Heading The release of dangerous chemical, biological, or physical substances into the world’s seas is not the only source of ocean pollution but it also includes an increase in underwater noise caused by a variety of human factors. The concept of underwater radiated noise first emerged during the Cold War, when its reduction had a strictly militaristic application, to ensure acoustic stealth of submarines. In the current scenario, however, Underwater Radiated Noise (URN) is a growing cause of environmental and socio-economic concern. However, unlike the other forms of ‘material’ pollutants, noise is a form of energy hence it is quickly dissipated and thus has to be quantified at the source. Another issue with imposing limits on the amount of emitted noise is that for the same intensity of noise severity may vary depending on the time and location. As a  result, there is a lack of research and regulation when it comes to noise pollution in comparison of air or water pollution. Shipping is the primary cause of underwater radiated noise. Global sea-borne trade has increased at a rate of 2.5% and 1.4% in the short and long term respectively. As a result, the underwater noise levels in some of the world’s oceans have nearly doubled every decade over the past 60 years and this is directly proportional to the Gross Domestic Product (GDP) of these nations. “The underwater radiated noise from a ship has got 3 main sources, namely the propeller machinery and the hull. The propeller is the dominant source, accounting for about 80%-85% of the generated noise, the reason being propeller cavitation. Cavitation contributes to both tonal and broadband noise. The noise from the machinery and hull is mainly produced due to vibrations. Noise levels at higher frequency (above some hundreds of Hz) will tend to decrease with increasing frequency. Therefore, the predominant noise is in the low-frequency band which affects the ambient noise over a large ocean area.” Higher levels of underwater noise impact the marine species as well as disrupts the economic development of people inhabiting the coastal regions. The audible range of marine fishes and mammals range from 5 Hertz to around 200 Kilohertz. Underwater radiated noise effects over 66 species of fish, almost all marine mammals and 36 species of invertebrates, and can severely hamper intraspecies and interspecies interactions. Hearing is the key sense for these creatures, and they rely heavily on it for navigation, communication, food acquisition, breeding, and threat detection. As a result, these animals are  more prone to be affected by an increase in ambient noise and it affects their interaction with the environment. This is termed ‘Acoustic Degradation’. ‘Acoustic Masking’ occurs when the presence of noise disrupts an animal’s capacity to hear a sound of interest, like the location of a prey through echolocation or a potential mate. Acoustic masking is thought to pose a hazard to marine life, particularly animals that communicate at low frequencies, such as baleen whales.  As a result, an excessively high amount of low frequency ambient noise might have a deleterious influence on their population, like reducing their ability to hunt and wandering too close to the shore. Apart from these, higher level of underwater noise also has severe socioeconomic impacts. It reduces the yield of fishing, impacts the marine tourism industry, and also has consequences on the health of divers. This disrupts the economic development of  people inhabiting the coastal regions and can lead of loss of employment. “As a result, in the recent times a number of organizations both governmental and private have concentrated efforts to find out ways to effectively manage underwater radiated noise. The International Maritime Organization (IMO) has introduced the MARPOL Convention to reduce the pollution from ships. The United Nation’s Environment Program (UNEP) has also made efforts in this regard such as linking it to the UN-SDGs and the formation of the Mediterranean Action Plan (MAP).” A number of countries across the American Continents and the European Union have also laid     down regulation to combat the rise of underwater noise. Researchers are also focusing to come up with technological interventions and determine the efficiency and cost effectiveness of URN management techniques. The Indian scenario of the underwater radiated noise management: However, when it comes to the Indian scenario much of the effort is still in progress. As India  shifts its focus to effectively utilize its oceans for economic advantage, sustainability becomes a concern due the predicted rise in pollutant levels including underwater noise. The primary reason behind this might be the lack of country specific research and knowledge hub containing scientific data establishing the ill-effects of underwater radiated noise and the uncertainty around the cost effectiveness and efficiency of management techniques. There is also the factor of noise being an ‘energy’ pollutant, absence of standardized methods for measurement of underwater radiated noise and lack of noise emission limits for shipping vessels, which have hindered the formation of appropriate policy framework “As a result, in the recent times a number of organizations both governmental and private have concentrated efforts to find out ways to effectively manage underwater radiated noise. The International Maritime Organization (IMO) has introduced the MARPOL Convention to reduce the pollution from ships. The United Nation’s Environment Program (UNEP) has also made efforts in this regard such as linking it to the UN-SDGs and the formation of the Mediterranean Action Plan (MAP).” Conclusion and recommendation: Since authorities like the Central Pollution Control Board already have policies in place to combat air and water pollution, similar methods  can be applied to manage noise pollution as well, such as a ship specific certificate for ‘noise emission in limit’. The policies can guide administrators, ship owners and operators on principles of design and operation to effectively reduce URN. Its aim is to encourage stakeholders to prevent, control, monitor, and manage underwater radiated noise. “This regulatory document can provide a set of ship-based noise limits, which could be phased in and implemented over time. The introduced technologies and policies should

