if measurements at low frequencies are desired. The con- 

 tainers should not be of too regular shape, in order to 

 insure a "diffuse" sound field with minimum volume of 

 sample. Spheres, as used by Moen, so and cubes have too 

 many degenerate modes to be well suited. Bubbles settling 

 down on the walls may actually be helpful under special 

 circumstances by coupling different modes together. 



In contrast to the reverberant chambers, the resonant 

 cavity involves the excitation of a pure normal mode, 

 characteristic of the particular cavity shape and size. The 

 resonant cavity method is the only practical method that 

 permits attenuation measurements at low frequencies on 

 relatively small samples. 



The reverberant chamber method was employed by 

 Knudsen for air-acoustic investigations. 21 It has been 

 extended for use with water by Leonard, and by Wilson 

 and Liebermann. Mulders used the method near 1 Mc/s, 

 and needed 2 to 3 liters of liquids for the measurement. 2 ' 

 The smallest absorption he could measure was of the order 

 of 100 db/kyd. Moen 20 employed bottles of spherical 

 shape ranging from 1 to 12 liter capacity for attenuation 

 measurements of water. The smallest frequency investi- 

 gated by Moen was 140 kc/s. 



Glotov 9 has suggested a reverberation method for use 

 in the field. The volume of sea water required is 500 liters, 

 and the lowest frequency utilized is 15 kc/s. Clearly it 

 would be difficult to obtain homogeneous plankton samples 

 of such large size, even though it is quite possible to utilize 

 this type of equipment to measure the sound attenuation in 

 natural sea water. Glotov indicates that even for this type 

 of equipment, the bubbles on the walls pose a problem, 

 since the equipment has to be conditioned at a depth of 60 

 meters before every measurement. 



There are essentially two types of resonant chambers: 

 the hard wall cavity and the soft wall cavity . The hard 

 wall cavity has been employed extensively for determining 



