Here is a do-it-yourself Hydrophone we made during a workshop in Istanbul. All of the materials cost us about 3 Turkish lira for each Hydrophone (about €1.50 or $2) and the quality is good. Istanbul is great for materials especially in the Karaköy district near the Galata bridge.
The material list is as follows: shielded stereo cable (at least 2m), 2 plastic ‘feet’ (for table or chair legs. make sure one is slightly smaller than the other so as to fit inside the larger), 2 small piezos (21mm), stereo mini-jack plug (use whatever plug you need), 1 or 2 nuts, epoxy (for plastic). Besides this you will need a soldering iron and some solder to connect the cable to the plug and piezos.
To start with you need to solder the piezo to the cable. The center goes to the positive and the ground goes to the outer ring.
The first question most people will have is, why am I trying to make a stereo contact mic? The answer is, I’m not. Most portable recorders use a stereo mini-jack input but not all allow mono recording (at least my Sony PCM-D50 doesn’t) so wiring this in stereo is better than making a mono one and only monitoring in one ear. Besides, for this simple design, maybe 2 surfaces is better than one. If you have a mono input on your recorder than you can simply use one piezo. Before you move on, test the piezo to make sure it works.
Once the piezos are properly soldered the hydrophone is ready for assembly. Epoxy is great but its toxic so be sure to work in a well ventilated space. I put things together in this order to insure a solid construction.
1) epoxy the piezos to the bottom surface of the plastic feet. Push it down firmly as to have as much full contact as possible. Allow the epoxy to harden
2) mix a little bit of epoxy and cover the backs of the piezos to make sure the solder connections are covered. Allow the epoxy to harden
3) Epoxy a metal nut (or lead fishing weight) inside the smaller foot. Allow the epoxy to harden
4) Use a generous amount of epoxy to cover the inside surface of the larger plastic foot. Then carefully slide the smaller foot into the larger one until the two are firmly sealed together.
5) seal the outer edge with a small amount of epoxy. Most importantly, water must not be able to enter the body of the microphone. If it does the solder connection at the piezos may short and you will not get any signal.
6) Once all the cracks or holes are sealed and the epoxy is set, the hydrophone is ready to use!
Original pair of hydrophones I decided to build (but then lost) after hearing some good recordings and discussing with others their experiences. There are several design plans offered on the web. Due to simplicity and availability of materials I settled for the Hydrophone Construction Manual. Below are two recording samples I made recently. While my results may not meet any scientific standards I’m still fairly impressed with what I was able to hear.
It is spring time in Estonia so the local lakes provide a good source of sounds. Hydrophone recording of Mooste lake:
While in Riga this past weekend, I dropped the hydrophones into the Daugava river. Sounds of motor boats in the Daugava river: