M, P. Fodch 123 
DR. D.E. WESTON: During the discussion, mention was made ofatechnique 
of hydrophone calibration in which the instrument is moved up and down in the 
water. The method was at a disadvantage if the hydrophone was sensitive to 
acceleration, but Dr. Weston drew attention to two variants of the procedure, in 
one of which the problem of hydrophone acceleration did not occur. In this vari- 
ant, the water, in a suitable container, is moved up and down past the hydrophone 
when the total acoustic pressure will be dueto the changing hydrostatic pressure 
plus the known pressures exerted by the acceleration of the water. 
DR. FOACHE: There are so many methods of calibrating hydrophones at very 
low frequencies that it would demand too much time to try each of them. The 
suggestion by Dr. Weston is certainly interesting, but we have never used that 
method. 
