Chap. 12] MISCELLANEOUS GEOPHYSICAL METHODS 947 



mosaics of such plates between two steel plates are used. In the Langevin- 

 Florisson transmitter/^' the quartz plates are 2 mm and the steel plates 

 each 3 mm thick. The diameter is 25 cm, the frequency 38 kc. Virtually 

 the same transmitter has been built in England and Germany for the 

 purpose of echo-sounding (37.5 kc, driven by peak voltages of 6000 volts, 

 transmitting damped impulses of about 1/1000 second duration). 



Magnetostrictive transmitters make use of the Joule effect, that is, 

 changes in length of a rod when it is magnetized longitudinally. The 

 drivers are solenoids (see Fig. 12-24) or iron yokes, surrounded by coils 

 and used with a biasing field. The armatures are nickel rods, coupled to 

 aluminum diaphragms. Driving power is suppUed by V.T. oscillators 

 in regenerative arrangement or by high-tension generators discharging 

 through a condenser into the field coils surrounding the rods. The latter 

 scheme is used in the (20 to 30 kc.) echo-sounding transmitters built by 

 Atlas-Werke and by the Electroacoustic Company"* and in the Hughes 

 echo-sounder (Fig. 12-24). 



3. Submarine sound receivers (hydrophones). Subaqueous sound re- 

 ceivers fall into two groups: (1) stethoscopic listening devices, and (2) elec- 

 trical microphones. In the former, the application of the stethoscopic 

 principle (ampUfication by reducing the section of a receiving chamber to 

 that of an ear tube) is necessitated by the energy loss occasioned by the 

 tremendous contrast in the acoustic resistivities of water and air. Were 

 the sound to pass directly from water to air, only 0.12 per cent of the 

 incident amount would be transmitted. A stethoscope with a 15:1 ratio 

 of base to tube diameters raises this to 2.4 per cent,"^ that is, it effects a 

 twentyfold improvement. To obtain unit yield, a 60 : 1 ratio in the base 

 to tube diameters would have to be realized. This means that with the 

 normal ear-tube size, hydrophones of impracticably large diameters would 

 have to be built. 



The diflSculty may be overcome by interposing another medium between 

 air and water. Unit transmission may be accomplished if the acoustic 

 resistivity of this medium is the geometric mean of the acoustic resistivities 

 of air and water and if its thickness is one-quarter of the wave length of 

 the sound in it. This has led to the adoption of listening devices with 

 rubber shells fashioned in the form of a Broca tube, that is, a spherical 

 receiver attached to an ear tube. On small vessels such receivers have 

 been used on both sides of the ship, and the direction of sound has been 

 determined by aiming the ship for equal sound intensity or phase, making 

 use of the binaural effect. For larger vessels, mechanical or electrical 



"' Illustrated' in Bergmann, op. cit., p. 196. 

 "* Illustrated in Bergmann, op. cit., p. 198. 

 "* Stewart and Lindsay, loc. cit. 



