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Sounder performance - background and diagnosis
08-26-19, 09:04 PM (This post was last modified: 09-09-19 02:03 AM by Tom - Raymarine - Moderator.)
Post: #1
Sounder performance - background and diagnosis
Introduction
We are often asked why a fishfinder/sounder/sonar might be producing a poor-quality image, or not achieving the depth performance that the owner expects. It might be that the sounder or transducer are faulty but usually they're not, and simply replacing hardware like-with-like usually won't fix the problem (unless the cause is incorrect setup on the sounder being replaced.) Here are some of the potential problems, causes and solutions distilled from more than a dozen years of successfully diagnosing these problems.

First steps
First of all, we need to ask a couple of simple questions about the problem:

  1. Does the problem still happen when the sounder is in its default/auto setup?
    Try doing a sounder reset through the display's menu, going to Sensitivity Settings and All to Auto, and making sure that Range is on Auto too. How does that change things? A lot of the time image problems are just setup so going back to defaults is a good sanity check.
    If the sounder is constantly cycling through a range of depth scales which are significantly different from the current water depth then it's in bottom search mode, where it's looking at various ranges to reacquire the bottom lock in Auto Range:
    [Image: attachment.php?aid=3491]
  2. Is the sounder operating within its expected capabilities (range)?
    Not all sounders and transducers are equal. Some are aimed at high detail in shallow water, some at value-for-money, and some at offshore performance. Do you have the right system for the depth of water you want to fish in? The below gives example absolute maximum depths for some Raymarine sounder technology types, and you can find transducer-specific information by searching for your transducer type on Airmar's cross-reference, clicking through the Description column to the product page and then opening the transducer's Brochure link (Airmar make most of the transducers we use). I would suggest halving these numbers for depths where you'll get a nice image for fishing as opposed to just seeing a thin bottom trace:
    [Image: attachment.php?aid=3489]

    If you're getting nowhere close to the expected level of performance for the sounder and transducer that you have, read on.


What a sounder needs in order to perform well - some background
All sounders, of whatever brand, require as large as possible a difference in the signal level (voltage on the transducer cable) and the background noise level. Every electronic system contains noise (unwanted randomly fluctuating voltages in parts of the circuit) to some degree, and both the sea and boats are relatively noisy places so there will always be a background noise level: the sounder needs to be able to clearly hear the echoes from fish and the bottom above this noise. If you imagine the amount of energy contained in a sound echo coming back from a single fish 100m down, or the bottom 500m or more down, and then picture that echo moving the transducer element and that movement then inducing a voltage onto the signal cable, I think that gives a bit of an idea of the low level of the electrical signals on the transducer cables, which can be less than 0.00001V. It doesn't take a lot of either sound or electrical noise to start to cause problems.

The larger the difference in signal level between the wanted echoes and the noise (what's known as the signal-to-noise ratio, SNR), the better the sounder image.

A low SNR will always lead to a poor image:
  • smaller/weaker targets being missed
  • excessive clutter in the water column
  • failure to track the bottom in deeper water (when the signal levels are lower)
  • failure to track the bottom at higher speeds (when the noise levels are higher)
  • ...

When a sounder isn't performing to the level it was designed to, then, the cause is either lower than normal signal level or higher than normal noise. Because what matters for the sounder is SNR rather than simply overall signal level, low signal and high noise have exactly the same effect. It's like trying to have a conversation in a busy bar or a good party: it can be hard for your to hear what your mate (or date) is saying clearly even when they're yelling. By repetition the general idea of what they're saying might come across (the bottom echo), but the fine details (fish) is lost in the noise.

Do you have low signal or high noise?
At the start of this guide we advised always using Auto sensitivity for normal use on any modern sounder. This is because our automatic sensitivity algorithms do a great job, in normal conditions, of adjusting to changing conditions from one moment to the next in order to give a clear, clean, easy-to-read picture. The problem with this is that this constant auto adjustment masks the actual signal levels, so if you get a poor result (a cluttered image with a weak bottom mark for example) then you can't see whether that's because the signal level is low (causing the system to increase the Gain in order to still show a weak bottom echo, and also inadvertently show background noise as well) or because the background noise is high (causing the system to drop the Gain in order to try to keep the noise off the screen, and inadvertently hiding all but the strongest bottom echo at the same time.)

What we need to do instead is use all-Manual setup.

Before we start: you should never use Manual gain for normal day-to-day use (unless you are an expert charter skipper who spends 200+ days a year on the water), but it's the only way to see for sure whether the cause of your problem is low signal level or high noise.

