ABSTRACT

Underwater noise radiated (URN) from ships represents a growing concern for the international community. The Acoustic Black Hole (ABH) combined with viscoelastic materials (VEMs), is a promising new solution for mitigating URN. In this paper, we present the characterization of an acoustic black hole for marine applications. The proposed procedure combines a method for the evaluation of VEM properties of high-damping materials and the analysis of the parameters that control the ABH performance. The results show the effectiveness of the ABH in mitigating low-frequency (50 – 200 Hz) ship's radiated noise.

INTRODUCTION

Ocean pollution is a concern for national governments and international organizations. In the 2015 A/Res/70/1 resolution (2015) adopted by the United Nations, the subject "Life below water" is identified as one of the seventeen sustainable development goals (SDGs) to be reached by 2030 and the prevention and mitigation of marine pollution are the first targets, which should be reached by 2025. Anthropogenic underwater noise, and underwater noise from shipping in particular, is a form of pollutant that consists in the direct or indirect introduction of energy in the marine environment. From an engineering point of view, reducing the impact of anthropogenic underwater noise on the environment is challenging because of i) the variety of sources, ii) the propagation of noise in the ocean, and, iii) the variety of impacted species, which are sensitive to different frequency bands.

In 2021, a report by the European Maritime Safety Agency - EMSA (Cruz, Lloyd, Bosschers, Lafeber, Vinagre and Vaz, 2021) reviewed the current state-of-practice of ships' underwater radiated noise (URN) management, which is a framework based on four criteria: policy, noise sources, environmental impact and mitigation sources.

The present paper will focus on the fourth of these points, as the aim is to investigate the possibility of developing a metamaterial solution for the reduction of noise transmitted from inside a ship to the outside environment. Although the propeller cavitation represents the major source of underwater noise radiated by ships, low-frequency noise generated by onboard machinery and systems cannot be neglected, especially when a vessel is sailing at reduced speed or berthed. Noise generated on board is transferred to machinery foundations and then to the hull plating, which acts as the speaker's membrane, radiating the noise in the surrounding environment.

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