C. Discussion of Direct Methods 



Tables III-l and III-2 show mostly instruments built around the 

 so-called "sing around" principle, which was first put to practical use in 

 oceanography by Greenspan and Tschiegg at NBS (Reference 26). The "sing 

 around" velocimeter uses a time-distance method which is made regenerative 

 by closing the loop from the receiving transducer through an amplifier to 

 the transmitting transducer. The resulting repetition rate forms directly 

 the telemetry output of the instrument. Only one instrument, the Dyna 

 Empire ANBQH-1, differs in principle — it measures without regeneration 

 directly the travel time between two transducers and produces an analog 

 output voltage proportional to the speed of sound. The market for velo- 

 cimeters in oceanography is necessarily rather small and there is hence 

 limited money available for development and for new designs. This may 

 be one of the reasons for the dominance of the "sing around" principle — 

 it was shown by NBS to work and to produce useable oceanographic data and 

 no commercial manufacturer wanted to argue with success. 



Other methods to measure velocity of sound in the ocean, based 

 on measurement of standing waves in resonant cavities, have been proposed 

 and tried (Reference 34, 35). These methods, however, did not establish 

 themselves in oceanography, even though some are quite successful in the 

 laboratory and some have been used experimentally in the ocean. 



There are two basically different methods of measuring the speed 

 of sound: by propagating a pulse or by setting up standing waves. These 

 two methods measure two different quantities : the group velocity and the 

 phase velocity. The group velocity is the quantity that describes the 



35 



3rthur ZD.lltttle.Knr. 



