As the speaker diaphragm compresses the air in the cabinet, the activated carbon absorbs more air molecules. The diaphragm consequently experiences less resistance to its motion, as if the volume of air in the cabinet were larger.


Andy Giles | KEF

It's All To Do With Size, says Andy Giles

As the speaker diaphragm compresses the air in the cabinet, the activated carbon absorbs more air molecules. The diaphragm consequently experiences less resistance to its motion, as if the volume of air in the cabinet were larger.

Now a world-famous name in loudspeaker design and manufacturing, UK company KEF was originally founded in 1961. From the beginning, KEF - borne out of the hi-fi boom of the sixties - was destined to become renowned to have a flair for the unusual and controversial in terms of loudspeaker engineering, design and use of materials. This tradition they would, perhaps justifiably, claim continues to this day. With the latest development from KEF - the introduction of a new technology that might just change the way loudspeakers perform in the home or studio - KEF seems to be on the right course. It's all to do with cabinet size. Does huge, spacial sound have to come from huge, space-hugging speakers? KEF says no - and most emphatically so.

KEF's take on this goes something like this. Ever since loudspeaker designers realized the acoustic benefits of partially or fully enclosing a loudspeaker in a cabinet they have sought to extract the maximum bass performance from a minimum cabinet size. It has become the ultimate aim of loudspeaker design to achieve 'big bass from small boxes'. Although it's not possible for us to break the laws of physics - KEF says we can bend them.

It makes sense. The old audio maxim that a good large loudspeaker will always beat a good small one reflects a core aspect of the physics of loudspeaker design: the interdependency of box internal volume, bass extension and sensitivity. Because a fundamental relationship interlocks these three factors, changing one always alters one or both of the others.

We all know the consequences: small loudspeakers have less bass extension and/or lower sensitivity than larger equivalents. With active loudspeakers there are techniques to offset these restrictions but with conventional passive loudspeakers all the designer can do is approach the theoretical limits as closely as possible. Ultimately the underlying physics acts as a shackle, which explains why there has been no breakthrough in this area down the decades. Until now, claims KEF.

Researchers at KEF Audio asked themselves: if improved bass extension and/or sensitivity require an increase in cabinet volume, is there any way to achieve this other than by making the cabinet larger? It may sound like a crazy question but it isn't - actually there is a means of making the internal volume of a speaker cabinet appear to be much larger than it is. KEF calls the technique Acoustic Compliance Enhancement (ACE), although it's more evocatively known as the virtual loudspeaker cabinet. The invention is patented, and the first products employing it are already in production.

Key to ACE is the unique surface action of activated carbon, which is widely used in industry - and a few domestic products also - as a means of 'mopping up' certain types of molecules. Under high magnification, grains of activated carbon are seen to have a complex surface structure comprising numerous pores of different sizes. These pores confer a high ratio of surface area to volume, and provide a large number of sites at which molecules can become attached to the material surface or 'adsorbed'. (Adsorption is strictly a surface phenomenon, in contrast to absorption where a substance is carried into the body of a material.)

Different processing of the carbon can generate different sizes of pore, giving it an affinity for different types of molecule. Some forms of activated carbon have a high affinity for air, and it's this property that is exploited in ACE. A loose-weave bag of activated carbon granules is simply placed within the cabinet, and the speaker then assembled as normal.

This is what happens in use. As the speaker diaphragm compresses the air in the cabinet, the activated carbon absorbs more air molecules. The diaphragm consequently experiences less resistance to its motion, as if the volume of air in the cabinet were larger. As the diaphragm moves in the opposite direction, reducing air pressure within the cabinet, the activated carbon releases air molecules (a process called desorption) so that again it appears to the drive unit as if it is operating within a larger enclosure.

Although this increase in cabinet volume is apparent rather than real, the benefits are exactly as they would be with a physically larger cabinet. This means bass extension and/or sensitivity can be improved while maintaining the same cabinet volume. Or the cabinet volume can be reduced while maintaining bass reach and sensitivity.

KEF's research shows that the apparent increase in volume achievable in practice can be as large as 3x. Still greater enhancements are feasible but rendered impractical because the activated carbon then adds too much internal damping.

Increasing the apparent cabinet volume by a factor of three equates to:

  1. a 30 per cent (over a third of an octave) reduction in the bass corner frequency while keeping sensitivity and box volume unchanged;
  2. a 4.8dB improvement in sensitivity while keeping bass corner frequency and box volume unchanged;
  3. a downsizing of the cabinet internal volume by two-thirds while keeping bass corner frequency and sensitivity unchanged;

or a spectrum of trade-offs between these extremes.

Particularly now that DVD-Audio and SACD are poised to enhance the profile of multichannel audio, speaker manufacturers are more concerned than ever to create wide bandwidth, high sensitivity designs which remain compact enough to be domestically harmonious. By bending what was thought to be an immutable rule of loudspeaker design, ACE promises to give KEF a key commercial advantage in allowing these inter-related factors to be traded off in a way that was not possible previously. Whatever ACE claims, one thing is certain. Its incorporation into a loudspeaker will guarantee more bass response from a smaller cabinet.

And that has to be good for the consumer.

The content & opinions in this article are the author’s and do not necessarily represent the views of HomeToys

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