From Audio Pro's website:
Since 1978, we have designed and manufactured loudspeakers which have gained worldwide acclaim - specially within our specialty - active subwoofers. Not surprisingly, Audio Pro's speaker history started with a unique product. The B2.50, a rare subwoofer featuring built-in amplification and active crossover electronics. B2.50 incorporated the ACE-Bass (Amplifier Controlled Euphonic Bass) technique. It's a patented technique designed to reproduce low frequencies from small enclosures. The ACE-Bass in the B2.50 used dual subwoofer speakers, exactly monitored by servos in order for the electronics to make adjustments accordingly. Ace-bass was invented by Karl Erik Ståhl, a young Swedish engineer who refused to back down until he had created a way to achieve deep bass without filling the living room with speaker cabinets the size of telephone booths. The ace-bass technology was first presented at the 61st Audio Engineering Society Convention, New York, in November 1978. Audiophiles were amazed, and Audio Pro became world famous in HiFi circles. Especially impressive was the extremely low bass, which gave sonic sensations that few had experienced outside the concert hall. The ace-bass technology has been further deveolped during the years and is still used in all Audio Pro subwoofers today.
A more tecnical description of ace-bass
In Elektronikvärlden (a Swedish hi-fi magazine) No. 8, 1988, Bertil Hellsten wrote a very good and instructive description of the ace-bass technology. Below is an abstract of the article:
General
There are many ways to accomplish good bass in a loudspeaker. One is to have a large cabinet and a large woofer.
In practice, the lower limit for a woofer is set by the lower resonance frequency, which can be lowered by increasing the membrane's mass. By adding weights, you can go lower and lower in frequency. But, at the same time, the speaker will respond increasingly slowly.
In addition, the membrane's compliance and damping will no longer match, so the result isn't useful. To achieve a very low limiting frequency in a small woofer, a combination of mass, compliance and damping is needed, and this is impossible to achieve mechanically.
Audio Pro's solution - electronic mechanics
Seen from the amplifier's point of view, the loudspeaker is an electrical component with its mechanical attributes transformed into electrical magnitudes. The mass corresponds to a capacitance, and the compliance to an inductance that is in parallel with the capacitance. The attenuation corresponds to a resistance that is in parallel with both of the above.
By adding more inductances, resistances and capacitances, one should easily be able to change the loudspeaker's mechanical attributes. This is the main idea behind Audio Pro's ace-bass loudspeaker constructions.
But there is a catch. The drive unit's voice coil has a resistance that lies serially with the parallel 'mechanical' components. This means that we can't parallel-connect directly to the woofer's mechanical parameters. It is not connected to the amplifier firmly enough.
If we were to increase the damping by parallel-connecting the amplifier output with a resistance, we would still never get a lower value than what the voice coil's resistance gives, and that is not good enough. The same thing happens to the other attributes. They are 'hidden' by the voice coil's (and the speaker cable's) resistance.
Negative resistance
The solution is to serially connect the amplifier output with a negative resistance. But you can't buy that at your nearest electronics parts shop. It can only be achieved by positive feedback in the amplifier's output stage.
There are several sensitive factors that can easily cause the system to self-oscillate. However, correctly trimmed, you can achieve a system that exactly equalizes all the resistances that are in series with the drive units' properties. In this way, you can also reach and affect the loudspeaker's mechanical attributes electrically.
You can also choose to let this influence decrease with increasing frequency, where it's no longer needed.
By using this feature fully, Audio Pro maintains that you can decrease the speaker's physical volume by 90%, compared to a conventional bass reflex box!
ACE-BASS is an abbreviation for Amplifier Controlled Euphonic Bass.
