International audienceThe feedback effect is well known but unwanted, by sound engineers. It results from a feedback loop between a microphone and a loudspeaker. Recently, it has been shown that we can take benefit of this effect to estimate with a very good accuracy some parameters such as sound speed. More recently, some experimental results has shown the effect of a local perturbation on the top of an ultrasonic wavewguide. Here we generalize the concept to MIMO (Multiple Input Multiple Output) system where the feedback effect occurs between an array of emitters and an array of receivers. We propose to model the MIMO feedback effect by introducing a feedback matrix. Thanks to the singular decomposition of this matrix times the transfert matrix, we are able to predict the spatial dependence of the feedback effect either on the emitting array and on the receiving array. In a second part, we present experimental results that are obtained with an array of about 10 microphones and an array of about 10 loudspeakers. Several feedback matrices have been tested. One of them is inspired from time reversal. We have applied this technique to detect a person who goes across this acoustic barrier
