Real-time near-field acoustic holography for continuously visualizing nonstationary acoustic fields

Abstract

Near-field acoustic holography (NAH) is an effective tool for visualizing acoustic sources from pressure measurements made in the near-field of sources using a microphone array. The method involving the Fourier transform and some processing in the frequency-wavenumber domain is suitable for the study of stationary acoustic sources, providing an image of the spatial acoustic field for one frequency. When the behavior of acoustic sources fluctuates in time, NAH may not be used. Unlike time domain holography or transient method, the method proposed in the paper needs no transformation in the frequency domain or any assumption about local stationary properties. It is based on a time formulation of forward sound prediction or backward sound radiation in the time-wavenumber domain. The propagation is described by an analytic impulse response used to define a digital filter. The implementation of one filter in forward propagation and its inverse to recover the acoustic field on the source plane implies by simulations that real-time NAH is viable. Since a numerical filter is used rather than a Fourier transform of the time-signal, the emission on a point of the source may be rebuilt continuously and used for other post-processing applications.

Real-time nearfield acoustic holography for reconstructing the instantaneous surface normal velocity

“…The disadvantage of TDH is that its reconstruction process is not real-time as a result of backward propagation at all frequencies before coming back to the time domain. Real-time nearfield acoustic holography (RT-NAH) proposed by Thomas et al [12,13] needs no backward propagation in the frequency domain. It operates in the time-wavenumber domain by means of a discretized time convolution between the wavenumber spectrum of the pressure recorded by a microphone array and an inverse impulse response.…”

Reconstruction of nonstationary sound fields based on the time domain plane wave superposition method

“…TDH is particularly well-adapted to the study of short time events due to the use of the Fourier operators to the time-dependent pressure signals. Real-time near-field acoustic holography (RT-NAH) proposed by Thomas et al 18,19 has the advantage to continuously reconstruct time-dependent pressure signals whatever the size of the signals analyzed. It is based on a time convolution formulation in the time-wavenumber domain between the time fluctuating wavenumber spectrum and an inverse impulse response.…”

An interpolated time-domain equivalent source method for modeling transient acoustic radiation over a mass-like plane based on the transient half-space Green’s function

“…This technique allows one to reconstruct or predict the radiated harmonic acoustic field in the frequency domain [4][5][6][7]. For the purpose of inversely studying the transient acoustic problems [8,9] in the time domain, some timedomain NAH (TNAH) methods [10][11][12][13][14] have been developed during the last two decades. Such TNAH methods have the ability to model the transient sound field but are, in theory, confined to the sound sources of planar geometry.…”