Subwoofers for Music

Caution: Highly personal and opinionated verbiage follows -- but it comes from years of experimentation, not from thin air. Read at your own peril, and the peril of your wallet. This is half of a pair of articles for January on subwoofers. The other half appears at the Home Theater & Sound site and covers subwoofers for home theater.

I have to tell you that after decades of futzing with audio systems, I have come to realize that I will never again be satisfied with any system that does not include full-range bass performance -- flat to 20Hz or very close to it. Too much is lost when the deep bass is absent. Deep bass imparts a feeling of space, size, solidity and reality that no amount of midrange bloom or high-end air can ever hope to capture. Unfortunately, full-range bass is expensive, perhaps twice as expensive as you might think. Read on.

Manufacturers aren’t telling consumers that what they call a "subwoofer" is really three distinct products. Let’s understand what these are before we continue. The best definition I can come up with is that a subwoofer (for music, remember) is an add-on product that extends the bass response of the main system so that it is flat or very close to flat all the way down to the vicinity of 20Hz. If the product gives up at 30Hz or higher, it isn’t really a subwoofer. If the product can respond powerfully to bass that is even lower than 20 Hz, perhaps it is not actually a subwoofer but something else that has not been clearly defined or identified by manufacturers or the press -- yet.

Another point of understanding is that integrating a subwoofer with the main speakers takes some finesse. You don’t stop the main speakers at 100Hz and start the subwoofer at 99.999Hz. The main speakers and the subwoofer really must overlap each other for at least one full octave. This means that the main speakers must be flat for one full octave below the crossover point and the subwoofer must be flat and musically accurate for at least a full octave above the crossover point. If the crossover point is nominally 80Hz, the subs have to have excellent performance up to 160Hz while the main speakers have to be flat or nearly flat to 40Hz. Bend this rule and integration of the subwoofer with the main speakers will be compromised to one degree or another based on the best possible integration. So you can see that a true subwoofer requires some fairly serious main speakers that are probably going to have to be floorstanding speakers with a fairly decent-sized bass driver, 10"-12" perhaps or possibly a pair of 8" drivers.

The three

Now that we understand the product definition a bit better, let’s look at the three distinct types of "subwoofers" that exist in the marketplace and try to define them more accurately than the manufacturers have done so far.

Add-On Woofer – This is always called a subwoofer, but because it accompanies small main speakers that have little or no bass response below 60Hz, these devices can never be true subwoofers. With 60Hz or thereabouts as the bass limit of the main speakers, the crossover for a "subwoofer" has to be one octave higher, 120Hz in this case. And the "subwoofer" will have to have excellent sound quality up for at least one octave above the crossover point -- 240Hz in this case. You can’t tell where bass is coming from when frequencies are below roughly 100Hz, so when there is bass below 100Hz, having a single "subwoofer" is acceptable. However, in this case, the "subwoofer" is going to be producing sound at 240Hz or even a little higher. It is quite easy to tell where these frequencies are coming from. This tells us that you simply must have two of this type of "subwoofer" to retain excellent stereo imaging. Because two are needed they must be placed close to the main speakers to maintain stereo separation.

These are best referred to as "add-on woofers" or just "woofers" because they are definitely not subwoofers. To reproduce 240Hz or higher, you are going to be limited to a cone size of 10" at the most, while 8" may be more practical. This small of a driver will not reproduce 20Hz, and probably not even 30Hz in any enclosure of domestically acceptable size and shape. In fact, in many cases, these devices may have a hard time getting below 40Hz because they are usually very much budget products. Which is all the sadder because they always seem to be sold alone. You really must have a pair of add-on woofers in order to preserve any semblance of stereo imaging in the upper-bass and lower-midrange octaves. As I said earlier, good bass is expensive.

Subwoofer – This product is capable of extending the bass of the main speakers to 20Hz or very close to it. In that role, true subwoofers will more than likely have, at minimum, a single 12" driver or perhaps multiple 12" or smaller drivers. Of course, larger drivers are widely represented also, though you will probably not see drivers larger than 18" in this category due to the need to respond accurately up to 200Hz or so. Most subwoofers are going to have a crossover at something close to 80Hz. This is because the main speakers are not likely to be very strong in bass response below 40Hz and because you really do not want the subwoofer crossover point to be higher than 80Hz because you then get forced into smaller and smaller drivers to support the reproduction of higher frequencies. So 20Hz or close to it on the bottom end and around 160Hz-200Hz on the top end pretty much describes the operating range of the typical true subwoofer.

If you have been paying attention, you have noticed that the subwoofer will be operating above 100Hz and therefore a single subwoofer will never be an optimum bass solution. Subwoofers should still be used in pairs for optimum music reproduction. Now you can see why I said deep bass may be twice as expensive as you were thinking.

Subwoofers will generally include an internal or external crossover that removes bass below the crossover point from the main speakers -- not a sharp cut-off filter but a more typical loudspeaker crossover slope of 6, 12, 18 or 24dB per octave.

