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Simrad ER60 Operator manual
36
850-164692 / Rev.B
Noise measurements at sea
Purpose
The final result of the noise measurements should be a plot of
the acoustic noise in front of the transducer versus vessel speed.
This plot may be compared with similar plots for other
transducers on the same vessel, or plots from other vessels. It
may thus serve as an evaluation of the transducer location and
the vessel noise radiation. In addition, the noise plot may be a
guide in choosing the vessel speed during acoustic surveys.
Since the propeller pitch and revolutions per minute influence
the noise level, it is important to determine the most favourable
combination of these factors. Normally a slow rotation and a
high pitch give the loer6t noise.
Basic guidelines
The noise measurements should take place at least one nautical
mile off shore, away from other ship traffic and with calm sea.
The water depth should be 200 m or more to avoid propeller
noise reflections from the seabed. The important parameter to
evaluate is the noise directly radiated from the propeller into the
transducer. This noise should be measured at different vessel
speeds, from 0 to maximum speed in steps of 1 or 2 knots. The
vessel’s course must be kept steady during these measurements.
The noise level for 38 kHz with the vessel in deep water and
without sailing should be approximately -145 dBW or lower.
Minor increase can be expected for higher and lower
frequencies. When the speed increases the noise level will
normally increase as well, but a properly designed vessel should
only show a modest increase and mainly when approaching the
maximum speed.
With the settings specified below, the printer may be used to
produce an echogram as documentation for all the installed
transceivers. With some experience it should be possible to
reveal the acoustic noise source from looking at the echogram.
Typical sources may be propeller cavitation, small damages on
the propeller blade, the machinery, or flow-noise. It is a good
routine to save the echogram for comparison with later
recordings. The propeller noise will often be revealed by the
pattern shown on the echogram. The distance between
succeeding high intensity fields in the pattern should correspond
with the number of propeller blades and the rotation speed.
Electrical noise from the ship’s mains supply and electrical
motors will give a pattern with distinct lines on the echogram
determined by the frequency of the mains supply. By choosing a
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