ACOUSTICS (Gr. aleouo, I hear) is the science of sound. This part of physics is bften. treated in connection with the atmosphere—an arrangement that seems inappropriate; for the atmosphere is only the most common conductor of sound: and every substance, whether solid or fluid, is capable, as well as air. of sounding itself, or of conveying the sound of other bodies. A. is rather a part of the science of motion. All motion is either rectilineal circular, or vibratory; and when a vibratory motion is quick enough to affect the sense of hearing—for which at least thirty vibrations in a second are required—it constitutes a sound. A definable, uniform sound is a note or tone, and the rapidity of the vibrations is its pitch: a confused indeterminate sound is a noise. The chief subjects treated of in A. are: 1. Musical sounds, or notes (q.v.). Here the question is concerning the absolute and relative velocities of the vibrations, and those modifications, called tem perament, to which their original proportions are subjected for the practical purposes of music. 2. The origin of sound (q.v.), and the laws which guide the vibrations of sound ing bodies, and which give rise to different phenomena in different substances. In all sounding bodies, it is elasticity that is to be looked upon as the moving power. The elasticity of a sounding body may arise from stretching, as in the strings of a violin or the head of a drum; or from its own stiffness, as in rods, bells, etc. 3. The propagation of sound, as well through the air and other gases as through solids and liquids; and the reflection of sounds or echoes. All elastic bodies conduct sound, many much more powerfully than air. In water the conducting power is four times stronger than it is in air; in tin, seven times; in silver, nine times; in iron, ten times: in glass, seventeen times. 4. Perception of sound, or the structure and functions of the ear (q.v.).
The ancients had made attempts to cultivate A. Pythagoras and Aristotle were aware of the way that sound is propagated through the air, but as a science independent of its application to music. it belongs almost entirely to modern times. Bacon ancl•Galileo laid the foundation of this new mathematical science: Newton showed by calculation how the propagation of sound depends upon the elasticity of the atmosphere or other conducting medium. He observed that a sounding body acts by condensing the portions of air that
lie next it and in the direction of the impulse. These condensed portions then spring back by their elasticity, and at the same time impel forwards the portions lying next them Each separate portion of air is thus driven forwards and backwards; and thus all round the sounding body there is an alternate condensation and rarefaction of air, constituting, as it were, waves of sound. In determining the velocity of sound, Newton, Lagrange and Euler erred in their calculations; the best researches on this subject are those of Laplace. Chladni first raised A. to en independent science. In recent times, compara tively little has been done in this bninch of physics. Savart has determined more exactly the number of vibrations in a second necessary to produce an audible sound; and Ca gniard de Latour invented the siren, and discovered many of the conditions under which both solids and fluids sound. The sounding of heated metals, when laid on cold metallic supports, has occasioned much discussion. See Min& Phil. Journal. Faraday and Marx have examined the figures of sound; Wheatstone, the phenomena of sympathetic sounds; and Willis, the formation of vowel-sounds by the human voice.
While the principles of A. are well known in theory, they are seldom carried out to a satisfactory result in practice. We allude more particularly to the many instances in which costly assembly halls and churches are defective as regards public speaking; it being seemingly a mere chance that new edifices of this kind exhibit proper acoustic qualities. In some cases, the sounds uttered cause echoes and reverberations, perplex ing alike to a speaker and his auditory, and in others the sounds are dispersed at a high elevation and are lost. This subject urgently demands consideration in connection with architecture. As a general rule, the ceilings of halls should be at a moderate elevation; the lowering of a ceiling and the removal of chandeliers have been known to improve the speaking and hearing properties; and the hanging up of flags and draperies has, in a variety of instances, had a similarly good effect. The whispering gallery of St. Paul's, London, offers an interesting example of one of the phenomena in acoustics. See Ecuo.