Sound-Ranging.—The location of hostile guns on land, and of submarine explosions at sea, constitutes one of the most im portant practical applications of sound during the war. Ordinary directional receiving devices such as those already considered are of little service when the sound consists of a sudden impulse or shock. The location of the explosion originating the impulse requires a special technique which is generally called sound ranging. We shall deal first of all with the war-method or mu/ tiplc-point sound-ranging. In this method three or more receivers arc mounted at known positions on surveyed base lines. The spherical explosion wave WF passes over, say, four receivers, o, I, 2, 3, in succession, the times of passage being recorded at instants respectively. The construction shown in fig. 17 indicates in a simple manner how the explosion E is located. If we draw circles of radii ch and with centres at 1, 2, 3, respectively, c being the appropriate velocity of sound, then a circle WF which passes through the receiver o and is tangential to the three small circles will have the point of the explosion E at its centre.
Army Sound-Ranging.--Six microphones were generally used, spaced along a base about 9,000 yards long and 4,00o yards behind the front line, a central recording station, to which the microphone leads were brought, being situated 5,000 or 6,000 yards from the front line. For hostile gun-ranging a special type of microphone, particularly sensitive to the low frequency disturb ance of the sound of discharge of the gun, was devised. This microphone, known as the resonant hot-wire microphone, invented by Tucker, has already been described (see p. 31). The hot-wire grid was mounted in the opening of a large Helmholtz resonator of about 16 litres capacity; the system responding only to very low frequency gun-sounds, whilst ignoring shell-wave (onde de choc), detonation of shell, and noises such as speech, traffic, or rifle fire. The resonance set up by a gun-sound produced cooling of the grid with consequent varia tion of the electrical current flowing through it. This current _ _ change was photographically recorded at the central station by means of an Einthoven string galvanometer fitted with six strings, one for each microphone. The resulting record, which was automatically developed and fixed, showed the six gun "signatures" on lines running parallel to the film. Across these lines, i.e., at right angles to the film, were a series of time marks every hundredth and tenth of a second, providing a direct means of measuring the time intervals with an accuracy of o•ooi second, if required (see the illustration on the Plate, fig. 6a). The record was started when a forward
observer, or a "sentry" microphone, heard the gun, i.e., a few seconds before the sound-wave reached the first receiver on the base. The measured time intervals were corrected for tempera ture and wind, which have a considerable influence on the velocity of sound (see p. 5). Variation of these factors with height results in troublesome refraction corrections, and under certain conditions of wind gradient, sound-ranging became im possible. The sound-ranging method could, under certain condi tions, be employed to locate both an enemy gun and the answering shell-burst, in which case the comparative records eliminate uncertainties arising from atmospheric conditions.
Navy Sound-Ranging.—The method employed at sea during the war was based on the same principles, but the receivers were essentially different. Under water, the sound of an explosion was received on a steel diaphragm on which was mounted a sensitive granular microphone. The diaphragm formed the cover of a watertight case mounted on a large tripod which rested on the sea bed. Four or more of such "tripod hydrophones" were laid in accurately surveyed positions on a 12 mile base line. They were connected by means of cables to a six-stringed Einthoven gal vanometer in the recording station on shore, the subsequent procedure to obtain photographic records of an explosion being essentially the same as outlined above. Specimen records are illustrated on the Plate, figure 6b. During the war, explosions of mines, depth charges and torpedoes could be located with great accuracy at distances of so or 6o miles from the base line of hydrophones. Since the war the method has been applied to the accurate location of buoys and light-vessels, a small charge being fired under water in the vicinity of the point to be surveyed. For such purposes it was necessary to know the velocity of sound in sea water at different temperatures and salinities, with great accuracy (see p. 20). As an alternative to the Einthoven photo graphic recorder system of measuring time intervals, A. B. Wood and J. M. Ford (see Jour. Sci. Instr., Mar. 1924) have devised a "Phonic Chronometer." This consists essentially of an electro magnetically operated stop-watch with three sets of dials for measuring three independent time intervals. The speed of the chronometer is governed by a tuning fork and phonic motor giving an accuracy of 1 in io,000. Time intervals are indicated directly on the dials to o.00i second. Special diaphragm shock receivers were designed for use with the chronometer, which was started and stopped as the explosion-wave passed the various receivers, the time intervals being read directly on the dials.