How Sonars Work: Key Aspects to Know

Understanding the basics of how sonars work and how to read your fish finder can be the difference between a blank session and landing a PB. Our short tutorial on how sonars work will teach you the basics about your fish finder and give you handy tricks to make reading it easier.

How Sonars Work

SONAR stands for SOund NAvigation Ranging. A sonar device sends pulses of sound waves down through the water. When these pulses hit objects like fish, vegetation or the bottom, they are reflected back to the surface. The sonar device measures how long it takes for the sound wave to travel down, hit an object and then bounce back up. It’s the same echo-location system bats and dolphins use. This information enables the device to judge the depth of the object it reflected off. It also measures the strength of the returning pulse – the harder the objects, the stronger the return pulse.

Once a returning pulse is received, another one is sent out. Because sound waves travel at roughly one mile a second in water, sonars can send multiple pulses per second. The Deeper PRO, Deeper PRO+ 2 and Deeper CHIRP+ 2 send 15 pulses per second. The returning sound pulses are converted into electrical signals and then displayed, showing anglers the depth and hardness of the bottom and any objects in between.

This information enables the device to judge the depth of the object it reflected off. It also measures the strength of the returning pulse – the harder the objects, the stronger the return pulse.

Four things to remember

  • Sonars scan in cones, not lines.

  • A scrolling screen doesn’t mean a moving sonar (or lots of fish).

  • Thicker lines and second sonar returns mean harder bottoms.

  • Spot the arches and you’ll find the fish.

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1. Sonars scan in cones, not lines

When we read data from our fish finder, we usually imagine that the information we see on our screen is all happening directly under our sonar. So, if we see a fish on the screen, we think it must be exactly underneath our sonar. In reality, the readings we see are taken from a wider area underneath our sonar. And even more importantly, the sonar receives data from a wider and wider area, the deeper you scan. This is all because sonars scan in cones.

Here’s how it works.

Sonars send out pulse of sound to locate objects. Sound travels in waves, not straight lines, and these waves expand in cones, getting wider and wider.

Most sonars can control the range of the sound wave cone by changing the scanning beam frequency. This is important because in different fishing situations different scanning beams will be more or less effective.

Wide beam scanning (usually 40° to 60° angle) is good for quickly scanning large areas and geting overall information on depth and bottom structure, but the accuracy and detail will be lower. Wide beam scanning is best suited for shallower waters because the cone covers a wider area, the deeper it scans. This means if you are scanning at a depth of 45ft / 13,7 m you will see objects in an area that has a 47ft / 14,3 m diameter.

Narrow beam scanning (around 10° to 20°) gives a more precise picture but covers a smaller area. It is better for finding the exact location of fish. Narrow beam scanning is also better suited for deeper water, as the cone does not spread as wide.

Surface clutter and blind zones

Another factor to consider related to your sonar cone width is that in some cases you will not be able to detect objects just below the surface of the water.

The cause of this is what’s known as surface clutter, which is common to all sonars. Surface clutter appears because the water close to the surface will reflect some of the sonar waves, and these reflections are much too fast for the sonar to process correctly. This reflection has numerous causes, the most common being waves on the surface, bubbles, currents and algae. The result is lots of “sonar noise” close to the surface. This creates a “blind zone“ in which it is not possible to identify fish.

The amount of clutter, and the size of this blind zone, can be reduced if the sonar frequency is higher. So, if you have a Deeper PRO and you are experiencing a lot of surface clutter, switch to scanning with the higher frequency (Narrow beam at 290kHz 15°). In the case of the Deeper START, its 120 kHz sonar frequency means surface clutter can reach down to 1 meter / 3.3 ft. below the surface of the water.

The Deeper PRO+ 2 and CHIRP+ 2 have the least amount of surface noise and clutter, providing accurate readings up to 15 cm / 6" from the water's surface.

