Researchers at the universities of Sussex and Bristol in the UK are working towards making scenarios like these – and more – possible by employing "acoustic metamaterials" to manipulate sound in the same way lenses do with light. Think, sniper-scopes and narrow-beam torches, but with sound.
The research team has demonstrated the very first dynamic metamaterial device with the zoom objective of a varifocal lens for sound, dubbed "Vari-sound." The team also built a collimator (a device which produces a parallel beam of rays or radiation) capable of transmitting sound as a narrow, directional beam from a standard speaker. An acoustic collimator could be used for the individual-in-the-crowd scenario mentioned above, or perhaps to aim a signal up the aisles of a cinema, to deter people from sitting on the aisle steps.