SOUND PROPAGATION IN LIQUID CONTAINING MANY BUBBLES 



475 



10 20 30 40 50 60 



TIME IN SECONDS AFTER BEGINNING OF PULSE 



RAOII IN CM OF BUBBLES IN SCREEN 

 >OJ370 0.040 0.030 0.025 0.020 0.017 0.015 aOI4 0.013 0.012 OODM 



|ll I 11 — I 1 1 I I I — r 7 



ui -50 



"^ O 10 20 30 40 50 60 TO 



TIME IN SECONDS AFTER BEGINNING OF PULSE 



Figure 4. Acoustic data taken with bubble pulse 

 screen at 20 kc. 



hydrophone was placed symmetrically with the pro- 

 jector, so that the sound reflected specularly from 

 the screen could reach the hydrophone. However, 

 scattered sound could also reach the reflection hydro- 

 phone, and presumably contributed to the so-called 

 reflected sound which was measured. 



As the resonant bubbles passed by the level of the 

 acoustic measuring instruments, the transmitted 

 sound intensity showed a sharp dip. The reflected, or 

 scattered, sound showed a very much broader maxi- 

 mum, in agreement with the constant scattered sound 

 intensity predicted by equation (73) whenever the 

 transmission loss through the region is appreciable. 

 However, the reflected sound showed much greater 

 fluctuations than the transmitted sound. 



Sample records of transmission through and re- 

 flection from bubble screens are reproduced in Figure 

 4. The curves show the output of the transmission 

 and reflection hydrophones as a function of time 

 elapsed after a 1-sec pulse of bubbles was formed 6 ft 

 below the acoustic equipment. Also shown are the 

 radii of the bubbles arriving at each time. These 

 radii were measured directly by visual means. The 

 upper diagram in Figure 4 shows three transmission 

 runs at 20 kc, superposed on each other. The radius 

 of the resonance peak in this diagram agrees well with 

 the theoretical value of 0.017 cm found from equa- 



<0 



_l 

 m I 



13 O 



OD 



03 "■ 



z 



0.12 

 0.10 

 0.08 

 0.06 

 0.04 

 0.02 



SOUND FREQUENCY IN KC FOR 

 40 30 20 



RESONANCE 

 10 



0.01 0.02 0.03 0.04 



BUBBLE RADIUS ON ACOUSTIC AXIS IN CM 



CD 



a 



3 o 

 z en 



< 3 

 O 



X 



0.01 002 5!!o3 oo* 



BUBBLE RADIUS ON ACOUSTIC AXIS IN CM 



SOUND FREQUENCY IN KC FOR RESONANCE 

 40 30 20 10 



0.01 0.02 0.03 0.04 



BUBBLE RADIUS ON ACOUSTIC AXIS IN CM 



Figure 5. Resonant attenuation and reflection with 

 bubble pulse screen. 



tion (18). The lower diagram shows a reflection run 

 at 20 kc. The measured reflection coefficient is the 

 difference in level between the incident sound at the 

 bubble screen and the sound measured with the re- 

 flection hydrophone, placed 2.5 ft from the center 

 of the screen. 



Each set of observations was repeated at least 

 three times at each of several frequencies. Before and 

 after each group of acoustic measurements, detailed 



