494 viGOUKEtrx and heksey [chap. 12 



D. Vibration Generators 



Explosive charges are useful for the study of propagation because they are 

 simple and they provide all frequencies simultaneously, but they end by be- 

 coming expensive and each one produces waves for a fraction of a second only, 

 which means that results cannot be averaged over a long time. When, therefore, 

 interest is concentrated in a few frequencies, it may be more convenient to 

 employ a vibration generator, of which there are many types available — 

 piezoelectric, magnetostrictive, electromagnetic, etc. The best known are 

 sonar transmitters, which cover a range of 10 to 30 kc/s and are now fitted not 

 only to naval vessels but also to large fishing vessels, and are fairly easily 

 obtainable. 



The transmitters are tuned instruments, meant to transmit a power of some 

 50 to 100 W. The associated electronic apparatus generates electric power 

 which is applied to the transmitter as an a.c. pulse of a few milliseconds every 

 3 or 4 sec ; the rest of the time the instrument, called transducer, acts as a 

 receiver, being switched over to a heterodyne amplifier and a recorder which 

 registers the reverberations as well as the echoes from submarines and other 

 bodies, if any. 



E. Pulse Generators 



Both explosives and vibration generators have been accepted instruments 

 for underwater acoustical research from the first. Recently broad-band, short- 

 pulse generators have been employed increasingly. Another section of this book 

 is devoted to their application to seismic studies. Instruments such as Sparker, 

 RASS, and Boomer will be very useful because they have a broad spectrum 

 somewhat like explosives, but they can be actuated repeatedly, thus sharing 

 some of the advantages of the vibration generator. 



F. Propagation Research 



Propagation is normally studied by observation of the signal received from 

 a distant transmitter, thus the transmitter and receiver are separate, but 

 when sonar apparatus is used for transmission, it is advantageous to use 

 similar apparatus of the same frequency for reception. First, sonar transducers 

 that are directional, on account of their large active surface, receive only a 

 fraction of the noise an omni-directional transducer would receive ; they are 

 trainable, and since in research of the sort envisaged here the bearing of one 

 ship would be known to the other, the transmitter and receiver can be set to 

 face each other. The technique then is to transmit short pulses and record the 

 signals received, taking averages for each range to allow for fluctuations. 

 Some modification of the receiver output stage may be necessary for measure- 

 ment of the amplitude of the received signals but presents no difficulty. Surface 

 vessels, however, are not particularly suitable for this study because, although 

 the transducers are stabilized in azimuth and sometimes in elevation, the 



