Home Theater Review


How to Choose a Subwoofer for Surround Sound or Stereo

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Subwoofers-thumb.jpgEvery audio system, from a $250 garage stereo to a $2,500 home theater to a $250,000 mega-system, can benefit from a subwoofer. Everybody knows a subwoofer can deliver cleaner, deeper, more powerful bass than most main speakers. Subs also have an advantage in that they can be placed wherever the bass response is optimum in your room, whereas your main speakers have to be placed for the best midrange and treble response.

Buying a sub isn't so simple, though. They're available with woofers sized from six to 24 inches, amps rated from 50 to 3,000 watts, and frequency responses in some cases dropping well below what humans can hear (but not below what you can feel). Every sub comes with some sort of performance specifications, but the specifications almost never include an explanation of how they were determined.

Having reviewed and measured hundreds of subwoofers since 1990, and having put dozens of them through blind tests with a panel of listeners, I've learned a lot about what's important and what's not...and what you can and can't ignore in the specs.

In this article, I'll discuss the features you should look for, and the ways in which you can and can't judge the performance of a sub from its specs.

Almost every subwoofer (except perhaps a few in-wall and in-ceiling models) includes a built-in amplifier with a built-in crossover. If you have an AV receiver, surround processor, or stereo preamp with a built-in crossover, you probably won't use the one in the subwoofer. If you have a stereo system, though, you'll probably need it. Almost all subwoofer crossovers offer a suitably steep slope and a wide enough cutoff-frequency adjustment range that they can mate pretty easily with almost any main speaker.

Practically all subwoofers have a line-level input with an RCA jack. This is all you need if you're using an AV receiver, a surround processor, or a stereo preamp with a built-in sub crossover. If you're using the sub with a stereo system that doesn't have any sort of sub crossover, look for a sub with speaker-level inputs. Using these inputs, you can hook up the sub using extra speaker cables connected from the amp or the main speakers. Your main speakers will still receive deep bass signals; however, if you set the subwoofer's crossover point at or a little above the speakers' rated bass response (usually around 50 to 80 Hz for bookshelf speakers, 20 to 40 Hz for tower speakers), the sub and speakers should blend pretty well. A few companies--most notably, REL and Sumiko--include extra controls that let you blend the speaker-level input with the line-level input; I find that these subs tend to blend smoothly with the main speakers, even if they don't usually have quite as much output as their more home-theater-oriented competitors.

Most subs give you a switch to let you flip the sub's phase 180 degrees. Some give you a knob that adjusts phase from zero to 180 degrees. The switch usually works okay, but I prefer the knob because it makes it easier to fine-tune the blend with the main speakers. (To do this, I play pink noise through the system, then I turn the sub's phase knob to the setting that makes the bass loudest.)

Some more expensive models include an automatic EQ function that uses an included microphone (or a mic built into a smartphone) to adjust the sub's response to suit the acoustics of your room. If you already have a receiver or surround processor with an auto EQ system such as Audyssey, you probably don't need this. With a stereo system or a surround system without auto EQ, having this function in the subwoofer can make a big difference in the sound. Some of these systems (such as Paradigm's Perfect Bass Kit and Velodyne's Digital Drive Plus) work extremely well, but they're costly. Less-expensive subs usually have auto EQ systems that adjust at only four frequencies, and usually over a range of only plus or minus a few decibels; they can improve the sub's sound somewhat, or they might not have much benefit at all.

Many subwoofers now offer a remote control; I personally love this feature because it lets you tweak the sub level to suit the movie or music you're listening to. Some include music and movie modes, or even EQ modes intended to improve the sound with certain types of music. I don't usually think much of these modes, but if you dig 'em, that's your business...and if you don't like them, you can just choose the best-sounding mode and stick with that one.

Lots of subs now offer wireless capability, usually in the form of a built-in receiver and an optional transmitter that connects to your system. All of the ones I've tried in the last few years worked well, with no loss of fidelity. However, they do tend to introduce latency, or delay, which could affect the distance settings in your receiver or surround processor. Each millisecond of delay added by a wireless system is like moving the subwoofer one foot farther away, and some wireless systems can add as much as 50 milliseconds. The phase adjustment I described above should fix the problem; you can also experiment with the distance settings in the processor or receiver. Or just ignore it; often a wireless rig will sound fine despite the latency.

