Unsteady acoustic imaging in adverse environments

Time reversal imaging of acoustic sources in reverberating and noisy environments

Superbvised student : Stéphanie Lobréau

Supervision : Éric Bavu , Manuel Melon

Project duration : 4 ans (2011 - 2015)

Funding : Bourse ministérielle

Résumé : In a large number of industrial applications, it is necessary to inspect radiating structures using unsteady acoustic source characterization and localization techniques. In the last decades, several acoustic imaging techniques have been developed, based on the use of measurements of a set of acoustic quantities (pressure and/or particle velocity) on microphonic antennas (structured or not).

Figure : Double layer pressure-pressure probe used in this project

The work carried out in this project focuses more specifically on unsteady imaging techniques by time reversal. We are particularly interested in optimizing the performance of these unsteady imaging techniques insensitive to the measurement conditions (reverberant and noisy environment).

For this purpose, various improvements have been proposed, thanks to measurements performed on a double-layer hemispheric array.

Figure : Microphone array geometry, with 36 double layer probes and allowing to project the acoustic field on spherical harmonics up to order 7. Given the geometric dimension of the antenna, this approach allows us to de-reverberate and denoise the measured field up to a frequency of about 3150 Hz.

In particular, quantitative acoustic time-reversal imaging is achieved by a time-reversed Helmholtz-Kirchhoff integral calculation, thanks to “double layer” measurements performed on the antenna and a Lebedev quadrature allowing to project these measurements on the basis of spherical harmonics. These double layer measurements are used to suppress room effects and contributions from interfering sources, thanks to a field separation method, which consists in projecting the measured data on the basis of spherical harmonics and then separating the “incoming” and “outgoing” contributions.

Finally, in order to refine the imaging resolution obtained by time-reversal, we have introduced the automatic use of a time-reversal sink, whose analytical formulation takes advantage of the double-layer structure of the measured data.

Figure : Comparison of acoustic imaging in a noisy and reverberant environment of two acoustic sources separated by 25 centimeters. On the left, conventional imaging fails completely to locate and quantify the contributions of the two sources. On the right, the approach developed in this project allows to find the two sources with an excellent resolution, and to quantify precisely the field of the two sources, by backpropagation in the plane of the sources.

The quality of pressure field reconstruction by the full double-layer time-reversal imaging process has been studied through numerical and experimental studies, ranging from an ideal configuration to particularly complex cases of measurements in highly reverberant and noisy environments.

Figure : Measurement of the radiation of a Steeldrum in a reverberant and noisy environment (Clément Auzou’s internship) and demonstration of a non-linear coupling between the notes of the Steeldrum.

Publications and communications related to the project

• S. Lobréau, É. Bavu, M. Melon, Hemispherical double-layer time reversal imaging in reverberant and noisy environments at audible frequencies, The Journal of the Acoustical Society of America, 137 (2), 785-796, 2015. doi
• S. Lobréau, É. Bavu, M. Melon. Acoustic imaging in confined and noisy environments using double layer time reversal and field separation methods. Proceedings of the 7th Forum Acusticum, Krakow, Poland, September 7-12, 2014.
• É. Bavu, C. Auzou, M. Monteil, M. Melon, C. Langrenne, A. Garcia. Time-Reversal Imaging and Field-Separation-Method applied to the study of the steelpan radiation. Proceedings of Acoustics2012, Nantes, France, April 23-27, 2012.
• É. Bavu, M. Melon, C. Auzou, S. Lobreau, C. Langrenne, A. Garcia. Sonic Time-Reversal Imaging optimization in reverberating, confined or noisy environments. Proceedings of Acoustics2012, Nantes, France, April 23-27, 2012.
• S. Lobréau, É. Bavu, M. Melon. Imagerie acoustique instationnaire par retournement temporel en environnement complexe. Actes du 22èmeCongrès Français de Mécanique, CFM 2015, Lyon, France, 24-28 août 2015.
• É. Bavu. Imagerie de sources acoustiques par renversement du temps - Optimisations et stratégies de dépassement de limites, du théorique à l’expérimental. Les Journées spécialisées SFA - AMTA - Applications des Méthodes Temporelles en Acoustique, Le Mans, France, 24-25 novembre 2011.