Page 241 - Electromechanical Devices and Components Illustrated Sourcebook

Chapter 11 Acoustic Devices 203

Hydrophones
Terminal
Plastic Cap
Rolled Seal Hydrophones are microphones that are specifically designed
Fixed Magnet to operate under water. Typically, a hydrophone is a standard
microphone that is housed in a waterproof housing. A simple
Case
hydrophone can be made by stretching a condom, or balloon,
Floating Coil
over a small standard microphone and sealing it around the
cable. Care should be taken when using one of these home-
made units, as they will not have very good resistance to
Figure 11-39 Geophone
depth. A hydrophone made in this fashion is probably good to
about 1 atmosphere or 33 feet of depth.
Figure 11-42 shows an inexpensive hydrophone that is
intended for general purpose underwater listening. It consists
O-Ring Sealed Cap of a crystal microphone that is molded into a watertight hous-
Waterproof
Cable Terminations ing. The unit is connected to an amplifier with a set of head-
phones and lowered into the water. These types of hydrophones
Bend Relief
are popular for listening to marine life and conducting acoustic
inspections of underwater equipment.

High Impact
Plastic Case Field Cable

Cable

Ground Spike

Figure 11-40 Geophone Field Assembly Molded Plastic Housing

Figure 11-42 Hydrophone Assembly

For locating underwater targets, hydrophone arrays are
Clamp Bolt
deployed as shown in Figure 11-43. A ship will tow a string
of hydrophones and then generate a sound pulse. The sound
Cable Bolt-On Cap propagates through the water and bounces off the object to be
located. As information from the hydrophones is fed into a
Three Geophone Elements
Positioned on Opposing Axis shipboard computer, a bearing to the object can be instantly
determined.
High Impact
Plastic Case

Ground Spike Ship

Figure 11-41 Three Element Geophone Assembly Towed Hydrophone Array

Sound
Source Reflected Sound
higher accuracy data and are typically laid out in arrays con-
sisting of several hundred to thousands of units over broad
geographic areas. A single explosive pulse is detonated and
the geophones pick up the reflected sound and transmit it to Transmitted Sound
field recorders. The recorded data is later analyzed on a cen- Target
tral computer system. Figure 11-43 Towed Hydrophone Array