A General Purpose Fiber-Optic Hydrophone Made of Castable Epoxy

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Fig. 8 Measured acoustic sensitivity of the hydrophone capsule in air and water. The solid line represents
a best fit to the measured data and has a value of 0.277 0.005 rad/Pa. The water measurements are
represented by the filled circles and were made at 23° C. The air measurements are represented by the
open diamonds and were made at 22° C.
4 . 4 Temperature Coefficient of Acoustic Sensitivity
The hydrophone capsule was calibrated in water over the temperature range from 7.5° C to 45° C
(45.5° F to 1 1 3° F) and the sensitivity was found to change at the rate of +1 .1% /°C . That is, the
sensitivity of the capsule increases with increasing temperature (+0. 10 dB/°C). This result includes the
correction for the temperature coefficient of the sensitivity of the Bruel and Kjer Type 8103 which is given
by the manufacturer as -0.03 dB/°C.
Based on the modulus measurements of the epoxy18'23 shown in Table 1, one would expect a
temperature coefficient of acoustic sensitivity that was equal to the measured temperature coefficient of the
Young's modulus which was only +0.4 %/°C. In an attempt to check this result, the temperature
coefficient of the elastic moduli were remeasured using a variation of technique and reaffirmed the original
results.
At the present time, the best available explanation for this discrepency is that the temperature
dependance of the Young's modulus of the epoxy also effected the boundary conditions (and the
resonance frequency) and therefore had a greater effect than would be predicted from consideration of
equations (1) or (2). The finite element model in Figure 5a tends to reinforce this hypothesis since the
elastic response of the junction of the two plates is clearly also flexing. Further studies will be attempted
to resolve and quantify this discrepancy.
24 / SPIE Vol. 1367 Fiber Optic and Laser Sensors V/li (1990)