Broadband acoustic holography reconstruction from acoustic intensity measurements. I: Principle of the method

Abstract

A method, broadband acoustic holography from intensity measurements (BAHIM), is developed for the reconstruction of sources using nearfield acoustic intensity measurement. Nearfield acoustic holography (NAH) is used, but data concerning complex pressure on the hologram are not acquired directly. These data are obtained by processing measurements of active intensity and potential energy density (quadratic pressure). These quadratic values are measured independently at each point on the hologram and therefore do not require a phase reference signal corresponding to the source. This method is thus applicable to sources emitting a broadband spectrum and, more generally, to industrial sources, for which the excitation is not known. The principle of the method is described and the advantages of this type of processing are illustrated by examples.

Extracting the Green’s function from measurements of the energy flux

“…The situation is different for the measurement of U AB for two simultaneous sources, because this measurement relies on the interference of waves excited at r A and r B . Repeating the derivation leading to expression (9), and assuming that the amplitude and phase variations are independent, gives in this case…”

“…In holography, phase information is restored from intensity measurements of the interference of a reference wave with waves reflected off an object. 9,10 In our work the phase information is retrieved from the energy flux through a closed surface of waves that are excited by different time-harmonic sources.…”

Acoustic intensity measurements in the presence of low Mach number flow

“…The level, in dB, of this bias error for the case of a plane progressive wave would be, The level of the on-axis bias error for a dipole source has been shown to be, 24 Ll= 101og,0 3 Ar r~r 2 r~r 2 j where qp = tan' (-I / kr,) and (2 = tan' (-1 / kr 2 ). The measurement accuracy for the dipole source is more sensitive to the parameters kAr and Ar/r, than is the measurement accuracy for a monopole source.…”

“…Examples of intensity measurement applications which are less developed and are the focus of more current research include structural intensity'' 2 (or structural power flow), near field intensity measurements for broadband acoustic holography, 3 intensity measurements in the presence of flow,"'7'6. 7 in-situ evaluation of the acoustic impedance and sound absorption properties of materials, and underwater sound 2 intensity measurements.…”

Parameter Analysis and Experimental Investigations on Cylindrical Broadband Acoustic Holography

“…Broadband acoustic holography from intensity measurements (BAHIM) [5][6] was first introduced in 1987. In BAHIM method, the main advantage is the measurement practicability in comparison with the technique in which the pressure phase is measured directly without an array of referenced microphones or by use of the reference signal to the source signal.…”