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.

A procedure for calibrating pressure-velocity (p-v) sound intensity probes using a progressive plane wave as reference field is presented here. The procedure has been checked for a microelectromechanical system technology-based Microflown(®) match-size probe by comparing the calibration results with the nominal correction curves available from the manufacturer. The reference field was generated along a wave guide by means of a dual cone loudspeaker supplying acoustic energy in the range 20 Hz-20 kHz through an impedance adaptor. Different from the current in-field procedures, the one proposed here allows the calibration of probes under test to be executed at once up to 10 kHz without any change in the experimental setup. After a detailed review of the general principles of calibration, the procedure has been finalized with three main stages: (a) determination of the full coherence calibration bandwidth of the probe, (b) comparison calibration of the probe built-in pressure microphone over the full coherence frequency range, and (c) relative calibration of the velocity sensor over the calibrated pressure one. Calibration results for the probe under test have been best fitted against the calibration filters modeled by the manufacturer and the direct comparison of the obtained data with the factory ones has been reported.