CuAP. 12] MISCELLANEOUS GEOPHYSICAL METHODS 945 



In water, the losses due to heat conduction are negligible. Contrary to 

 air, water is suitable for both audio- and high-frequency transmission. 

 Theoretically, for 1000 cycles the sound intensity decreases to 0.81 of its 

 initial value at a distance of one kilometer in air, whereas in water approxi- 

 mately the same ratio (0.75) obtains for ten times the range (10 km) at 

 ten times the frequency (10 kc). Ultrasonic frequencies are, therefore, a 

 highly suitable means of signaling. A wave of 100-kc. frequency travels 

 in air but 5 m to its lo/e value while in water the same wave would travel 

 3.6 km, or more than 700 times the distance."^ Actually, the ranges are 

 very much less than the theoretical values because of scattering, refrac- 

 tion, and reflection, H. Hecht ^ places the average practical range of 

 subaqueous (audio-frequency) signaling at 20 km with 150 km as the 

 maximum possibility. 



2. Marine sound transmitters. Sources of sound in marine transmission 

 vary greatly with application. Their construction depends primarily on 

 the directional characteristics desired. As pointed out on page 937, the 

 latter depend on the ratio of transmitter diameter and wave length. Since 

 in water the wave lengths, for the same frequencies, are 4| times greater 

 than in air, it is seen that correspondingly greater transmitter dimensions 

 are required to obtain the same directional characteristics as in air. Such 

 dimensions are usually impracticable; therefore, transmitters intended for 

 signaling are built for high or ultrasonic frequencies. Where directional 

 characteristics offer no advantage, low frequencies are satisfactor3^ 

 Hence, the frequency range of subaqueous transmitters covers the entire 

 band from detonations to ultrasonics, depending on purpose. Detona- 

 tions may be produced by depth bombs (as in radio-acoustic position- 

 finding) or by the firing of cartridges (as in the Behm echo-sounder). For 

 submarine telegraphy, a frequency is selected which is sufficiently removed 

 from the noise produced by the propellers, shafts, engines, and the like. 



When transmitters are used in conjunction with direct listening devices, 

 this frequency is usually close to the frequency for which the human ear 

 is most sensitive. Originally, the transmitters in the audio range between 

 500 and 1000 cycles were simple bells or sirens driven by jets of water. 

 They were later abandoned in favor of electromagnetically driven dia- 

 phragm transmitters. On lightships, two or more twin-diaphragm trans- 

 mitters are employed. They are rotated with respect to one another for 

 uniformity of directional coverage and are mounted one above another 

 with diaphragms vertical. These transmitters are lowered through a shaft 

 in the vessel to a point 10 to 15 feet below its keel. A transmitter of this 

 type (frequency, 525 c.p.s.; power, 800 watts; efficiency, 63 per cent) has 



"8 E. Grossmann, Handb. Exp. Phys., 17(1), 498 (1934). 

 "9 Loc. cit. 



