SEISMIC DETECTORS 243 



The compressed air loud speaker consists of an electrically actuated valve 

 which interrupts or modulates an air stream. Thus, the output consists 

 of a series of puffs, the envelope of which corresponds to electrical impulses 

 which actuate the valve. Horns are usually coupled to the system to im- 

 prove the efficiency. In these systems the sound power output may be 

 several times the electrical input to the valve. As a consequence, it is 

 possible to obtain very large acoustical outputs with relatively small elec- 

 trical inputs (small power amplifiers). 



10.7. Supersonics^*'^^. — Supersonics, in general, refers to acoustic waves 

 of a frequency higher than those which may be heard by the ear (about 

 20,000 cycles). Supersonic frequencies are used for directional signalling 

 in water, location of the depth of the ocean bed by the echo method, as a 

 light valve in television projection and various other applications. It is 

 beyond the scope of this book to consider the subject of supersonics. 



Piezoelectric and magnetostriction oscillators are the systems most used 

 for the production and reception of supersonic waves. Piezoelectric micro- 

 phones and telephone receivers have been considered in Chapters IX and 

 X. Piezoelectric supersonic generators and microphones usually employ 

 either Rochelle salt or quartz crystals. Magnetization of magnetic ma- 

 terials such as iron, nickel and nickel alloys produces a small change in the 

 dimensions. A rod of these materials surrounded by a coil carrying an 

 alternating current will experience a change in length corresponding to the 

 magnetic field produced by the coil. If the frequency corresponds to one of 

 the resonant frequencies of the rod, the amplitudeof the resulting vibrations 

 will be relatively large. The same system may be used for a microphone. 



The Galton whistle is another means of generating supersonics in air. 

 Sounds of frequencies up to 100,000 cycles per second have been generated 

 by the Galton whistle. 



10.8. Seismic Detectors^^. — The variation of the velocity of sound in 

 the various strata comprising the earth's crust forms the basis of geo- 

 physical investigations in prospecting for oil. The detonation of a charge 

 of dynamite creates an acoustic wave which is reflected from the various 

 strata of the earth's surface. These reflected waves are picked up by 

 microphones connected to recording oscillographs and located in strategic 

 positions on the earth's surface. From the geometrical configuration of the 

 apparatus, the oscillograph record, and the velocity of sound in various 



^* Bergmann, " Ultrasonics," G. Bell and Sons, Ltd., London. 

 ^^ Olson and Massa, " Applied Acoustics," P. Blakiston's Son and Co., Philadel- 

 phia. 



26 Silverman, Daniel., Jour. A.I.E.E., Vol. 58, No. 11, p. 455, 1939. 



