I haven't been following your hydrophone design effort very closely, but I had a look at the photos you posted, and I was wondering why you would mechanically / electrically select a particular hydrophone to listen to? A multiple receiver array of directional hydrophones like that seems ideally suited to computerized monitoring whereby every transducer signal is monitored simultaneously, and target direction could be determined from SNR of every hydrophone in the array, FFT performed to identify particular targets, configurable filters applied, and an audio stream for user monitoring provided by mixing all signals - again shapable by software filter, so you could listen in a particular direction if you wanted to, but significant sound sources would be automatically located. The hardware to do this wouldn't be much more than a single board computer with analog input data aquisition, and having the processing ability available means you could filter out the boat traffic above to listen to the whale song below - good cruising music.
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