Notes on measurement of radiation efficiency

Radiation efficiency is defined as the radiated power normalized by the equivalent power for a baffled piston having a velocity equal to the average mean square vibration. As a general rule of thumb, radiation efficiency will be greatest when the structural wavelength is on the order of or greater than the acoustical wavelength. There will be a steep rise in the radiation efficiency as this critical frequency is approached, and structural modifications can sometimes be made to move this upswing higher in frequency. This normally involves increasing the compliance of the structure. ISO-7849 describes a common procedure for determining the radiation efficiency. A sound intensity scan is used to determine the sound power and an accelerometer array is used to determine the surface vibration. This approach is compared to direct measurement using a P-U probe where surface vibration and sound power are measured simultaneously. The methods are applied to determine the radiation efficiency of a 3.175 mm aluminum and 1 mm thick stainless steel plate.