There’s an exhibition or installation by Bruce Neuman at Tate Modern at the moment. The web page in itself is worth looking at as an interesting way of documenting an exhibition on the web.
According to a post on the Max mailing list the installation uses some kind of wall-mounted planar speakers to give tightly localised beams of sound in the vast and reverberant space of the Turbine Hall. Ten metres away from the speaker the width of the beam is still only a couple of metres or less almost silent outside that zone.
