Experimental identification of force radiation modes

The location of a vibration source within a machine is sometimes found to have a significant impact upon the machine’s radiated sound power. Radiation mode analysis is known to be a powerful tool for interpreting and predicting sound radiation from a vibrating object since radiation modes are independent of a structure’s surface vibration. However, knowledge of the radiation modes alone cannot be used directly to understand the relationship between vibration source location and sound power radiation. Previously, it was suggested that the radiation mode concept could be extended to understand the relationship between the sound power and the driving force distribution by combining a structure’s mobility matrix and its radiation modes to form force radiation modes (frad-modes). It was demonstrated by simulation that the radiated sound power can be reduced by choosing the driving force location to be at or near the nodes of the force radiation modes. Here, to further validate the usefulness of the force radiation mode concept, the acoustic transfer vectors and the mobility matrix of a thin steel strip mounted in a rigid baffle were first measured. Then, the force radiation modes were calculated by using the experimental data and it was confirmed that the force radiation modes can serve as a practical guide for choosing a vibration source location that minimizes the radiated sound power, especially at low frequencies.