The use of surface power for characterisation of structure-borne sound sources of low modal density

The use of the surface power methods for source characterisaiton of vibrating machinery of low modal density is investigated in this paper. It was demonstrated by Ohlrich and Larsen that this relatively simple, but very useful measurement technique for quantifying the vibratory strength of machinery, is very suitable in cases of high modal density, especially with respect to overall evaluation of machinery vibration characteristics and for estimation of the power produced by internal source mechanisms of the machine.

Thus, it is envisaged that the method can be used in the development stage of new machines, in comparison studies of different machines, and in factory quality control to ensure that vibro-acoustic specifications are met. Carefully controlled experiments with an instrumented 3/4-scale structural model of a helicopter gearbox of low modal density, show that the surface power is a robust measure of the machinery vibration, and of the total power injected by internal source mechanisms (despite of their slight dependence on gearbox mounting condition, ie, resiliently suspended or connected to the helicopter fuselage).