Calculation of the structure-borne sound power emission from machines

Abstract

The structure-borne sound power emission from vibration sources is the closest analogue to airborne sound power and has tremendous potential benefit in prediction of sound levels in buildings and machines. However, the concept has yet to find wide application because the large number of terms needed to calculate power makes practical implementation difficult. In the paper, some simplifying assumptions are investigated, aiming to reduce the number of terms while maintaining acceptable accuracy. In general, the power through one contact point between a machine and its foundation depends on the velocity contributions from other points. The phase angles of these coupling contributions are investigated by measurement of free velocities and the mobility matrix at the seven mount points of a large, resiliently mounted fan. The assumption is that any phase angle is equally likely and this is shown to be reasonable at the frequencies considered. It is shown that a simplified calculation of sound power, using just seven terms causes little loss in accuracy compared with the full 28-term formulation.