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Sounder Fundamentals

Dual-frequency vs chirp technology

October 14, 2020
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A hand pointing to a sonar radar marine electronics screen at the helm of a boat.
Although scanning sonars are popular, dual-frequency and chirp models still produce. Courtesy Furuno USA

With omnidirectional and sector-scan sonar systems seeing a significant increase in use, especially in ­tournament fishing, it is important to note that this is not an option for everyone. Cost, physical size and installation requirements are just a few limiting factors that prevent many boat owners from owning them. However, plenty of fish have been—and will continue to be—caught without an expensive high-tech sonar.

Although marks on your machine do not always translate into guaranteed bites, without a sounder you would essentially be blind to what’s under the water’s surface. And to accurately stay on a ledge or track a thermocline definitely can’t be done by just looking at the water itself.

Sounders show us quick, notable information. The seabed line is drawn with varying thickness based on the strength of the returned signal. In general, rocky ­seabeds produce a thicker return than softer sand or mud, which is represented as a narrower return. Looking at the opposite end of the scale near the surface, there is typically a certain level of clutter or noise, and you might notice that running through leftover prop wash shows only surface noise, essentially wiping out the rest of the display as the sounder signals bounce off these bubbles. This is due to leftover air pockets in the water column. A smooth, uninterrupted flow of water is needed to allow the pulse to travel from the transducer to an object, bounce off it and return to the transducer, creating a picture of what’s below. Air bubbles stop theses pulses in their tracks—as does motoring in reverse—throwing turbulent, bubble-filled water over the transducer and interfering with the signal.

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Watch: Check out our video of Mama C, a Bayliss 60.

Frequencies and Pulse Length

Conventional sounders typically possess dual frequencies with a low and a high: 50 kHz and 200 kHz, respectively. The frequency represents the waveform of the pulse that is generated. For example, a 50 kHz frequency generates 50,000 cycles per second during the entire transmitted pulse. This is important in understanding the difference in the image drawn on the screen. Frequency determines the wavelength, and wavelength determines the response.

The pulse’s wavelength directly ­correlates to the type of object it will bounce off. A 200 kHz pulse has a wavelength near a quarter-inch in physical size, while a 50 kHz wave is a little over 1 inch, and the shorter wavelength is able to bounce off smaller objects with higher resolution. This has its drawbacks, though, merely because lower frequencies are able to travel farther through salt water.

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The 50 kHz wavelength is much ­better suited at transmitting through salt water, but it may travel around smaller objects and not return back to the transducer, or it may draw individual responses together as a single return.

A flock of birds swarming over the surface of the sea. A bait ball of fish can be seen just under the surface of the water.
Should a feeding event like this one be unfolding underwater, it’s possible a conventional, ­dual-frequency sounder might misinterpret the individuals as a single mass. © Scott Kerrigan/www.aquapapaprazzi.com

Change the Pulse, Better the Return

Chirp technology changes the waveform of the pulse so it is no longer a single frequency but rather a sweep of various frequencies. Depending on ­manufacturer and model, the chirp pulse would be a sweep somewhere within the specifications of the installed transducer. A low chirp pulse may sweep from 28 to 60 kHz, compared to the fixed 50 kHz of traditional sounder; a high pulse may sweep from 130 to 210 kHz, compared to a traditional sounder’s 200 kHz pulse.

The pulse lengths of chirp ­sounders can also be much longer, investing more power into the water, without ­losing resolution as a fixed-frequency unit would. These differences translate into better display resolution and target separation. Where a school of bait witha larger feeding pelagic nearby might show as one solid mass on a ­ conventional ­fixed-frequency system, a chirp sounder has an improved ­likelihood of showing larger predators both below and around the baitball. As the sound wave sweeps a frequency range, each pulse is given the opportunity to acquire target responses based on the frequency that is returned.

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Chirp technology has no doubt improved today’s sounder capabilities and image quality, but as with any of the tools we use on board, it is up to the operator to be sure he is using the system for the intended purpose. For example, when trolling in deep, open water, it might be necessary to bring up the range to 500 feet—or less—to produce an image large enough to see the responses. And, when deep-dropping, using the bottom lock and zoom features can provide a better view of the targeted area. Similar to a chart or radar display, zoom and range are important for the best viewable picture.

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Extremely common across all the ­current manufacturers’ offerings, chirp is not just a buzzword or sales gimmick. Not having the technology in your arsenal does not necessarily mean you are missing fish by not having it, but it does mean you will not have the increased resolution or target separation you’d get by using it. Chirp simply offers you a more detailed response—allowing individual targets of varied sizes to be drawn separately.

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We all want to know what is ­swimming in the depths, especially when we are on a fishing mission. With the ­variable-frequency wavelength we get in chirp technology, we just get to see it more clearly.

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