Subwoofers: The Guts and the Glory
Sound and Vision Magazine | Feb 24, 2014
Five Subwoofer Masters Explain How They Work Their Magic
Subwoofer design has undergone a revolution. No, the physics of woofers, amplifiers, and enclosures haven’t changed. But new technologies have made it possible to push the bass-ic laws of nature to their limits, such that the best of today’s inexpensive subwoofers can outperform many of the top models from 15 or 20 years ago.
Affordable digital audio processing lets designers tune subwoofers in ways the engineers of the 1990s could only have imagined. High-efficiency amplifiers pound out powerful bass from boxes hardly bigger than a basketball. New speaker drivers use high-tech materials to produce sound levels that would have pushed older models way past the breaking point.
Since things are so different now, we thought this would be a great time to revisit some of the fundamentals of subwoofer design and examine how the old rules might have changed in the digital age.
Of course, we have some insight into this at Sound & Vision, having tested many of today’s best subwoofers. But we can’t observe how all these new technological twists affect a designer’s day-to-day decisions. So I consulted a handful of the hundred or so people on Earth who possess this in-depth knowledge.
The experts I talked with were Andrew Welker, R&D director for Axiom; Dr. Poh Hsu, founder of Hsu Research; Tom Vodhanel, president of Power Sound Audio; Ed Mullen, director of technology and customer relations for SVS; and Chris Hagen, acoustic system development engineer for Velodyne.
In the interest of brevity, our experts’ comments are heavily edited here.
OK, time to go deep—literally and figuratively.
Sealed, Ported, or Passive?
To kick off my conversations with the experts, I decided to revisit the defining characteristic of a subwoofer’s design: whether the enclosure is sealed or ported and whether it uses any passive radiators. A ported (vented, or bass reflex) box has one or more ports, tuned with tubes that usually run into the enclosure. A passive radiator replaces the port with what is, in essence, a large woofer with just a suspended diaphragm and no voice coil connected to the amplifier or magnet. There are other types of subs, but they’re rarely used in home theaters.
These acoustical alignments determine the way the natural bass response of a subwoofer decreases below the inherent resonant frequency of the enclosure/driver system. With sealed-box subwoofers, this rate is generally –12 decibels per octave. With subs that use a port or a passive radiator, the rate is steeper, usually –24 dB per octave. But a sub with a port or passive radiator will typically deliver deeper bass extension and more output from the same amp power.
Why choose one or another? “A bass-reflex sub typically has much higher maximum output at the deepest frequencies than a smaller, sealed sub at the same price point,” said SVS’s Mullen. However, he also pointed out that a sub with super-deep bass extension may overwhelm a smaller room. This is because of a function called room gain, in which the natural acoustics of a room reinforce a sub’s low bass output. As Mullen explained, “a sealed sub can work better in a small room because its rolloff characteristics are often a better match for the room gain.”
Power Sound Audio’s Vodhanel agrees. “Under 3,000 cubic feet of effective room Volume—including the room space it’s open to—a sealed enclosure may be all you need. From 3,000 to 5,000 cubic feet, you can go either way. Above 5,000, we lean toward larger ported models—or, if the budget is there, our new Triax sealed model, which has three 15-inch drivers.”
For Velodyne’s Hagen, it’s often a matter of price. “If the subwoofer needs to be cost effective and doesn’t have much power, then you lean toward having assistance devices like a port or a passive radiator,” he said. “If cost is no object, it’s typically better to get a less steep rolloff using a sealed box, then throw a lot of power at it to get deeper bass extension.” In their higher-end subs, Hsu and SVS include foam plugs and switchable equalization so the customer can choose to use the sub sealed, with one port open, or with two ports open.
All of these experts feel that passive radiators have their place—mainly in smaller subs—but none is enthusiastic about them. “The advantage is that you don’t have port noise,” Hsu noted, “and when you have a very small cabinet, the passive radiator is the easiest way to get good low-frequency response. You probably don’t have enough room for a port tube, and you’d get too much port noise anyway. But for a passive radiator to be tuned for low-frequency output from a small cabinet, you need a really heavy diaphragm and very high excursion, so you could have a reliability problem.”
