A time reversal metasurface for mimicking the cocktail party effect

The cocktail party effect is the capability to focus one's auditory attention on particular audio sources while ignoring other audio sources. We propose an experimental setup reproducing the cocktail party effect by designing a time dependent metasurface composed of independent active mirrors. Each active mirror is a programmable acoustical unit cell capable of hearing, computing, and emitting acoustic signals: each of them acts as a convolution filter. The proper metasurface temporal filters configuration allows us to establish acoustic communication between groups of individuals immersed in a noisy environment: a MU-MIMO acoustic system. The experiment consists in recording a set of Green's function between Ne emitters and Nj active mirrors, and a second set of Green's functions between the Nj active mirrors and Nr receivers. To increase the spatio-temporal degrees of freedom (Lemoult et al., PRL 103 2009), we place a forest of steel rods between the active mirrors, the emitters, and the receivers. These data are then used to compute each active mirror's temporal filter using time reversal properties to establish a predefined MU-MIMO configuration. The Ne emitters now emits simultaneously uncorrelated noises. These signals propagate through the disordered medium to the metasurface, then each active mirror hears, convolutes with his filter and emits another signal, which propagates back through the disordered medium towards the Nr receivers. In the end, we establish a correlation matrix by comparing emitted and received signals.