ABSTRACTS 749 



Measurement of High Q Cavities at 10,000 Megacycles^ R. VV. Lange, 

 Known methods of measuring Q in high Q resonant cavities, together with 

 their accuracies and sources of error, are discussed. For relatively low 

 values of Q and of frequency, it is shown that band width methods are more 

 accurate than decrement methods. For values of Q above 30,000 at fre- 

 quencies above 3,000 megacycles the reverse is true. The significant feature 

 of the present method, the wide range heterodyne decrement method, is that 

 the accuracy is improved by observing the decay over a relatively long 

 interval of time. An absolute accuracy of plus or minus three per cent and 

 a relative accuracy of plus or minus two per cent are achieved. Design 

 features and performance are discussed and constructional details are 

 presented. 



Absorbing Media for Underwater Sound Measuring Tanks and Baffles.^ 

 W. P. Mason and F. H. Hibbard. By using absorbing walls surrounding 

 a small body of water, measuring tanks have been produced which will de- 

 termine the directional properties of underwater sound instruments down 

 to a level of 25 db below the direct beam. These absorbing media are con- 

 structed by inserting fine mesh screen or packed copper wadding in a viscous 

 liquid such as castor oil. These obstructions result in an enhanced viscous 

 action which is nearly independent of the frequency above 10 kilocycles. A 

 six-inch wall can reduce the reflections by 20 db. Tanks using such ab- 

 sorbing media were used for testing transducers at the manufacturing plant 

 and were used for determining the approximate characteristics of small sized 

 instruments. Absorbing media were also used in the sound transparent 

 dome housing the transducer and in the back of the QJB transducers. 



Calculation of the Directivity Index for Various Types of Radiators.^ C. 

 T. MoLLOY. This paper gives the derivations of the "directivity index" 

 formulas for several t3^es of sound radiators. The "directivity index" is 

 defined as "the ratio of the total acoustic power output of the radiator to 

 the acoustic power output of a point source producing the same pressure at 

 the same point on the axis." The utility of the directivity index concept is 

 that it permits power calculations to be made for all radiators in the same 

 manner as for point sources. Directivity index formulas, together with 

 graphs covering practical cases, are given for the following types of radiators: 



1. General plane piston in infinite baffle, 



2. Circular plane piston in infinite baffle, 



3. Rectangular plane piston in infinite baffle, 



4. Sectoral horn, 



' Trans. A. I. E. E., vol. 66, 1947 (pp. 161-166). 

 ^ Jour. Acous. Soc. America, July 1948 (pp.476-483). 

 ^Jour. Acous. Soc. America, July 1948 (pp. 387-405). 



