Case Studies of Machine Bases as Structure-borne Sound Sources in Buildings

The appearance of an international standard for sources of structure-borne sound has highlighted rather than solved the problem of estimating structure-borne emission from and source characterisation of vibrating machines and machine components. The standard gives procedures for measuring the velocities at machine contact points. Although it is limited to resiliently mounted machinery, the velocities obtained often closely approximate those of the free source i.e. obtained when the source operates under ‘normal’ conditions but is not attached to supporting and connected structures. These data are a subset of those required but they cannot give a complete source characterisation or sufficient information for structure-borne emission. The additional data required will be obtained through an as yet incomplete set of international standards for the measurement of mechanical mobility. The term characterisation has been used in three ways: 1) when representing the internal mechanisms that produce the vibrations, 2) as a measure of the energy of the vibrating machine and 3) of the ability of the machine to emit power to connected and supporting structures. An example of the first is the pseudo-force method of Verheij et al. where the internal mechanisms that produce the vibrational field on the machine casing or at the contacts are modelled by a substitute set of external point forces. The approach has the advantage of being applicable to cases where sources are strongly coupled and integrated into larger machine assemblies, and cannot be considered in isolation. An example of the second is the surface equivalent source power of Ohlrich, also applicable to integrated assemblies where the source cannot be treated in isolation. The surface power is the product of the equivalent forces and the corresponding real part of the point mobilities. This is a descriptor of the power generated in the machine by the internal mechanisms but does not lead to transmission predictions. An example of the third is the terminal source power, of Olhrich et al., which has been used to predict transmission from a main gearbox to a helicopter fuselage. Two primary issues are identified. The first is that the structural dynamics of the receiving structure must be known in some form for a description of the transmission process and the second concerns the large amount of data required for characterisation and prediction of emission.