DIY DUNKING HYDROPHONE DETAILS|
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
www.jaycar.com.au 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
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
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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