Transducer Bandwidth; The Sonar Image - Raymarine a65 Installation And Operation Instructions Manual

15.6 Transducer bandwidth

The fishfinder application displays the fishfinder frequency, center
frequency or CHIRP mode depending on the connected sonar
module and transducer.
Example of CHIRP sonar module in CHIRP mode.
• When using a CHIRP sonar module set to CHIRP mode and a
wide band transducer the CHIRP mode is shown in the fishfinder
application title bar and the center frequency is displayed
on-screen.
• When using a CHIRP sonar module set to a non-CHIRP mode and
a wide band transducer the center frequency for the transducer
is shown in the fishfinder application title bar and is displayed
on-screen.
• When using a CHIRP sonar module connected to a non-CHIRP
(traditional) transducer the CHIRP sonar module will behave the
same a non-CHIRP (traditional) sonar module.
• When using a non-Chirp sonar module the transducer's operating
frequency is displayed in the fishfinder application title bar and
is displayed on-screen.
Note: When using a CHIRP sonar module in non-CHIRP mode
only the transducer's center frequency is displayed, this maybe
different to the actual frequency transmitted.
Using the fishfinder

15.7 The sonar image

Interpreting the seabed using sonar
It is important to understand how to correctly interpret the seabed
structure represented in the fishfinder display.
The seabed usually produces a strong echo.
The following images show how different seabed conditions are
represented in the sonar display:
1
2
3
Item
1
2
3
The dark layers indicate a good echo; the lighter areas indicate
weaker echoes. This could mean that the upper layer is soft and
therefore allowing sound waves to pass to the more solid layer
below.
It is also possible that the sound waves are making two complete
trips – hitting the seabed, bouncing off the vessel, then reflecting
off the seabed again. This can happen if the water is shallow, the
seabed is hard, or the gain is set to high.
Factors influencing the sonar display
The quality and accuracy of the display can be influenced by a
number of factors including vessel speed, depth, object size,
background noise and transducer frequency.
Vessel speed
The shape of the target changes along with your speed. Slower
speeds return flatter, more horizontal marks. Higher speeds cause
the target to thicken and arch slightly, until at fast speeds the mark
resembles a double vertical line.
Target depth
The closer the target to the surface, the larger the mark on screen.
The depth of individual targets can be displayed by switching on the
Target Depth ID in the fishfinder menu Menu > Presentation. The
number of target depths displayed is influenced by the fish alarm
sensitivity level.
Description
A hard bottom (sand) produces a thin
line.
A soft bottom (mud or seaweed cover)
produces a wide line.
A rocky or uneven bottom or a wreck
produces an irregular image with
peaks and troughs.
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