Basic guidelines for noise measurements
This section explains how to perform noise measurements at sea and also outlines the concepts.
Noise measurements are performed to ensure the system performs as expected and to evaluate system installation. Noise measurements are also performed to measure and document how system performance is affected by vessel operation. The measurements may therefore be used as a guide when choosing vessel operational parameters for acoustic surveys. This test procedure focuses on measuring the system self noise and vessel noise affecting the system.
Note
The procedure does not focus on measuring interference from other acoustic instruments on the system.
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. 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 lowest noise.
Description
The noise measurements must take place at least one nautical mile off shore, well away from other ship traffic, and with calm sea. The water depth must 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.
The noise must 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 the noise measurements.
All other acoustic instrumentation which may affect the measurements shall be turned off during the measurements.
Important
The ME70 system must be calibrated prior to the noise measurements. A sound velocity profile shall be taken at the time and location of the noise measurements, and archived.
Noise levels shall be measured in both passive and active mode. Raw data files shall be recorded during all measurements and archived.