P.M. Kendig 253 
wave. This explanation is, of course, by no means a quantitative theory as yet, 
and such factors as the energy loss due to viscosity have to be considered, but 
Dr. Weston pointed out that it was encouraging to hear Professor Skudrzyk state 
(Lecture 12) that the Kolmogorov spectrum lawisquite general and is applicable 
to both ripples and ambient noise. 
DR.H. A. J. RYNJA commented upon the statement concerning the efficiency 
of hydrophones and said that their efficiency approaches 100% only at resonance, 
and below the resonant frequency will drop to a much lower value determined 
by the square of the coupling factor (k). The maximum value of k for the ma- 
terials now available is between 0.50 and 0.60 so that the efficiency of the 
hydrophone below resonance cannot be more than 30%. This means that for 
maximum sensitivity in the measurement of thermal noise a set of hydrophones 
with different resonance frequencies should be employed. 
DR. KENDIG: At low frequencies, which are well below any resonance and 
for which the wavelength of sound in the medium is large compared to the hy- 
drophone's dimensions, the efficiency will generally be low. In the case of typical 
ferroelectric materials, the low efficiency in this low-frequency region is due 
primarily to the dielectric losses. Thus, the loss tangent (dielectric losses) is 
just as important as the electromechanical coupling coefficient. On the other 
hand, if pure mechanical losses are large compared to the radiation losses, the 
efficiency will be low no matter what values exist for the loss tangent and the 
coupling coefficient, because the ratio of pure mechanical losses to radiation 
losses is independent of the loss tangent and the coupling coefficient. 
At low frequencies, it is usually possible to measure the ambient noise of 
the sea even if the hydrophone efficiency is quite low, because the sea ambient, 
even for zero sea state, is very much greater than the thermal noise. For 
example, at 1000 cps the zero sea-state ambient is about 60 db higher than that 
due to the thermal noise. Thus, it is quite possible to measure the sea ambient 
at all sea states with a nonresonant hydrophone up to at least several thousand 
cycles per second. 
