166 MEASUREMENT OF PRESSURES 



where A = to^ is the gauge area. The relative response R{f)/KAPm 

 is plotted in Fig. 5.7 as a function of {a/c)J and is seen to fall to zero for 

 a frequency /« = 0.61c/a and to pass through a series of maxima and 

 zeros of response for higher frequencies. The useful frequency range 

 of such a disk, as limited by the transit time 2a/c across its face, thus 

 extends to an upper limit somewhat less than the value /«. For a gauge 

 ^ inch in diameter in water this frequency is 150 kc./sec. 



1.5 



"t^ 0.5 



o 



> 1.5 



I- 

 < 



-I 



UJ 

 OL 



1.0 



05 



0.1 



10 100 



FREQUENCY (KC) 



1000 



Fig. 5.8 Measured frequency responses of circular disk tourmaline gauges. 

 The dashed curves are calculated from the geometrical size effect. 



Steady state response measurements have been made on 3^ and 

 Yz inch circular disk tourmaline gauges at the Mountain Lakes station 

 of the Underwater Sound Reference Laboratory (Division 6, NDRC) 

 by an experimental method developed for calibration of underwater 

 sound hydrophones, which depends on the so-called reciprocity relation 

 between the functioning of an acoustic transducer as a source and de- 

 tector.^° The results of these tests are slioAvn in Fig. 5.8, where for 

 comparison the responses predicted by Eq. (5.12) are indicated by 

 dashed lines. The general agreement, apart from irregularities in the 

 experimental curves, indicates that the very simple concept of transit 



^° The theory of reciprocity calibrations is given in a report by Dietze (27), and 

 the measurements described here are from another report by the same author (28). 



