326 EARLE C. GREGG, JR. 



Wood and Loomis have recorded a value as high as 35 watts per 

 square centimeter but this is an unusual case. 10 watts per square 

 centimeter is the usual limit although even here there is danger of 

 fracture. As a general rule, the sound radiation pressures are suf- 

 ficient to produce fountains of oil 3 to 20 cm. above the free surface 

 of the liquid. 



Crystal efficiencies range from 75 to 90%. This means that 75 

 to 90% of the electrical energy supplied to the crystal is actually con- 

 verted into sound energy. On the other hand, the over-all efficiency 

 of the system including oscillator and crystal runs about 20 to 40%. 

 Efficiencies of magnetostriction sources are about the same order of 

 magnitude. 



3. Sound Field Measurements 



As with any research, a knowledge of the magnitude of the factors 

 involved is of paramount importance. Some of the discrepancies in 

 the interpretation of biological action of ultrasound today is a direct 

 result of not recording or measuring the character of the sound field 

 used. To measure the electrical quantities alone is not sufficient. 

 The three most important parameters in sound fields of the type used 

 in biological and chemical research are the frequency, amplitude 

 (either displacement or pressure), and intensity. A measure of any 

 two allows the calculation of the third. 



The frequency is most simply determined by measuring the fre- 

 quency of the driving oscillator with a standard radio wavemeter. 

 The amplitude presents, on the other hand, a very difficult problem. 

 Salisbuiy and Porter (6) developed a special circuit that measures the 

 amphtude of vibration of a magnetostrictive tube. This method con- 

 sists simply of arranging the tube to form one plate of a two plate 

 air-spaced condenser and measuring the alternating voltage produced 

 when the distance between the two plates is changed in a sinusoidal 

 motion {i.e., vibration of the tube). An instrument of this type can 

 be calibrated on an absolute scale. It will only measure, however, the 

 amplitude of the source and not that at any given point in the sound 

 field. To measure the amplitude at any point in the sound field, re- 

 course must be made to devices that convert the sound energy to some 

 other measurable form of energy (i, p. 373; ^, p. 39; 5, p. 28; 27). 

 A simple and accurate instrument for this is again a piezoelectric 

 crystal, but in this case using the converse of the effect discussed pre- 



