Prediction of Far-Field Sound Radiation Using Blocked Pressures and Reciprocally Measured Vibro-acoustic Frequency Response Functions

Abstract

In industry, the use of blocked forces for the characterisation of structure borne sound sources is now common practice, whereas the sound power is typically used for airborne sound sources. In principle, it should also be possible to use blocked forces (or blocked pressures) to predict airborne sound radiation using a similar approach and measurement methodology as the in-situ blocked force method. This is achieved by discretising the surface of the source into small unit cells, and measuring the vibration velocity whilst operational. In order to convert these velocities to blocked forces, the mobility of the source for each of the positions must also be known: and to predict the radiated sound, a measured vibro-acoustic frequency response function is also required. Typically vibro-acoustic FRFs are measured by exciting the structure with an instrumented hammer or shaker, but they may also be measured reciprocally using a volume velocity source. Described in the paper is an experiment conducted in a fully anechoic chamber to investigate the feasibility of the above approach, i.e. using blocked pressure and reciprocally measured vibro-acoustic FRFs to characterise and predict airborne sound. The method is validated by comparing the directly measured radiated sound pressure to that which is predicted according to the above.