Underwater ship noise characterisation with sound intensity

Paul Boughton

Growing consideration of the environmental impacts that underwater noise can have makes assessing the noise characteristics of ships a necessary part of their development.

Since Japan has no large-scale acoustic underwater ranging system, JRC Tokki invested in a portable sound intensity system that enables field measurements to be made on large vessels.

This static ranging system uses technology that benefits from decades of successful use in the automotive industry, and applies it to underwater use. It uses hydrophones mounted on a floating raft to make underwater Noise Source Identification (NSI) measurements.

JRC Tokki specialises in system-support engineering for installations on ships and the manufacture of peripheral equipment. The company also provides repair and overhaul services of defence electronics for ships and aircraft. For JRC Tokki, it is important to be able to measure the underwater noise radiated from ships at sea.

In order to get a complete characterisation for the whole of the underwater structure of a ship, JRC Tokki needed a portable system that would enable in-situ measurements to be made. As the curvature of ship’s stern and bow often varies, JRC Tokki required a solution that can account for this by following the hull closely, at a uniform distance.

After deciding on sound intensity, JRC Tokki needed to integrate Brüel & Kjær hydrophones into their sophisticated underwater measurement system. Underwater sound intensity measurements are problematic due to the difficulty of determining particle velocity. In order to do this, JRC Tokki needed carefully selected, phase-matched hydrophones in combination with specified cable lengths and special connectors.

JRC Tokki created a portable system to position the measurement transducers at the correct point, which doesn’t require a permanent setup. A floating raft is moved along the sides of the ship, and stops at a planned point where a measurement is made. After a measurement has been done, the floating raft is moved on to the next measurement point. Taken together, all of the points that are measured create a virtual ‘mesh’ that covers the entire underwater surface of the vessel. The process is repeated for both sides of the vessel, as well as the bottom, in the same way.

The full case study is available to read at: www.bksv.com