Depth Transducer Pros and Cons

What They Do
Depth transducers perform a dual role: they act as speakers that transmit a burst of energy down to the bottom of the body of water, and they act like microphones to listen for the returning echo. This information is then fed to the display/control head of a fish finder or depth sounder.

How They Work
Depth transducers use a crystal to produce a sharp pulse (usually at 200,000 cycles per second), then listen for the sound to return to the crystal. The control head measures the time which is translated into water depth. They are generally made from a polycrystalline ceramic which resonates at a very specific frequency.

How to Select

Water depth
If you are consistently operating in shallow inland waters, deep water performance may not be critical, and bottom detail may be the highest priority. However, along coasts or in deep mountain lakes, maximum water depth readings may be precisely what you need. For maximum depth readings, you want a powerful low frequency transmitter, a large diameter transducer, and/or a narrow beam angle. Maximum readable depths in saltwater will be 25%-50% less than freshwater depths, since salt water attenuates (reduces) the signal more.

Frequency
Refers to the "pitch" of the sound that the transducer sends and receives. Water absorbs high frequencies more than low frequencies, so low frequency transducers provide the deepest readings. (50kHz frequencies in water over 400' deep). High frequencies provide better resolution and detail.

If you examine 50kHz and 200kHz displays simultaneously, you'll notice better detail on the high frequency display. This detail could show you bottom feeding fish, bait, structure, or other critical details. Lower frequency displays will show targets such as fish as wider fish arches on the display because of their longer residence time in the wider sound beam.

Dual frequency fishing sonars combine low and high frequency operation in a single display/transducer. This gives serious fishermen the advantages of both, and can combine a narrow and wide beam angle so that anglers can determine if targets are directly below the boat.

Beam width
This is the approximate angle at which the signal spreads out under water. Narrow beamwidth transducers "look" directly below the transducer. Wide beamwidth transducers spread out to the sides and fore and aft. By spreading the beam out over a larger area, the signal is not as strong as it would be with a more focussed beam. Generally, when probing deep water where good resolution is required, a narrow beamwidth transducer is desirable. In shallower waters, a wider transducer will look at a larger area under the boat to locate fish over a greater area. Higher frequencies have narrower beamwidths, all other considerations held constant.

Style and Shape
We refer to depth sounder transducers by several names, but is it difficult sometimes to visualize how those transducers are installed in your boat. By far the most common models are the transom-mount and the low-profile styles. These are usually made of tough plastic. Stem-type transducers are usually used with thick hulls, or when the deadrise angle is too steep for the shorter versions, and are made of bronze. Flush transducers require a countersinking cutter to install, but the result is a beautifully fair, low-resistance installation.

Transducer diameter
High-frequency transducers don't have to be as large as low-frequency versions (think of a woofer vs. a tweeter in a stereo speaker). Also, given the same frequency, larger transducers focus their energy in a tighter cone, resulting in a narrower beamwidth, deeper readings, and greater ability to pick up small targets. Our Sea View fishing sonars are sold with a 2" Skimmer transducer, since they are intended for deep blue-water fishing. The nearly identical Eagle sonars are sold with a 1" Skimmer transducer for greater beamwidth in shallower, inland waters.



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