The Influence of Internal Resonances From Machinery Mounts on Radiated Noise From Ships

Machinery vibration is a significant source of noise from marine vessels. Consequently, problematic machinery is often decoupled from the hull using resilient mounts. When radiated noise is a priority, two-stage mounting arrangements can be used which provide superior performance. This improvement, however, comes at a much higher weight and cost. A commonly overlooked issue with isolation systems is that the individual mounts suffer from internal resonances which reduce their effectiveness. This paper broadly evaluates the influence of these on the underwater noise levels from a typical ship diesel. The effect of nonrigid structural components, that is, the supported machine, the machine foundation, and the intermediate mass in two-stage mounting arrangements, is also discussed. The structureborne and airborne transmission paths are resolved individually using approximate analytical methods, and the source levels are estimated from empirical relationships for typical marine diesels. It is shown that, by mitigating the internal resonances of the mounts, a two-stage mounting arrangement may be unnecessary because of the contribution to the underwater noise by airborne transmission.