Hydrophones

ERDC TN-DOER-E18

February 2005

Widely used fixed and mobile antennas include the 4-, 7-, and 12-multi-element General Purpose

Yagi. The Yagi antenna is the standard directional receiving antenna and is the most commonly

used antenna in radio telemetry. The basic model consists of a boom, generally about 1 meter in

length, with three elements of equal length. The length of the elements, as well as the spacing

between them, is the inverse function of the frequency for which the antenna was designed. The

three elements are referred to as the reflector, driven, and director. The addition of each

subsequent director element increases the overall range of signal detection. The signal is passed

through a coaxial cable equipped with a BNC connector, from the driven element to the

receiver/datalogger.

Hydrophones. A hydrophone is an electronic receiver for detecting or monitoring sound

traveling through water by converting acoustic energy (sound energy) into electromagnetic

waves (electrical energy) by use of a ceramic transducer. High quality hydrophones contain

internal low noise pre-amplifiers to increase the amplitude of signals at their source and to

reduce noise as the signal is transmitted along the cable. Potential sources of noise include the

power supply and electrical interference, although in well-designed units this is usually not a

significant problem. An external source of noise can result from water flow if the hydrophone is

attached to a moving vessel. Assuming that the hydrophone is mounted in an area in which

water flow is smooth, this noise is dominant when compared to noise generated by the vessel

itself. For a typical hydrophone, the effect of flow noise will be insignificant up to a vessel speed

of a few knots. Beyond this point noise will increase dramatically (~18 db for every doubling of

vessel speed).

There are different types of hydrophones to match different receivers, users and applications.

Hydrophones may be either directional or omni-directional. Omni-directional hydrophones

receive signals from all directions with equal sensitivity. They are typically used for data

telemetry from captive animals or stationary transmitters. Some models, such as the Sonotronics

DH-3, are typically used in small pools with a detection radius of tens of meters. Directional

hydrophones have a greater sensitivity to signals from a particular direction, and are used in

tracking to determine the direction of the transmitter. Directional hydrophones can use either a

single transducer element with a dish or conical-shaped sound reflector to focus signals or a

more complex array system, which uses transducers of one or more elements in an array. For

frequencies used in tracking, a "line array" provides a manageable size hydrophone. Note that

the beam width of a "line array" hydrophone is a function of the signal frequency and is

therefore only suitable for a range of frequencies. For example, the VH10 hydrophone,

manufactured by VEMCO, is designed for use between 50 and 80 kHz and can only be used for

transmitters within this frequency range (VEMCO Models V8 and V16 transmitters).

Transmitters with lower frequencies, such as their V22 and V32 line, would require a

hydrophone designed for that specific frequency range.

Hard-wired hydrophones can be used where the study site permits. Bottom mounting of hard-

wired hydrophones tends to provide better position accuracy. Hard-wired systems are also less

expensive when compared to radio-linked systems. A comparison of one model of wired-

hydrophone (LHP-1) manufactured by Lotek Wireless averaged under $700, or approximately

six times less expensive than the wireless models ($4500). The most frequently cited