A Subwoofer Saga: The Story of the EP800
by Alan Lofft
It began as a kernel of thought about deep bass from a subwoofer. It’s a known fact that to extract and reproduce the most subterranean low frequencies, you have to move a lot of air. If you want to replicate a convincing sense of a thunderous explosion or other earth-moving event in a domestic living room or home theater, then setting the alternating pressure waves into motion has to take place. Moreover, compared to midrange sounds, our ears aren’t very sensitive to deep bass, so to be effective, a speaker for very low frequencies must have huge reserves of power to generate the powerful air pressure necessary to convince us that something deep is happening.
Axiom had already developed the EP500 and the EP600 subwoofers, large, ported subwoofers with enough amplifier power and digital control of a single driver to reproduce the lowest musical frequencies with convincing authority. But to Ian Colquhoun, a challenge remained: how to take the technology just that much farther. Clearly, the box size couldn’t get bigger. There’s a practical limit to the size of large boxes that can be placed into domestic spaces and not appear ludicrous. Ian decided to use the box of an EP600.
Catching the Back Wave
However, the existing EP500 and 600 push the limits of port size. (A subwoofer port or vent is used to funnel the back wave from a big driver out of the enclosure “in tune” with the wave from the front of the woofer diaphragm, or at least in a constructive fashion where the response is aided and enhanced at the lowest frequencies.)
Nevertheless, if you want to explore even lower frequencies, all that air rushing out a port begins to create noises of its own. So he decided that trying to get deeper frequencies from the same size box in a vented design would result in excessive port noise.
Still, the challenge was to produce deeper output to frequencies as low as 12 Hz. If you can’t make the box bigger, then there are alternatives and one is to seal the box. Sealing the box eliminates the problem of port noise since all that energy is dissipated inside the enclosure. Another benefit of a sealed enclosure is a gentler roll-off of the deepest bass, so slightly deeper extension is a side benefit of a sealed box. On the other hand, the efficiency of the subwoofer has dropped because there is no back-wave energy to help out. So more power will be required to make up for the losses of a sealed box. Ian immediately decided to use two 12-inch drivers, working in tandem. And because he knew the digital amplifier could produce more and more output into ever-lower impedances, two voice coils on each woofer would take the impedance to almost 1 ohm, so the amplifier could produce 800 watts, hence the designated model name, the EP800. And unlike some subwoofer manufacturers who make exaggerated claims of subwoofer amplifier output power (some subwoofer specs cite figures that exceed the actual power available from a standard AC wall outlet!), Axiom’s EP800 amplifier produces 800 watts continuous output coupled with real dynamic headroom.
Measuring the EP800
The next challenge was to measure the 800 to see if it really produced frequencies to 12 Hz. Axiom has a big anechoic chamber (anechoic means “no echoes”; a chamber where all reflected sound is absorbed so you know exactly what is coming out of the subwoofer, unaided by any room gain or reflections). But trying to measure frequencies of 12 Hz requires a chamber of huge dimensions; so we went to the tower in Axiom’s back lot to do a free-air measurement. On a quiet summer morning at dawn, before any wind came up, the EP800 was hoisted to the top of the 90-ft tower, a microphone suspended in front, and the EP800 was measured. The EP800 did indeed produce output to the 12-Hz region. At this stage an optional filter was added to the digital amplifier that would enable an owner to limit deep bass to 20 Hz, intended for playback of vinyl recordings as well as any recordings that had anomalous subway rumble, air-exchange system noise and the like. (When a subwoofer is genuinely able to reproduce such low frequencies, it’s surprising what turns up on some recordings.) When the filter is switched off, the EP800 extends its deep bass to 12 Hz for playback of some extraordinarily deep effects on some movie soundtracks.
Beta-Testing with Customers
After some final tweaks in design, Axiom put the EP800 out for some early testing with Axiom customers in the Owner’s Club before a formal launch on the Axiom web site. Those early results are in, and here are some of the comments:
Steve Roode wrote: “Fantastic, blown away, highly musical”; “Compared to my previous subwoofer, the EP500, the EP800 is as musical if not more. So much more powerful; will really shake the room.”
SatKar noted: “Favorably pleased; very realistic; previously didn’t have a subwoofer.” “Made system provide greater presence; a very strong offering; tested with movies only.”
Michael M. stated: “Compared with my previous subwoofer, an AR12, no comparison. Heard things that were not heard before. Tested with U571, Saving Private Ryan, Gladiator, and 300.”
Omelan N. said: “Compared to my previous subwoofer, an EP500, feel [the bass] more than hear [it].”
So far in this ongoing story, the EP800 is a success. When I asked Ian Colquhoun about the new sub, he said, “I’ve written new custom digital code for the EP800, which has resulted in a sub with the smoothest, most linear (+/- 1 dB) deep bass I’ve ever produced (to 13 Hz; 12 Hz in a room) or, for that matter, which anyone has ever produced!”
Those are no small claims, but on the evidence of this anechoic frequency response curve, the EP800 has set a new standard for linearity to ultra-deep frequencies.
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The consensus at Axiom is that the EP800 is ready for launch on Axiom’s web site. Click Here
Thanks to all the early EP800 customers who sent in their comments.