No abstract

From the energy density and the intensity vector of a general acoustic field in the linear adiabatic approximation, the corresponding time-averaged quantities, the active and reactive intensities, are derived and the inherent ambiguities and some of their properties are discussed. The concepts of pressure-velocity phase relation and of velocity of acoustic energy transport are also introduced for a general acoustic field. It is shown that this velocity in modulus cannot exceed the speed of sound, it vanishes only if acoustic pressure p and velocity v are in quadrature, while it reaches its maximal value only if p and v are in accordance, or in opposition of phase.

A kind of energy polarization is shown to occur in certain acoustic fields, because of energy oscillations due to the instantaneous reactive intensity. The time-averaged behavior of such oscillations is described by a second degree symmetric tensor, which is identified with the time-independent reactive intensity, whose conventional vectorial definition is obtained for particular fields, including monochromatic and spherical ones.

Following recent advancements in the study of time-averaged properties of energy propagation in linear acoustic fields, the well established concept of power factor known from the electric AC circuits analysis, is here extended to acoustics. This allows our outline of a complete acousto-electro-mechanic analogy, where the fundamental physical concept of energy trajectory is assimilated to a continuous line network of electric circuits, and the complex intensity vector field is defined by means of three special spatial directions: the tangent, the principal normal and the binormal direction at each point of any energy path. The notions of sound energy conductance and susceptance are then introduced and their relationship with complex intensity is highlighted. Finally, the frequency distributions of the defined quantities are measured in different acoustical contexts, thus illustrating their practical utility for advanced intensimetric metrology.