The repetition rate of the pulses is directly a measure of the 

 speed of sound in sea water, according to the following relations: 



f = r-Td^ (2) 



e 



where 



f = repetition rate 



t = electrical delay 

 e 



d = acoustical path 



v = speed of sound 



The following relation holds normally: 



t ^0.1ys<<- =a00us ,-,. 



e v (3) 



This reduces (2) in first approximation to the more convenient expression: 



v - f . d (1 + ft ) (4) 



e 



The frequency f is transmitted up the cable to the deck where the computa- 

 tion according to (4) can be performed in a digital frequency counter directly 

 or a small correction according to (4) can be added to the measured frequency 

 to obtain the velocity. 



The alternative to the pulse propagation methods is the measure- 

 ment of standing waves, and these latter methods, in contrast to the former, 

 are particularly suited to lower frequencies. A resonating cavity is usually 

 employed which is either excited by a wideband pulse and observed after all 

 but the resonant frequencies have died out or made part of a manual or automatic 

 feedback loop with only its resonant modes excited (Reference 34, 35). 

 Suitable shape of the resonator and selective amplifiers are used to suppress 



37 



artbur ZD.IMcKnr. 