Notice I haven't said anything about the amplifier size or type, or the size or construction of the driver? That's because you can't generalize much from those specs. I've tested 350-watt subs that nearly shook my house down. I've tested 3,000-watt subs that sounded like they were about to fall apart when asked to reproduce deep bass tones at high levels.

The same goes for driver size. An eight-inch model probably won't ever beat out a 15-incher, but 10-inchers often deliver more and deeper output than 12-inchers, and 12-inchers can outperform 15-inchers. However, I do find that subs with smaller drivers are often (but by no means always) easier to blend with bookshelf speakers and small tower speakers.

Sealed vs. Ported vs. Passive Radiator
This is the place where some enthusiasts seem to have lots of misinformation. Sealed subs have a reputation for sounding tight and punchy, with less deep bass output. Ported subs have a rep for sounding loose and boomy, with more deep bass output. Passive radiators perform essentially the same acoustical function as a port, but I've encountered enthusiasts and reviewers who assume they're more like sealed subs.

According to every subwoofer designer I've talked with, and to my own experience, it's unwise to generalize in this area. I've heard boomy sealed subs and tight ported and passive radiator subs. Nevertheless, I often recommend two-channel audiophiles lean toward sealed subs and home theater enthusiasts lean toward ported or passive radiator subs. Saying something like "ported subwoofers have too much group delay [or phase shift] to sound well-defined" marks you as a non-expert in this case, but saying something like "I tend to prefer the sound of sealed subs" is entirely defensible. (For the record, I used to prefer sealed subs, but all of my current favorites are ported models. I would speculate that's partly because of the tuning flexibility of the digital signal processors used in most subs, and partly because speaker designers have gotten better at port tuning and other acoustical aspects of subwoofer design.)

The downsides of ported subs are that they tend to be much larger and that the air moving through the port can cause a "chuffing" sound if the port's not well designed. Passive radiator subs with equivalent performance can be made much smaller, although sometimes the passive radiator can introduce clicking and banging noises under extreme stress. Sealed subs have neither of these problems, but they rarely match the deep-bass power of a ported or passive radiator sub with the same-size driver and amp.

Some of the larger ported subs allow you to plug their ports in different combinations, which in conjunction with an EQ switch allows you to fine-tune the sub's sound. I find this feature invaluable, and I recommend it for anyone who's serious enough to spend some time experimenting with the different options.

While we're on the subject, I tend to take subjective assessments of a subwoofer's sonic character with a big grain of salt. So much of a subwoofer's sound is determined by where you place it in your room and how skilled you (or your installer) are at getting the sub to blend well with the main speakers. I also find a disturbing tendency for some reviewers and audiophiles to be influenced by manufacturers' reputations or claims of "musicality." All things being equal, some subs do sound more precise and tuneful than others, but it takes a careful, knowledgeable assessment to make sure the comparison is done fairly and not influenced by branding, marketing, cosmetics, or price.

Here's where buying a sub gets tough. While a really good sub will deliver great measured performance under almost any conditions, a lesser sub can often be measured in ways that make it seem just as good as the really good sub.

The problem is frequency response measurements, which are usually performed at low signal levels. I've tested feeble subs that had flat measured response down below 20 Hz, and I've tested impressively powerful subs that started to roll off below 35 Hz. The problem is, the feeble sub can't deliver 20 Hz at loud levels. Meanwhile, the powerful sub might deliver just a few decibels less output at 20 Hz than it does at 35 Hz. Try to play deep tones through the feeble sub at high levels, and either the tones will be attenuated or the sub will distort, rattle, or produce port noise.

This is why I ignore most manufacturers' frequency response measurements. Even if the manufacturer is above-board in its testing, the frequency response measurement won't tell you what the sub can do under stress--and subs are often under stress. One thing I can say, though, is that frequency response measurements done using ground plane technique tell you more than measurements done by placing the microphone close to the driver (and port or radiator, if there is one). In a ground plane measurement, the microphone is one or two meters from the sub, and the measurement will probably be taken at an average level of 90 dB or so; if the sub delivers usable output at low frequencies under these conditions, it's probably pretty good.

