Hi. I've been designing and building my own loudspeakers for about seven years, and have held a job at a manufacturer of professional loudspeakers in Philadelphia, PA. I'd like to clear up for your readers exactly what is happening in the Altec Lansing subwoofer, because I don't feel it is adequately explained in the article.
To tune a subwoofer to a low frequency, typically a large enclosure is needed, but a small enclosure with a very long bass port can be used to achieve the same tuning frequency. However, this small enclosure tends to produce a much higher acoustic impedance on the back-side of the driver, restricting the cone motion and requiring a large amount of force from the driver's motor. When the enclosure used is too small, the response curve appears to gradually roll off at a higher-than-desired frequency instead of exhibiting the proper vented box cutoff characteristics at the desired tuning frequency. Simply, the bass response won't be there, even though the enclosure is tuned to a low frequency, because the air volume is too small to let the woofers do their thing. By taking two drivers and coupling their diaphragms together, it is possible to produce a driver with twice the motor strength and suspension stiffness acting on the same amount of diaphragm area, allowing the required enclosure size for a given tuning to be reduced by half. This method of coupling two drivers together is called Isobaric* loading, because it creates a tiny chamber of constant pressure between the two driver cones, which is about as effective as coupling the voice coils of both drivers. Sometimes both drivers are facing forward, and an extra chamber must be built into the enclosure. However, another method of achieving Isobaric coupling is by taking the second driver and mounting it face-to-face with the first driver. Then, the second driver is wired out-of-phase with the first one, so that when the electrical signal is applied, both cones move in the same direction, even though both drivers are facing each other. This is called Isobaric Push-Pull, because one driver pushes while the other driver motor pulls. This has the added effect of canceling out asymmetries in the motor's magnetic field or the compliance of the drivers' suspension, lowering the distortion produced by the drivers when they are driven hard, for bass that's both deeper and cleaner. Isobaric push-pull doesn't double the effective diaphragm size, so it doesn't double the efficiency, but it does double the thermal power handling. Isobaric push-pull isn't useful for midbass and midrange drivers, but it can be very useful especially in situations where a small enclosure is a design requirement but deep bass is desired. Isobaric loading is also fairly uncommon because the much more expensive drivers used in high-fidelity audio have the powerful motors necessary to drive a low-tuned, small box. Isobaric clusters double the cost of the drivers used, and they're not very aesthetically appealing. For a multimedia subwoofer, an Isobaric cluster is a very practical way of achieving deep bass output while keeping the enclosure size small.
The use of the dual woofers in the FX4021 subwoofer isn't to produce louder bass, but to produce bass that is deeper and lower in distortion. Unfortunately, those who have an untrained ear might be accustomed to lots of extra energy around 60-80 Hz and the distortion that comes from conventional driver configurations involving small drivers, and suppose that the FX4021 isn't producing much in the way of bass. But listen again -- there's real high-fidelity bass there, and it goes deep. That's the thing that struck me most when listening to these speakers - the quality and extension of the bass when compared to similarly-priced offerings from other manufacturers. Altec Lansing has seriously hit upon something here. My only personal beef with the FX4021 subwoofer is that the vent size (~1.5" diameter) is too small to support the amount of air that the enclosure is capable of moving at that frequency, so there is some wind turbulence noise produced at each end of the vent. This could be ameliorated by the use of radiused 'flares' at each end of the port tube.