X. ULTRASONIC VIBRATIONS 327 



vioiisly, that is, the production of charges on certain faces when the 

 crystal is mechanically strained. RochcUe salt, ammonium dihy- 

 drogen phosphate, quartz, and tourmaline have all been used for this 

 purpose. 



While care must be taken to see that the crystal and holder are 

 small enough so as not to interfere with the sound field, in most bio- 

 logical investigations the whole liquid volume is filled with sound 

 energy and the effect of a small pick-up will be negligible. Rochelle 

 salt has been used considerably for low sound energy detection while 

 for high sound levels, quartz and tourmaline have proved the most 

 useful. Such units may be used for relative sound measurements at 

 one frequency or calibrated and used on an absolute basis (27). 

 Magnetostriction pick-ups have also been devised for this purpose 

 (9). Other sound receivers based on thermal and pressure effects 

 have been used but have not proved as useful or accurate as the piezo- 

 electric devices mentioned above. 



In some cases, it is desirable to know the total energy delivered 

 to a given volume. This is done most simply by measuring the tem- 

 perature rise for a given exposure time and calculating the heat en- 

 ergy delivered to the volume. The usual precautions should be 

 taken to insure that no heat is lost by radiation or conduction. This 

 heat energy will then represent the total acoustic energy delivered 

 by the crystal since only a negligible amount of. acoustic energj^ is 

 delivered to the surrounding air. 



As a simple example assume that 500 ml. of oil of specific heat 

 0.5 and density 0.8 is raised 2°C. in one minute by a 2 cm. diameter 

 quartz disc vibrating at 500 kilocycles. Assuming that all the energy 

 of the quartz is delivered to the oil, we see that the total energy added 

 is 400 cal. or 1670 joules. The total power is 28 watts and the inten- 

 sity at the quartz surface is, then, 8.9 watts per square centimeter. 



This value of intensity is only approximate since it assumes that 

 the surface is vibrating uniformly and that the energy dissipated 

 within the crystal did not heat the oil. It also assumes that no heat 

 energy was delivered to the container walls, crystal, and holder. If 

 the electrical energy delivered to the disc is measured at the same 

 time, an approximate value may be had for the conversion efficiency 

 of the crystal. More precise measurements would take into ac- 

 count the effect of the walls and other objects. Once the intensity of 

 a sound field is determined at a given i)oint or for a given geometrical 

 arrangement, it is desirable not only to know its value at anj^ other 



