Technical Description

The AIRSOUND™ Loudspeaker - Spring 2006

The History

Back in 1931, the scientist and inventor Alan Blumlein described a method of recording and reproducing 'stereophonic' sound: His arguments and theories involved the idea that as we have two ears, the ability to recognise where a sound is coming from is to do with how the sound reaches our ears; that if one side is louder than the other, then the sound is obviously located on the loud side.

Blumlein however recognised that there was a lot more to stereo listening than the positioning of the sound. He went on to describe in theory how it might be possible to record and reproduce 'spatial' sound, using a principle called 'sum and difference' or 'middle and side' (M/S). This involved using an omni-directional microphone to record the main signal information, and a figure-8 response (or 'dipole') microphone set across the 'field of sound' to record the spatial information. By adding and subtracting the 'omni' or 'sum' signal with the figure-8 or 'difference' signal, it is possible to arrive at a conventional 'left' and 'right' audio signal; although, there are significant flaws in the argument that creates a stereo signal from an 'M/S' signal.

In spite of the inaccuracies, the M/S system can be used as the basis of reproducing sounds with space; the M signal represents the majority of the signal content, while the S contains all the spatial information.

The First Principle

In sound reproduction this M/S recording system can be reversed, such that a sum of left and right information from the stereo pair can be reproduced by a normal monopole loudspeaker and a difference (left minus right) signal may be reproduced through a coincident dipole loudspeaker across the field of sound. By doing that, the combination effect produces a representative image. However errors in relation to what is really left and really right combine to produce a muddled spatial image.

The Second Principle

As well as the scientific principle of M/S reproduction, a further scientific principle is relevant and important to the AIRSOUND™ loudspeaker; this is the 'surface effect' of acoustic propagation adjacent to a flat surface. When a sound is reproduced in close proximity to a flat surface, being a surface that is of greater linear dimension than the wavelength of the most significant frequencies within the sound, then the reflections from the surface have the effect of reinforcing the wave motions across the surface, such that the volume of the sound is maintained at a considerable distance from the source loudspeaker. This effect is easily understood by thinking of being on the banks of a lake when the air is quiet and still; under these conditions it's easy to hear quiet conversations right across the water. This is an example of the 'surface effect'.

The AIRSOUND™ Combination

AIRSOUND™ is the combination of electronic processing, the application of the M/S principle combined with the surface effect principle, and the detail design of the loudspeaker arrangements; such that the errors inherent in a simple M/S reproduction system are eliminated. The system has several Domestic and International Patent Applications.

The AIRSOUND™ loudspeaker system is a significant improvement over conventional loudspeakers in four respects:

1. In a domestic environment it is no longer necessary to position a pair of loudspeakers accurately to optimise stereo reproduction.
2. The depth/width effect can be appreciated over a very wide listening area compared with conventional stereo, where deviation from a fixed listening position at the apex of an equilateral triangle made by the pair of loudspeakers and the listener degrades the stereophonic effect.
3. By virtue of the single point sound source, the audio path lengths to the ears of the listener are proportionally the same, irrespective of the position of the listener with respect to the loudspeaker unit. This provides improved clarity of sound by the elimination of path length phase distortion common with all left/right loudspeaker systems.
4. A single speaker enclosure eliminates the frequency node and cancellation 'comb filter' effects resulting from separate left and right loudspeakers