252 Lecture 13 
logarithm of the wind speed andthere isa frequency dependence with a maximum 
effect at approximately 600 cps. These results are similarto observations made 
by N.E.L. and groups in Canada and Britain. 
The data below 150 cps showed variations which could be associated with 
tidal current flow around the hydrophones. More recent measurements, in the 
same locality but at 25 cps using a 3-axis seismometer, can be related to the 
lunar tides, but also display a sensitivity tothe solar tides. In order to eliminate 
the noise arising from water flow, a systematic series of low-frequency meas - 
urements are planned with the seismometer buried in the bottom. 
DR. W.N. ENGLISH said that he would like to draw attention to the ambient 
noise measurements made by A. R. Milne of the Pacific Naval Laboratory under 
ice in the Canadian Arctic. The initial measurements * were made under smooth 
new ice and the continuous noise level was extremely low and well below the 
Knudsen sea-state zero shown in one of Dr. Kendig's slides. Below 30 cps, some 
noise effects were obtained, attributed to wind and seismic influence, but ob- 
servations generally were limited by instrument noise. A second operation was 
carried out in the spring of this year under old, rough, shore-fast ice in a water 
depth of 1300 ft, using greatly improved instrumentation. As before, the ambient 
noise level was found to be extremely low provided the ice was not undergoing 
cooling. On calm clear nights and on other occasions when the ice was contract - 
ing, very sharp transients were heard which echoed back and forth in spectacular 
fashion before disappearing. Depending on the rate of cooling, the transients 
were infrequent or gave rise to almost continuous noise. Dr. English thought 
that this form of impulsive noise was sufficiently different from anything found 
in the open ocean to provide some interesting problems. 
DR.D.E. WESTON doubted the existence of any connection between ambient 
sea noise and turbulent density variations and thought a more likely cause to be 
the variations of water height, i.e., surface waves, ignoring the effect of break- 
ing waves which could occur at higher sea states. Although the first-order 
pressure amplitudes due to surface waves will fall off exponentially with depth, 
yet, if standing waves exist, the second-order (or Miche) pressures will not 
show appreciable amplitude diminution. At the time of the maximum displace- 
ment in a standing wave, quantities of water have been raised from below to 
above the mean water level and expressed physically the Miche forces cor- 
respond to the vertical acceleration of these water masses, at a frequency double 
that of the surface wave. Longuet Higgins’ has given a theory of microseisms 
based on the Miche pressure variations. 
Dr. Weston then said that, for the explanation of ambient noise, it was 
necessary to consider the high-frequency or capillary waves and to demonstrate 
the occurrence of standing wave ripples. The latter may arise from the large 
beamwidth (approaching 180°) of waves generated by wind, especially in gusty 
conditions of variable wind direction. Reflection of ripples at the sharp crests 
of the gravity waves could also give a standing wave ripple "riding" on a gravity 
*J. Acoust. Soc. Am. (August, 1960). 
tPhil. Trans. Roy. Soc. London, Ser. A, Vol. 243, 857 (1950). 
