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EARLE C. GREGG, JR. 



centimeter, coupled with the density effect of the media mentioned 

 previously, it is obvious that other techniques and sources must be 

 employed than those usually encountered in air acoustics. 



B. PRODUCTION OF LARGE AMPLITUDE ULTRASOUND 



IN LIQUIDS 



1. Magnetostriction Devices 



One of the better methods for the production of large power ultra- 

 sound in liquids involves the use of magnetostrictive materials. 

 The phenomenon of magnetostriction is the change in length of a rod 

 or tube of ferromagnetic material when it is introduced into a mag- 

 netic field parallel to its length. This change in length, like most mag- 

 netic phenomena, is reversible. Hence if an initially unmagnetized 



TIME 



TIME 



Fig. 3. Relative change in length of a magnetostrictive rod as a function of 

 time: at left, without a biasing d.c. field; at right, with a biasing field. T is 

 the period 1// of the alternating magnetic field. K is the relative stretching of 

 the rod produced by the biasing field. 



rod is brought into an alternating magnetic field, it will contract and 

 expand with twice the frequency of the field. On the other hand, if 

 the rod is initially magnetized, it will vibrate with the same fre- 

 quency as the field. This is illustrated in Figure 3. If the natural 

 period (period = 1//) of the premagnetized rod is the same as that 

 of the alternating magnetic field, the amplitude of vibration will be a 

 maximum and since the vibrations of the rod are longitudinal, sound 

 waves will be emitted from the ends of the rod. 



Figure 4 shows typical curves of magnetic field versus relative 

 change of length for a few magnetostrictive materials. While nickel 

 shows a relatively large effect and is recommended for ultrasonic 

 sources, other materials such as Permalloy and Invar have been used 

 (5). The relative change of length as plotted is for a free bar of the 



