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Successful Depth Sounder Great Transducer

Behind Every Successful Depth Sounder Is A Great Transducer

by Dave Alston

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.
OK, we now know the transducer converts electrical energy from the depth sounder into acoustic (sound) energy then back again.  The depth sounder produces pulses of electrical energy that matches the characterics of the transducer.  The most common frequencies used are 50kHz and 200kHz.  Inside the transducer is a device called a piezoceramic disc that vibrates at the designed frequency.  By measuring the elapsed time between transmitting the signal and receiving it back, the depth sounder is able to calculate the distance from the transducer to the object reflecting the signal.  Just in case you were wondering how fast all this happens.  Sound travels about four times faster in water that it does in air.  While sound we can hear in the air is covering 768 miles per hour, in water it’s more like 3,320 mph.
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 beamwidth, 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 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 sounder 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 sounder.