III-44 



screen of bubbles having a cross section of about 17 inches by 3 - 6 inches ex- 

 tending various lengths in the vertical direction, depending on the experimental 

 conditions . The incident waves were in a direction which made an angle of 35 

 degrees with the normal. A reflection and transmission hydrophone were placed 

 on opposite sides of the screen in mirror image position; thus, reflection and 

 transmission could be measured simultaneously. 



Bubbles were allowed to rise either uniformly or in short pulses. In 

 the latter case, differing rates of rise for different bubble sizes separated the 

 pulse so that at any one time all the bubbles on the acoustic axis would have ap- 

 proximately the same radius. Bubble radii and rates of rise were measured 

 by determining the resonance frequency and elapsed time from the time the 

 pulse was released. This data was then used to determine distributions of bub- 

 ble sizes and the amount of air entrained in bubbles for various screen condi- 

 tions. Typical data is shown in Figure III- 16. Note in particular the small 

 amount of air in the bubbles and the smaller number of bubbles on the axis at 

 any one time for the pulses relative to the continuous release. 



The experimental data was compared with the theory. To obtain 

 theoretical values for the reflection coefficient, it was necessary to consider 

 specific wave forms. These were taken as configurational averages, in the 

 sense of Foldy's theory. Except for reflection, only the coherent terms were 

 considered. In order to simplify the expressions, several assumptions were 

 made concerning bubble distribution and the complex propagation constant. The 

 net result is that the expressions used to calculate theoretical results are several 

 approximations removed from the already approximate expressions of Foldy's 

 theory. 



Very few experimental results are presented. Those that are given 

 show a fairly good correspondence between theory and experiment for the screens 

 of essentially uniform bubble size. For such screens, attenuations and reflec- 

 tions were measured at frequencies corresponding to the theoretical resonance 

 frequency of the bubbles along the acoustic axis at the time the sound was trans- 

 mitted. Figures III- 17a and III- 17b illustrate the results obtained, giving a com- 

 parison of the theoretical and observed values. An estimate for the probable 

 error is indicated for the experimental points. Carstensen and Foldy remark 

 that the agreement between observed and calculated values "must be considered 

 highly satisfactory in view of the difficulties of measurement and the approxima- 

 tions made in the theory." 



The results of measurements of attenuation and reflection for continu- 

 ous-flow screens are illustrated in Figures III- 17c and III-17d. In the case of at- 

 tenuation, the maximum measurable attenuation, due to equipment limitations, is 

 also shown. The theoretical and experimental agreement here is not very satis- 

 factory. A partial explanation for the reflection results possibly lies in the ap- 

 proximation that the screen has sharp boundaries. In the experiments, there was 



artbur B.lLittk3nc. 



S-7001-0307 



