74 



EXPERIMENTAL PROCEDURES 



270' 



90° 



180° 



Figure 5. Response pattern of the CN-8-2 No. 597 

 hydrophone at 60 kc in horizontal plane. 



The degree of discrimination is reported in a manner 

 analogous to the statement of the directivity pattern 

 of a projector. The ratio of sensitivity in a given 

 direction to the sensitivity on the axis is denoted by 

 6', which is often plotted in decibels relative to axis 

 sensitivity. The degree of discrimination of the 

 hydrophone may also be reported as a single quantity, 

 its directivity index, defined by the equation 



T>' = 10 log 



(lifH- 



(3) 



which is completely analogous to equation (2). 



Cable hydrophones cannot be trained since they 

 are freely suspended from their cables. It is, there- 

 fore, extremely important for a cable hydrophone to 

 be nondirectional in the horizontal plane; otherwise 

 appreciable unknown errors in the received intensity 

 result. Several models, which have been used in re- 

 search, come fairly close to nondirectionaUty. Figure 

 4 shows the horizontal directivity pattern of the 

 CN-8 crystal hydrophone, iLsed extensively for trans- 

 mission runs at both UCDWR and WHOI. This pat- 

 tern was determined at a frequency of 24 kc. Figure 5 

 shows the horizontal directivity pattern of the same 

 hydrophone at 60 kc. It will be noted that at this fre- 

 quency the CN-8 is quite noticeably directional in 

 the horizontal plane. More recently the HUSL 



Figure 6. Horizontal directivity pattern of the Har- 

 vard B19-H hydrophone at 20, 60, and 100 kc. 



B19-H magnetostrictive hydrophone has found 

 favor because of its great stability and high degree of 

 nondirectionality in the horizontal plane at a wide 

 range of frequencies. Figure 6 shows the horizontal 

 directivity of the B19-H at 20 kc, at 60 kc, and at 

 100 kc. 



The response of a hydrophone is a measure of the 

 strength of the electrical signal which will be passed 

 into the cable with a given intensity of incident sound. 

 Since the cable and subsequent amplifying stages will 

 produce a certain amount of instrumental back- 

 ground noise, the response alone may, under exceed- 

 ingly favorable external conditions, determine the 

 level of the minimum detectable signal. The thermal 

 noise in a 1-c band is determined by the receiving 

 response and by the effective resistance G of the 

 hydrophone according to the following equation.' 

 AT = 10 log G - s - 195. (4) 



G is measured in ohms while iV represents decibels 

 above 1 dyne per sq cm. 



The output of the hydrophone, or preamplifier, is 

 transmitted through the hydrophone cable into the 

 receiving sound stack. There the signal is filtered, 

 amplified, possibly rectified or heterodyned, and then 

 fed into the recorder. Most commonly used for re- 

 cording are cathode-ray or galvanometer oscillo- 

 graphs with a very nearly linear'' response, which re- 



■• The circuit is said to respond linearly if the amplitude of 

 the recorded signal is proportional to the amplitude of the 

 incident sound field. 