Infra-subwoofer – This is the bass monster of the audio world. It will generally operate from something a bit below 16Hz up to perhaps 40Hz, and never much higher. This will have one or more 18" or larger drivers, and its cabinet tends to be very large. Occasionally you may see an extremely tall cabinet with a vertical array of 15" or 16" drivers, but larger drivers tend to be more common for this class of product. Infra-subs generally do not limit the bass response of the main speakers because they are designed to be used with main speakers that are already full-range performers. Because infra-sub response rarely goes much higher than 40Hz, the infra-sub is really the only bass-extension product you can use in mono and not suffer some reduction of upper-bass/lower-midrange imaging capability. And did I mention that infra-subs are usually very expensive indeed?

For the ultimate in music reproduction, integration of the subwoofers with the main speakers is very important. Crossovers generally have a very obvious sound that isn’t beneficial to the musical performance. The very best-sounding music subwoofers have either very simple crossovers (a single capacitor, for example) or no electronic crossover at all. If there is no crossover at all that removes bass from the main speakers, the subwoofer crossover itself will have exceptional amounts of adjustability in order to integrate with a wide variety of main speakers. This added complexity in the subwoofer crossover can add cost to the subwoofer as well as complexity that impacts overall sound quality perhaps more than the simple external crossover might.

Each manufacturer has to make a call as to which method to use, then devote a lot of time to making the most of their decision. I’ve personally found that I tend to like the musical performance of subwoofers that employ simple first-order electrical crossovers with additional mechanical slopes derived from cabinet design. A good designer can get 6dB per octave from the electronic crossover plus another 12dB per octave from the cabinet. These will add to make the effective crossover 18dB per octave. The companies that I know of that make music subwoofers this way are: Vandersteen, Bag End, and REL. If you have watched subwoofer-for-music reviews over the years, you may have noticed that these three companies consistently get high marks for musical performance.

The myth of the small driver in the small box as subwoofer

Most people do not know what high levels of bass distortion sound like. This has led to a growing number of (relatively) tiny drivers in (relatively) tiny boxes connected to massively powerful and massively equalized amplifiers. Small drivers are limited in the amount of air they can move. To produce visceral bass from a small driver, you must drive it harder and harder and harder at increasingly lower frequencies. This leads to inevitable doubling, tripling, quadrupling -- or even worse -- of bass distortion. Since people do not generally know what bass distortion sounds like, they assume that because these little drivers in little boxes shake the room that they are good subwoofers.

The problem arises not just from the driver size. The box size also dictates much of the physics of bass reproduction. These tiny boxes can’t support deep bass frequencies without driving the small driver massively harder at the lower frequencies. This is why these small "sub-cubes" tend to have amplifiers with 1000 watts or more of power.

People also don’t understand that in order to keep the amplifier small enough to fit in the tiny box and run cool enough not to start the whole thing on fire, the amplifier must be a switching amplifier. These create a virtually open circuit from the amplifier to the power line when the amplifier needs a lot of current. This means that these amplifiers modulate the power line at audio frequencies to a startling degree, actually generating large amounts of power-line noise. The noise generated by these amplifiers is easily measurable using a spectrum analyzer. The noise modulates in frequency with whatever sounds the sub-cube happens to be reproducing.

So these sub-cubes are a triple whammy on your music system. They have very high levels of distortion because of the small driver trying to move lots of air at low frequencies and even higher levels of distortion because of the tiny box size requiring the driver to be pushed even harder than it would in a much larger box. And the amplifier in these sub-cubes will inject audio-frequency noise on the power line because of the design and small space permitted in the box for the amplifier.

Conclusion

Home theater is driving today’s subwoofer market. Unfortunately, home-theater subwoofers sound distinctly inferior when placed in music duty -- and vice versa. I’ve had the opportunity to hear music and home-theater subwoofers from the same manufacturer, and even with all the adjustability of both products, I could never get home-theater results from the music subs that matched the sound of the home-theater subwoofer and vice versa.

Not only do you need stereo subwoofers to get optimum imaging for music, you need at least one separate home-theater subwoofer. Home-theater subwoofers are designed to move a lot of air in the 40-80Hz range to make explosions and the like have that solid, realistic character that can make home theater so satisfying. When you apply that to music, you end up with too much 40-80Hz energy and not enough below 40Hz. If you have to share duties due to budget limitations, you’ll have to decide on your own whether to get one or two music subwoofers and use them for movies too or get home-theater subwoofers and use them for music. Which way to go depends on your personal preferences and goals for your system.

Never trust a sales person who says a given subwoofer is equally good for music and movies. Either the person is clueless or the subwoofer is so compromised that it won’t be that good for either one. Most subwoofers being made today are made with an eye to home theater. If you are looking for subwoofers for music, be very cautious. Your real choices are limited to just a handful of different products these days.

...Doug Blackburn
db@soundstage.com

(Note: This article goes hand in hand with the January 2001 "Video Noise" column appearing in Home Theater & Sound titled "Subwoofers (and Bass) for Home Theater.")

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