Velocity of Sound.—The sound of thunder is not heard until some time after the lightning flash; the flash of a gun is seen before the report is heard; the puff of "white" steam from the whistle of a distant engine is observed before the sound arrives. These, and many other examples might be quoted as evidence that sound takes time to travel and that its velocity is small compared with the velocity of light (186,000 miles per second). In air, sound takes nearly five seconds to travel a mile, in water about one second, and in iron or steel about a third of a second, i.e., it travels about fifteen times as fast in iron as in air. The experiment is easily tried on a long stretch of iron railing or piping—an observer at one end, listening to the sound of a blow on the other end, hears two blows, the first through the iron, the second through the air.
All kinds of sounds, high and low, loud and weak (with cer tain limited exceptions) travel with the same velocity in the same medium,—otherwise the music of a distant band would become confused on arrival at the listener's ears. Just as ripples spread outwards in two dimensions from a stone thrown into a pond, so waves of sound spread, in three dimensions, from a simple vibrating body through the surrounding medium. The movement of any particle of the medium, is, however, purely local, each particle making small to and fro excursions in a man ner similar to that of the vibrating body itself. The local motion of the particles must be clearly distinguished from the motion of the disturbance which travels forward from layer to layer of the medium with an ever-increasing radius. It is the state of the minute to and fro motion which advances, the medium as a whole, after the disturbance has passed, being practically in its initial position. In this respect the motion is well exemplified by the ripples on a water surface. The sound waves, however, differ from the ripples in one important respect, viz. the vibra
tory motion takes place in the same line as the direction of ad vance of the wave, whereas in the case of the ripples the to-and fro motion is at right angles to the direction of propagation of the wave. The former type of motion, as in the sound wave, is termed longitudinal; the latter, transverse. The general character of longitudinal wave motion may be demonstrated by means of a long helical spring (e.g., 6 ft. long 4 inches dia. of wire •06 in. thick) supported horizontally at frequent intervals by strong threads. If one end be moved to and fro longitudinally a cor responding motion will travel, like a wave, to the other end. Various effects of reflection, etc., with which we are not imme diately concerned may also be demonstrated with this simple apparatus. It is a commonplace observation that any number of sound waves may cross the same airspace at the same time. No confusion arises due to this overlapping of the waves.
Receivers of Sound.—To complete the natural sequence of events a suitable receiver is required to collect the sound energy after transmission through the intervening elastic medium. When the sound travels through the atmosphere the ear forms a natural, though not always the most convenient, receiver. When liquids or solids transmit the sound other forms of receiver are generally to be preferred.
Order of Treatment.—From what has already been said it will appear that the physical basis of the theory of sound involves three fundamental considerations. First a vibrating body is essential to the production of sound; second, an elastic medium in contact with the vibrating body is required to transmit the vibrations to a distant point; and third, some form of receiver is necessary to absorb the energy from the medium and to re convert it into a form of vibratory motion convenient for observation. We shall follow this order of treatment as far as practicable.