5161 



HYDROGRAPHIC MANUAL 



Page 450 



striction. Where the magnetostrictive metal is in the field of a coil, should this coil be energized by 

 an alternating electric current, the metal will alternately contract and expand along the axis of the 

 coil in unison with the exciting current. Where this dimensional change is properly coupled to the 

 water it will produce acoustic waves. Conversely, a fluctuating pressure applied to a face of the mag- 

 netostrictive material parallel to the magnetic field of the coil will cause changes in the field to produce 

 an electromotive force across the terminals of the coil. Thus it is possible to use the same material 

 for both the transmission and reception of acoustic waves. Commercially pure nickel, because of its 

 high magnetostrictive property, its uniformity, and its chemical and mechanical stability, is most 

 frequently used in this type of acoustic unit. Numerous methods, differing principally in coupling 

 the vibrating unit to the water, are utilized to apply magnetostriction to the production of acoustic 

 waves. In some types of transmitters the vibrations are coupled to the water by means of a dia- 

 phragm and in others the vibrating magnetostrictive elements are in direct contact with the water. 

 The direct-contact units may utilize one surface of the magnetostrictive element in a manner similar 

 to an oscillating piston, or the acoustic energy from the vibrating surface may be concentrated in the 

 desired direction by the use of a reflector. 



The exciting energy for the magnetostrictive element may be alternating current, normally 

 generated by a vacuum-tube oscillator, but the method most frequently used involves the discharge of 

 a previously charged condenser into the exciting coils, so that the magnetostrictive element is set into 

 oscillation by the transient flux. This latter is known as shock excitation. 



Magnetostriction finds its greatest application in echo sounding at supersonic frequencies. 

 Figure 109 illustrates the assembly of a magnetostrictive unit which is described in detail in 5273. 



C. PIEZOELECTRIC UNIT 



Acoustic transmitting units that utilize the piezoelectric properties peculiar to certain crystalline 

 substances are known as piezoelectric oscillators. Where such an oscillator is properly coupled to the 

 water and excited, acoustic waves will be generated. Piezoelectric phenomena are exhibited by a 



-Piezoelectric supersonic acoustic unit. ^4. Quartz crystals. B. Steel diaphragm. C. Steel plate. D. Housing. 

 E. Hull mounting. F. Reinforcement ring. Q. Hull plate. 



number of crystalline substances. Those most frequently used for echo sounding are quartz and 

 Rochelle salt, although others are sometimes used. Quartz has a higher mechanical and chemical 

 stability, even though it may not be so piezoelectrically active as Rochelle salt. These crystals, 

 where properly cut and ground, will show a physical strain and distortion when subjected to an 