Delays, Delays, Delays
Another difference between sealed subwoofers and ported or passive-radiator models is in phase response. As a simple explanation, an open-back woofer cone radiates backwave energy that effectively cancels out what’s coming off the front of the driver; it is said to be in “reverse phase.” With a sealed enclosure, this energy is discarded—it does no work but is also not allowed into the room where it could destructively cancel bass energy from the front. In a bass-reflex enclosure, the port or passive radiator delays the backwave energy just long enough so that its effect is additive instead of destructive. But, since there are no free lunches, this added delay results in some sacrifice of performance in the time domain, which may affect our perception of how “tight” the bass sounds, among other things.
Another reality of speaker design is what’s called “group delay,” or the time difference between when one frequency coming from the speaker reaches you versus another. The more abruptly a sub’s output level changes with frequency—as it does at low frequencies, below the resonance of the enclosure—the greater the group delay and, some would say, the potential for audible, phase-related effects on the sound.
As SVS’s Mullen said, “If you look at a sealed-box sub like our SB13, the rolloff slope is shallower, and there’s less phase rotation and ringing in the time domain. Subjectively, people will perceive that as faster/tighter bass with less overhang. A bass-reflex sub will typically have a much more abrupt transition at the rolloff . If you plot the response in the time domain, you will see distinct ringing at or near the corner frequency.”
Vodhanel pointed out that the deeper a sub’s response is, the less you have to worry about group delay because the effect is pushed down to lower frequencies, so it doesn’t involve as much of the audible band. “If you get it to extend as deep as possible, you’re gaining two things: extension and overall sound quality,” he said. “You’ll have less ringing, and the impulse response will be tighter.”
However, Axiom’s Welker dismissed the idea of group delay as a serious subwoofer issue. “There’s no good data showing perceivable levels of group delay at low frequencies. All the papers that have been written only deal with group delay at mid and high frequencies. We’ve simulated different group delays with digital signal processing, and in controlled blind testing we haven’t found any correlation between added group delay and a decrease in sound quality.”
Said Hagen: “Making a judgment [about sealed versus ported subs] based solely on alignment is unfair. I’ve heard ported systems that were very tight and had no phase issues. Just because a speaker has been designed with a port in it does not instantly equate to an awful or objectionable phase response.”
What Kind of Amp?
Many, perhaps most, of today’s subwoofers use a high-efficiency amplifier, which requires little or no ventilation and does not require large transformers and heatsinks. The most popular type of high-efficiency amp is Class D, in which the output transistors switch either fully on or fully off at ultrasonic frequencies, and a filter circuit at the output extracts the desired audio signal from the high-frequency switching pulses. Other types, Class G and Class H, employ a variant of a standard Class AB amp fitted with a power supply that uses multiple power rails (Class G) or continuously tracks the level of the incoming signal (Class H) so it can provide only as much rail voltage as needed moment to moment.
Another advantage of high-efficiency designs is that their compact size makes it practical to fit, say, a 1,000-watt amp into a small subwoofer. This is especially important in small subs that use DSP to boost bass output. For example, with a sealed-box sub that has an anechoic response that is –12 dB at 40 hertz, adding in the natural bass rolloff of –12 dB per octave means you need 16 times as much power to produce a flat measurement under anechoic conditions at 20 Hz as you do at 80 Hz.
Many audiophile-oriented subs use traditional Class AB amps, because some engineers and audiophiles believe the sound quality is superior. Not surprisingly, I heard some disagreement about this.
“The sweet spot is in high-efficiency topologies like D or H, because dealing with a high-powered Class AB amp inside a subwoofer enclosure is difficult,” Welker said. “There’s also a lot of concern about power consumption these days. With Class D, standby power can be easily a tenth of that of a similarly powered Class AB amp. So all the amps we use are Class D. I think they’re every bit the equal of a good Class AB amp, even used full-range.”
