cycles within the thickness of the plastic. A number of 

 transducers of various sizes were calibrated inside and 

 outside the capsule at psig. Very little capsule effect on 

 signal strength was revealed below 8 kc/s. As shown in the 

 chart, the transmitted signal is reduced smoothly and uni- 

 formly up to 100 kc/s, and fluctuates above that frequency. 

 These results seem quite predictable. Results of tests on 

 unknown transducers can be checked against duplicate mea- 

 surements made without the capsule, if necessary. 



DB 



KC/S 



1000 



Attenuation as a function of f requenc y , 



Since for frequencies below 8 kc/s the walls of the 

 capsule have little effect on the signal, measurements can 

 be steady state or pulse. At higher frequencies reflections 

 from the walls add to or subtract from the signal, and pulse 

 measurements are required. The vessel is large enough 

 compared to the transducer to provide adequate time delay 

 in the reflected signal for true evaluation of the incident 

 sound only. 



10 



