A technique for measurement of structure‐borne intensity in plates

A measurement technique to determine the structure‐borne intensity (power flow) in a point‐driven, fluid‐loaded, homogeneous thin plate is demonstrated. This intensity is uniquely determined from a knowledge of the normal velocity of the surface of a sinusoidally driven plate. A noncontact method is presented which uses a scanning hydrophone to measure the pressure in a plane very close to the source. It is shown how this pressure measurement is sufficient to compute the two‐dimensional structural intensity vector inside the plate. We call this technique SIMAP for Structural Intensity from the Measurement of Acoustic Pressure. Uses of the structural intensity vectors to compute the power injected into the plate from a point driver are shown. Independent measurement of injected mechanical power is shown to be in close agreement. SIMAP, which is an outgrowth of nearfield acoustical holography (NAH), also provides the normal acoustic intensity radiated into the medium from the same pressure measurement. A comparison of the structure‐borne intensity with this normal acoustic intensity indicates that structural intensity is much more accurate in locating the real source than the acoustic intensity, the latter often failing completely. The examples given in this paper are for an underwater source, but the technique applies without modification to plate sources in air.