408 



SUBMARINE TARGET STRENGTHS 



TARGET STRENGTH IN DECIBELS 



BEAM 



BEAM 



STERN 

 Figure 22. Dependence of target strength on frequency (San Diego). 



is somewhat obscured by the fluctuations encoun- 

 tered during these particular runs. 



In general, target strength depends on the signal 

 length for short signals although it varies less rapidly 

 than the signal length, or rather, less rapidly than 10 

 log T, where t is the signal length; a decrease in tar- 

 get strength is most marked at signal lengths less 

 than 10 msec and at aspects away from the beam. 



23.6 DEPENDENCE ON FREQUENCY 



Both sonic and supersonic frequencies have been 

 employed in echo ranging. Lower frequencies are de- 



sirable from the point of view of transmission, since 

 the transmission loss increases with frequency. On 

 the other hand, since high-frequency sound is more 

 directive, the bearings of targets may be located more 

 accurately at high frequencies than at low frequen- 

 cies. As a result, choosing a frequency for echo rang- 

 ing is always a compromise between these two char- 

 acteristics. 



Target strengths have been predicted and meas- 

 ured directly at frequencies between 12 and 60 kc. 

 Theoretical calculations have been based on a fre- 

 quency of 25 kc. Most of the direct measurements 

 have been made at 24 kc, since this frequency is 



