marine electronics

Chasing Chirps – Simrad BSM-2 Broadband Sounder Review

We cruised along at roughly 20 knots in a 2- to 4-foot chop, and the Simrad BSM-2 Broadband Sounder we were testing never lost the bottom and the screen never turned into a wash of rainbow-colored lines.

The bottom-lock was certainly impressive, but wait a second, was that some structure we just cruised over? No problem. The captain of the boat, Bill Dobbelaer, who is also the sales manager of Gray Taxidermy and a longtime offshore and deep-drop fisherman, hit the backtrack button, rewinding the image on the screen to the exact rock pile we had just run over. He placed the cursor on the structure, got a waypoint, turned the boat around and we had just found yet another possible fishing spot.

“I’ve been fishing here a long time, and I’m now finding stuff I never knew was out here,” Bill says. “This new technology is going to really change the game.”

During this Simrad BSM-2 Broadband Sounder review we noticed it doesn’t just use one frequency to pull up a bottom image, it uses Frequency Sweeping Pulse Compression technology — also known as “CHIRP.” Coupled with the new lineup of broadband transducers introduced by Airmar, this new platform creates an image that is free of any noise. The clear images create more separation on the marks, allowing you to spot individual fish from other targets and the bottom.

We pulled up onto one of Bill’s spots. It was a small wreck consisting of a downed Cessna plane. The wreck was tiny, even smaller than the 40-foot Topaz we were riding on. Bill drifted across the wreck and slowly motored along each side of it as we watched the screen on the sounder. With the BSM-2’s zoom feature we focused on the bottom 200 feet of water and could make out the wings of the plane and the fuselage as it sat on the bottom. It was also super evident which side of the wreck the fish were holding on. And here’s the best part, we were in more than 500 feet of water.

The BSM-2’s CHIRP function uses longer pulses, resulting in improved definition at greater depths. This eliminates the surface and turbulent water clutter as well as interference within the water column to give you an easier-to-interpret image. You never have to crank up the gain to get a good separation of targets. There simply is no need. You can pick out the predators from the bait and see if that’s a rock on the bottom or a big fish.

“CHIRP was actually first used in radar technology,” says Lucas Steward, Navico product manager. “It was used in commercial applications for sonar, but you never saw it in the recreational lineup because the original transducers were too large.”

The new line of Airmar transducers work with next-generation CHIRP, FM, and Spread-Spectrum fish finders. Frankly, it’s the transducers that have made these new sounders possible.


Garmin has also brought a CHIRP finder to market with the GSD26, a $2,000 unit that is tunable from 25 to 210 kHz. This creates a pulse across a range of frequencies for improved resolution. Other electronics manufacturers are not far behind. Geonav is planning to release a unit with CHIRP capabilities, and Furuno has the technology in place but hasn’t used it in any recreational sounders as of yet. Raymarine says it is also working on new sonar units that will use CHIRP, and expects them to hit the market by the end of 2011.

“Broadband transducers represent the next generation of technology that will help fishermen unlock new secrets in fish finding,” says Stephen Boucher, CEO and founder of Airmar. “While broadband transducers enhance conventional echo sounder performance, maximum benefit is when they are driven by next-generation CHIRP and Spread-Spectrum fish finders.”

When transmitting at dedicated frequencies, targets that are smaller than the 50 kHz or 200 kHz sound wave often go undetected on the display. With CHIRP, the transmitted signal covers a wide band of frequencies, providing improvements in the ability to detect all targets in the water column. More bottom and water column area is covered as the beam width constantly changes from narrow to wide while the transducer is “chirped” across the specified band.

According to Airmar, the new transducers send out 25 to 50 times more energy on targets to get the enhanced detail and resolution, precise location of fish, no noise interference, and extreme deep-water sounding below 10,000 feet.

Depending on the model, these transducers contain a low-frequency ceramic array capable of running between 28 to 60 kHz or 42 to 65 kHz. The high-frequency ceramics operate in the 130 to 210 kHz band and the medium-frequency ceramics cover the 85 to 135 kHz band, which has historically been used in commercial fishing, especially 88 and 107 kHz — certain fish species are better detected at these frequencies. This band has rarely been used in the sport fishing market, until now. The transducers come in various mounting styles to accommodate any vessel size and type.

The Simrad BSM-2 features dual transceivers, two depth transducer ports and supports independent speed and dual temperature sensors. The new sonar module is compatible with a wide range of Airmar transducers, including Broadband transducers, BSM-1 narrowband, and Simrad Combi-C models with frequencies of 28, 38, 50 and 200 kHz.

Housed in a die-cast aluminum waterproof black box, the BSM-2 mounts anywhere and is plug-and-play compatible with the full-featured Simrad NSE and NSO multifunction navigation systems via a high-speed Ethernet connection. The unit is 11.37 inches long by 13.38 inches wide and 3.94 inches high and weighs 10.6 pounds. The BSM-2 retails for $2,495 and you can combine it with multiple transducers.

If you’re in the market to upgrade your sounder, take a chirper out for a test drive. You have to really see one in action to believe it.

Manufacturers Mentioned

Furuno —

Garmin —

Geonav —

Simrad —

Raymarine —

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