Fast average prediction method based on the upper -lower limit theory for ship’s mechanical noise

The equipment loads include acoustic excitation and mechanical excitation which is characterized by acceleration, and a reliable prediction method of mechanical noise is important to control the hull vibration noise and improve the accuracy of hydroacoustic detection. To address the issue, an accurately and efficiently predicting method is proposed and verified by the hydroacoustic experiments which included the three different diesel engine working conditions. In the method, for the acceleration loads with complete information, an equivalent load model and a gradient meshing model are used to identify the generalized forces, and the mechanical noise is predicted by the identified load and acoustic BEM. For the acceleration loads with missing partly information, the interval theory and the energy superposition method are introduced to calculate the mechanical noise, and the upper-lower limit theory and the average method are proposed to determine the range of mechanical noise and average value. The effect of acoustic excitation generated by diesel engines on mechanical noise is also discussed in detail. The results of hydroacoustic experiments show that the identified forces and mechanical noise can be obtained by the present method, with an error nearby 1 dB, under the accelerations with complete information. For the average acceleration, the range of mechanical noise is obtained and the average sound power is used to represent the ship's mechanical noise, with an error of less than 3 dB. The experimental results also reveal that the effect of acoustic cavity resonance should be included in low frequencies.