERENCE.

Two sound waves moving simultaneously through the same medium will advance independently, each producing a disturbance as if the other were not present. If the two waves have the same frequency - in phase with each other - and are moving in the same direction, they are additive and are said to interfere constructively. If the two waves have the same frequency and are moving in the same direction, but out of phase with each other, they are subtractive and are said to interfere destructively. If these two subtractive waves have equal amplitudes, the waves cancel each other. This addition or subtraction of waves is often called interference.

Resonance

At some time during your life you probably observed someone putting his or her head into an empty barrel or other cavity and making noises varying in pitch. When that person's voice reached a certain pitch, the tone produced seemed much louder than the others. The reason for this phenomenon is that at that a certain pitch the frequency of vibrations of the voice matched the resonant (or natural) frequency of the cavity. The resonant frequency of a cavity is the frequency at which the cavity body will begin to vibrate and create sound waves. When the resonant frequency of the cavity was reached, the sound of the voice was reinforced by the sound waves created by the cavity, resulting in a louder tone.

This phenomenon occurs whenever the frequency of vibrations is the same as the natural frequency of a cavity, and is called RESONANCE.

Noise

The most complex sound wave that can be produced is noise. Noise has no tonal quality. It distracts and distorts the sound quality that was intended to be heard. NOISE is generally an unwanted disturbance caused by spurious waves originating from man-made or natural sources, such as a jet breaking the sound barrier, or thunder.

Q.26 What term is used in describing the science of sound?
Q.27 A sound wave that is reflected back toward the source is known as what type of sound?
Q.28 What is the term for multiple reflections of sound waves?
Q.29 A cavity that vibrates at its natural frequency produces a louder sound than at other frequencies. What term is used to describe this phenomenon?
Q.30 What do we call a disturbance that distracts or distorts the quality of sound?