M. Greenspan and C. E. Tschiegg 101 
phase velocity; but, in many cases, it is the signal velocity or perhaps the group 
velocity which is desired. In any case, the appropriate velocity will have to be 
measured at a frequency near that of interest. 
In view of what has just been said, the second part of Mr. Crease's question 
cannot be answered at present. 
MR. L. KAY, in connection with near-field irregularities, drew attention to 
the work by Christie* on the near-field characteristics of a circular transducer 
which had been set ringing by an applied impulse. This work had revealed 
variations in the near-field pattern which are not predicted by continuous wave 
theory; and, depending upon the position ofthe receiving transducer, the received 
Signal obtained was of varying wave shape. The fact mentioned by Mr. Greenspan 
that two instruments did not operate satisfactorily following a slight change in 
position of the transducers would tendto confirmthis. Mr. Kay, then, commented 
upon the stress laid by Mr. Greenspan on the use of an impulse rather than a 
driving wave as being better for the reliable operation of the system. He thought 
that there would be no difference inthe received signal, since the transducers are 
free to ring under the damping effect of the water only, giving a 9, say, of 15. 
Even with an impulse driving force, there will be several cycles of the resonant 
frequency which in the medium will decay exponentially but at the output of the 
receiver will appear as a typical rf pulse. 
MR. GREENSPAN: I believe that Mr. Kay has misconstrued my remarks on 
the relative merits of a video-pulse and pulsed-carrier drive. The velocimeter 
operates on the portion of the signal received earliest; what follows the first 
quarter-cycle is ignored. Either type of drive would give the same result; it 
is only that use of a video pulse simplifies the electronics. In either case, we 
have to cope with the fact that the receiving circuit recognizes the leading wave- 
front somewhat later than it would in the ideal case of zero attenuation and zero 
noise. Some of this delay is accounted for in the calibration; but there is a vari- 
able part, associated with the diffraction effects mentioned by Mr. Kay, which 
remains as a source of error difficult to estimate, but thought to be small. 
DR. W.N. ENGLISH commented upon the discrepancies in the values of the 
sound velocities calculated by various methods for the deep ocean. He pointed 
out that C. D. Maunsell, of Pacific Naval Laboratory, British Columbia, following 
a suggestion of M. Peterson (San Diego), has compared the observed and cal- 
culated positions of the first convergence zone for the Kuwahara and Wilson 
formulations. The former gives a range less than that observed while the latter, 
even after applying a sphericity correction, gives a greater range by two miles 
or so. Dr. English said that the discrepancy they have obtained is in the same 
direction as that found by Hays (Wood's Hole) in his experiments in the Mediter- 
ranean at a depth of 2200 meters. 
PROFESSOR T.S. KORN pointed out that in the standing-wave method it was 
the measured phase velocity of sound which, in certain circumstances, was ob- 
viously different from the required propagation velocity. 
*Progress in Nondestructive Testing, Vol. 1. 
