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ACE Technology

J.R.Wright Head of Research February 2001
Ever since loudspeaker designers realised 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 - the ‘holy grail’ - of loudspeaker design to achieve ‘big bass from small boxes’.

It is not possible to break the laws of physics.

But we can bend them!

Maintaining a 40-year tradition of genuine innovation in loudspeakers, KEF now have that enviable ability to bend the laws of physics and produce the Virtual Loudspeaker Cabinet.

By introducing a ‘magic dust’ into the cabinet interior we can make the box acoustically bigger without physical change to the cabinet.

Let us be clear. This is a real innovation: not an evolution, not a slight improvement, no tricks, just something totally new and remarkable. We call it Acoustic Compliance Enhancement (ACE).

To download a PDF copy of the Acoustic Compliance Enhancement,
please click here

INDEX

How does ACE work?

What is adsorption?

how does this improve a loudspeaker?

Performance issues

Applications

Technology Demonstrator

References

Given the potential competitive advantages, it is not surprising that numerous attempts have
been made to extend the low frequency performance of small loudspeaker systems. However,
thus far none have reached commercial viability. Most attempts have involved putting a
specialised gas contained within an impervious bag into the enclosure, the condensation of this
gas into its liquid phase providing an effective increase in acoustic compliance. These systems
[1-4] usually required the presence of an active heating element in the loudspeaker to maintain
thermal conditions critical to successful operation.

There are too many practical obstacles in these approaches. What if we could just use air and
avoid the need for active elements? Back to top

Figure 1. Progressively magnified views of an activated carbon granule.

For a larger image, please click on thumbnail images below.

Figure 2. Typical pore size distribution in two different types of activated carbon.

ACE is achieved by introducing granules of activated carbon into the enclosure.

Activated carbon can be produced from any organic material. The source material is first heated in an oxygen free environment, to prevent burning and to remove any volatile components. The carbon is then activated by additional heating in a controlled environment of oxygen and steam.

We can see from the magnified images (figure 1) that the surface of activated carbon contains a multiplicity of cavern-like pores. In fact these pores penetrate deep into the material and there are more than a million-fold range of pore sizes, from visible cracks to holes of molecular dimensions. Porosity is what distinguishes activated carbon from other carbon materials, and gives it amazing versatility.

Intermolecular attractions in these pores result in adsorption forces. Carbon adsorption forces work like gravity, but on a molecular scale. The pore size distribution is normally classified into macropores, mesopores (collectively known as ‘transport pores’) and micropores (figure 2). It is in the latter - also known as adsorption pores - that the key process of adsorption takes place.
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There are two forms of adsorption - physical and chemical adsorption.

Chemical adsorption occurs when molecules form a strong chemical bond. The process is irreversible - a compound is formed.

Physical adsorption occurs when molecules are weakly attracted to each other (van der Waals’ forces). Physical adsorption is reversible - desorption is possible. This is the process by which ACE works.
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When the loudspeaker cone moves backwards, the air in the box is compressed slightly. In a conventional loudspeaker this results in a pressure increase which acts to impede the movement of the cone. In an ACE system, the pressure increase is smaller because some of the air molecules are momentarily joined to the surface of the carbon granules (adsorbed). So the impedance to motion is significantly reduced. When the cone moves forwards the air molecules are desorbed by the resulting pressure decrease.

Technically, we can consider this as a reduction of air density. The acoustic compliance of air in
the loudspeaker cabinet is given by

	CA = VB / r c2
where	VB is the nett enclosure volume
	P is the density of air
	c is the velocity of sound in air

Therefore a reduction in density produces an increase in compliance, equivalent to enlarging
the enclosure.
This stiffness reduction or Compliance Enhancement can be as much as four times or more
under optimum conditions. Factors of 1.5 to 3 are readily achievable in practice. Back to top

For a larger image, please click on thumbnail images below.

Figure 3. Typical frequency dependence of Compliance Enhancement Factor.

Figure 4. Water adsorption isotherms for two different types of activated carbon

Effective Frequency Range

The ACE process is principally effective below about 100Hz
(figure 3). Above this frequency performance deteriorates because the cycle time becomes too short for adsorption and desorption fully to take place.

Moisture

There is a strong relationship between the tendency of an activated carbon to adsorb air and its tendency to adsorb water vapour. Adsorption of water vapour adversely affects the Compliance Enhancement because the water molecules block the pores and prevent air adsorption. Therefore we have two basic requirements of the carbon - 1) that it is kept as dry as possible,
and 2) that its ‘water uptake’ is minimal. The former is a function of the packaging design and the latter a design issue for the carbon chemist.
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It is a fundamental restriction of conventional direct-radiator loudspeaker system design that enclosure volume, efficiency and low-frequency extension are interdependent. Small [5] shows that

Improving any one of these parameters forces a degradation of one or more of the others. ACE allows the loudspeaker designer to break this apparently immutable principle.

There are therefore three possible applications of ACE:

1. Reduce volume, maintain efficiency and extension;
2. Increase extension, maintain volume and efficiency;
3. Increase efficiency, maintain volume and extension (requires changes to drive unit).

We shall illustrate the use of ACE in the exploitation of (1) above. Back to top

For a larger image, please click on thumbnail images below.

Figure 5. Difference in Sound Pressure Level between RDM1 lab reference and smaller ACE version.

The KEF RDM1 is a high performance bookshelf loudspeaker incorporating a UniQ array in a closed box, 8.6 litre internal volume. The UniQ array was removed from one of the lab reference pair and placed in a cut-down version of the cabinet, internal volume 5 litre. 2.2 litre of ACE material was introduced into the test cabinet. Figure 5 shows the difference between the lab reference and the ACE prototype. Note that the maximum deviation is 0.2dB - within the measurement tolerance. In listening tests the view was unanimous that the low frequency extension had been maintained: furthermore all listeners reported a preference for the bass ‘attack’ of the ACE version - possibly because of some measurable differences at higher frequencies, but more probably because of other acoustic properties of the activated carbon which require further investigation. Back to top

1. H.W.Sullivan, Loud Speaker, U.S.Patent no. 2,797,766, 1957.

2. J.H.Ott, Enclosure System for Sound Generators, U.S.Patent no. 4,004,094, 1977.

3. E.J.Czerwinski, Device for increasing the Compliance of a Speaker Enclosure, U.S.Patent
no. 4,101,736, 1978.

4. R.E.Marrs, Acoustic Energy Systems, U.S.Patent no. 4,450,929, 1984.

5. R.H.Small, Closed-Box Loudspeaker Systems Part 1: Analysis, J. Audio Eng. Soc. 20 (10), 1972.
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