Heading The ocean is a gigantic, complex ecosystem comprising life forms ranging from small enough to be invisible to the naked eye to giant enough to be compared with commercial jets. The oceans cover ~70% of the Earth’s surface, and to nobody’s surprise, have a tremendous variation in geology across the globe. This  incredible variety in marine life and bathymetry, coupled with precipitation and winds, generates random underwater noise that cannot be modelled deterministically. Anthropogenic activities like commercial shipping, coastal activities, and seismic explorations add to the noise. Why must we measure underwater noise? Outside the realm of textbooks which have the freedom to make ideal assumptions (often dubbed ‘the real world’), no signal can be ‘noise-less.’ Communication systems are cursed with the problem of addressing noise and finding clever solutions to minimise them. To establish effective underwater communication, we must understand the noise soundscape we are working with. Research suggests that for tropical littoral waters like that of the Indian Ocean Region (IOR), commercial shipping is the most significant contributor to the ambient noise profile of the ocean. Anthropogenic noise originating from sources  like these is generally in the frequency range of < 100Hz and harms marine life. For instance, whale strandings have been reported with origins traced to noise generated by commercial ships. “Effective noise measurement is vital for creating  technology and policy interventions to contain noise and protect the marine environment.” Concerns for the Indian Ocean Region Electromagnetic waves are easily absorbed by water and practically useless for long-range signal transfer underwater. Acoustic waves become the default option  for underwater communications. Commercial off-the-shelf (COTS) sonar technology was designed and prototyped during the Cold War era. Naturally, it is suited for the deep, temperate waters of the Atlantic. “The IOR is a tropical, shallow water body, and COTS sonar is ineffective for acoustic measurements. Research and development to create custom sonar sensors capable of performing in tropical shallow waters is necessary.” How do we measure noise? Mathematical models have been developed to approximately map ambient underwater noise-based on data from commercial ships. Shipping data can be   obtained from the Automated Identification System (AIS). However, these ambient maps are theoretical and require validation through experimental measurements. Hydrophones are underwater analogues to microphones and can measure sound underwater. They can be deployed from boats or moors, but surface waves intrinsically introduce noise. “Surface waves can be avoided by deploying hydrophones via Remotely Operated Vehicles (ROVs) or Autonomous Underwater Vehicles (AUVs). AUVs are not encumbered by tethers and provide an intrinsic advantage over ROVs.” Autonomous Underwater Vehicles (AUVs) An AUV is a self-propelled, unmanned, untethered underwater vehicle capable of  being utilized as a survey platform to map the seafloor or characterize the water’s physical, chemical, or biological properties. AUVs can scan shallower waters than boats can. AUVs can also venture into areas unsafe for human divers. Due to their modular design, various sensors can be attached or removed, giving the designer the ability to use them for various purposes. AUVs have been used to search for crashed aero planes like the Malaysian Airlines MH370. Oil-drilling companies also use them commercially to map the bathymetry before beginning operations. “AUVs are an attractive choice for ambient noise measurement. They can perform identical experiments multiple times, are shielded from bad weather, and perform underwater for an extended time.” Designing an AUV for noise measurement An AUV uses a variety of sensors for dynamics (IMU, DVL, Depth Sensors), acoustics (Hydrophones), and vision (Cameras). This data is used for localization, navigation, experimentation or any specific task that an AUV might be designed for. The vessels (hulls) that house the sensors and electronics are waterproof (obviously) and extremely hydrodynamic for deep-sea missions. Methods for designing hydrophone arrays for high-resolution ambient noise mapping are actively researched. Literature suggests the possibility of using an array of hydrophones attached to a streamer, which can be towed using an AUV.  Using this apparatus allows us to take multiple measurements in the same locality, and interpolating the data can help in receiving higher resolution maps. “Another method that researchers are looking into is using multiple AUV formations (leader-follower formations or swarm formations). Inspired by swarm-forming animals like ants, bees, fishes, etc., hydrophones can be deployed on numerous AUVs designed to work in synergy and record high-resolution maps of underwater ambient noise to validate theoretical data.” Sarthak Raj About Author Sarthak has completed his internship at MRC, Pune. His research project revolved around the design and development of Autonomous Underwater Vehicles (AUVs) for underwater ambient noise validation. He is currently completing his studies at Indian Institute of Technology (IIT), Bombay.

