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JASCO ShipConsult’s project Development of Noise Reduction Measures for Conventionally Propelled Whale Watching Vessels highlighted how cavitation noise often dominates at higher speeds, but low-frequency engine and machinery noise takes centre stage at lower speeds. With low-frequency noise travelling farther than high-frequency sound, tackling these noise sources is as important as tackling cavitation.
Controlled Environment Test Platform for Evaluating Noise and Vibration Mitigation Measures Generated by Ships
Innovation Maritime’s project Plateforme de Tests en Milieu Contrôlé pour L’évaluation de Mesures de Mitigation du Bruit et Vibrations Générés par les Navires (Controlled Environment Test Platform for Evaluating Noise and Vibration Mitigation Measures Generated by Ships), in partnership with Université de Sherbrooke and Davie Shipyard, looked at designing a platform that will allow researchers and engineers to test different ways of reducing noise generated by machinery in a controlled environment like a pool. The advantages of using a platform rather than testing noise mitigation measures directly on a vessel include cost-effectiveness, more control over the factors being measured, and faster results.
The platform is comprised of a steel structure, representing the section of a ship to which different structures can be attached, and instrumentation like accelerometers and microphones to measure inside the structure. Hydrophones and other instruments can be added directly into the pool at different locations to give a complete profile of the noise radiating outside the structure. Rather than attach actual machinery to the structure, the platform is fitted with vibrators and speakers to simulate the noise and give control over the exact noise and vibration levels being produced, as well as different weights to simulate the machinery.
The platform has undergone several tests with different mitigation measures, both traditional solutions (vibration isolators, acoustic insulation and dampening tiles) and innovative solutions based on resonant metamaterials such as acoustical and mechanical resonators.1,2,3 In the case of vibration isolators – devices designed to reduce the transmission of vibrations from engines, generators, and other machinery to the vessel’s hull (and therefore into the water) – the results demonstrated that the platform can accurately assess noise levels and give insights into the different paths through which vibrations are transmitted through the vessel and into the water column.
The project developed a methodology for assessing the capacity of mitigation measures to reduce underwater noise, airborne noise and vibrations. This information can help guide the most appropriate types of vibration isolator and their placements.
Dampening all Noise with Active Underwater Noise Cancelling
While many of the Quiet Vessel Initiative projects focused on quieter components, Rising Tide BioAcoustics Inc. took a different approach in their Active Underwater Noise Cancelling project. They developed an underwater active noise cancellation system that targets low-frequency noise generated by propellers and engines.
The system has two principal components – a signal processing algorithm and a set of low-frequency projectors (speakers) attached to the outside of the vessel. Developed by Riding Tide’s partner GeoSpectrum, these C-BASS speakers produce very low-frequency noise at high sound pressure levels. The algorithm analyses the noise the vessel is making, and then the speakers produce a specific frequency to cancel out or absorb that noise. This works similarly to noise-cancelling headphones.
The system can’t completely cancel out all noise, but tests so far indicate that continuous, low-frequency propeller and engine noise levels, which is the type of noise that travels furthest through the ocean, can be reduced by around 20 decibels – a factor of 100.
The system doesn’t have to be restricted to vessels. Ports, for example, could be fitted with a similar portable setup to dampen noise from ships’ generators when at berth. Generator noise can transmit through the water into the seabed and from there into the ground, causing low-frequency noise pollution in nearby buildings. Reducing underwater noise in ports is a benefit to surrounding communities as well.
The underwater noise cancelling system is complementary to other noise reduction measures, including the bubble curtains (or air lubrication) used on some vessel hulls or during in-water construction to muffle high-frequency noise. It has the additional benefit of reducing biofouling on ship hulls when the vessel is docked or moving at slow speeds. There is potential for other applications as well, including offshore renewable energy, offshore oil and gas, and possibly pile driving activity.
Work on Rising Tide’s system is ongoing. Once ready for commercial operation, it should be suitable for new builds and retrofitting alike.
This article is part of a five-article series on ship design to limit underwater vessel noise.
Continue learning about the new discoveries and challenges in making vessels quieter with the other topics in this series here
The Quiet Vessel Initiative is a federally funded program through Transport Canada. Industry partners and researchers interested in potential research and development collaborations to advance innovative solutions in marine technology are invited to contact the Quiet Vessel Initiative team at Marine-RDD-maritime@tc.gc.ca.
Accelerometer: a device that measures the vibration, or acceleration of motion, of a structure. The force caused by vibration or a change in motion (acceleration) causes the mass to “squeeze” the piezoelectric material which produces an electrical charge that is proportional to the force exerted upon it. Since the charge is proportional to the force, and the mass is constant, then the charge is also proportional to the acceleration. These sensors are used on a variety of platforms – from space stations to handheld devices like smartphones.
Algorithm: A process or set of rules to be followed in calculations or other problem-solving operations, especially by a computer. Algorithms act as an exact list of instructions that conduct specified actions step by step in either hardware- or software-based routines.
Biofouling: The growth of marine life on the hull and other underwater parts of a ship that results in increased “drag” or friction when the ship is underway. This drag increases the amount of noise and decreases the energy efficiency of the ship’s operation.
Decibel: A unit used to measure the level of sound pressure (intensity of a sound) or the power level of an electrical signal. It is a relative unit, not an absolute one, and is used to express a relative change. Decibel is used to describe sounds in terms of their loudness. For underwater ocean sounds, a reference pressure of 1 microPascal (μPa) is used to describe sounds in terms of decibel.
Hydrophone: An underwater microphone that can be deployed individually or in groups. Groups of hydrophones can be arranged either horizontally on the seafloor or vertically at different depths in the water column. Hydrophones detect pressure changes in the water caused by sound waves. These sensors convert the underwater pressure fluctuations into electrical signals, which can then be analysed to determine the properties of sound, such as volume and frequency.
- Robin, O., Guy, M. A., Vulliez, M., Kesour, K., & Gauthier-Marquis, J. C. (2024, October). Developing and testing systems for the attenuation of ships’ machinery noise. In INTER-NOISE and NOISE-CON Congress and Conference Proceedings (Vol. 270, No. 3, pp. 8236-8245). Institute of Noise Control Engineering. ↩︎
- Guy, M. A., Kesour, K., Vulliez, M., Gagnon, S., St-Jacques, J., Tremblay, R., … & Robin, O. (2024). Assessment of the effectiveness of ship machinery noise reduction measures using a test platform in a water basin. Ocean Engineering, 313, 119380. ↩︎
- Guy, M. A., Kesour, K., Robin, O., Gagnon, S., St-Jacques, J., Vulliez, M., … & Marquis, J. C. G. (2024, May). Effectiveness of standard mitigation technologies at reducing ships’ machinery noise using a small-scale ship-like structure. In Proceedings of Meetings on Acoustics (Vol. 54, No. 1). AIP Publishing. ↩︎