Here are recommended levels for diagnosing problems in all current (as of late-2019) sounder models. (If you have something older, contact us to check the right settings for your system).
    From the Home screen:
  • First, please set up a sounder page with at least two sounder channels displayed (for example, if you have a CP470, a two-way split with both Low Chirp and High Chirp at the same time would be great. If you have an RV system, Sonar and Downvision would be good. The idea is to compare the results from the two channels (frequencies.)
    From your newly-configured fishfinder page, Menu > Settings > Page settings > Edit data overlays
  • Please set up overlays for SOG, Vessel Position and Depth (so that we can see how fast you were going and from a chart, what the correct depth ought to have been compared to what the system is measuring)
    In Adjust Sensitivity, please set exactly the same Sensitivity and display configuration on each sounder channel, as below:
  • Gain: Manual, 65% (if you only see -50% to +50% you're still in Auto, make sure you untick the Auto box at the bottom of the gain slider)
  • Intensity (Contrast, Colour Gain): Manual, 15% (again, make sure you're not still in Auto)
  • Surface Filter (Noise Filter, TVG): Manual, 0%
    In Menu > Settings > Sonar Display:
  • A-Scope: Right
  • Bottom Lock: Off
  • White Line: Off
  • Bottom Fill: Off
  • Colour Threshold: 100%
    In Menu > Settings > Sounder > Ping Control:
  • Power Mode: 100%
    If you have the right Sensitivity setup then your sounder picture ought to look something roughly like this:
    [Image: attachment.php?aid=3493]

    Once you have the system set up correctly, it's time to go for a run and take some screenshots. Please take lots of images on the water, ideally in the conditions you normally use it. If the sounder normally works ok when going slowly and the problem only occurs when going faster, please take images at a range of speeds from idle/engine-off up to normal cruising speed. Please use exactly the same setup (above) throughout so that we can directly compare images.

    Common causes of poor performance
    As we've discussed, if it's not simply the limits of the technology or non-default setup, poor performance is always caused by either low signal, high noise or some combination of the two. Here are some common causes:

    Low signal level at all speeds
    • Failed or failing transducer. If a transducer acoustic element has a problem then it can fail to resonate correctly and output low power. Common causes are water ingress into the transducer (incorrect installation, through-hull transducers not bedded in sealant according to installation instructions), damage from submerged objects, grounding, crane lifting straps etc., or being operated out of the water (*never* run a sounder dry.) If a transducer needs to be tested, good dealers will have an http://airmartechnology.com/tdt1000/]Air...test tool.
    • Failed drive output in the sounder. This usually comes from the sounder having been run out-of-the-water, which lowers the impedance of the transducer, drawing excess current from the sounder and blowing the drive FETs.
    • A poor contact in a transducer cable. For example a missing locking collar on an extension or adaptor cable.
    • An in-hull wet-box that has dried out/leaked
    • A transducer installed in-hull in the wrong hull material (e.g. cored fibreglass, aluminium, or above a rib, stringer or similar)
    • A transducer glued in-hull. In-hull transducers must only ever be installed a tank filled with fluid. Gluing a transducer inside the hull is just asking for early failure (changed impedance, excess current, lack of cooling) and poor performance (glues and sealants do not effectively transfer sound energy from the transducer to the hull and then out into the water, and also spoil the designed beam-pattern of the transducer.) Never glue a transducer in-hull.
    Example: an in-hull transducer installed in a double-skin fibreglass hull with a coring material
    [Image: attachment.php?aid=3494]

    Low signal level only at higher speeds
    This is the second-most common sounder performance problem we hear about, and the one that most people know about: air bubbles across the face of the transducer. Causes include:
    • Low-profile through-hull transducer installed in a boat that is too big. Low-profile transducers are only suitable for smaller and/or slow vessels which don't produce much of an aerated, turbulent boundary layer next to the hull.
    • Transducer installed behind a hull feature that's causing bubbles. Strakes, steps, intakes and outlets, rivets, thruster tunnels, props/shafts will all do this. Try doing a tight turn at speed: if you turn one way or the other and the image clears up, the cause of the problem is a stream of bubbles coming off a (small) hull feature upsteam. Of course this won't make any difference if the cause of the problem is something big like a step.
    • Transom transducer installed in a boat with shafts. Transom transducers are only suitable for outboards or sterndrives, not shaft-drive or pod inboard engines.
    • Through-hull transducer poorly faired-in to a fairing block or fouled with weed or barnacles.
      Example: a low-profile transducer incorrectly installed in a 42ft flybridge powerboat, at 8kn in light harbour chop. It is very characteristic of aeration that it affects low frequencies (50kHz in this case) worse than it does higher frequencies (200kHz here).
      [Image: attachment.php?aid=3495]

    High noise level at all speeds
    High noise levels that are present at all boat speeds are relatively uncommon. Make sure you're doing your testing in open water because marinas are noisy places (electrically and acoustically) and a high noise level is normal inside a marina. If the problem is still apparent in open water then the cause is going to be either a break in the shielding of the transducer cable (this is one of the many reasons why we say never to cut/join transducer cables) allowing normal boat electrical noise levels to be picked up on the transducer cable, or a boat system that is causing the problem. The noise could be acoustic (actual sound at the transducer's frequency, which remember is way above the frequency range of human hearing) but is more likely to be electrical. It could be being picked up on the transducer cable, but is equally likely to be coming into the system via the sounder's power supply.
    Noise signals that produce a repetitive, herringbone pattern in the background are always vessel systems: something that operates with a frequency, whether it be electrical or something like a constant engine vibration.
    The best clue as to the source of this kind of noise signal is the timing: does it coincide with when your live-bait tank pump, or 'fridge compressor, or power-steering pump or ultrasonic anti-fouling systems are active, for example? Go around the boat and power systems down until the noise disappears.
    Example: strong pulsed noise from a power-steering pump
    [Image: attachment.php?aid=3496]
    Example: continuous noise signal from boat systems (source not confirmed)
    [Image: attachment.php?aid=3498]