The image below illustrates 2 different situations where surface clutter can affect your sonar readings – (in these examples the surface clutter extends to approximately 1 meter / 3.3 ft. below the surface):

  1. The fish is below the surface clutter zone. In this scenario, the sonar reflection from the fish is strong enough to be identified by the fish finder (in the case of Deeper fish finders, the Fish Deeper App uses an algorithm to work out if it’s a fish). As a result, the fish is displayed on the app.
  2. The fish is within the surface clutter zone. The sonar reflection from this fish is mixed together with the surface clutter, making it too weak to be identified by the fish finder. As a result, the fish is not displayed on the app.

Why this matters for …

  • Fish finding

    For fish finding, don’t assume every fish you mark is directly under your sonar. Instead, think that they are somewhere within a cone spreading out under your sonar. And remember, the deeper the depth it marks at, the wider the area it could be in. If the fish is shallow, then you know it’s more or less directly under your sonar, especially if you’re using a narrow beam. If the fish is deep, then it could be in a much wider area, and much further from the location of your sonar.

    Deeper Sonar Tip: When hunting for fish, use the broad beam first to find the general area of the fish, then switch to narrow beam and scan that area a few times to get the exact location.

  • Finding structure and features

    An additional point you should understand when feature finding is the concept of what’s known as a dead zone. Your sonar will use the first bit of bottom it detects as the level for marking the bottom on your screen. But if the cone is scanning a drop off, there might be a deeper section underneath which doesn’t get included in the scan – this area is the dead zone (see diagram).

    Deeper Sonar Tip: Using a narrow beam will minimize the chances of their being a dead zone on your display. When you find a drop off or feature, scan it a few times using the narrow beam setting.

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2. A scrolling screen doesn’t mean a moving sonar (or lots of fish)

On the Fish Deeper App and many other sonar displays the data on screen is shown scrolling from right to left. The far right of the display shows the most recent results, the oldest are on the left. You should remember that your screen will keep scrolling even if your sonar isn’t moving, because the unit is constantly sending and receiving sound pulses. Understanding how this scrolling display works is really important for making sense of the sonar data you receive.

Why this matters for …

  • Fish finding

    One of the easiest mistakes to make when analyzing your sonar readings is to mistake one fish for many fish. Here’s how it happens. You cast out and there is a stationary fish suspended in the water. If you don’t move your sonar, and the fish stays still, you will see a constant stream of fish icons on your screen. It’s a natural reaction to get excited and think there are 4 or 5 huge monsters down there. In fact, there is only one, but the scrolling display makes it look like there are more.

    Deeper Sonar Tip: If you are finding the scrolling display confusing, try adding the vertical flasher display (Settings – Sonar – Vertical Flasher: On). It’s just like the Ice fishing display, and goes on the right of the screen. This display is a live feed that doesn’t scroll – it shows what’s happening right now under your sonar.

  • Finding structure and features

    Imagine you’ve cast out your sonar, and now you are reeling it back in to get a picture of the underwater structure. You stop reeling in for a few seconds, then start again. Afterwards, you look back at the scan and see a steady incline, but with one level, flat area in the middle. So, is there a flat section to the water bottom?

    The answer is no! That’s because the horizontal axis of your display is showing time, not distance. The “flat section” you see is when you stopped reeling in. The sonar kept scanning and scrolling, so it looks like the bottom is flat when it isn’t.

    Deeper Sonar Tip: To avoid this, maintain a steady speed when reeling your sonar in. You can also use the mapping features from boat or shore instead. These use GPS to add depth levels onto your map, so there’s no problem if the speed you reel in changes.

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3. Thicker lines and second sonar returns mean harder bottoms

Your sonar is capable of telling you not just what the bottom structure looks like, but how hard the bottom is as well. Here’s how.

Sonars measure both the time it takes for a sound pulse to return, but also the strength of the signal that returns too. This enables it to show how hard or soft the underwater objects are. Soft, low density objects return a weaker signal, whereas hard, high-density objects return a stronger signal.

