106 Lecture 6 
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phones in the 100 to 3000 cps range which cannot be easily obtained under free- 
field conditions. Free-field calibration is carried out in a range from 2 to 10 
keps. It is thus possible, by using the value measured in the 2 to 3 kcps band, to 
obtain the over-all response between 100 cps and 10 kcps. Provided some pre- 
Cautions are taken, this method works with a good accuracy. 
Fig. 6.2. Closed-tank 
pressure calibration. 
6.1.4, Closed-Tank Pressure-Calibration Technique* 
Although preferring on the whole the method of electrostatic calibration, we 
also apply the well-known closed-tank measuring technique for occasional checks. 
Our closed-tank system consists of a small cylindrical tank, thick-walled in 
order to minimize the excitation by flexure modes. It is filled with water and a 
small volume of air of known value. The hydrophones having been placed in the 
water, the air is compressed mechanically by a small piston. This device works 
in the range below 50 cps. In the very low frequency region (3 cps), the heat ex- 
change between tank wall and air would call for correction factors to the adia- 
batic law, but we have never exploited this technique to its utmost limits. We 
are, however, aware of the fact that other laboratories have succeeded in im- 
proving this calibration procedure in a very satisfactory manner. In Fig. 6.3 are 
summarized the results of various methods for the calibration of a H6Tcl-type 
hydrophone, 
6.1.5. Calibration by Comparison Using Filtered Noise 
The hydrophones used in acoustic ranging are calibrated in the range 2000 to 
100,000 cps by comparison witha standard hydrophone calibrated by the reciproc- 
ity method. The most highly perfected anechoic tank still presents a certain 
*Figure 6.2. 
