Multipath-enabled private audio with noise
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We address the problem of privately communicating audio messages to multiple listeners in a reverberant room, using a set of loudspeakers. We propose two methods based on emitting noise. In the first method, the loudspeakers emit noise signals that are appropriately filtered so that after echoing along the multiple paths in the room, they sum up and descramble to yield distinct meaningful audio messages only at specific focusing spots, while being incoherent everywhere else. In the second method, adapted from wireless communications, we project noise signals onto the nullspace of the MIMO channel matrix between the loudspeakers and listeners. Loudspeakers reproduce a sum of the projected noise signals and intended messages. Again because of the echoes, the MIMO nullspace changes between the different locations in the room. Thus the listeners at focusing spots hear intended messages, while the acoustic channel of an eavesdropper at any other location is jammed. We show using both numerical and real experiments, that with a small number of speakers and a few impulse response measurements, audio messages can be indeed be communicated to a set of listeners while ensuring negligible intelligibility elsewhere.
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