Patent GB-146350-A (GB)
Title: Improvements in or relating to the detection and production of compressional waves
IPC: G01S1/72
Filed: 1920-07-02
Abstract: 146,350. Fessenden, R. A. Dec. 14, 1918, [Convention date]. Void [Published under Sect. 91 of the Act]. Audible signals; signalling proximity of vessels.-Sounds and vibrations of super-audible frequency are transmitted and detected by condensers in which the dielectric has a density and elasticity of the same order as the surrounding medium. For example, in subaqueous signalling the dielectric may be rubber, glass, mica, or paper soaked in oil. Vibrations of super-audible frequency, generated by a condenser transmitter or by the oscillator described in Specification 18765/13, are propagated, on account of their short wave-length, in a directional beam, and may be employed for detecting submarines, either by observing the screening-effect of the submarine or the echo reflected from it. The super-audible signals may be detected by a second oscillation, or by a receiver of the condenser type, and may be rendered audible by a rectifier or made to beat audibly with vibrations of a neighbouring frequency. In the echo method this may be effected by varying the speed of the alternator which feeds the generating oscillator and combining the currents produced in the detector by the echo with currents taken direct from the alternator. For example, the speed of the alternator may be varied cyclically or may be arranged to fall off when the signal transmitter key is opened; in both cases the beat frequency increases with the distance of the submarine. Fig. 1 shows four receivers 17 - - 20 in the quadrants of a cross-shaped sound-shield 16 in an oil or water filled tank 11 which extends across the width of a ship. The top of the tank 11 may be below the water-line, as shown in Fig. 2, and neighbouring compartments 12, 13 may act as sound screens. The receivers 17, 18 are polarized by a battery 28 and separately connected to the telephones 29, 30 of a head-set, so that the direction of sounds ahead can bo estimated from the relative intensity of the sound in the two telephones, or by swinging the ship until the intensities are equal. The receivers 19, 20 are connected to act in opposition through a transformer 31, 33, 32 on a condenser telephone 34. When a sound arrives from one side or the other the receivers are unequally affected and currents are produced in the circuit of the telephone 34 which beat with currents of different frequency from an alternator 35. Owing to the balanced arrangement of the circuits, currents from an alternator 25, which is used for transmitting signals, produce no effect on the telephone 34. Polarizing batteries 24, 24<1> are included in the circuits of the receivers 19, 20. A receiving-apparatus of magnetic type is shown t the left of Fig. 1. A soft iron or nickel wire 38 is stretched between diaphragms 37, 39 at the ends of a box 36, and is surrounded by a magnetizing-coil and a coil energized by alternating currents from a generator 54. A similar coil in a box 50 which is not exposed to the sound signals is also energized by the alternator 54, and the circuits of the two coils include the primaries 44, 45 of a differential transformer. On the arrival of a signal the wire 38 is vibrated and its magnetization altered, so that the balance of the circuits is upset and a sound is heard in a telephone 48. It is stated that any of the forms of magnetic detector used in wireless telegraphy may be employed, by arranging that the magnetic circuit is set in vibration by the sounds to be detected. In order that a detecting vessel may escape the acoustic observation of a submarine, it may be propelled by aerial propellers 56, 57, Fig. 2, driven bv motors 55, 58 mounted in davits. The invention is not confined to marine purposes, but is applicable for signalling to or from aircraft.