Acoustic imaging in confined and noisy environments using double layer Time Reversal and Field Separation Methods

Stéphanie Lobréau (1) , Eric Bavu (1) , Manuel Melon (2)

1 LMSSC - Laboratoire de Mécanique des Structures et des Systèmes Couplés
2 LAUM - Laboratoire d'Acoustique de l'Université du Mans

Many imaging methods cannot localize precisely unstationary sources in confined and noisy environments. In this paper, the use of a Time Reversal acoustic sink (TRS) method is proposed, in conjunction with a Field Separation Method (FSM). The proposed time reversal (TR) process is based on the measurement of the sound pressure field and its normal derivative on a double layer hemispherical antenna, which bounds the region of interest (ROI). These data are time-reversed and numerically back-propagated to a surface, 0.5 cm away from the source plane. As most imaging methods, the efficiency of this process relies on the use of the most suitable Green functions, which depend on the propagating environment. A way to improve the TR process is to transform numerically the confined space problem into a free field case, for which the Green functions are well-known. The proposed FSM consists in expanding the measured fields on the spherical harmonics functions, thus allowing to compute the outgoing waves. This process allows a precise localization and characterization of the source placed under the antenna, using free-field Green functions. Thanks to this method, the influence of reverberation and acoustic fields radiated by sources outside the ROI can be suppressed. The measurements presented in this paper are performed in an anechoic room, using two acoustic sources. The first one to image in the ROI emits a filtered pulse and the second one, placed outside the ROI, is driven by a Gaussian white noise. In order to assess the reconstruction quality of the proposed imaging process, a reference field is measured in an anechoic room on the back-propagation surface, corresponding to the pressure values when the source laying in the ROI is radiating alone. Comparisons with back-propagated pressures using TRS in conjunction with FSM show a good accuracy both in space and time domains.

Domaines: Acoustique [physics.class-ph]

Stéphanie Lobréau, Eric Bavu, Manuel Melon. Acoustic imaging in confined and noisy environments using double layer Time Reversal and Field Separation Methods. Forum Acusticum, Sep 2014, Krakow, Poland. ⟨hal-02088332⟩