Blocking; Check The Noise Yourself - Simrad SH90 - REFERENCE MANUAL REV A Reference Manual

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The echoes from the bottom will vary with how uneven the
bottom is, what kind of bottom you have under your vessel,
(rock, mud, sand etc.) as well as your current tilt setting. The
echo from the sea surface will vary with the current sea state and
your current tilt setting.
The illustration demonstrates how the bottom and the sea surface
return echoes back to your sonar transducer. In situation (A) you
will see several weak echoes from the waves and from the rock on
the bottom, you will still be able to identify the school of fish. In
situation (B) you will be able to identify the upper school of fish,
but the lower school will be hidden in the shadow of the rock.
Be aware of that other vessels close to you will also generate
reverberation due to the water disturbance caused by the wake,
the propellers and the cooling water spill.

Blocking

The movement of your vessel through the water will often –
especially in higher sea states – cause air bubbles to be generated,
and these bubbles will move along the hull. This is called
blocking. In order to prevent this noise from disturbing your
sonar operations, it is important to lower the sonar transducer as
far down below the hull as possible.

Check the noise yourself!

If you are bothered with acoustic or electric noise, you can easily
run a few simple tests to identify the cause of the problem.
How to check the noise
Click the tab to open the menu.
1
Setup
2 Click the button to open the
Test
Click the
3
Test config
parameter dialog.
Click the button in the parameter dialog to close it.
4
Close
5 Observe the
Echo level
6 Read the noise level.
The level your read in the button is the current noise in the
direction of the cursor. The noise is provided in dB, and the
number should be as small as possible.
In order to reduce the noise, try to adjust the propeller pitch and
the number of revolutions.
How to check for electrical noise
1 Switch off all electric equipment that can cause electrical
noise.
System test
button, and select
Noise and VR
button on the
System test
Basic theory
menu.
in the
menu.
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