Developments in Ceramic Materials Research

Progress in Porous Piezoceramics 235
A custom made parametric array is used as the acoustic source. The hydrophone
sensitivities are measured by comparing the hydrophone outputs with those of a calibrated
Brüel and Kjær 8103 hydrophone (B and K 8103, Denmark). The outputs of the hydrophones
are amplified by 60 dB with a Brookdeal 9452 precision a.c. amplifier and displayed on a
Lecroy 9304C digital oscilloscope remotely controlled by a computer for data acquisition.
The input impedance of the pre-amplifier is very high (~5 GΩ), so that it does not affect the
voltage developed by the standard hydrophone. Further, the same amplifier is used for the
piezocomposite and standard hydrophones, so that the common errors are cancelled out when
we take the frequency response, which is a ratio of two outputs. The hydrophone is evaluated
in a far-field condition. For each acquisition, the waveforms are averaged over 100 sweeps
and uniformly sampled over 1000 points. The angular response of the hydrophones is
measured by mounting the hydrophones on a rotating table that is remotely controlled by the
computer.
Figure 17 shows the results of acoustic measurements of the dense PZT hydrophone, the
piezocomposite hydrophones (11%PZT-polymer and 22%PZT-air) and the BandK 8103
hydrophone in the form of time-domain signals. The dense PZT and piezocomposite
hydrophones have significantly different responses to the acoustic pulse. The dense PZT
hydrophone showed a ‘ringing’ behaviour with a long tail (Figure 17a) and is not useful for
high-resolution applications, whereas the piezocomposite hydrophones, as well as the BandK
hydrophone produced short clean output pulses (Figure 17b) characteristic of a broadband
transducer. The voltage output from 22% PZT-air composite is higher, while at the same
time, retaining a relatively clean short pulse. The high output from the piezocomposite
hydrophones for a given acoustic input signal is also reflected on the measured sensitivity
graph, shown in Figure 18, which illustrates that radial resonances are suppressed in the
composites when compared with the sharp resonance peak obtained for dense PZT.
Suppression of the radial resonances gives a flatter response to the hydrophone.
Figure 17. (Continued).