ACOUSTICS, n-kon'stiks or (Gk. aeov akoust ikos, relating to hearing, front cm, nkoucin, to hear). The name applied to the science of the phenomena of sound. The name "sound" is given to the sensation perceived by the auditory nerves, and it is a matter of every day experience that the immediate cause of the sensation is some vibrating body, e.g., a violin string, a drum head, a hammer when striking a nail. This was early recognized, and, so far as acoustics is considered as a science (healing with the vibrations of matter and with the waves pro duced in the air by this motion, the history of its development is identical with the progress of mathematics and dynamics from the time of Galileo and Newton to the present. Few dates van be assigned to definite discoveries. The laws of vibrations, of a stretched string were first deduced mathematically by Brook Taylor in 1715 and by Daniel Bernoulli in 1755, although they had been discovered experimentally by Mersenne in 1636. Longitudinal and torsional vibrations of bars were first investigated by Chladni (1756-1827). Daniel Bernoulli was the first to attack the problem of the lateral vibra tions of bars; but the mathematical treatment of the question is still of interest. Poisson (1S29) was the first to give a correct mathematical so lution of the free vibrations of a membrane, and good experimental work on the subject has been done by Savart, Bourget, and Elsa s. The vibra tions of plates have been studied mathematically by Poisson. Kirchhoff, and more recent writers, and experimentally by Chladni. Sa•art, and Wheatstone. A full account of the history of the mathematical side of acoustics will be found in Rayleigh's great work on the Theory of ottnd.
The history of that portion of acoustics which considers the phenomena of the sense of hear ing, harmony. discord, pitch, etc., begins un doubtedly with the earliest days of civilization. It was known to Pythagoras (sixth century it.c.) —and to whom before him no one can tell—that sounds were in harmony when produced by two stretched strings of the same material, cross-sec tion and tension, provided their lengths were in the ratio of 1 : 2, 2 : 3, or 3 : 4. Mersenne discov ered in 1636 that the frequencies of such vibrat ing strings varied inversely as their lengths, and so proved that for two notes to be in harmony it was necessary for their frequencies to bear sim ple numerical relations to each other. No ex planation of this fact was given until the great research of Ilelinholtz, begun in 1854, the results of which were published in 1862 in his classical work on the Sensa lions of Tone. Helmholtz was the first to discover the existence of summa tional tones, although the differential tones were discovered probably by %Intim in 1743, and cer tainly by Sorge, the court organist at Loben stein, in 1745. Ifelmhoitz's theory of vowel sounds is still under discussion. Most interest ing work on audition has been done in recent years by Rudolf li;;nig of Paris and Professor Mayer of Hoboken.
of the physical properties of sound are matters of common experience and can readily be appreciated. In the first place, it is well known that an interval of time elapses between the vibration of the body and the perception of the resulting sound if the vibrating holy is at a considerable distance; thus the flash of a gun is seen before the sound is heard. It. was shown
by Otto von Guericke that if a bell is set ringing in a glass jar from which the air has been ex hausted no sound is heard; so that the presence of some material medium between the vibrating body and the ear is essential for the production of sound. This medium need not be air, but may lm water, or, in fact, any gas, liquid, or solid which can carry waves. The whole mecha nism is, then, as follows: The vibrations of the body, e.g., a drum-head. produce waves in the medium in contact with it, e.g., the air; these waves spread out through the medium and, after a certain interval of time, reach the ear; in the ear the waves prodnee motions of the ear-d•um and corresponding effects in the internal ear where the auditory nerves have their endings. It should be noted that not every vibration will produce waves in a fluid medium; because if the number of vibrations per second is too small, the fluid will simply flow' around the body as it vibrates, and so will not he compressed; conse quently, in order to produce waves in a fluid, the frequency of the vibrations of the body must exceed a certain number, which depends upon the viscosity and density of the fluid. Further, it is evident that, since fluids can carry only coin pressional (i.e., longitudinal) waves, the pro duction of the sound-sensation is due to waves of this kind. Tha difference between the longitu dinal and the transverse wave can he appreciated by reference to the accompanying diagram, Fig. I.
In this illustration 1 represents a row of par ticles at rest; these particles displaced to form a simple transverse wave are shown in 2, while a longitudinal wave is shown in 3. Here each particle moves to and fro in the direction of the line of propagation of the wave, and the ampli tude of the wave is the distance that each par ticle moves from its position of rest, while the wave-length is the distance between similar points of condensation and rarefaction. as from 4 to 4. Although sound is produced by longi tudinal waves, there is no reason for believ ing that all compressional waves will produce sounds; some may he too long or too short to affect the nerves of the ear.
Our sense of hearing distinguishes between two great classes of sounds: noises and musical notes. A noise is recognized as being abrupt, discontinuous, and exceedingly complex; a mnsi cal note is smooth,continuous.and with a definite, regular character. We distinguish, further, be tween different musical notes as being simple or complex, meaning, by the latter, a note in which we can recognize the presence of several simple tones. Thus, if a piece of paper is torn, or two blocks of wood struck together, we call the re sulting sound a noise. The vibrations of a tun ing-fork cause a simple musical note; while if a banjo string is plucked we hear a complex note. Complex notes differ greatly in their character. They are said to have "quality" or "timbre;" thus, a sound produced by an organ pipe has a quality entirely different from one produced by a piano or by a drum. Simple notes may differ in loudness and in shrillness or "pitch;" thus, a note of a definite pitch may be loud or feeble, and the pitch of a piccolo note is quite different from that of a note produced by a flute.