Sweet Space

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.

Of course, the audio was mono, and Blumlein noticed that the actors' voices didn't always seem to come from their onscreen positions. This sparked the idea of recording the audio with two microphones and playing them on two speakers, allowing voices to "follow" the actors as they moved around on the screen.

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. The company was founded by Ted Fletcher, a longtime engineer who also started Joemeek, a well-respected maker of professional audio electronics and microphones.

To address the drawbacks of two-speaker playback, Fletcher devised a way to reproduce two-channel stereo from a single speaker enclosure. The system works much like M-S (mid-side) recording in reverse. M-S recordings are made with a central microphone aimed at the sound source, which captures the main part of the sound, and a bidirectional or figure-8 mic pointing to the sides, which records the spatial information. Interestingly, this idea was first proposed by Blumlein in the 1930s.

Fletcher applied the same idea to reproduction. His Airsound speaker cabinet includes a central speaker driver aimed forward and two drivers facing sideways, each oriented at 90 degrees to the central driver as shown in the illustration above. The forward-facing driver reproduces the main signal, which is the sum of the left and right channels (L+R), while the side drivers reproduce a difference signal (L-R), with one of them being 180 degrees out of phase with the other. These drivers provide the spatial information necessary for perceiving a stereo soundfield.

Used in isolation, this technique can result in a somewhat muddled spatial image, so the Airsound concept also relies on something called the "surface effect." By placing the speaker cabinet against a flat surface, reflections reinforce the sound waves, allowing the volume to be maintained at a considerable distance from the speaker and enhancing the clarity of spatial image. Obviously, placing the cabinet against a wall is the easiest way to accomplish this; alternatively, a flat surface can be mounted to the cabinet to allow more flexible positioning.

The advantages of this approach are said to be dramatic. Perhaps most importantly, there is no sweet spot—the stereo effect is balanced and clear no matter where you are in front of the cabinet. In addition, the placement of the speaker is not critical (other than needing a boundary surface). And because the sound comes from a single source, there are no frequency, phase, or time anomalies, which results in a more well-defined and intelligible sound than two separated speakers can typically produce.

This technology could be a boon for consumer sound systems, studio monitoring, PA systems, and many other audio applications. Alan Blumlein would be proud that his theoretical ideas have found such fertile ground in which to bloom.