332 EARLE C. GREGG, JR. 



at 600 kilocycles. The crystal was mounted with an air backing in an 

 oil bath and the bacterial suspensions were introduced into the sound 

 field by means of a thin-bottomed flask. After a given exposure time, 

 the suspensions were centrifuged and analyzed. They found that 

 the number of bacteria disintegrated increased with exposure time, 

 leveling off somewhat at 56% disintegrated for ten minute exposure 

 and 63% for fifteen minute exposure. A ten minute exposure was 

 accepted as a reasonably standard interval for subsequent measure- 

 ments and extractions. They further found that the viscosity of the 

 suspension and the shape of the container greatly influenced the re- 

 sults. In particular, thick suspensions and pastes showed very little 

 disintegration even though they became heated. In this process, 

 heat has no disruptive effects. 



While they made no observation of the actual sound pressure in 

 the suspensions or cavitation conditions, a plausible explanation is 

 that the sound pressures involved were large enough to produce cavi- 

 tation in the thin suspensions and not in the much more viscous ones. 

 As mentioned in the section on cavitation, heavy viscous liquids re- 

 quire two to four times the sound pressures to produce cavitation due 

 to their larger cohesive pressures. They also found that the degree 

 of disintegration was independent of frequency and dependent only 

 on the intensity — a characteristic of cavitation. If the power were 

 raised beyond a certain point, denaturation and inactivation of the 

 labile proteins occurred rapidly so that ultrasonic disintegration 

 (production of cell-free enzymes) took place only in a very narrow 

 range of power. Investigation of various types of bacteria showed 

 that some were easily disintegrated while others were completely re- 

 fractory. 



Recently some patents have been granted (33) on frequency modu- 

 lating a sound field by moving a reflector back and forth in front of 

 the sound radiator. Since the moving reflector constitutes a second 

 source of the primary ultrasonic radiation, it follows that the wave- 

 length of the sound in the medium will be changed alternately from a 

 certain maximum value to a certain minimum value due to the Dop- 

 pler shift in frequency by a moving source. While this was proposed 

 as a means of irradiating organisms with various frequencies simul- 

 taneously in case there were critical lethal frequencies, the latter 

 have not been shown to exist. However, this technique has been 

 used previously {27, pp. 46 and 112) to ehminate standing waves in a 

 volume filled with sound energy and might prove of value in biological 



