IAP - Measuring the acoustic energy flow

Acoustic Poynting Vector

One of the most awarding acoustical experimental technique of the last decade is the vectorial intensity (Poynting vector, energy density flow, power per unit area) measurement method. By means of this method, visua lisation of an acoustic field can be achieved in terms of energy-flux lines, thus adding to analytical possibilities not found in more conventional procedures. Intensity is a much more informative noise emission measurand than, e.g., the scalar acoustic pressure in air-borne acoustic fields. The intensity concept allows the discrimination between a reactive part of the energy flow, which is dominating in a high quality acoustic resonator, and the active or net energy flow, which is the interesting v ariable in damped complex real structures. Sound sources and sinks, as well as main propagation paths, are clearly indicated by this physical quantity.

The existing theory regarding the acoustic Poynting vector in the free surface of a structure which up to now has only been established for the case of isotropic structures, was generalized to non-isotropic structures. It was shown that, in this general case, the measurement of the acoustic Poynting vector in the surface can be reduced to a measurement of the three in-plane strain and the two in-plane velocity components. Based on this formulation, a sensor head concept for the determination of the acoustic Poynting vector, using multipoint optical in-plane displacement measurement was developed. The concept was studied experimentally with a simple case of one-dimensional structural vibration (Fig. 1). The experimental results obtained with a sensor head designed for one-dimensional acoustic energy flow demonstrate the feasibility of the technique.