The transition from monaural to two-channel stereo in the 1930s is undoubtedly one of the most important innovations in the history of recorded and reproduced sound. The idea first came to Alan Blumlein when he went to the movies, which had only recently been enhanced with synchronized audio.
Since then, of course, two-channel stereo has become the de facto standard of music recording and reproduction, but it's not without its own drawbacks. For example, the sound from two separated speakers is optimal only at the point that forms an equilateral triangle with them—the so-called "sweet spot." Anyone at a different location might hear a very different sound, thanks to the cancellation or reinforcement of certain frequencies that arise when the distances from the listener to the two speakers are not equal.
A British company called Airsound has come up with an ingenious solution to this problem.