liquids of high purity, which can be obtained only in rela- 

 tively small quantities. Such measurements are of great 

 interest to the physicist, since they offer one of many use- 

 ful approaches to the investigation of the nature of matter. 



The laboratory methods divide naturally into two 

 groups. The first utilizes one-dimensional (plane wave) 

 propagation, and the second utilizes three-dimensional 

 wave propagation. 



The instruments belonging in the first group include 

 acoustic interferometers, and equipment resembling inter- 

 ferometers, but utilizing pulsed rather than continuous 

 signals. Plane waves are usually obtained by using trans- 

 ducers with a flat actuating surface, large compared with 

 the wavelength of the sound. For this purpose a quartz 

 crystal is usually employed, especially for the high-fre- 

 quency acoustic waves. 



Interferometers measure the sound velocity by utilizing 

 a reflector and create a one -dimensional standing wave sys- 

 tem. A change in the distance between reflector and trans- 

 mitter by an integral number of half-wavelengths will not 

 disturb the standing wave pattern. The sound velocity can 

 therefore be determined from the frequency and the half- 

 wavelength, The detection of the standing wave pattern can 

 be done by one of several methods: (a) measurement of 

 the drive current to the transmitting crystal at constant 

 voltage, (b) employment of a second crystal as a reflector 

 and also as a receiver, and (c) utilization of a suitable 

 optical system. 10-12 



The sound velocity can also be evaluated by impressing 

 pulses of the desired frequency on the transducer and 

 measuring the time of flight from transmitting to receiving 

 crystal. 13 A transistorized version of a pulsed interferom- 

 eter is being used for sound velocity measurements in the 

 field. 14 



Both types of equipment (those using continuous and 

 pulse signals) are also suited for attenuation measurements, 



