SOUND. The definition of the word sound depends upon the point of view adopted. Subjectively it may be regarded as the sense impression of the organ of hearing; objectively, the vibratory motion which produces the sensation. The physiological and psychical aspects of sound are treated in the article on HEARING. In what follows we shall deal mainly with the physical aspects of sound, that is, with the phenomena which occur outside the ear. Just as it is customary to regard "light" as including invis ible radiations in the ultra-violet and infra-red, so we shall regard sound as including mechanical vibrations of all frequencies audible or otherwise. It may, in fact, be regarded as a branch of mechanics which deals with alternating or vibratory motion. As we shall see, the study of alternating mechanical motions is closely analogous to that of alternating electrical currents; the mathematical theory of one is often of much assistance in the consideration of the other. Alternating currents provide also a convenient means of exciting the corresponding mechanical vibrations, of whatever frequency. Recent developments in the production of alternating currents of very high frequency have their counterpart in the production of corresponding me chanical vibrations.
Sound Due to Vibratory Motion.—It is common observa tion that all sounding bodies are in a state of vibration. In the more obvious cases (e.g., a vibrating stretched string) the vibra tions can be seen directly as a blurred outline, or optical mag nification may be necessary to reveal their existence. The vibra tions of a body which is emitting sound can sometimes be felt if the finger touches the surface lightly; any appreciable pressure by the finger is often sufficient to stop the sound and the vibra tion instantly. The simple experiment with a lightly suspended pithball brought into contact with the surface of the body emit ting sound, proves in a convincing manner that the body is in vibration—a resonant glass or metal vessel, or a tuning fork, serve excellently as a source of sound for this experiment.
Medium Necessary for Sound Transmission.—When a body, e.g., a tuning fork, vibrates in an elastic medium such as
air, the latter is compressed and rarefied by the " to and fro" motions of the vibrating body. These compressions and rare factions are passed on from one layer of the medium to the next and in this manner the vibrations of the body are transmitted through the medium to a distant point. The necessity for an elastic medium to transmit the sound from the source is best illustrated by an experiment first performed effectively by Boyle and later .by Hawksbee (1705). In this experiment a bell is sus pended by a silk thread inside a glass globe connected to a vacuum pump. When the globe is full of air and the bell is excited, the sound can be heard clearly. As the air is withdrawn by the pump, however, the sound weakens and ultimately, when the vacuum is very good, the bell becomes inaudible. The ex periment is more effective when an electric bell is set ringing continuously in an atmosphere of hydrogen reduced to an ex tremely low pressure by an efficient quick-acting vacuum pump.
We are naturally inclined to regard the atmosphere as the universal medium for the transmission of sound. Speech, music, and all the familiar noises of every day life are conveyed to the ear through the surrounding air. Solids and liquids are, how ever, excellent media for sound transmission as may be observed in numerous ways. The water-pipes in the house often carry the " singing" and "hammer" sounds which travel along the pipes from one room to another. The " string-telephone " exemplifies the passage of sound through a long wire tightly stretched. Certain "deaf cases" have been made to hear via the bones of the head, by holding one end of a stiff rod between the teeth, the other end being pressed into contact with the vibrating source, e.g., a clock or a piano. Listening near the hull in a ship's cabin below the water line, the sound of the propeller of a neighbouring ship can be heard clearly—exemplifying the transmission of sound through the sea. If further proof is desired submersion of the observer's head in the water will be even more convincing.