Test Report: Sunfire Atmos Subwoofer

For subwoofer designers, the laws of physics boil down to: Small box, low cost, high output — pick any two. You can always shrink the enclosure, but to get decent output from it, you’ll need a high-powered amp and a beefy driver. And if you shrink the box way down, as Sunfire did with its new Atmos subwoofer, you’ll need to go even more extreme.

The first thing you notice about the Atmos is its size: The longest side measures just 10.1 inches. It’s smaller even than some of those cheapie “subwoofers” that come with desktop audio systems. But Sunfire contends that the Atmos can go hertz to hertz, decibel to decibel with full-size home theater subs. The next thing you notice is that its enclosure is made from thick extruded aluminum, which contributes much to the sub’s 32-pound weight. Sunfire chose aluminum in order to contain the intense pressure generated inside the Atmos. You might also notice the ultra-mega-beefy asymmetrical rubber surrounds (the part that connects the diaphragm to the speaker frame) on the woofer and the passive radiator. Sunfire says the size and unique shape of the woofer’s surround gives it a maximum excursion of 1.8 inches. That’s a feat few 18-inch drivers can match.

What you can’t see is the 1,400-watt amplifier inside. Why so much power in such a small sub? Because the power demands of small-box subs increase radically with every octave below the subwoofer’s resonant frequency — by a factor of 16 for a sealed box, and even more for a passive radiator design like the Atmos. The amp uses Sunfire’s tracking down-converter technology, basically a Class H design where the power supply’s output is continuously adjusted so that it provides just enough to meet the sub’s power demands. The advantage of Class H is that it produces no excess energy that would need to be flushed out through a big heat sink.

Even with such muscular internal components, the Atmos can’t match the output of a much larger subwoofer. That’s why Sunfire strongly recommends placing it in the corner, where it’ll get a huge boost in output. Unfortunately, corner placement also tends to create huge resonant peaks and a really boomy sound, so Sunfire added an automatic room EQ circuit that’s intended to smooth out the big peaks.

Setup

Hookup for the Atmos is straightforward: line-level connections only, and I needed just one of those jacks to connect to my receiver. A nice feature that I didn’t use but that could be handy for stereo systems is a line-level stereo output with a defeatable 85-Hz high-pass filter. Connect your preamp to the Atmos’s line ins, run the Atmos’s line outs to your amp, activate the 85-Hz high-pass fi lter, and you have a fully functional subwoofer crossover — which is necessary because so few stereo preamps have built-in crossovers.

Calibrating the auto EQ circuit is simple. Just place the included microphone on the back of your favorite listening chair, then hold down the EQ button on the back of the sub for 5 seconds. A low-level calibration tone will emerge. Wait 10 seconds, then hit the button again to trigger the next test tone. Repeat the process twice more and you’re done.

Obviously, an auto EQ circuit that uses only four test tones can’t be all that sophisticated, but to my surprise, the results I got from the Atmos’s auto EQ were above average. Using TrueRTA software and an Earthworks M30 microphone, I measured the before-and- after results with the auto EQ and found that it had correctly identified the major peaks in my room, reduced the biggest one by 4 dB and the next-biggest one by 2 dB, and actually raised the level of the third one by 12 dB to match the level of the others. Normally, boosting frequencies this much with an auto EQ circuit is a bad idea because it causes distortion, but with a driver this robust, it’s no problem. The result was much smoother, more even bass, and elimination of the nasty resonant sound that corner positioning tends to produce with subs.

I used the Atmos with a variety of speakers, from my 200-pound Krell Resolution One towers to Definitive Technology’s new SM45 mini-monitor, using crossover points ranging from 80 to 120 Hz, depending on the speaker.

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