Production and Transmission of Sound

Nature of Sound. - The source of sound is always in a state of vibration. As the vibration dies down, the intensity of the sound di­minishes. If a ringing bell is touched with the fingers, the sound ceases because the vibrations are stopped by the fingers. When a weight falls to the floor, the weight as well as that part of the floor which is struck is set in vibration, and sound waves are pro­duced. If a stretched guitar string is plucked, it gives a musical note owing to the vibrations set up in it. These vibrations take place too fast for the eye to follow them, and the string seems to be drawn out into a ribbon in the middle. In a vibrating tuning fork the prongs alternately approach and recede from each other. These movements of the prongs can be felt by touching the prongs with the fingers. They produce compressions and rarefactions in the surrounding air that travel forward as sound waves.

Velocity of Sound. - The velocity of sound depends on the density and the elasticity of the medium. The greater the elasticity and the less the density, the greater is the velocity. The relation between the velocity, the density, and the elasticity of the medium is expressed by the formula

v = √ e / p,

where v = the velocity of sound;

e = the modulus of elasticity of the medium;

p = the density of medium.

The Intensity of Sound. - When sound waves spread out in every direction from a source of sound, the intensity varies inversely as the square of the distance from the source. In this case, the sound spread out as spheres. The same amount of energy is transmitted across every spherical surface having its center at the source of sound. The larger the surface of these spheres, the smaller the energy that goes through each square centimeter of surface. The surfaces of these spheres increase as the squares of their radii. Hence, the energy that passes through unit area decreases as the squares of the radii increase.

Vibrations of Wires and Strings. - When the center of a stretched string is displaced and then released, the disturbance is handed on from one element of the string to the next and a transverse wave is produced. These waves are reflected at the fixed ends of the string, return to the center of the string where they pass each other, and go on to the ends where they are again reflected. They combine to form stranding waves in the string. When the string is plucked in the middle, the whole string vibrates as one segment. When the string is held lightly at the middle point and plucked at a point midway between the middle and one end, the string vibrates in two segments. In a similar way, the string may be made to vibrate in three, four, etc., segments. The frequency of the sound is increased as the number of segments increases.

If plucked at random, the string may vibrate as a whole and at the same time be vibrating in segments. The note from the string then consists of the fundamental together with one or more overtones. In this case, the stationary wave in the string is the resultant of the fundamental vibration and all the overtones.

Velocity of Transverse Waves in Strings. - The velocity of a trans­verse wave along a flexible stretched string is constant for a given tension. Whatever the nature of the wave, its velocity of propagation is the same so long as the tension is unchanged [10, https://en.wikipedia.org/wiki/Sound].

Main statements:

1. Sound intensity depends on the vibration frequency of the source.

2. The properties of medium determine the velocity of sound.

3. The larger the distance, the weaker the sound intensity while moving away from the source.

4. The stranding waves in a string produce the transverse waves.

5. The velocity of transverse wave in a string depends on tension.

2.13.3 Look through texts 2.13 and find the English equivalents for the following Russian phrases and word-combinations:

чем более туго веревка натянута, тем быстрее рывок; также, как и та часть пола; скорость звука зависит от плотности; чем больше упругость и меньше плотность, тем больше скорость; обратно пропорционально квадрату расстояния.