Fish Finders – Understanding CHIRP, DownScan Imaging, etc. in Depth


Depth finder descriptions are jam-packed with figures and jargon – what to the technical details really mean, and what features do you actually need?

A lot has happened since the physicist Alexander Behm submitted patents in 1916 and 1920 for his sonar technology. These days, manufacturers like Lowrance, Garmin, Humminbird and others keep outdoing one another with tech developments of their own. These constant improvements to the technology give us better and better insights into the world beneath the waves. Where we once had only rudimentary dots on our fish finder screens, we could now be forgiven for thinking we were looking at actual photographs of what goes on down there.

There are numerous specialized technologies like CHIRP, DownScan Imaging™, Down Imaging (DSI)™, Side Imaging and Structure Scan HD™ – but what do they all mean? That’s exactly the topic we’re going to dig into in this article.

How does a depth finder work?

Before we get into the functionality of CHIRP and photorealistic imaging, we need a fundamental understanding of how different frequencies work. We’ll just do a quick overview of the subject here, as we’ve already done an in-depth explanation of the topic in this article. If you want even more comprehensive information, we recommend taking a look at The Ultimate Guide to Fish Finders.

When a transducer sends a soundwave into the water, it’s called a “ping”. The soundwave (ping) is an audible tone created by a vibrating crystal and has a specific frequency. Most fish finders operate in the 2D sonar range: frequencies between 50 and 200 kHz. A 200 kHz wavelength is significantly shorter than one at 50 kHz (ultrasonic wavelengths range from 0.3 inch to 1.2 inch (7.5 mm to 30 mm), which means that an image produced at 200kHz will be much more detailed than one produced at 50 kHz: more soundwaves come into contact with the object in question and are reflected back up to the transducer.

At high frequency, individual soundwaves are shorter than those at low frequency.

The frequency utilized has a significant impact on how deep the ping can penetrate into the water. It’s also a determining factor in whether a given spacing between fish results in them appearing to be a single fish on-screen. The difference can be massive: at 50 kHz, fish that are closer together than 50 cm simply can’t be distinguished from one another. This is directly dependent upon distance resolution and should be a point of interest for anyone who owns or is looking to buy a fish finder. In our book, we go into great detail on the subject of distance resolution.

Individual soundwaves impact a given object much more often at higher frequencies than at lower ones. In terms of imaging, this results in much more detailed scans of fish.

What is CHIRP sonar?

Conventional transducers emit sequential soundwaves of a single frequency and convert the returning echoes into an image. Things are different with CHIRP. Here, the transducer simultaneously emits multiple soundwaves of differing frequencies and analyzes the returning echoes – the higher frequencies are more sensitive, and the lower ones penetrate farther into the depths. Taken together, the differing frequencies enable CHIRP to create a much more detailed image. CHIRP stands for Compressed High-Intensity Radiated Pulse, and it represents the state-of-the-art when it comes to depth finder technology. With its Elite 7 Ti Touchscreen fish finder, Lowrance has done a fantastic job of implementing CHIRP.

sonar traditional
sonar chirp

Lowrance Elite 7 Ti Touchscreen fish finder/map plotter

Manufacturer’s specifications

  • Exclusive high-resolution Lowrance 7-inch color display with LED backlighting
  • Quick access to all functions of the Elite 7 Ti via touchscreen
  • CHIRP sonar, StructureScan® and DownScan Imaging™
  • Premium sonar technology delivers the best possible underwater imaging
  • StructureScan requires the optional TotalScan transducer
  • TrackBack™
  • Scroll through your SideScan or DownScan history to take another look at bathymetry or target fish and set location waypoints
  • Precision integrated GPS antenna
  • Optional worldwide map upgrades, including Navionics+ and Jeppesen C-MAP MAX-N
  • Integrated wireless connectivity
  • GoFree Cloud access
  • Memory card slot (microSD)
  • Mounting bracket with quick-release


So, you want to know why CHIRP depth finders deliver images 1,000 times better than those with fixed frequencies? Get these 20 free pages and find out! They’ll fill you in on the incredible capabilities of CHIRP and how you can optimize your fish finder for real-world situations. The Ultimate Guide to Fish Finders

What are Down Imaging, DownScan Imaging (DSI), ClearVü and DownVision?

Down ImagingDownScan Imaging, ClearVü and Downvision provide photorealistic representations of objects (or the bottom) directly below your boat/transducer. This photorealistic imaging is made possible by the use of higher frequencies and specialized software which interprets and converts the signals produced and collected by the depth finder. Usually, dual-beam fish finders operate with 2 different frequencies in the 50-200 kHz range.

