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Understanding Marine Electronics - Behind Every Successful Fish Finder is a Great Transducer

By Dave Alston – Technical Consultant http://www.navshack.com

What is a transducer?

Wikipedia says:  “A transducer is a device, usually electrical, electronic, electro-mechanical, electromagnetic, photonic, or photovoltaic, that converts one type of energy or physical attribute to another for various purposes including measurement or information transfer.” Examples of common transducers are microphones and speakers.  Just consider the transducer for your depth sounder to be a combination of both.  When the depth sounder transmits a signal the transducer coverts the electrical energy into acoustic energy, like a speaker.  In the receiving mode the transducer reverses the process and converts the received acoustic energy back into electrical energy, like a microphone.  Once you draw the comparison it’s much easier to understand just how the design and quality of the transducer effects the performance of the sounder.  Even the very best depth sounders are no better than the quality of the transducer it’s connected to.  If it’s not starting to glow yet, as you learn more about transducers that light will go on that tells you maybe I should have been paying more attention to what transducer I selected with that new sounder.

How a Transducer Works

The most basic type is called a single element transducer.  That means it has one disc that vibrates at both 50kHz and 200 kHz.  Higher performance transducers have multiple elements.  On the higher end, transducers may have up to 15 individual elements.  In these cases they would typically have one larger element for the 200kHz signal and 14 smaller elements working at 50kHz resulting in a much higher degree of sensitivity.

With regards to the transducer, it’s the combination of the amount of power transmitted, the size of the cone (beam angle), and the sensitivity of the transducer that determine how well any given sounder works.

Beamwidth and Frequency of a Transducer

A key element of any transducer is it’s beamwidth at a given frequency.  Beamwidth is the size of the cone measured in degrees.  Where a floodlight covers a wide area and therefore has a large beam width, a spotlight is more focused and has a narrow beamwidth.  Not all of the signal is within the stated beamwidth of a transducer just as there is some dimmer light surrounding the main beam of a floodlight or spotlight.  In case you were wondering.  When the signal deminishes to half or less of the main beam, it’s no longer considered part of the beamwidth.  That part of the cone outside the main beam is called the sidelobe.  Targets that enter a sidelobe and reflect enough energy for the transducer to detect are still shown on the fish finder or depth sounder display.

It’s important to know about beamwidth and even sidelobes to properly use and interpet targets shown on your sounder.  For example:  With a beamwidth of 40 degrees, the signal cone is 36 feet wide at 50 feet below the boat.  That means a fish detected in the main beam may be directly below you or 18’ away from center.  If that same fish is large enough and is detected within the outer limits of a sidelobe it could be 50’ to one side or another.  Extend those numbers to 100 feet deep and the fish, or structure, you think you’re on top of may be 100 feet or more away.  That’s when knowing how to operate and interpet your fishfinder really makes all the difference.

The reason behind using sounders and transducers that operate on both 50 kHz and 200 kHz is the inherent characteristics of the specific frequency.  Lower frequencies (50 kHz) can detect targets at greater distances and higher frequencies (200 kHz) have greater resolution.  Lower frequencies use wider beamwidths than do the higher frequencies.  A basic transducer may have a 50 kHz beamwidth of 46 degrees and 12 degrees at 200 kHz.  Higher quality transducers focus the beamwidth to say 20 degrees at 50 kHz and 8 degrees at 200 kHz.  More power to the target means a better operating fish finder.  If you’re interested in running the numbers on your particular transducer, you can find a transducer beam angle calculator tool by clicking HERE.

About the Author

should my fish finder beep when a fish is located?

I view fish on my fish finder ..but shouldn't I get a beep when a fish or the transducer picks up something? I get a visual but no sound..I own a fisherman's habit portable fish finder model 94511. 45 transducer angle with a 350 khz frequency. thanks

The finder should have a menu in it, open it and there should be something in there to turn the sound on and off. I think its called fish id or beep or something.

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