    High noise only at higher speeds
    This is the single thing that probably causes most complaints about poor sounder performance, but one which is much less well known than aeration across the transducer face. Far too often we hear I've moved the transducer 20 times and it's no better.
    A noise source that gets worse as the boat speeds up can essentially come from one of two things:
    • Hull vibration. Actual sound, acoustic noise, at the operating frequency of the transducer. The source will be the engine but what the transducer's picking up will be the resonant vibration of the particular part of the hull where the transducer is mounted. Shorter, more rigid structures resonate at higher frequencies than larger or more flexible ones, so although your motor might only be running at 1200rpm, the bit of the hull where the transducer is might be resonating at a much higher-frequency harmonic that's within the frequency range that the transducer is operating. This sounds fanciful perhaps, but I have seen boats where the sounder was unusable until the transducer was stood off the transom on an anti-vibration (rubber) mount.
    • Engine electrical system noise. Either from the charging or ignition systems, and either being transferred from cabling running in parallel or through the power or grounding wiring, an engine electrical noise signal can severely affect your sounder image.
    Example: Crippling speed-related noise.
    [Image: attachment.php?aid=3502]
    Example: interference from a faulty alternator
    [Image: attachment.php?aid=3500]
    And the same system after the alternator had been replaced
    [Image: attachment.php?aid=3501]
    Here are two views of the same sounder channel in Auto gain and in Manual. In Auto, the broken bottom gives impression is that there's air going under the transducer, but Manual proves that high background noise (in this case hull vibration) is the cause of the poor performance:
    [Image: attachment.php?aid=3549]
    Here is the same system in Manual only, showing how noise increases with boat speed:
    [Image: attachment.php?aid=3550]

    Other common problems and questions - TO BE EXPANDED
    There are a range of behaviours that can occur when you have a sensitive sounder and pick up an echo that's coming back from deeper than the current display range. This is because the sounder only 'listens' for long enough to receive echoes from the current depth range, but of course the sound energy from the ping doesn't stop travelling then. If it's still travelling back from the bottom when the sounder starts listening for the next ping, you can get a situation where unwanted echoes are received and displayed. This isn't a product fault, just a combination of circumstances.

    First echo interference
    This is the simplest example, and occurs when you're set to a manual range that is shallower than the current true water depth. It won't occur in all depths or on all ranges, it depends on the exact timing of the ping-rate (governed by range) and the travel time to the bottom and back in the current depth. If the timing is just right (wrong?) then you pick up the last ping's echo, as it travels back from the bottom, whilst listening for the next ping.
    First-echo interference, showing the effect of changing range. This proves that the cause is 1EI rather than real returns in the water (thermocline, fish) or interference from another sounder.
    [Image: attachment.php?aid=3503]
    First-echo interference on an Axiom Pro MFD (1kW transducer, shallow water, hard, sandy bottom exacerbating the effect)
    [Image: attachment.php?aid=3504]

    Second echo
    Second echoes (or even third echoes) are visible in good conditions, when ranged to at least double the current water depth. The sound energy returning from the bottom can reflect off the underside of the water surface and back down, making a second trip to the bottom and so be picked up again as a (usually fainter) echo at double the real water depth. This is actually a sign that your sounder is working really well (the second echoes are understandably weak), and if you range in a little you won't see them any more.

    Second echo interference
    Second Echo interference occurs when the second echo described above is picked up whilst listening for the subsequent ping, in much the same way as First Echo interference:
    [Image: attachment.php?aid=3505]

    For all these 3 echo interference behaviours, the remedies include:
    • Setting the Interference Rejection controls to a higher level in your sounder's setup menu
    • Changing range slightly to break the synchronisation between pings
    • Setting a lower Ping Rate Limit in your sounder's setup, to leave greater time gaps between pings and again, break the synchronisation.
    If you don't mind a slightly slower scroll rate, this last option is the most complete fix. Use the highest Ping Rate Limit that you can that removes the interference.

    Colour palettes
    Colour palette selection makes a big difference to perceived image quality. Here is the same bait school, with the same gain setup, on two different colour palettes as an example:
    [Image: attachment.php?aid=3488]

    I generally recommend the Sunburst colour palette because:
    • The white background shows up very well in bright sunlight
    • Strong echoes appear in dark red that contrasts very well with the background for visibility
    • Weak echoes are in pale yellow that does not make the screen look cluttered
    • Since all of the echoes are in the same range of colours, it's more intuitive which returns are stronger and which weaker than in the multi-coloured palettes.


    RAYFAQ#


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Raymarine since 1999.
Interests: Diagnosis of problems in sonar/fishfinders, NMEA2000, ethernet comms, autopilots, thermal cameras
Location: Sydney, Australia.

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