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  1. The study of sound, including its production, transmission, and effects.
  2. Those qualities of an enclosure that together determine its character with respect to distinct hearing.
  3. The branch of science and technology that is devoted to the production, transmission, control, processing, transformation, reception, and effects of sound, longitudinal waves, particularly as vibration, pressure, or elastic waves and shock phenomena in material media. [After Weik '96] Source: http://www.its.bldrdoc.gov/projects/devglossary/_acoustics.html
  4. The science of sound. It can also refer to the effect a given environment has on sound. Source: http://www.owenscorning.com/around/sound/glossary.asp

What Were They Thinking?

From the 1882 edition of The American Universal Cyclopædia, published by S. W. Green's Son, New York.

Acoustics (Greek akouo, I hear) is the science of sound. This part of physics is often 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. Acoustics 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 acoustics are: 1. Musical sounds, or notes. Here the question is concerning the absolute and relative velocities of the vibrations and those modifications, called temperament, to which their original proportions are subjected for the practical purposes of music. 2. The origin of sound, and the laws which guide the vibrations of sounding 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.

The ancients had made attempts to cultivate acoustics. 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 and 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 to 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 acoustics to an independent science. In recent times, comparatively little has been done in this branch of physics. Savart has determined more exactly the number of vibrations in a second necessary to produce an audible sound; and Cagniard de Latour invented the sirene, and discovered many of the conditions under which both solids and fluids sound. The soundings of heated metals, when laid on cold metallic supports, has occasioned much discussion. 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 acoustics 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, perplexing 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 the 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 (echo).

The velocity of sound has been determined by ascertaining the time intervening between the flash and report of a gun, as observed at a given distance, and dividing the distance by the time. After many experiments in various countries, Van der Kolk assigned 1091 ft. 8 in. per second, with a probable error of 3.7 ft. as the velocity of sound in dry air at 32° Fahr. More recent experiments by the astronomer royal at the cape of Good Hope, give 1096 ft. To this velocity may be added 1.11 ft. for each degree Fahr. But air is not a perfect gas, and the variations of elastic force caused by a wave of sound passing through it are not uniform; so these measures, though approximately, amy not be absolutely, correct. Furthermore, the rapidity of transmission depends upon the loudness of the sound; and capt. Parry found, in the polar regions, that the discharge of a cannon at a distance of 2 ½ miles was heard perceptibly sooner than the word ordering to fire, which, of course, preceded the discharge. There is also a gradual falling off in the speed of sound; and Regnault determined that a sound decreased in speed by 2.2 ft. per second in passing from a distance of 4000 ft. to one of 7500 ft. He also found that the velocity depended upon the pitch, the lower notes traveling faster than the higher ones; thus, the fundamental note of a trumpet travels faster than its harmonies. Sound travels faster in liquids than in air, and faster in solids than in liquids. In the river Seine, at 59° Fahr. the speed was 4714 ft. per second. Through iron, sound travels ten and a half times faster than through air. Experiments on telegraph wire produce almost identical results. Different metals transmit sound in widely different degrees. Wertheim assigned 16,822 for iron and 4030 for lead, at a temperature of 68° Fahr. Except in a few cases, the loudness of a sound is less as the distance increases between the source of the sound and the ear. In an unlimited and uniform medium, the loudness of the sound proceeding from a very small sounding body varies inversely as the square of the distance. But to verify this fact it would be necessary to make a test at a considerable elevation above the earth’s surface, the ear and source of sound being separated by air of constant density. As the density of the air diminishes, it would be found that the loudness of a sound at a given distance would decrease. The decay of sound due to this cause is observable in the rarefied air of high mountain regions. De Saussure found that the report of a pistol at a great elevation appeared no louder than would a small cracker at a lower level. But it must be stated that when air-strata of different densities are interposed between the sound and and the ear placed at a given distance, the intensity depends only on the density of the air at the source itself; whence it follows that sounds proceeding from the surface of the earth may be heard at equal distances as distinctly by a person in a floating balloon as by one situated on the surface itself; whereas any noise originating in the balloon would be heard at the surface as faintly as if the ear were placed in the rarefied air on a level with the balloon. This was exemplified by Glashier, the aeronaut, who, at an elevation of 20,000 ft. heard with great distinctness the whistle of a locomotive passing beneath him. The prolonged roll of thunder, with its manifold varieties, is partly to be ascribed to the reflection of the sound by mountains, clouds, etc., but is mainly due to the comparatively low rate of transmission through air. The explanation will be more easily understood by noting the case of a volley fired by a long line of troops. A person at a given point in the line would hear the sound of the nearest musket first, and of the others in the order of distance, and the effect would be a prolonged roll, concluded by the musket most remote from the hearer though all were fired at the same instant; and the roll would gradually decrease in loudness. If he stood exactly opposite the centre of the line, the reports from either end would reach him simultaneously and the effect would be more nearly a loud crash. If the soldiers formed a circle, the listener in the centre would hear a single explosion, since the report of every gun would reach his ear at the same instant, and the whole explosion would be equal to that of the sum of all the separate discharges. By varying the form of arranging the troops, corresponding variations in the sound would be produced. Keep in view, then, the fact that flashes of lightning may be regarded as representing lines of troops, at the points and along the ranks of which explosions are generated at the same instant of time; then consider the variety of distance and position relative to the electric discharge of the listener, and we find no difficulty in accounting for the rolling peals of thunder. In a mountainous region this rolling is greatly augmented by reverberations or echoes from the steep declivities.