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 SOUND (see 25.437).&mdash;The increase in our knowledge of the subject of acoustics (the science of Sound) during recent years has been largely associated with the war conditions which prevailed from 1914 to 1918. As a consequence of the war the development of this science has been abnormal, and research has been directed towards the rapid realization of practical acoustic devices and methods for immediate use in warfare, both on land and sea. A general survey of the work done shows that the advances consist of applications of well-established principles, rather than the discovery of new phenomena. Generally, the observations made have proved to be in accordance with previous theoretical investigations, mainly due to the late Lord Rayleigh. This war work falls naturally under two headings, viz. (1) the detection and perception of direction of sounds in air, and (2) the detection and perception of direction of sounds in water. Theoretically, these two problems have much in common, but, practically, there are important differences which make it desirable to treat them in separate sections. A special section (3) is devoted to the important advances recently made in auditorium acoustics, and the remaining section (4) deals briefly with miscellaneous outstanding features of modern work on sound, not essentially military in character.

Detection.&mdash;The human ear itself is a remarkably sensitive detector of the air vibrations which constitute sound. It is still much superior in this respect to any mechanical device which has yet been produced for recording the vibrations visually. Thus the perception of feeble sounds of necessity depends upon the limitations of audibility, either indirect listening, or with the ear aided by the intervention of an electrical device such as a microphone. The audibility of a feeble sound can be very largely augmented by making use of the principle of resonance, provided that the sound itself approximates to a pure tone. This can be secured, for example, by the use of a Helmholtz resonator applied to the ear in the case of direct listening, and in addition, by tuning the diaphragm receiver when microphonic listening is adopted. It has happened fortuitously that one of the chief sounds in air which it is important to be able to detect, viz. those emitted by aircraft, do contain predominant notes which enable the application of resonance, as above indicated, to increase largely the range of audibility. Typical predominant frequencies (apparently due to engine exhaust) are given in the following table, which relates to the engine running at the usual speed:&mdash;

The following frequencies have been detected in the sounds from the Maybach engines of a Zeppelin airship:&mdash;

The operation of the Doppler effect, arising from the relative motion between the aircraft and the observer, prevents the possibility of the identification of the machine by means of the observed frequency, this being liable to change by as much as 20%, according to the speed and direction of flight. An interesting observation which has been constantly made is that the notes of low pitch continue to be heard at ranges where those of high pitch have ceased to be audible. This is in accordance with the theoretical expectation that damping increases with frequency.

The determination of the direction whence a sound arrives is theoretically possible by a variety of methods dealt with below, several of which have been tried in aircraft localization.

(a) Binaural Listening. &mdash;Lord Rayleigh's experiments (Collected Papers, vol. 5, p. 347) have shown that low-pitched sounds are determined in direction by the observation of the phase difference between the vibrations arriving at the two ears. This principle has been applied in direction-finding, and the effect has been exaggerated by increasing the distance between the two points of reception. The sound is received by two equal trumpets or horns rigidly connected together and capable of rotation about an axis perpendicular to the line joining them. Separate and exactly equal tubes lead from the trumpets to the two ears, respectively, and the apparatus is rotated until the sound under observation appears to come from directly in front. The line joining the sound receivers is then perpendicular to the incident sound stream. An alternative method which dispenses with the necessity of rotating the apparatus is to use a compensator or phase-measurer, which consists of tubes, adjustable in length, inserted between the sound receivers and the appropriate ears, so as to provide a path difference equal to that between the distant source of sound and the tworeceivers. Adjustment of the tube lengths is made until the impression received is that the sound is neither to the right nor to the left, and the determination of direction is then a matter of simple geometry. In practice the compensator is graduated to give direct angular readings.

The practice of binaural listening has verified theoretical conclusions in several important respects. It has been found that it is easier to perceive the direction of a mixed sound, or noise, than a pure note. Apparently it is necessary that the wave train should contain more or less isolated special characteristics whereby the phase difference can be readily appreciated. In the regular sine wave corresponding to a pure tone each vibration is exactly like those which immediately precede and follow it, and the ears are unable to identify corresponding displacements. It is apparently also necessary for successful binaural listening that the two portions of the incident wave which enter the two receivers should be free from subsequent distortion; in particular, that the sound receivers should be as nearly as possible non-resonant for the vibrations in question. Any amplification of the sound which depends upon resonance, therefore, such as the use of Helmholtz resonators already referred to, is incompatible with efficient direction-finding by observations of phase difference.

The construction and arrangement of the receivers used has varied very much in practice. As a typical system, that commonly used in the British army may be quoted, namely, circular cones, 2 to 4 ft. long and of semi-angle 20, as receivers, placed about 7 ft. apart&mdash;a distance which proved to be sufficient for attaining nearly the maximum practical accuracy of setting.

The method is subject to many errors, chiefly those arising from the motion of the sound source, refraction due to temperature inequalities in the air, and the effect of winds. The necessary corrections are tabulated for use in practice.

(b) Sound Mirrors.&mdash;Some success has been attained in direction-finding by means of concave sound reflectors. The chief limitations have arisen from the question of size, and, consequently, of