KEF : ACE
Obtaining good bass performance from small speaker boxes has always been the ‘holy grail’ of loudspeaker design. Typically a small box will have less bass performance than a larger one. However, after years of research KEF has overcome this problem with the development of Acoustic Compliance Enhancement (ACE), where activated charcoal, just like in a water filter, is placed inside the loudspeaker cabinet.
Tiny holes in the porous ACE material act like the small air sacks in a lung by absorbing and giving out air molecules as the drive unit moves in and out. This helps the bass driver ‘breathe’ more easily allowing improved bass performance. In fact, a speaker using ACE technology can have the same bass performance as a conventional speaker of twice the size.
The tiny holes in the porous ACE material act like the small air sacks in a lung by absorbing and giving out air molecules.
The bass response from a loudspeaker is critically dependent on the internal cabinet volume. If you want good bass extension, to hear all those powerful low notes from organ fundamentals to life-like movie explosions, you need a physically large enclosure. It all comes from the basic physical laws that determine how a conventional bass drive unit interacts with the air inside the cabinet.
ACE technology allows us to circumvent these basic restrictions and gain the freedom to adjust the ‘effective’ internal volume by filling the enclosure with activated carbon.
ACE works by a process called adsorption – the highly porous carbon granules take in or give out air molecules in response to the change in air pressure caused by the inward and outward movement of the speaker cone. This flow of air molecules is always acting to reduce the internal pressure variations – making the enclosure appear to be much larger than it actually is. The more activated carbon we put into an enclosure the bigger the effective internal volume becomes – up to a point where losses start to dominate. Typically, we can double the internal volume whilst still maintaining a high quality bass roll-off characteristic.
Using ACE to reduce the enclosure volume whilst maintaining the same bass response. The first public demonstration of ACE was based on the RDM monitor – the ACE loaded version maintained the same bass performance with 40% less volume. The volume increase can be utilised in two main ways: (1) reducing the size of a particular speaker while keeping the performance the same or (2) increasing the bass extension from a given speaker whilst keeping the size the same. In general, it’s a bit of both as we will custom design the bass driver for each application and look for the best overall compromise. In sound quality terms ACE has some major advantages – principally that it allows us to design a speaker with the bass extension of a reflex system but with the superior transient response of a closed box. In addition to the measurable parameters, the sound quality of ACE enclosures is consistently well liked by audiophiles, with a precise and detailed bass reproduction.
It has an energy generating device (drivers). It has walls with a floor, ceiling, and side walls. It has unwanted pressure (modes) that resonate at certain frequencies based upon the dimensions of the speaker cabinet. The room modes are responsible for what you hear within the “room”. They are responsible for what you hear outside the cabinet or room. The sound that comes out of a speaker is sound from the drivers, the electronics or amplifiers, and the sound from inside the cabinet that exits through the drivers themselves, especially the low-frequency driver.
You can measure the frequency response inside the speaker cabinet. It will look similar to the frequency response of a small room. It will have a peak below 100 Hz. that is double digit over baseline (flat). After all, it is really just a very small “room”. Instead of using feet to measure the distances inside our speaker cabinet, we use inches. You can measure the frequency response inside the cabinet and you can measure the frequency response of the cabinet itself. The first measurement is the airborne energy within the speaker cabinet. The second is the vibrational signature of the cabinet.
The low-frequency energy generated inside our small room or speaker cabinet leaves the cabinet through the drivers in the front wall. The sound is a combination of the driver and amplifier along with the internal cabinet resonances. We want the sound from the amplifier and the drivers but we don’t want the distortion from inside the speaker cabinet. We also do not want the vibrations from the cabinet mixing in with the sound of the amplifier and the speakers or drivers themselves. How do we minimize the internal cabinet distortions without breaking into the cabinet or “room” itself?
The first thing we do is measure the internal cabinet response or frequency response of our small room. Next, we measure the vibrational signature of the cabinet itself. Remember, we have two response curves to consider. We have the internal cabinet or room response and we have the vibrational signature of the cabinet itself. The low-frequency peak of the “room” response must be treated and the low-frequency vibrational signature of the cabinet must also be treated with our carbon technology.
After we measure both signatures, we go about designing the proper carbon treatment to “wrap” the speaker cabinet in. The miniature diaphragmatic absorbers are designed to deal with both the “room” resonances within the cabinet and the vibrational signature of the cabinet itself. Each “wall’ of the speaker cabinet “wrap” is designed to address both frequency and amplitude of the internal cabinet response and the vibrational signature of the cabinet itself. Let’s see how we did as we send our two way and subwoofer designs to KRK speakers for testing and voicing.