Cookson R., Qi N.

The structure-borne sound power of installed machines requires a measurement of both the vibration activity of the machine and the structural dynamics (mobility or impedance) at the contacts with the building structure. The source activity can be obtained as a free velocity, which requires the source to be removed from the installation whilst operating in otherwise normal conditions. There are practical difficulties in duplicating the operating loads on the machine and the free velocity must be measured at each contact point and often for more than one component of excitation. A practical simpler alternative is described where the machine under test is attached to a thin plate, of high mobility, and the spatial average response velocity of the plate is recorded. This allows an estimate of the vibration power received by the plate, which equals the power delivered by the machine. The power is proportional to the sum of the squares of the source free velocity, over the contact points. Results are presented from numerical models of thin reception plates excited by machines. An experimental determination of reception plate power also is described and directly and indirectly measured free velocities are compared.