Over the past number of months, I’ve been experimenting with various hydrophone designs with varying degrees of success. I don’t really know why I became interested in hydrophones but I guess you don’t know what can be heard if you don’t listen.
I attempted to follow along with Zach Poff’s designs although I had an issue with reassembling the electret microphone capsule once I had drilled a hole in the top of the casing. For the life of me, I just couldn’t get it to work again. I’m sure it’s down to clumsiness on my part but I decided to investigate other means of capturing sounds underwater.
One method was to use a pair of piezo discs on the inside of a tuna can. I fixed the discs in place at the bottom of the can with hot glue and then filled the can with silicone to protect the circuitry and discs from water. I then sealed the can and dipped the entire enclosure in PlastiDip. This worked well but was incredibly bulky. It also got a few knocks when testing it off a local pier resulting in some of the plastic coating tearing and a little water got into the enclosure. It still worked but there was some intermittent noise caused by moisture inside the enclosure.
The next method I attempted was to place an electret microphone into a small bottle filled with oil. The idea was that the vibrations traveling through the water would be transmitted via the oil onto the diaphragm of the electret capsule. The theory was that the capsule would not be damaged by the oil as the oil is not conductive. As it turns out, the oil I used was more like perfumed paraffin so needless to say the electret stopped working soon after being tested. I tried again a couple more times with a different kind of oil before arriving at the design pictured below.
The entire capsule and connections are first dipped in PlastiDip before being placed in a small bottle (I think it originally contained eyewash), although this greatly attenuated the strength of the signal. I also stuck about half a dozen washers to the bottom of the bottle to weigh it down in the water. I was sure that this version would stand the test of time but unfortunately, that wasn’t the case, and after about a month or so this too has stopped working.
I did manage to get some nice recordings before the entire thing stopped though. Below is a short clip of the hydrophone being pushed in and out of a bunch of kelp by the tide.
Finally, I admitted defeat with this particular design (if anyone reading this knows what the issue was please get in touch). I returned to the piezo design but this time I used a smaller disc and set it into a brass plumbing fitting. I encased the disc in copper tape so as to create a balanced connection and then set it in place using epoxy resin, careful not to allow any epoxy to get onto the face of the disc.
Once the epoxy had set, I gave the mic a few coats of PlastiDip and then that was that. The PlastiDip will attenuate the signal a little but it is flexible enough to still work well and the signal to noise ratio is workable.
The mic needs a preamp as piezo discs tend to be quite tinny-sounding when used on their own. I use a little preamp that I put together using this design here. I have yet to have a chance to test it out in the sea but I did get out in a kayak recently. The river was quite shallow but here’s an excerpt of the hydrophone being dragged in the wake of the kayak. Some interesting almost formant-like sounds at times. More experiments to follow.