RECOMMENDATIONS FOR IMPROVEMENT 



137 



trie instrument. This assumption or preferably an 

 exact knowledge of the phase constant phis the phase 

 measurements on the three possible combinations of 

 a projector, hydrophone, and reversible transducer, 

 would be sufficient to obtain the phase shift between 

 the incident pressure of the hydrophone and its gen- 

 crated voltage. The time delay in the acoustic medi- 

 um woidd have to be taken into account, but once 

 such a standard had been calibrated, an unknown 

 could be determined by comparison. Measurements 

 of this nature have not been made to date by the 

 Underwater Sound Reference Laboratories. 



Improved Variable Band-Pass Filter 



An instrument which proves of value in both the 

 analysis of noise and the characterization of transduc- 

 ers is a continuously variable width band-pass filter 

 centered at any given frequency. The filter of this 

 nature described previously requires too much auxil- 

 iary equipment and is awkward to use. A simpler and 

 self-contained one is most desirable. For example, it 

 is often important to describe a hydrophone's per- 

 formance in terms of the response to a given band 

 width of noise centered at a specific frequency. Such 

 a filter could be readily used in conjunction with a 

 noise source to provide the necessary signal. 



Improved Pulse Recorder 



One of the inconveniences of the present pulse 

 recorder is the fact that its sensitivity and perform- 

 ance are not independent of the pulse length and 

 rate of repetition. A recorder to overcome these diffi- 

 culties is very desirable. 



Recording fMPEDANCE Bridge 



A continuous-recording impedance bridge would 

 be of great value to any laboratory concerned with 

 electric and acoustic measurements. The recording 



wattmeter already described can be used for imped- 

 ance measurements and with high accuracy if com- 

 bined with an insertion-type phase bridge mentioned 

 above. Both the ratio of voltage to current and the 

 phase could be determined by the amount of attenua- 

 tion called for bv the two null-balancinsj circuits. 

 Recorders of the same general type as those used in 

 the electric systems of USRL laboratories would pro- 

 vide a continuous record of the attenuation inserted 

 and hence a continuous record of the phase and mag- 

 nitude of the impedance. 



Absorbing Materials 



A perfect absorbing material would provide almost 

 ideal testing conditions when operating in small 

 space. It is felt that there is a possibility of producing 

 better absorbing materials or combinations of mate- 

 rials than have been developed up to the present 

 time. Any improvement along these lines would be 

 of aid not only in calibration techniques but also in 

 the design and application of acoustic devices. 



Directivity Tndex Measurement 



The directivity index of a projector is necessary 

 for the computation of its efficiency. In general, this 

 index can be obtained only by computation from the 

 directivity patterns and even then with limited ac- 

 curacy. Because of the large amount of time required 

 for these computations, the University of California 

 Division of War Research has built a system for mak- 

 ing direct measurements of the directivity index, ft 

 consists in rigging the projector so that it can be 

 rotated and the acoustic output integrated over the 

 surface of a surrounding sphere by means of the watt- 

 hour meter. The directivity index is the ratio in db 

 of this integrated power to the area of the sphere 

 times the acoustic power per unit area on the acoustic 

 axis. 