Different manufacturers, different names

  • DownImaging – Lowrance
  • DownScanImaging (DSI) – Humminbird
  • ClearVü – Garmin (in Europe)
  • DownVision – Raymarine

With Down Imaging/DownScan Imaging, high frequencies of 400+ kHz are used to capture exceptionally detailed information and convert it into a high-definition image. As such, the soundwaves transmitted downward in narrow bands as opposed to a cone. Have a look at the picture below to get an idea of how this setup works.

SideScan sonar transmits its signal horizontally from both sides

The exit angle is usually less than 45°, and these high frequencies make it possible to turn the resulting information into photorealistic images. The higher the frequency and display resolution (number of pixels) the fish finder/depth finder has, the more realistic the imaging.

Fact is, photorealistic images are a huge help in analyzing bathymetry, which can lead to the discovery of previously unknown hotspots and – most importantly – more fish!

One of the most affordable fish finders with this new technology is the Lowrance HOOK 4 X Sonar

Manufacturer’s specifications

  • Exclusive high-resolution, vibrant Lowrance 4-inch color display
  • CHIRP sonar plus DownScan Imaging™ – the leading fish finder technologies together in one device give you the best possible view of the world beneath your boat
  • DownScan Overlay™ technology displays DownScan Imaging on top of CHIRP sonar
  • Advanced Signal Processing (ASP) reduces the need to manually adjust settings in order to get a clear view of fish and bathymetry
  • TrackBack™ lets you review your collected sonar history
  • Display multiple CHIRP zones with one transducer
  • CHIRP sonar performance with various affordable transducers, including the Lowrance HDI Skimmer®
  • Display multiple CHIRP sonar settings all on one screen

What are the drawbacks of high/photorealistic frequencies?

The higher a soundwave’s frequency, the shorter its range. It’s safe to assume that a 400 kHz frequency can be used at depths of between 130 and 230 feet (40 and 70 meters). This means that the soundwaves

  1. reach the bottom
  2. aren’t eliminated by disturbances in the water
  3. produce an echo with enough energy to return to the boat/transducer

Higher frequencies, like those emitted by Humminbird Mega Imaging, have even shorter range. The newest fish finder series from Humminbird utilizes an unbelievable 1,200 kHz frequency, which creates ultra-sharp images at a depth of up to 100-130 feet (30-40 meters). These numbers should be sufficient in most European bodies of water, as no one uses only photorealistic imaging while fishing.

One our favorites in the sub-$2,000 price segment: The Helix 9, 10 and 12 CHIRP SI Mega Imaging series (2nd generation)

Image courtesy of – you can even see the fishes’ shadows!

The new 7-element MEGA transducer can produce the following transduction cones and frequencies (Side Imaging 180° coverage, Dual
Beam PLUS™ with SwitchFire and Down Imaging):
– 2 x 85° cones; 1,200 kHz @ -10dB (MEGA),
– 2 x 55° cones; 800 kHz @ -10dB
– 2 x 85° cones; 455 kHz @ -10dB
– 20° & 60° cones; 200 & 83 kHz @ -10dB

The Helix 9, 10 & 12 series differentiate themselves based on their display sizes (9, 10 and 12-inch, respectively). This is an extremely well-executed group of new depth finders, and we can give them a wholehearted “best buy” recommendation!

Manufacturer’s specifications:

DI transducer:

  • Lower 2D frequency range: 185 kHz – 225 kHz
  • Higher 2D frequency range: 440 kHz – 490 kHz
  • Lower DI frequency range: 420 kHz – 520 kHz
  • Higher DI frequency range: 790 kHz – 850 kHz

SI and 2D transducer:

  • HIGH 2D frequency range: 175 kHz – 225 kHz
  • MEDIUM 2D frequency range: 75 kHz – 95 kHz
  • LOW HB 2D frequency range: 40 kHz – 60 kHz
  • DI/SI frequency range: 420 kHz – 520 kHz

Helix 9, 10 and 12 CHIRP SI Mega Imaging series (2nd generation)

What are Side Imaging and Side Scan?

With Side Imaging, soundwaves are emitted out from both sides of the boat. To work properly, the transducer needs to have a clear view – that means it shouldn’t be blocked by the motor or other parts of the boat! Especially if you have a small boat that doesn’t have a lot of extra real estate, you should check out how this kind of transducer needs to be mounted before buying one.

