Acoustics

Acoustics, the science relating to those effects called sounds, their causes and transmission, qualities and analysis. Drawing a bow across a violin-string causes it to vibrate. A certain effect is produced on the ear, an effect varying with different ears, or at different distances with the same ear. This effect is transmitted from the string to the tympanum or drum of the ear by a vibratory motion of the particles of the air, or other elastic medium, which may intervene. Without an elastic medium to transmit this effect no sound would be heard. Thus a bell ringing inside the exhausted receiver of an air-pump cannot be heard. The velocity of transmission depends on the nature of the medium, varying with its elasticity and with its density. If the elasticity be increased the velocity will increase; if the density be increased the velocity will decrease. The rate at which sound travels in the air is 1,093 feet per second at 0° C, increasing about 2 feet per second for every degree Centigrade. The velocity of sound in liquids is as a rule much greater than in gases, and much greater in solids than in liquids, the elasticity increasing more rapidly than the density. Thus in water the velocity is 5,000 feet per second, and in iron 16,000 feet per second. The chief laws of acoustics are thus stated: - (a) The intensity or loudness of a sound varies inversely as the square of the distance of the sonorous body from the ear. If the distance is doubled the intensity is diminished to 1/4. (b) The intensity increases with the amplitude or extent of vibration of the sonorous body; (c) it diminishes if the density of the medium is diminished, and (d) it is strengthened by the neighbourhood of other sonorous bodies. Hence the use of sounding-boxes in stringed instruments, and of sounding-boards for the voice. Sounds vary in pitch or acuteness if the frequency of the vibration varies; thus, if the number of vibrations per second be increased we obtain a higher note, if diminished we have a lower. If the number be doubled a note is heard that produces a certain physiological effect of sameness. This note is the octave, or first harmonic. If trebled we get the second harmonic, and so on. The limits of hearing of the ordinary human ear are from about 34 (Helmholtz) to 34,000 vibrations per second, but the range varies considerably with different individuals. 261 vibrations per second are recognised by our musical sense. A tuning-fork used on a sounding-board gives us a nearly pure note such as this, but as a rule we never get simple notes corresponding to definite frequencies of vibration. Thus in sounding C on a pianoforte it is easy to recognise some of the harmonics, especially when the keys of the harmonic notes are kept down. This admixture of other notes to the fundamental gives us the quality or timbre of a sound, and we are thus enabled to distinguish between the voices of different people or the sounds of different instruments. Like other wave motions, sound waves may be reflected or refracted; they may augment each other or they may interfere.