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Various types of hydrophones may be constructed, including some which could be used at great depth, but for shallow water use the cheapest and most sensitive are called "bender" hydrophones.

The following  diagram and photo provide information on construction of a simple DIY  bender  hydrophone, in a housing suitable for dunking over the side of a stationary boat - or even a wharf. It uses the piezo-ceramic element and attached brass disc and wires, extracted (taking great care not to scratch or flex it!) from an inexpensive audio transducer as illustrated below. Such transducers are available from electronic hobby suppliers (e.g. Audi Transducer catalogue # AB-3440 from for Au$3.60). Similar piezos  are also found within  electronic buzzers and even some talking greetings cards , but  I don't know whether those buzzers having three rather than two connections to the piezo are suitable. As a buzzer or enunciator the piezo is made to vibrate by applying a rapidly varying voltage to it - but all these piezo elements  can also do the reverse - produce tiny voltages that vary as sound waves "bend" the piezo.

To make a transducer the brass shim has to be bonded over a shallow sealed air cavity, so that it can minutely bend in response to sound pressure waves striking its outer surface. I made my cavity by machining a shallow well in a suitable piece of round aluminium, but something similar could also be fabricated by bonding a ring, or perhaps even a loop of wire onto a flat metal disk. After bonding (with rapid set araldite epoxy) the disk in place ( to seal the cavity with the ceramic side out) I "potted" the whole thing in a runny grade of epoxy resin, with the wires protruding. The curing rate was accelerated using a dinner plate warming oven set to minimum heat, with the items (bender elements and pre-amplifiers - see later-)  suspended by their wires over a disposable aluminium foil tray.

 The voltages produced are extremely small, so two stages of amplification are needed to make the variations loud enough to hear with headphones or a very small loud speaker. The electronics involved are cheap and not difficult to assemble, and the second stage amplification can even be a general purpose audio amplifier or pre-amplifier purchased "off the shelf".

In the example below,  a transparent film canister has been used as the housing, but many other flexible containers are also suitable, including sauce squeeze bottles. In this instance the canister has a snap-seal, but the the rubber-weld tape prevents it opening if dropped. The optional "deflection limiter" protects the disk against cracking if the hydrophone is used too deep, but if a relatively short cable (3-5 meters) is used the limiter should be unnecessary.

The various electronic components within the canister and inside the project box form a pre-amplifier circuit,  with the output signal and the power supply to the pre-amp using the same pair of wires in the cable. The diagram shows an external 12 Volt supply to the project box, which could come directly from a boat's battery, or via a voltage regulating chip. The pre-amp circuit will work over a range of voltages, and the unit in the photograph is self-contained, powered by a single 9 Volt battery within the project box.
The optional pair of 1N914 diodes help to protect the pre-amp circuit against voltage spikes that may  arise  if the hydrophone is dropped on deck for example.
 When working correctly and coupled to an amplifier and headphone as shown, the hydrophone is sensitive to small sounds in water, but does not function effectively as a microphone in air. One way to test is immerse the hydrophone in a bucket of water, and pour cupfuls of  water into the bucket from a height of  300mm or so. The water noises should be clearly audible.

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