  • SideScan – Lowrance
  • SideImaging – Humminbird
  • SideVü – Garmin
  • SideVision – Raymarine

The SideScan depth finder frequently operates at frequencies in excess of 4,000 kHz, which are emitted horizontally to create a sort of underwater bird’s-eye view. The high frequency of the depth finder’s transducer captures rich details and offers horizontal ranges of up to 250 feet (75 meters) (depending on the frequency used). The biggest benefit of SideImaging is that you have to boat around significantly less than you would with a conventional 2D or DownImaging transducer – it’s simply a much quicker way to find hotspots.

When using this feature, it’s important that you keep your boat speed to a maximum of 9 mph (15 km/h) (lower is better). Likewise, you should follow as straight a path as possible unless you want inaccurate side imaging – swerving too much can cause details to be lost completely or return frequencies to overlap (which can result in skewed information). Anyone who picks up a depth finder with SideScan or similar technology should familiarize themselves with depth finder shadows – you don’t want to miss out on the best hotspots, do you? (Ultimate Guide to Fish Finders p.124, etc.)

Here’s a quick look at the Humminbird Helix 7 SI (it’s got a built-in map plotter).

Humminbird Helix 7 SI

Manufacturer’s specifications

  • Max. depth: 1,500 ft (450 m), 590 ft (180 m) in salt water; 330 ft (100 m) with DI; 115 ft (35 m) with SI
  • Side Imaging, Down Imaging®, DualBeam PLUS™ with 20°, 60° and 85° cones, 200/83/455 kHz @ -10 dB, Output: 4,000 W PtP (500 W RMS), water temperature
  • 7-inch TFT color display, 800 x 480 pixels, 256 colors, LED backlighting
  • Fish ID+™, displays fish icons with depth values
  • Structure ID® bathymetry display
  • SWITCHFIRE technology
  • Adjustable scroll speed
  • Large, easy-to-read digital numbering
  • Depth alarm, fish alarm (for all three sizes)
  • Manual and automatic settings for zoom, sensitivity and depth zones
  • Compact unit with tiltable bracket mount
  • Internal GPS antenna
  • Nautical map plotter for Navionics maps (SD/MSD format)
  • Waypoints, routes, tracks: 2,500 waypoints, 45 routes, 50/20,000 tracks
  • Compatible with Humminbird Autochart™ und Autochart™ PRO

Want to catch more fish? Download over 20 free pages and learn how to get the most out of your fish finder!

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What is StructureScan HD?

Structure Scan HD™ is a registered trademark of the depth finder manufacturer Lowrance. It’s a separate module that combines the benefits of DownScan Imaging™ (DSI) and SideScan  to create a comprehensive, photorealistic representation of the underwater world below and to the sides of the boat. The tech also uses dual 455 and 800 kHz frequencies to capture 3D data, which it then converts to a 2D image for display on-screen. It’s able to create realistic imaging of the water all around the boat at depths of up to ca. 160 feet (50 meters) and a radius of ca. 250 feet (76 meters). It gives you a comprehensive overview of what’s going on below and around you, making it easier than ever to find hotspots. A submerged tree looks like a tree, and a boulder looks like a boulder – you can also see the depth and distance of displayed objects. Now, you don’t have to master the art of depth finder interpretation before getting down to business!

The depth rating of 160 feet (50 meters) is entirely sufficient for bodies of water, and the tech is usable at boat speeds of up to 9 mph (15 km/h) – just remember to mount the transducer in an interference-free location on your vessel.  Of course, the fun and privilege will cost you more than a conventional fish finder. The HDS-7 Generation 3 model from Lowrance is an ideal starter unit. 

Lowrance HDS 7 Gen. 3 CHIRP

Manufacturer’s specifications

  • 7-inch widescreen multitouch display with LED backlighting
  • Improved Lowrance interface with touchscreen or button operation
  • Simultaneous CHIRP sonar and StructureScan® HD imaging
  • Integrated wireless connectivity with Lowrance GoFree app and other onboard devices
  • Plug-and-play compatibility with Lowrance performance modules: Broadband Radar, SonicHub Marine Audio, Class B AIS and DSC VHF, as well as industry-leading technologies like SmartSteer for MotorGuide PinpointGPS and the Lowrance Outboard Pilot.
  • Interfaces with HDS Gen2 Touch und HDS Gen2 multi-function displays
  • Dual ethernet network ports
  • Micro-C port for NMEA 2000® devices
  • Video input via optional adapter cable
  • Shares the same flush-mount cutout and bracket with the HDS-7 Gen. 2 Touch (supported via Lowrance Advantage Service Program)


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