A good example for use in air, is W ente' s condenser microphone (see MICROPHONE) which employs a tightly stretched diaphragm of high natural frequency and a considerable amount of air damping. Piezoelectric receivers are perhaps the best examples of the non-resonant type, for their natural frequencies are always very high to Ie.). Rochelle salt, quartz and tourmaline suit ably mounted give electrical effects which are an extremely faithful counterpart of the incident sound vibrations, over the whole sonic and a considerable region in the super-sonic range of frequencies. As Boyle, Langevin and others have shown, however, such receivers are most suitable for use in a medium like water. In air they are extremely insensitive, although Nicholson has obtained fairly good results with Rochelle salt receivers (see also p. 12). A multi-resonant receiver with a number of fairly flat overlapping resonance peaks, due to the various components which make up the receiver, could be designed to have a fairly uniform high sensitivity over a wide frequency range. The prin ciple has been applied with success to gramophone recorders and reproducers (see p. 34). (See GRAMOPHONE.) Sensitive Flames are very convenient detectors of high fre quency sounds in air. A long narrow gas flame, issuing from a "pinhole" orifice and adjusted to the point of flaring, becomes unstable, flares and shortens when high pitched sounds fall upon it. The action of such flames and sensitive jets of gas is discussed in Tyndall's and Rayleigh's Sound. The flame is commonly used to demonstrate the presence of nodes and antinodes in stationary-waves of high frequency, and to indicate the position of the sound focus of a concave reflector.
Under-Water Receivers. Hydrophones.—During recent years rapid progress has been made in sound-signalling under water, and the allied navigational problems of sotind-ranging, echo-depth-sounding, submarine detection, etc. The earliest and most simple of all subaqueous receivers was known as the "Broca" tube which consisted of a long metal tube with a thin metal capsule fitted with a diaphragm stretched over the end. A simi lar, but less resonant, device employs a thick-walled rubber bulb in place of the diaphragm. The arrangement is fairly sensitive but is very inconvenient in use—observation at a distant point (on a ship, or at a shore station) being almost impossible. Elec trical devices have replaced such tubes, both on the grounds of sensitiveness and convenience. Microphonic and electro-magnetic devices, attached to diaphragms and enclosed in watertight cavities have proved very satisfactory. Such subaqueous sound receivers are generally known as hydrophones. Generally speak ing, the diaphragms for use under water are much more massive than those used in air, on grounds of strength, durability and efficiency. A microphone or magnetophone (an electro-magnetic device like a telephone earpiece) attached to a metal diaphragm and immersed in water forms a system which may have one or more resonant frequencies. Such a receiver is therefore suitable where sensitivity is more important than "faithfulness "—i.e., for reception of under-water signals of a definite frequency, a suit ably tuned diaphragm and microphone are desirable. Com paratively non-resonant receivers have been constructed of thick rubber—the sensitivity being correspondingly low—these being found valuable in discriminating between various types of ships' noises under water. Receivers of the diaphragm type are essen
tially pressure receivers. Another type of receiver known as the "light body " displacement receiver (see Wood and Young, Proc. Roy. Soc., zoo, 1921) is essentially responsive to dis placement or velocity of the water particles and is relatively insensitive to pressure. The two types may be compared with voltmeters and ammeters as electrical pressure and current measuring devices (see Drysdale in Mechanical Properties of Fluids, p. 293. See also Bragg, World of Sound, pp. 161-177). Reference has already been made to the use of piezo-electric devices for production and reception of high frequency sounds under water (see p.
Geophones.—A simple device first used to detect enemy tunnelling operations in the war, serves to detect sound pulses travelling through the ground. It consists essentially of a cylin drical wooden box of about 3 in. diameter and 2 in. deep, divided into three compartments by two mica discs. The space between the discs is filled with mercury, whilst the two air compart ments are connected to the ears by stethoscope tubes. The geophone " is laid on the ground and vibrations are detected by the relative motion of the box and the mercury. The air spaces are alternately expanded and compressed, the sound pulses being conveyed to the ears via the tubes. Two such instruments, one compartment of each connected to each ear, give a sense of direction as in binaural audition.
Diaphragms.--A single diaphragm or membrane mounted in a heavy annular ring and carrying a microphone at its centre forms a bi-directional receiver. A section of such a receiver is shown in fig. 15 (a) with its "figure of 8" polar curve of sensitivity. The microphone is unaffected when the sound pressure falls sym metrically on both sides of the diaphragm, whilst the response is a maximum when the sound-waves fall broadside on the diaphragm. The mode of action is discussed by Wood and Young in a paper dealing with the Admiralty pattern of "Single Plate Direction Finder" for use under the sea (Proc. Roy. Soc., zoo. 1921). A disc of tightly stretched parchment mounted on a ring, with a light "button" microphone at the centre, gives excellent results in air. By means of a suitable screen ("bias" or "baffle plate ") on one side of such a diaphragm it is possible to obtain a uni directional instrument having the polar curve of sensitivity shown in fig. 15 (b). This baffle plate, when used under water, consists of a compound hollow disc containing air and lead shot (or some other suitable damping material to prevent excessive vibration of the walls of the cavity). Its exact function is some what complicated, but no doubt it acts partly as a sound screen and partly as a phase-shifting device with respect to the sound vibrations reaching the diaphragm. See Plate, figure 5.