DEPENDENCE ON PULSE LENGTH AND FREQUENCY 



433 



where the relative echo level in decibels is plotted 

 against the pulse length for four freighters. Little 

 dependence on pulse length is evident for pulses more 

 than 10 msec long, in qualitative agreement with 

 the results described in Section 23.5.2 applying to 

 submerged submarines. However, for pulse lengths of 

 less than 10 msec, the echo level drops rather sharply. 

 More data are required, however, to show how great 

 this dependence will be for any actual vessel. 



No information is available on how surface vessel 

 target strengths vary with the frequency of the echo- 

 ranging beam employed. The only tests were made 



at San Diego at 24 kc and at New York at 27 kc ; any 

 difference in the target strengths at these two fre- 

 quencies would probably be very small, from theo- 

 retical predictions, and the actual measured differ- 

 ence is too small to verify any such dependence. For 

 still vessels, if the echo comes from the hull, very 

 little variation of target strength with frequency 

 would be expected (see Sections 20.2 and 20.3). For 

 moving vessels, however, with sound scattered from 

 a layer of bubbles, the target strength would be ex- 

 pected to vary with frequency in accordance with 

 the acoustic properties of small bubbles. 



