**2. Underwater radiated noise**

The underwater radiated noise (URN) is the acoustic signal emitted out of a marine platform as a by-product of its operations. These operations include

#### *Acoustic Habitat Degradation Due to Shipping in the Indian Ocean Region DOI: http://dx.doi.org/10.5772/intechopen.90108*

propulsion, maintaining habitability on-board, any other mission specific activities and more. The platforms may be deployed in marine environment like the coastal waters or the high seas and the freshwater systems including rivers, lakes and reservoirs. Further, we could be looking at surface platforms like warships for naval deployments and merchant vessels for commercial deployments and also subsurface platforms including submarines for naval deployments and other sub-sea submersibles for undersea explorations.

The URN emissions are broadly categorized into three components including propulsion systems (comprising of the propellers and other machinery associated with the propulsion of the platform). The second component is the auxiliary machineries required for maintaining habitability on-board, power supply and distribution, mission specific activities and more. The third component is the hydrodynamic noise associated with the flow of fluids within and outside the hull of the platform. The spectral characteristics of the URN can be defined as narrow band due to the reciprocating (corresponding to the cylinder firing rate) and rotating machineries (corresponding to the rotation rate) and broad band due to the cavitation effect in the propellers and the non-laminar flow of fluids within and outside the hull of the platform. The URN spectrum is largely low frequency with narrow band tonals seen up to 500 Hz; however, at higher propeller speed, the broadband cavitation spectrum masks the narrow band machinery tonals and is largely seen up to 1000 Hz [11].

The URN management comprises of three distinct communities. The first is the naval community for whom acoustic stealth is of paramount importance, and they are willing to deploy higher resources to achieve higher stealth standards. The second is the commercial merchant marine who need to maintain low levels of URN to comply with regulatory norms for acoustic habitat degradation. The third community comprises of the ship designers and shipbuilders who need to make sure that the newer marine platforms are compliant of the regulatory norms whether for acoustic stealth or for acoustic habitat degradation. They need to establish a direct link of every stage of the ship design and building activity to the overall URN value, at the end. The acoustic stealth has been a very well-developed area of science and technology for the naval community for a long time; however, it never percolated to the other communities as it remained a closely guarded secret for ensuring military supremacy [12].

The acoustic stealth requirement has multiple dimensions to it. The acoustic signature management as it is called comprises of three distinct stages—the first stage translates to measurement and analysis to be able to maintain enhanced levels of stealth or minimal levels of acoustic signatures to avoid detection. The second stage is the precise prediction of the acoustic signature of any platform (own or from the adversary) based on the available information in the open source regarding the design and machinery details. This can greatly enhance our tactical capabilities of initiating counter measures. The third is the deception, where we fake the actual signature of the platform and try to deceive the adversary given the advent of intelligent mines that can precisely target platforms. The acoustic signature management capabilities require high resource deployment right from infrastructure of an underwater range to measurement and analysis of hardware and software. The know-how is also highly specialized in terms of algorithm design for real-time implementation to advanced signal processing capabilities. The acoustic signature management will have similar corollary for the acoustic habitat degradation assessment as well. The measurement and analysis to minimize the URN is the first stage, and the second stage will be prediction that will be required for effective policy formulation and regulatory compliance. The third stage is the deception in some form, which will be required to ensure minimizing the acoustic habitat degradation for specific species [8, 12].

The second dimension of acoustic stealth management is impact on the performance of our own sensors. The self-noise of the platform that has its origin in the noise and vibration on-board (similar to the URN) impacts the performance of the own sensors. Thus, containing the self-noise is equally important to enhance the effectiveness of the platform. The third dimension is the condition-based preventive maintenance (CBPM) that has its implication on the health of the equipment and fatigue failure. Regular noise and vibration measurement and analysis on-board the platform is an integral part of the planned maintenance schedule to enhance operational efficiency and minimize failures of running machineries. Thus, noise and vibration is an important prerequisite for all the three dimensions with its origin remaining the same; however, the manifestation is different for all the three. There is a need to establish clear linkages of the noise and vibration measurement data for each of the three dimensions, while we undertake the analysis [13].

The URN management needs to be ensured right from the design stage and beyond. The naval warship design has evolved over several decades of effort, and now most of the advanced navies have very mature design and manufacturing capabilities to ensure very high acoustic stealth of its platforms. The stealth requirements for naval platforms are so stringent that it is not enough to have a good design, but also during the operations as well, the regimes of operations are so chosen that the platforms emit minimal URN. The naval platforms undergo regular stealth assessments, to evaluate the exact status of the acoustic signature under multiple machinery regimes and also to monitor any deterioration due to mechanical wear and tear. The redundancies on-board are used effectively toward ensuring enhanced stealth standards during operations based on the exact stealth assessment. The maintenance schedules and routines are planned based on diagnostic ranging to identify causes of poor stealth during operations. Post refit, the effectiveness of the maintenance schedules is evaluated based on the stealth assessment. Any mid-life upgradation of equipment and systems on-board has to undergo acoustic stealth assessment to evaluate its impact on the overall acoustic signature of the platform.

The URN measurement and analysis, normally referred as underwater ranging, is a very complex and involved process, with significant infrastructure requirement and also analysis capabilities. The NATO Standard STANAG 1136 is a framework defined by the NATO for undertaking underwater ranging of warships. The URN measurement of naval platforms is a highly classified activity, and navies have defined their own protocols of measurement and analysis that are not available in open source. The STANAG 1136 is just a broad guideline [14]. The underwater measurement has its own complexities in terms of deployment of the sensors to far field measurement requirements. The Acoustical Society of America (ASA) has its own standards for vertical sensor arrays and the measurement protocols [15]. The underwater ranges in earlier days were the fixed over-run ranges where sensors were laid horizontally, and the vessels were made to pass over them at certain depth and specified regimes of operations. The regime-wise recordings are subsequently analyzed, and inferences were drawn. The fixed sensors have their advantages of high accuracy and effective mitigation of environmental distortions; however, the infrastructure cost is prohibitive. Portable vertical ranges are being increasingly used with the advantage of low cost and reasonable deployment ease [16].

**Figure 3** presents a detailed framework for URN management. The three stakeholders, namely the navy with their interest in acoustic stealth, the marine conservation community with their requirement to contain acoustic habitat degradation and the blue economic entities related to ship building and ship design, are represented by the three horizontal faces of the prism. Policy, Technology and Innovation and the Human Resource Development will remain the pillars of any

*Acoustic Habitat Degradation Due to Shipping in the Indian Ocean Region DOI: http://dx.doi.org/10.5772/intechopen.90108*

initiative toward URN management. The basic steps are measurement and analysis, prediction and alteration (naval community will call it deception as part of their acoustic signature management efforts). Acoustic capacity building will remain the core requirement.
