Sonic Time Reversal Imaging optimization in reverberating, confined or noisy environments

Time reversal (TR) is a powerful method for the imaging and localization of sound sources. Classical TR is based on the recording of the pressure field on a time reversal mirror (TRM), followed by a numerical backpropagation of the time-reversed signals in a simulated propagation environment. To achieve accurate imaging, Green functions (GF) describing the environment must be well-known. When dealing with reverberating environments, precise numerical backpropagation is a rather complicated problem to solve. In order to avoid this situation, we propose a field separation method (FSM) in order to recover data that would be measured on the TRM in free-space, corresponding to the well-known free-field GF. This method consists in measuring the acoustic pressure on a double-layer hemispherical TRM. Outgoing waves are separated from ingoing waves by using spherical-harmonic expansions. The outgoing contribution is then time-reversed and numerically backpropagated using the free-field GF, allowing to achieve accurate imaging. This FSM also allows to separate contributions from sources outside the region of interest. This new method is illustrated by simulations and measurements in a car-trunk mockup and in a reverberating room. Imaging resolution will be discussed using several TR schemes, taking advantage of the double layer p-p measurements.