Your sonar display will show you how hard the object is using color and brightness: the more vivid the color, the stronger the signal and therefore the harder the object. This is particularly important when scanning the bottom.

You may notice that the bottom display gets thicker and more intense in some places (hard bottom), then thinner and weaker in others (soft bottom). You might also notice a second sonar return for the bottom. Here the bottom is so hard, the sonar beam has reflected up to the surface, bounced down again, reflected off the bottom and been picked up by your sonar.

  • Soft bottom

  • Example of second bottom

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Why this matters for …

  • Fish finding

    Being able to analyse bottom consistency is all part of learning to work with the raw data. It might take a bit of your time to master, but you’ll save lots of time in the long run, because you’ll understand exactly what’s down there.

    Deeper Sonar Tip:  If you want to improve, it’s important to go from knowing (“I know where the fish are”) to understanding (“I understand why the fish are there”). Start looking for relationships between the bottom hardness you see on your sonar and where the fish are usually located. For example, you might notice that in certain conditions or seasons your target species is always found where there is a soft bottom. This is a valuable piece of understanding that will significantly improve your catch rate.

  • Finding structure and features

    This factor doesn’t impact too much on fish finding directly. But it can be interesting to look for patterns in terms of bottom hardness and fish location.This data on bottom hardness is very useful as part of the overall picture you build up. Telling the difference between brush piles and rocks, muddy bottoms and hard bottoms, can be crucial in finding the right spots to hunt your target species.

    Deeper Sonar Tip: Once you have found an interesting spot, use the narrow sonar beam to get the most detailed and accurate readings of bottom hardness. Make sure you are using the detailed rather than basic display in the Fish Deeper App (use the left hand menu to choose) to see bottom hardness readings.

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4. Spot the arches and you’ll find the fish

Using fish icons is a great way to get started with a fish finder, but you’ll be able to identify fish, and fish size, more accurately using the raw data. So, when you’re ready, switch off your fish icons and start looking for arches.

Why Arches?

In most cases, fish will show up on your display as an arch. The reason why is very simple. If a fish swims right through your sonar cone, it will return impulses from the edge of the cone, the middle and then the other edge. The returns from the two edges of the cone have traveled slightly further than the returns from the middle. So an arch or “fingernail” shape will show on your display.

Spotting the fish

Some important things to remember about fish arches:

  • You’ll only get arches from moving fish (or if your sonar is moving over them).
  • If your sonar and the fish are both stationary, you will see a line, not an arch.
  • You’ll only get a full arch if the fish moves through the full sonar cone.
  • If a fish swims through part of your cone, it will show as a half-arch or a thick dash – look out for these.

Think vertical, not horizontal

Long arches mean big fish, right? Wrong. Long arches mean a fish was in your sonar cone for a long time.

And don’t forget, depth matters here – fish at lower depths will create longer arches or lines, because the sonar cone is wider so they remain in it for longer. A huge fish near the surface might only make a short arch or line.

So how do you work out fish size?

The answer is thickness. If an arch or line is thick, you’re marking a big fish. So think vertical, not horizontal.

The first image is a perfect illustration. These big fish haven’t made a full arch, but the lines are vertically thick, so we know they are big.

And spotting bait fish works the same way. Don’t look for how long the lines are, look for thickness and how clustered the marks are.

Why this matters for …

  • Fish finding

    Learning to work with the raw data will give you the most accurate fish finding. It might take a bit of your time to master, but you’ll save lots of time in the long run, because you’ll know exactly what’s down there.

    Deeper Sonar Tip: Switch off fish icons on the Fish Deeper App (Left hand menu – Fish icons), then remember to think vertical, not horizontal. Look for thick arches or half-arches, and don’t worry about how long they are. Plus, remember that depth will affect length. You’ll get longer arches from fish that are deeper. Again, focus on line thickness rather than arch length.

  • Finding structure and features

    While you are marking structure, you can keep an eye out for fish arches and schools of bait fish. This can help you to better understand which kinds of features hold which kinds of fish.

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