Heading The Noise and Vibration management (N&V) for marine vessel has multiple dimensions, right from the identification of vibration sources, transmission paths through the diverse vessel structure and then the coupling with the water medium as noise transmitted to the environment. Multiple stakeholders have their specific application related requirements and thus it is important to understand these varied requirements to satisfy all parties involved. Firstly, the human resource on-board marine vessels i.e., the passengers and the crew are significantly affected because of the excessive noise and vibration levels produced by the propulsion and auxiliary machinery. On a global scale, 16% of the disabling hearing loss in adults is attributed to occupational noise of which around 5% is due to the shipping industry. According to official statistics, an estimated $242 million is spent annually on workers’ compensation for hearing loss disability. Thus, it is crucial to maintain a safe work environment to ensure proper safety for all. Secondly, fatigue failure is the formation and propagation of cracks due to a repetitive or cyclic load. Propulsion-induced loads and vibrations are among main causes of fatigue on ships. Higher frequent loads caused by engines and propellers result in forced vibrations with high number of load cycles, are main cause of fatigue. Thirdly, in naval applications, excessive noise can cause a ship to be detected by enemies and unnecessary vibrations may pop up on radar detection systems. Acoustic mines are in place at various locations underwater and they may be triggered by unwanted noise levels. The navies throughout the world give very high priority to acoustic stealth and after World War II, massive progress has been made in this field. Finally, the most critical application of these studies is the Underwater Radiated Noise (URN). A detrimental, low-frequency ambient noise radiated by maritime sub-systems because of different machines and experimental activities carried out to the surrounding aquatic environment is called URN. This anthropogenic ocean noise is the cumulative result of the human maritime activities including seismic exploration by the oil and gas industries, military and commercial use of sonar, recreational boating and shipping traffic. This noise harms the peaceful aquatic eco-system for all living beings underwater, reduces the available dissolved oxygen and creates a plethora of impactful problems. A brief understanding on how noise and vibration gets generated : Energy is found in nature in various forms such as mechanical, thermal, electrical etc. Sound is a medium through which energy propagates by means of oscillation of particles. Vibrations are mechanical oscillations or the intermittent motion of a particle or body, resulting when it is moved from its equilibrium condition. The necessary conditions for oscillatory motion are elasticity and inertia. Elasticity is the ability of a body to return its equilibrium position, after it is displaced while inertia is the measure of tendency of a body to resist change in its current state of motion. In order for the particles to fluctuate around their default position, the medium in which sound waves propagate must have both inertia and elasticity. Noise is defined as being unwanted sound and is generally a result of vibrations. By measuring the amplitude of noise, we can identify the precise force or energy of the sound wave and the measure of amplitude indicates the intensity of noise. Noise and vibration (N&V) analysis is a combination of computational and experimental procedures to measure noise and vibration levels for a system, compare the obtained values with a standard reference and monitor these real-time values over a period of time to detect any possible failure within the system. The marine vessel represents a very complex system of N&V sources determined by the operation of numerous on-board installations and specific activities of crew and passengers. Source: Marine Insight – Underwater Radiated Noise. The above figure shows the different sources of noise which are radiated below the water surface “Vibration in ships not only causes structural fatigue but also ruins the experience of the passengers as well as the crew due to discomfort. Thus, it is extremely important to understand what are the sources of these vibrations.” Classification of vibrations The classification of vibrations is done on the basis of the components which cause these vibrations. The first classification is machinery vibrations. Engines, propulsion shafts, gearboxes, propellers, pumps, diesel generators etc. have various moving parts that can induce vibration while operating. These machinery vibrations may be further classified into the following three types. Torsional vibration can be defined as the angular vibration of an object along its axis of rotation. The main propulsion system of a ship consists of the main engine and a marine shaft which consists of an intermediate shaft and a propeller shaft, which are connected by means of coupling flanges. The presence of connections, like coupling flanges, thrust block, engine connection flange, and the cylinder-piston system in the main diesel engine creates torsion in the rotating shaft system and thus it creates an excitation. Axial or longitudinal vibrations of the propulsion system are one of the most interesting cases of machinery vibration and also possibly the likeliest to cause vibration. The velocity of the water incident to the propeller blades determines the thrust generated by the propellers also called as wake. Since the hull has a curvature at the aft, the wake on the top propeller disc is different from that on the bottom propeller disc. This change is repeated with every revolution of the propeller. Thus, this periodic thrust generated by the propeller is known as alternative thrust which acts as the exciting force resulting in the axial vibration of the propulsion system. Lateral or transverse vibration is perpendicular to the axis of the shaft’s rotation. Because of the curvature of the shafts, the ideal centreline of the shaft and its center of gravity do not coincide with each other. Hence, when the shaft rotates its shifts away from the ideal center line because of the centrifugal force on the center of gravity resulting in a vibratory motion called whirling of shafts. The second main classification