Subwoofers: The Guts and the Glory Page 3

“My process is to go for flat response in a known environment, such as an anechoic chamber or 2-pi [ground plane] measurements,” said Velodyne’s Hagen. “Then we get it into our sound room and listen to it, and we make some adjustments based on that. We don’t necessarily just stick with flat all the time. It depends on what we think the expectations are of people shopping for that kind of a product. That’s why we put four sound-mode presets in our low-priced subs, and six presets and auto EQ capability in the Digital Drive Plus line. We don’t want to impose a particular response curve on anyone, but flat in an anechoic environment is always our starting point.”

Velodyne’s Digital Drive Plus subwoofers offer six different sound modes plus a built-in auto EQ circuit.

Axiom’s Welker agreed that flat is the place to start but added that the room can have its own pleasing effects. “We EQ subs to be flat anechoically, which we decided based on blind listening tests. But in a normal room, we tend to like a lift at the bottom end. We’ve found that when you EQ the sub to be flat, room gain gives you a lift in the low frequencies. If you put an artificial hump in the bottom end, in most cases you’ll find that to be overbearing.”

According to SVS’s Mullen, there’s a lot more to digital tuning than just frequency response. That’s because the limiter/compressor that protects the driver from being damaged due to overloads is also typically in the digital domain. “Our limiter/compressor algorithms are frequency-dependent,” he said. “At deeper frequencies, the question becomes, ‘How much further can you push the sub?’ There’s not a lot left in the tank after you hit the CEA-2010 threshold [discussed below]. Typically what we’ll do is draw the line with the limiter just a little past CEA-2010 at lowest frequencies.”

Which Measurements Matter?
When it comes time to evaluate an audio prototype, the basics of the process with conventional speakers is long-settled: Measure the frequency response on and off axis with a 2.83-volt (1-watt) signal. But with subwoofers, there’s an active debate.

Consider our recent roundup of $800 subwoofers. The tiny Paradigm Monitor SUB 10 measured –3 dB at 19 Hz, but it averaged only 100.4 dB of bottom-octave output. Meanwhile, the hulking Power Sound Audio XV15 had –3-dB output down to only 21 Hz but average bottom-octave output of 116.2 dB. Why? Using DSP, Paradigm’s engineers boosted the low bass of the SUB 10 so it was flat to 20 Hz—at least at the low levels at which frequency response is usually measured. Push it to higher levels, and it can’t keep up with larger subs.

Axiom’s large subwoofers, such as the EP800, combine a traditional, linear power supply with a high-efficiency Class D amplifier.

Whereas measuring frequency response is important—“If you get the frequency response wrong, it’s never going to sound right,” Mullen said—obviously there’s a need to measure the output of subs, too. That’s why the CEA-2010 standard was created. CEA-2010 measures how loud a sub can play at six frequencies (20, 25, 31.5, 40, 50, and 63 Hz) without exceeding certain distortion thresholds.

All of our experts agree that measuring distortion is important. Vodhanel said that at Power Sound Audio, “we’ve done a lot of listening tests, and we think the tone bursts used in CEA-2010 are closely related to what someone’s going to hear in transient material.”

However, our experts don’t agree that CEA-2010 is the best way to measure distortion. “The idea is excellent,” Welker said. “If I could guarantee that those measurements are repeatable within 0.5 or 1 dB, I could approve of it. But I can easily get 1 or 2 dB of variation just by moving around our parking lot. It also doesn’t tell you anything about how a subwoofer is going to sound, only how loud it will play within a given frequency range.”

Hagen felt that CEA-2010’s thresholds—allowing as much as 30 percent distortion for the second distortion harmonic—are too lax. “Our ideal is under 10 percent total harmonic distortion at maximum output. We’re trying to make reproducers, not producers, and anything with a lot of distortion is producing a sound of its own.”

The Bottom Line
Building great subs is easier and less expensive than it once was, but it still demands a delicate balance of size versus output versus deep bass extension versus cost. “It all has to be designed to work together synergistically,” Hsu summed up.

Yes, our experts disagree on some aspects of subwoofer design. Members of online forums disagree about which design is ideal for a certain application. Reviewers disagree about the best ways of evaluating subs. Yet there’s one thing we can all agree on: With so much knowledge out there, and so much powerful audio technology available at such low costs, these are the best days ever for bassheads.

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COMMENTS
mdanderson's picture

Great article. Good to hear from the experts about sub design and setup.

designmule's picture

This is a great article! The full interviews are also extremely informative. I felt that the interviews were very neutral in terms of the questions asked. Sometimes there can be real nasty "two channel" or "home theater" divide amongst enthusiasts and the article and particularly the interviews gave equal time to both groups.

I have a question after reading through all of the material. When we talk about DSP on-board the subwoofers does that mean that the analog signal which arrives at the subwoofer is converted to digital for processing and then back to analog for amplification?

If the answer is "yes" does that mean that it is time for digital subwoofer outputs on the receivers/pre-pros and for digital inputs on the subs?

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