946 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



been described by Hecht/^" In echo-sounding, single units may be made 

 to do double duty as transmitter and receiver. These are then mounted 

 on the keel of the ship with diaphragm in horizontal position. Where 

 separate units are preferred for transmission and reception, the transmitter 

 is on one side of the ship and the receiver is on the other side and slightly 

 forward from the transmitter. 



An electrical transmitter of high efficiency, designed like a dynamic 

 speaker, has been constructed by Fessenden. There are two fields at 

 each end of the electromagnet; the diaphragm is a steel plate; and the 

 moving coil is a copper cylinder in which eddy currents are induced by 

 two stationary windings in push-pull arrangement. The range of this 

 transmitter is about 30 miles for telegraphy and one-half mile for speech. 

 Another widely used transmitter of unique construction is the Hahnemann 

 "Tonpilz." Since, in a subaqueous transmitter, one part of the vibrating 

 system is actuated in air whereas the other vibrates in contact with water, 

 it is advantageous to employ a mechanical step-up transformer because of 

 the considerable difference in radiation impedance between water and air. 

 This is realized by converting the large displacement and low-pressure 

 oscillation of the electromagnetic driver into the small displacement and 

 high-pressure oscillation of the water-bounded diaphragm. Mechanically, 

 the transformation is effected by coupling a heavy diaphragm to a driver 

 of small mass by means of a solid elastic rod. 



Ultrasonic transmitters are less useful in position-finding and sound- 

 ranging but are well suited for communication and echo-sounding because 

 of their directional characteristics. Three types are in predominant use: 

 electromagnetic, piezoelectric, and magnetostrictive. The electromag- 

 netic transmitters follow in design the Fessenden or Hahnemann type 

 previously discussed and cover the lower ultrasonic range (10,000 to 20,000 

 cycles). A transmitter consisting of six elements about 15 cm in diame- 

 ter, mounted one above another on a vertical tube which can be withdrawn 

 by a hydraulic lift into the ship's hull, is described by Hecht. An echo- 

 sounding transceiver operating electromagnetically at a frequency of 

 17,500 cycles is illustrated in Fig. 12-23. The piezoelectric (crystal type) 

 transmitters are generally used at between 30 and 40 kc, which gives 

 ample range (10 to 20 km) and sufficient directional discrimination (about 

 25° to 30°, see footnote on page 937) for signaling between shore stations 

 and ships and between ships in motion, and also for echo-sounding. Since 

 a large radiation area would not be obtainable with thin quartz plates, 



«« Ibid., p. 413. 



1" Illustration in G. W. Stewart and R. B. Lindsay, Acoustics, p. 249, Van Nos- 

 trand (1930). 



«2 Op cit., p. 413. 



