Hydrophone fun (not radio)

A diversion today, with construction of a simple hydrophone, to see if we can hear the sounds of our resident pod of Harbour Porpoises.

This is very easy to make, using a cheap and readily-available piezo speaker, which you can buy from Ebay or recycle from those musical greetings cards; both are remarkably sensitive.

All you need to do is glue the piezo sounder to the face of some plastic (I used perspex, 3mm thick), place an 'O' ring seal around it, and bolt the top plastic to this. Don't overtighten.  In fact, I recommend you use brass or copper plate, rather than plastic, as plastic is prone to cracking.

Sealing the hydrophone.  The gap between the perspex edges was later sealed with silicone as a first-layer barrier.  I also added silicone under the bolts.  But the O ring probably is enough on its own.

To pass the wire to the outside world, I cut a small 'nick' in the 'O' ring and added plenty of silicone sealant to restore the seal as the bolts clamp the housing shut - you can see this in the photo.

UPDATE: The perspex, somewhat predictably, is a little too fragile, and tends to crack.  My second version is two food can end pieces, one from the top, one from the bottom (they will tend to give sizes slightly different, that fit, one inside the other.  A small nick to allow the cable out, and sealed between plates with plenty of silicone, makes a much simpler, cheaper and robust microphone housing.

Piezo mic is between the two can lids.  You can varnish or paint this (front) side to protect from corrosion.

I further siliconed the mic assembly to an old perforated aluminium backing, onto which I can attach weights and a rope for deployment.

Reverse of the mic assembly, silicone coming through the holes to provide  secure attachment (I also soldered  copper hanging hook to the mic assembly before I inserted the mic (obviously!))

No interesting recordings beyond the kitchen sink yet (it's raining with a 90km/h wind again), but the hydrophone works well and has very low noise.  It's simply connected to a cheap handheld office sound recorder, the input to which can be monitored using some headphones.

Great fun!  Costs were: perspex £3, piezo (5 pack) £3.95, bolts, cable, audio jack about £3.  A resonable quality commercial version is a minimum of £60.