Kongsberg Simrad SN90 Reference Manual page 269

c Ask the bridge to abruptly turn off all engine power and - if possible - change
the pitch of the propellers to zero.
d Read the noise value from the noise circle.
e For every knot the speed is reduced, read the noise.
(If you find it more convenient, type the data directly into the spreadsheet.)
f Fill in the result table.
9 When all the measurements have been made, type the data (speed and noise) into a
spreadsheet to create the curve.
10 If applicable: Hoist the transducer back up to its upper position.
Result
Speed
20
19
18
17
16
15
14
13
12
11
10
381294/E
Speed/Frequency/Noise
Frequency
Maintenance procedures
Noise level
267