Tech Page: Sweet Space

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The transition from monaural to 2-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. That sparked his idea of recording the audio with two microphones and playing it on two speakers, allowing the actors' voices to “follow” them as they moved around on the screen.

Since then, of course, 2-channel stereo has become the de facto standard of sound 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.

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FIG. 1: The airSound concept uses a single main speaker driver in the front of a cabinet to reproduce the main sound, which is the sum of the left and right signals (L + R). The two side-firing drivers, 180 degrees out of phase with respect to each other, reproduce the difference signal (L – R) to provide the spatial information.
Photo: Chuck Dahmer

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 manufacturer of professional audio electronics and microphones.

To address the drawbacks of 2-speaker playback, Fletcher devised a way to reproduce 2-channel stereo from a single speaker enclosure. The system works much like M-S (middle-side) recording in reverse. As you may already know, 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 sound 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 (see Fig. 1). 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 the difference signal (L - R), with one driver being 180 degrees out of phase with the other. These drivers provide the spatial information necessary for perceiving a stereo sound field.

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 the spatial image. 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.

According to Airsound, the advantages of this approach are dramatic. Perhaps most important, 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.

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