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  A Subwoofer Saga: The Story of the EP800
  Easy Answers to Confusing Specs: Sorting Out “Impedance”
  Wild Times in Axiom Country: Fun in the Snow

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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!”

Click for a Larger View

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.

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.

Easy Answers to Confusing Specs: Sorting Out "Impedance"
by Alan Lofft

You’ve seen references to “impedance” and “ohms” in various loudspeaker specifications or in your owner’s manual for an AV receiver. But what is it? Do you have to “match” speaker impedance to your AV receiver or amplifier?

Let’s first get a couple of things clear: Impedance has nothing—I repeat, nothing—to do with sound quality. It is an electrical measurement of a loudspeaker’s resistance; its opposition to the flow of electric current (the audio signals) from your AV receiver or amplifier through your speaker cables to the speaker drivers and the fine wire in the driver voice coils. It’s a kind of electrical "friction" to the movement of electrons through the copper wires in your speakers. We measure impedance in "ohms," named after George Ohm, the German physicist.

In a loudspeaker, current does all the work; voltage is the “push” behind the current, kind of similar to the way water pressure (voltage) forces the water (current) through a hose. If you have a narrow hose (a high impedance), not as much water (current) flows. Use a larger diameter hose (lower resistance) and more water (current) flows.

Current has to flow through your loudspeakers, but we certainly don’t want them or the speaker cables to heat up and waste all your amplifier power! We want the electric audio signals to drive the speakers to produce great sound. Loudspeakers have impedances of 8 ohms, 6 ohms or 4 ohms (those are “nominal” or approximate values, because the impedance of a speaker changes all the time with the different frequencies of music). A 4-ohm speaker draws more electric current through your AV receiver’s output transistors, and since more current equals greater power, 4-ohm speakers tend to have greater dynamic range and play louder more easily than 8-ohm speakers. AV receivers also produce more power into 4-ohm speakers than 8-ohm speakers, as much as 50% more. There isn’t any way you can lower the impedance of your speakers--that’s set by the designer and the voice coil windings and crossover parts, but you can check the impedance of any speaker by looking at the identification plate on the speaker's rear panel, where its impedance will be stated in ohms.

Your AV receiver has essentially zero output impedance (0 ohms) so you do not have to match the impedance of your amplifier to the speakers. The amplifier does not expect to “see” speakers of given impedance and you can connect speakers with different impedances (8 ohms, 6 ohms, 4 ohms) to an AV receiver with no negative effects so long as the impedance of any of your speakers doesn’t go below 4 ohms. If speaker impedance is too low, too much current will run through the AV receiver's output transistors, causing the receiver to overheat and shut down. If you get 4-ohm speakers, make sure your AV receiver is able to drive them easily without overheating. Some brands of AV receivers have no problems driving 4-ohm speakers, others cannot.

Finally, you do not want your speaker cables to raise impedance or resistance and waste your AV receiver's power on its way to your speakers. So use 12-gauge speaker cables between your AV receiver and any loudspeakers and you won't have problems of increased resistance. You can run 12-gauge speaker cable to lengths of 50 feet or more without problems.

Wild Times in Axiom Country: Fun in the Snow

Those who work at Axiom’s plant in Northern Ontario are a hardy bunch, but it’s not all loudspeaker building and enclosure design.

Axiom’s factory is located near Dwight village, in Muskoka, a beautiful summer recreational area dotted with big interconnected lakes and rivers about 175 miles north of Toronto. I emphasize “summer,” because during the long winter, snow falls to depths of 4 and 5 feet or more and it’s darn cold. So what do the year-round residents do in the winter (besides shovel snow and ice fish), you ask?

Well, “water skipping” for one. A sure sign that spring is coming is an annual excursion to Dorset, a nearby village that’s a gateway to Algonquin Park, a huge wilderness area full of thousands of lakes, moose, bears, deer and loons.

Water Skipping in Dorset ON

The idea of “water skipping” –it’s not an activity for the faint of heart--is to ride a snowmobile at full throttle across a stretch of frozen river, gaining enough speed to cross a section of open water beneath the Dorset bridge, hydroplaning until the snowmobile reaches the safety of firm ice at the other end of the channel. One key point to remember is that snowmobiles don't float. And unless drivers can maintain an open throttle, there's a good chance they'll sink!

The bridge over the open water is usually crowded with spectators to watch the daredevil “water runners.” The lure, of course, is that the odd one may not quite make it across. It has become such a spectacle that contract divers stand by ready to jump in the water to retrieve sunken snowmobiles for those who don’t make it!

Here’s a YouTube video of the annual snowmobile “water-skipping” event held in Dorset, Lake of Bays:
http://www.youtube.com/watch?v=N4z6oAditGQ