Sound-Signalling.—In the section on sources of sound, reference has been made to various types of siren and the diaphone (p. 15) for producing powerful sounds in air, mainly for use in fog-signalling from light-houses and light-ships. Under the sea, large bells are used for a similar purpose. A more power ful under-water signalling device, due to Fessenden, employs a large diaphragm forced into resonant vibration under water. The apparatus consists essentially of a transformer, the secondary of which is free to move in a strong radial magnetic field. The primary is supplied with alternating current, and the resulting periodic motion of the secondary (a copper cylinder) is com municated to the submerged steel diaphragm which gives rise to sound waves in the water. The Fessenden oscillator is illus trated on the Plate, figure 4. The same device operates, conversely, as a receiver of sound-waves, these being converted into electrical oscillations by the vibration of the copper cylinder in the magnetic field, the induced currents in the primary coil being amplified and passed through telephones. Fessenden has used the apparatus to transmit signals through water to dis..
tances exceeding 3o miles and has even used it to transmit speech for a distance of half a mile. The oscillator has also been applied to detect icebergs and to take continuous depth soundings by echo methods (see below). In another form of oscillator, which is manufactured by the Signal Gesellschaft. Kiel, Hahneman has obtained powerful signals by means of a diaphragm excited in a different way. The diaphragm forms one member of a coupled system of two masses connected by a stiff spring (metal tubes). A powerful alternating current magnet causes periodic stretching of these tubes with corresponding vibrations of the diaphragm. A transmitter of this type weighing about 5 cwts. and having a diaphragm i8 inches diameter, gives a sound output of 30o to 400 watts, the mechanical efficiency being about 50%. Reference to supersonic directional transmitters of the Langevin type, and to directional receivers, has already been made (see pp. 31 and 32 respectively).
Echo Depth Sounding.—Since the war considerable progress has been made in the development of new systems of sounding at sea. These new methods all employ, most appropriately, sound-waves as a means of measuring depths. The saving of time and labour is enormous and such methods are steadily replacing the old lead and wire systems which have survived so long. A sounding in 4,00o fathoms can be made in about io seconds by the new "echo" methods, whereas many hours labour arc in volved to obtain a somewhat uncertain value by the old "wire" system. An additional advantage lies also in the fact that ob servations of depth can be made whilst the ship is in motion, up to speeds of 15 or 20 knots. The echo method involves the measurement of distance in terms of the time interval between the initiation of a sound impulse and the reception of an echo.
This requires a knowledge of the velocity c of the sound waves in the medium, water or air; and depends on the fact that sound is reflected from the sea-bed in the same way that it is reflected in air from buildings and cliffs. The distance travelled by the wave in the time I is 2D, the depth D is therefore equal to c/2t. Various systems have been devised in America, France, Germany, and Great Britain, differing in the manner of producing and receiving the sound impulse and in the measurement of the time interval. In the Behm system, a small detonator is fired under water thereby operating a microphone and relay and setting a graduated disc in motion; the arrival of the echo stops the disc which is engraved in "depths. " The angle through which the disc has revolved is a measure of the time interval t and conse quently of the depth D. The system developed by the British Admiralty depends on the indirect measurement of the time interval. The transmitter consists essentially of a steel diaphragm which is struck a powerful blow by an electromagnetic hammer, thereby emitting a heavily damped sound-wave of about 2,000 p.p.s. The receiver is an ordinary "button" carbon granule microphone mounted on a small diaphragm and enclosed in a watertight container. The transmitter and receiver are mounted in water-filled tanks fitted inside the ship's plates at points screened by the hull. A small motor drives two commutator switches at constant speed. One of these commutators actuates the hammer three times per second whilst the other short cir cuits a pair of brushes across the listening telephones except at one particular moment during which they can "listen. " The position of these brushes may be displaced by hand relative to the corresponding brushes in the transmitter circuit, so that a short time interval, proportional to the angular displacement of the brushes, separates the initial sound impulse and the moment when the telephones listen. Nothing is heard therefore unless this moment coincides with the moment of arrival of the echo. The angular displacement of the brushes measures the time interval (36o° = 1 second). The dial attached to the rotatable telephone brushes is graduated in feet, or fathoms, and depths may be taken continuously whilst the ship is in motion. Two types of apparatus have been developed, (a) The shallow water set (described above), up to i so fathoms, and (b) Oceanic depth apparatus, up to 4,000 fathoms or so. Both types are in regular service in numerous British and foreign commercial vessels and in the survey ships of the British Navy. Fessenden (U.S.A.) has devised a depth sounding system similar to the above, using his "oscillator" for the purpose of generating the short sound im pulses. He claims in addition to depth sounding, to have used the apparatus to detect icebergs at distances up to 21 miles, a some what remarkable and surprising result, in view of the dimensions of icebergs, the wave-length of the sound (io ft. approx.) employed, and the poor reflecting properties of ice under water. Boyle and Reid, using a Langevin directional quartz oscillator at supersonic frequencies could not detect an iceberg beyond 200 yards. The echo from ice was found to be very feeble compared with that from a rocky shore. The supersonic method has also been employed by Langevin and by Boyle in depth sounding; a good echo from the bed of the sea being easily distinguishable at con siderable depths.