  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
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 echoes, the
MIMO nullspace changes across 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 indeed be communicated to a
set of listeners while ensuring negligible intelligibility elsewhere.