Asked if he had any concerns about the sound quality of high-efficiency amps, Vodhanel replied: “When we first tried a digital amp, circa 1999 or 2000, the big worry was sound quality. But in every objective test we did—impulse response, group delay, compression, everything we could think of—sound quality issues didn’t show themselves.”
Hsu disagreed, citing a concern often raised with Class D amps: damping factor, or the ratio of the speaker impedance to the amp’s output impedance. The higher the damping factor, the greater the “braking force” it can exercise on the moving woofer cone.
“Most o the Class D amps I’ve looked at have a poor damping factor, because they have a choke [an inductor, or filter component] at the output to keep high frequencies from getting out,” Hsu explained. “So I feel Class AB still makes the most sense. A switching power supply is fine, though, and I have no problem with Class G or H because those are just Class AB with multiple rails. If I come across Class D with a good damping factor, though, I’d be happy to try it out.”
However, Welker believes that designing a Class D amplifier, using a switching power supply with a switching amp, presents problems. “With a switching supply, if it’s rated for a certain power, that’s all you’re going to get. A linear supply can deliver peak current well beyond its rated power. It’s also proven that they last. You can find amps from the 1950s where the transformers are still working.” This is why Axiom uses linear power supplies for their Class D amps.
DSP Tuning Tricks
Not long ago, digital signal processing was too expensive for most subwoofers, but now you can find it even in budget subs. It’s used not only for tweaking frequency response, but also for compression and limiting, subsonic filtering, crossover functions, and special sound modes. I was curious to learn how, with so much processing power in their hands, our experts prefer to tune their subs.
“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.”
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.
In an era when you can find automatic room equalization on even a $250 A/V receiver, it might seem like there’s no need to bother with careful subwoofer setup. Won’t the receiver just do it for you? Not according to our experts.
“As good as auto setup systems are, they still make mistakes, sometimes egregious mistakes,” said SVS’s Ed Mullen. “It’s inexplicable that sometimes they’ll set an in-wall or in-ceiling speaker to full-range, but it does happen.” And auto EQ can’t fix everything. “You cannot fill in a valley [a dip in frequency response] caused by a room mode,” pointed out Axiom’s Andrew Welker. “You’re just going to waste amplifier power.” Added Velodyne’s Chris Hagen: “Sometimes if you put the sub in without doing any setup work, you’ll get a big lump at the crossover point. The auto setup in the receiver will tend to try to fix that by turning the sub down. Then we get the call, ‘What’s happening? I can’t hear your subwoofer!’”
Thus, if you want to ensure the best performance, you’re still going to need to put some thouht into the placement and calibration of your subwoofer(s).
If you’re using Audyssey or another auto EQ system, SVS’s Mullen and Power Sound Audio’s Tom Vodhanel have the same suggestion: Put the sub in the corner and let the receiver sort it out.
“Corner placement excites all possible room modes,” Mullen said. “The more modes you’re exciting, the less potential there is for a gaping hole in the in-room frequency response—i.e., a null. To the extent corner placement creates peaks in the response, that can be fixed by your receiver, provided it equalizes the subwoofer channel.”
“One of the challenges is getting as much headroom as possible for your budget,” Vodhanel said. “If you place the sub in a corner with no openings nearby, you’ll excite all the room modes. That may or may not be a good thing, but you will be getting maximum efficiency. Another option is nearfield placement, as close as possible to the seating position, maybe 2 meters away. This way the energy of the sub reaches the seating position before it has a chance to interact with the room, and the sub is so close to you that you’ll still see good system efficiency.”
If you don’t have or don’t want to use auto EQ—or if you just want the smoothest possible performance—the best solution is the “subwoofer crawl.” To do this, place the sub in your favorite listening seat, then play some music or movies and crawl around the floor until you find the place with the smoothest bass response. Then just put your sub there.
“I always suggest doing a subwoofer crawl to determine the best spot to place the sub,” Hsu said. “If your system has equalization capabilities, find the location that gives the highest output in the low bass without any major weakness in the upper bass. That would give you the highest headroom and lowest distortion. If you do not have EQ capabilities, find the spot that gives you the smoothest response.”