948 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



compensators take the place of the rotatable receiver system. In stetho- 

 scopic devices, mechanical compensation may be accomplished by varying 

 the length of the ear tubes in trombone fashion, or by inserting a rotatable 

 capsule which has variable air passages inside"* and allows the direction 

 of sound to be read directly. Greater accuracy in direction-finding is 

 possible by the use of multiple receivers. As many as twelve to eighteen 

 have been used on each side of the ship. 



Stethoscopic listening devices have been largely superseded by electrical 

 receivers. The latter consist in the main of diaphragms provided with 

 carbon-microphone, electromagnetic, piezoelectric, or magnetostrictive 

 transducers. Carbon microphones have been applied mainly in inertia- 

 coupled form as described below. Electromagnetic hydrophones are of 

 the inductive (moving coil) or reluctance (variable air gap) variety. 

 Representatives of the former are the Fessenden oscillator (see page 946) 

 and the Electroacoustic Co. detector.^^^ In this, the diaphragm carries a 

 piston, moving in a closely fitted ring, with oil in the gap to achieve 

 damping. Reluctance receivers are constructed very much like the reluc- 

 tance seismographs described in Chapter 9."^ One example is the Hahne- 

 mann Tonpilz transmitter (when used as a receiver), another the ordinary 

 headphone receiver, and a third is the balanced armature (Baldwin or 

 Westinghouse) speaker when suitably coupled to the diaphragm as dis- 

 cussed below. The piezoelectric and magnetostrictive receivers are usually 

 identical in construction with the transmitters previously described. 



Hydrophones may be readily constructed with available microphone or 

 speaker units. Three arrangements are possible: (1) mounting the micro- 

 phone to the orifice of a stethoscopic air chamber behind the diaphragm;"' 

 (2) combining the diaphragm with the moving coil of a dynamic speaker 

 or of a coil microphone, or with the armature of a reluctance phone, the 

 magnet unit being rigidly fastened to the case; (3) suspending the repro- 

 ducer in inertia or Tonpilz fashion from the diaphragm. The first arrange- 

 ment lends itself best to hot-wire, condenser, electromagnetic, and other 

 available microphones or diaphragm reproducers, but it is the least efficient 

 of the three. The second method is best suited for velocity (inductive 

 and reluctance) transducers and for quantitative reproduction, particu- 

 larly in connection with compensators. The third is probably the most 

 effective and is used with reproducers of light weight, such as crystal and 

 carbon button microphones. 



^^* Illustrated in Stewart and Lindsay, op. cit., p. 276. 

 ^''^ Illustrated in Hecht, op. cit., p. 429. 

 "8 See p. 611. 



^^' Such a use of Baldwin balanced armature reproducers is described by H. G. 

 Dorsey, U. S. Coast and Geod. Surv., Field Eng. Bull. No. 12, 212 (Dec, 1938). 