A much better alternative is CEA-2010 output measurements, which tell you how much sound the sub can output at 20, 25, 31.5, 40, 50, and 63 Hz. The biggest and best subwoofers put out somewhere around 125 dB at 63 Hz, falling to maybe 112 dB at 20 Hz. Drop those numbers to about 120 dB/105 dB for mid-sized/mid-priced subs, and maybe 116 dB/90 dB for smaller subs. Those are just rough numbers, though, and two or three dB here and there probably won't make a huge difference.

The great thing about CEA-2010 isn't just that it tells you how much a sub can shake your couch; it also tells you a lot about how clean and full the sub will sound. A big, powerful subwoofer usually produces much higher fidelity and more natural sound than a smaller sub that's running closer to its limits and producing more distortion. I learned this almost 20 years ago, when a sub with measured response down to 19 Hz lost out in a blind listening test to a sub that started to roll off below 30 Hz. I experimented with all sorts of measurements to try to find out why, and I finally got my answer when I did distortion measurements at 20 Hz. The former sub produced about 50 percent total harmonic distortion, while the latter sub produced around 10 percent. The difference was obvious to everyone who participated in the test.

Unfortunately, while many sub manufacturers use CEA-2010 measurements in their product development, few actually publish the numbers. That's a shame. But for now, you know that, if the manufacturer does publish the numbers, they're serious about what they're doing and probably benchmarked their products against those of their competitors.

Most reviewers haven't picked up on CEA-2010 yet, but I include it in all my subwoofer reviews, and a few others do, too. Most of the reviewers and many of the manufacturers who do CEA-2010 have collaborated behind the scenes to make sure all of our measurements are in the same ballpark; so, in most cases, you can compare subwoofer X with subwoofer Z by using Reviewer Joe's CEA-2010 measurements of subwoofer X and Reviewer Bob's measurements of subwoofer Z, and you can probably trust the manufacturers' measurements, too. Just be careful about making too big a deal about a difference of one or two dB; as much as we try to make this measurement perfectly accurate, it's probably not possible to do so.

By the way, you can do your own CEA-2010 measurements using the free Room EQ Wizard software package with a cheap measurement microphone such as the ones made by Dayton Audio. You can learn how to do so here.

One Sub vs. Two vs. Four
The last question I'll address is one I get asked all the time: should you spend your money on one big sub, two mid-sized subs, or four smaller subs? Adding more subwoofers tends to smooth out the response in your listening chair (and elsewhere). That's a good thing in general, but the ultimate answer depends on you.

A few years ago, I conducted a blind test in an attempt to find this answer. I built a single 15-inch sub, two 12-inch subs, and four 8-inch subs, all tuned to the same Q (or resonance bandwidth) and built using comparable drivers. I placed the 15-inch sub in my room's "subwoofer sweet spot," the 12-inchers in the front corners, and one 8-incher in each corner. Then I set up my testing switcher, covered all the subs with black fabric, called in some experienced listeners, and told them they would be hearing three different "subwoofer setups." Each listener did the test from my usual listening chair (positioned for the best possible sound) and from a seat farther back and a few feet closer to the left wall.

All of the listeners appreciated the smoothness of the four small subs, but none felt the small subs had sufficiently satisfying deep bass response. All of them loved the floor-shaking power of the big 15-inch sub, but they all complained about its uneven response at both listening positions (especially the second one).

Our conclusion was that using the two mid-sized subs was the best compromise...but that might not be the case for you. If it's only you listening or you really don't care much about the sound everyone else is experiencing, one nice option is to get a big, high-powered 13-, 15-, or even 18-inch subwoofer and use some sort of auto EQ (either built into the receiver, surround processor or sub, or provided by an outboard device) to smooth out the response at your listening position. You'll get gratifying power and smooth response. If you want good sound all around the room, get two somewhat smaller subs and place one in each front corner of the room. If you have the budget, by all means get two or four really good subs.

I've answered all the questions about choosing subwoofers that come to mind, but I'm sure I've missed some. If there's something you'd like to know that I didn't address, please let us know in the Comments section below.

Additional Resources
• Check out our Subwoofer category page to read our most recent subwoofer reviews.
Why Do Audiophiles Fear ABX Testing? at HomeTheaterReview.com.
The Good, Better, and Best AV Receivers on the Market at HomeTheaterReview.com.

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