propagation of energy (and hence information) , while phase velocity is 

 the product of wavelength and frequency in a single frequency wave. Only 

 in the absence of dispersion, i.e. only when the phase velocity is inde- 

 pendent of frequency, are the two velocities identical, and only then will 

 a measurement of a propagating transient yield the same result as a measure- 

 ment of standing waves. In other cases the group velocity has to be cal- 

 culated from a knowledge of phase velocity as a function of frequency, or 

 it has to be measured directly. 



All velocimeters that are presently commercially available 

 measure the group velocity of high frequency pulses. High carrier fre- 

 quencies are needed in this method to keep the length of the pulse (the 

 "group") small compared to the acoustical path, which in turn is limited 

 by the linear dimensions of the instrument. All "sing around" velocimeters 

 use pulses of 3-4MHZ carrier frequency, i.e. about 1000 times higher than 

 common sonar frequencies. A velocimeter does have the fundamental advantages 

 of a direct method over an indirect method, and of many possible principles 

 the "sing around" method seems at present the most attractive from an engi- 

 neering standpoint. It is, in fact, almost a text book example how in a 

 difficult and exact experiment the burden of accuracy can be placed on a single 

 rugged element, in this case the mechanical base for the acoustical path, and 

 the influence of all other elements made negligible. Effects of dispersion 

 and frequency response in the transducers and in the amplifier can be suppressed 

 by reshaping the pulse before it is transmitted again and the time delay in 

 amplifiers and transducers is easily made very small compared to the time it 

 takes the pulse to travel through the measured distance in sea water. 



36 



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