| The problem of noise, which is a key concern in the ship engineering, is related to the ship's stealth and comfort performance. In the multiple noise sources in ships, the noise of machinery and equipment is the main one in the cabin as well as underwater at low frequency. All along, the noise of machinery and equipment has been studying as a whole. However, such practice neglects the difference between the partial sound fields which constitute the overall machinery and equipment sound field. And is not conducive to the development of targeted noise reduction strategy. Based on such understanding, the noise of a main engine of a 300 ton patrol boat is studied. Besides, a numerical method was developed to distinguish the airborne sound field and the vibrating structure radiating sound field and their contribution to the overall sound field is analyzed. The result shows that the contribution of the airborne sound is significantly smaller than the structure-borne sound. Which means that vibration isolation is far more important than the sound isolation in the engine room.There is a close relationship between the ship's underwater noise and cabin noise. The airborne sound of equipment in the cabin will transmit into seawater through the hull and the hull excited by the structure-borne sound of equipment will also radiate sound into seawater too. So, the underwater sound field can be subdivided into the one caused by airborne sound and the one caused by structure-borne sound. In essence, these two kind of sound field is radiated by the hull excited by sound and by force. Follow such separation rules, the underwater sound field of the 300 ton patrol boat induced by the main machine is subdivided into two parts and their characteristics was studied. These characteristics includes intensity, directivity and radiation efficiency and so on. The analysis result shows that the underwater sound intensity caused by airborne sound is significant smaller than the one caused by structure-borne sound. Besides, the directivity of the former is weaker than the later while the radiation efficiency is higher than the later. In general, the underwater sound field caused by airborne sound possess more plane wave property.The far field sound radiation is closely related to the stealth performance of warship, and has a negative impact on marine life. The localization of far field sound radiation hot spots has important significance. The hull of a warship is an irregular geometry with complex phase relationship between different locations, which means that the near filed of sound field will be very different from the far field. In the near field, part of the energy propagate in the surface tangential direction in the form of evanescent wave, which makes no contribution to the far field. However, this energy flow is contained within the near field active sound intensity, makes the localization not accurate enough. In view of this, we try to locate the far field sound radiation hot spots of the 300 ton patrol boat by means of surface contribution method. The surface contribution method is different from the panel contribution method in boundary element method. It design and achieve a positive quantity on the structure surface, which means that this tiny area has a positive contribution to the overall radiated sound field. And the process to achieve the quantity is the process to obtain surface non-negative intensity, which make a positive contribution to the far field sound radiation. Using this method, we reveal the far field sound radiation hot spots on the hull precisely, which shows an application prospect in ship noise reduction engineering. |
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