sound attenuation in pure liquids at low frequencies. The 

 shape of the container is almost always spherical, since 

 this shape eliminates losses due to shear at the boundary 

 for the radial modes. The slightest contamination of the 

 cavity wall, however, will upset the results, and it is 

 hardly possible to get reliable measurements with anything 

 but carefully purified and degassed liquids. A development 

 of the theory for the spherical resonator can be found in 

 reference 2 3. The practical application of this method is 

 demonstrated in references 11, 24, and 2 5. 



Since a few bubbles on the hard wall will completely 

 negate the results, it is virtually impossible to use this 

 method for natural sea water, where the measurements 

 must be performed rapidly and cleaning must be relatively 

 easy. The most promising solution for sea water appears 

 to be the use of soft wall cavities. The surface of water 

 is an excellent reflector of sound waves, and a small 

 bubble trapped in the surface film will have no effect on 

 the reflection of sound. Making a box- shaped cavity of 

 thin sheet stock will give almost complete pressure release 

 at the boundaries, since very little force is required to 

 bend the thin metal sheet. A bubble attached to the boundary 

 will therefore have only a minor effect. 



SOFT WALL CAVITY RESONATORS 



Soft wall cavities were first constructed by W. J. Toulis 

 at the Navy Electronics Laboratory in the summer of 1953, 

 and the best available in 1957 had a Q factor of approximately 

 2000 at a frequency of 5 kc/s. This implies a reverberation 

 time* of approximately 0. 9 second. This method was con- 

 sidered very promising for the present investigation. Fur- 

 thermore, some equipment of this type was already avail- 

 able, so a development program of this method was under- 

 taken, as described in the preceding section. 



*The reverberation time is the time in seconds it takes the 

 sound intensity in a cavity to diminish 60 db. 



