FACILITIES, EQUIPMENT, AND METHODS 



The equipment and facilities of the U.S. Navy 

 Electronics Laboratory, Sweetwater Dam fa- 

 cility, San Diego, Calif., were used to deter- 

 mine the target strength of the various fishes. 

 Target strength is defined as 10 log the ratio 

 of the acoustic reflectance of the specimen re- 

 ferred to a perfectly reflecting sphere of 4 m. 

 diameter at a range of 1 m. It is expressed in 

 db (decibels). Figure 1 shows a block diagram 

 of the test setup used to nnake our measure- 

 ments of target strength. The fish target was 

 placed appropriately at either 6 or 12 m., de- 

 pending on the frequency, from the trasmit- 

 receive transducer so that near-field effects 

 were not evident. A 1 -millisecond pulse was 

 transmitted at the carrier frequencies of 20, 

 40, 50 or 280 kHz. At an appropriate time, the 

 receiver gate was opened, to permit the echo 

 pulse to be recorded in amplitude on the polar 

 plot equipment. The pulse repetition rate was 

 adjusted to minimize the interference in the 

 water. The fish target was mounted on mono- 

 filament rigging which was rotated by means 

 of a servo system at about l°/second. The 

 recorder response was capable of 50 db/second, 

 20 db/half second, and 10 db/third of a second. 



To support the fish specimen properly, the 

 rigging took the shape of an inverted "A" with 

 a weight at the bottom (fig. 2). Wire gave 



TO TRANS 4 

 SHORTING REIAY 



SCANNING RATE - 1° PER SEC 

 RECORDER RESPONSE = 50 db PER SEC 



20 db PER 500 rr 

 to db PER 300 n 



5"., 



TO REC GATE 



y 



TO TRANS GATE 



Figure 1. — Block diagram of the test setup used to measure 

 the target strengths of fish. 



MOTOR 



BEARING 



■-''■■ ^■■^■- ^■■^'■■ 



ROTATION 



WATER SURFACE^ ^ 



-.^--.y-.y'-y'.. 



MONOFILAMENT RIGGING 



^^•. -^ ^■. 



HYDROPHONE 



PROJECTOR ( 



WEIGHT 



•iiwsiSS^SiSSsS^^SfSS; 



Figure 2.— Diagram of the apparatus used to hold and rotate the fish for target strength measurements. 



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