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Subwoofer Basics

Excerpted and adapted from The Complete Guide to High-End Audio (Fifth Edition). Copyright © 1994–2015 by Robert Harley. hifibooks.com. To order call (800) 841-4741.

Subwoofers are often misunderstood, both in name and in application. While they have a reputation for providing hefty bass lift, they need not be proverbial bulls in the china shop of your listening room. Although proper setup and integration require some attention and patience, the results can enhance the sound of a wide range of systems—without being “in-your-face” obvious about it. Put simply, it is possible to greatly improve a system’s performance by adding a subwoofer (or a pair of subs).

First, the basic definition: A subwoofer is a loudspeaker that produces low frequencies that augment and extend the bass output of a full-range loudspeaker system. The term subwoofer is grossly misused to describe any low-frequency driver system enclosed in a separate cabinet. But subwoofer actually means “below the woofer,” and the term should be reserved for those products that extend bass response to below 20Hz. A low-frequency driver in an enclosure with an output extending to 40Hz and used with small satellite speakers is more properly called a woofer.

You’ll also see full-range speakers with a built-in “subwoofer” powered by its own amplifier. Most of these products actually employ woofers that are simply driven by an integral power amplifier. Such a design relieves your main amplifier of the burden of driving the woofer, but the speakers must be plugged into an AC outlet.

Read on to find out about different types of subwoofers—and their pros and cons—as well as their setup, placement, and integration.

Active vs. Passive Subwoofers
Subwoofers come in two varieties: passive and active. A passive subwoofer is just a woofer or woofers in an enclosure that must be driven by an external amplifier. In one variation of the passive subwoofer, the same stereo amplifier driving the main speakers also powers the subwoofer. In this least desirable method of connecting a subwoofer, the full-range output from a power amp is input to the subwoofer, and a crossover in the subwoofer removes low frequencies from the signal and outputs the filtered signal to the main loudspeakers. This technique puts an additional crossover in the signal path, to filter out bass from the speaker-level signals driving the main speakers. Some subwoofers are designed to extend the system’s bass response without filtering bass from the signal driving the main speakers. Although this technique doesn’t add a crossover to the signal path, it doesn’t increase the power handling of the main speakers, either. Keeping low bass out of your main speakers has many advantages, including a much cleaner midrange.

A better way of driving the passive subwoofer is with an electronic crossover and separate power amplifier. This method separates the bass from the signal driving the main loudspeakers at line level, which is much less harmful to the signal than speaker-level filtering. Moreover, adding a separate power amp for the subwoofer greatly increases the system’s dynamic range and frees the main-speaker amplifier from the burden of driving the sub. Adding a line-level crossover and power amp turns the passive subwoofer into an active subwoofer, and also makes the system bi-amplified.

A self-contained active subwoofer combines a subwoofer with a line-level crossover and power amplifier in one cabinet, eliminating the need for separate boxes and amplifiers. Such a subwoofer has line-level inputs (which are fed from the preamplifier), line-level outputs (which drive the power amp), and a volume control for the subwoofer level. The line-level output is filtered, removing low frequencies from the signal sent to the amplifiers driving the main loudspeakers. This crossover frequency is adjustable to allow you to select the frequency that provides the best integration with the main speakers. (I’ll write more about integration methods in subsequent sections.)

Subwoofer Pros and Cons
Adding an actively powered subwoofer can greatly increase your system’s dynamic range, bass extension, midrange clarity, and ability to play louder without strain. The additional amplifier power and low-frequency driver allow the system to reproduce musical peaks at higher levels. Moreover, removing low frequencies from the signal driving the main loudspeakers lets them play louder because they don’t have to reproduce low frequencies. The midrange often becomes clearer because the woofer cone isn’t furiously moving back and forth, trying to reproduce low bass. This improvement in a small speaker’s performance can be dramatic. The small woofer’s excursion is no longer a limiting factor in how loudly the system will play when low frequencies are filtered from the signal driving it. The midrange is much cleaner, and the overall system sounds like a large, full-range speaker.

Now for the bad news: More often than not, subwoofers can degrade a playback system’s musical performance. Either the subwoofer is poorly engineered (many are), set up incorrectly, or, as is increasingly common, it’s designed to reproduce explosions in a home-theater system, not resolve musical subtleties.

 

Subwoofer Challenges (and Rewards)
Let’s look at the theoretical problems of subwoofers. First, most subwoofers—passive or active—add electronics to the signal path. The active subwoofer’s internal crossover may not be of the highest quality. Even well-executed crossovers can still degrade the purity of very-high-quality source components, preamplifiers, and power amps. This drawback can be avoided by running the main loudspeakers full-range (no roll-off), but you then lose the dynamic advantages and additional midrange clarity conferred by keeping low frequencies out of the main speakers.

Second, the subwoofer’s bass quality may be poor. The subwoofer may move lots of air and provide deep extension, but a poorly designed subwoofer often adds a booming thumpiness to the low end. Rather than increase your ability to hear what’s going on in the bass, a subwoofer often obscures musical information.

Third, a subwoofer can fail to integrate musically with the main loudspeakers. Very low frequencies reproduced by the subwoofer can sound different from the midbass produced by the main speakers. The result is an extremely distracting discontinuity in the musical fabric. This discontinuity is manifested as a change in the sound of, for example, acoustic doublebass in different registers. Ascending and descending bass lines should flow past the crossover point with no perceptible change in timbre or dynamics.

Another factor that can make integrating a subwoofer difficult is matching a slow subwoofer to taut, lean, articulate main speakers. Put another way, the sound from an underdamped subwoofer won’t integrate very well with that from a pair of overdamped speakers.

Fourth, subwoofers often trade tight control, pitch resolution, and lack of overhang for greater sensitivity or deeper extension. This is particularly true of subwoofers designed for home theater. Consequently, many subwoofers sound bloated, “slow,” and lacking in detail.

Finally, a subwoofer can fill the listening room with lots of low-frequency energy, exciting room-resonance modes that may not have been that bothersome without the subwoofer. This problem of room-mode excitation can be ameliorated by using two (or more) subwoofers; each subwoofer will excite different room modes, substantially smoothing out the room’s low-frequency response. Placement is therefore crucial—you can’t put a subwoofer just anywhere and expect musical results.

All of these problems are exacerbated by the common tendency to set subwoofer levels way too high. The reasoning behind this is that if you’ve paid good money for something, you want to hear what it does. But if you’re aware of the subwoofer’s presence in the sound, either its level is set too high, or it isn’t positioned correctly, or it has been poorly designed. The highest compliment one can pay a subwoofer is that its contribution can’t be directly heard. It should blend seamlessly into the musical fabric, not call attention to itself.

Having said all that, my experience suggests that a subwoofer/satellite system can outperform a similarly priced, full-range loudspeaker system.

Here are some compelling reasons why:

  1. The satellite speakers can be positioned for the best soundstage without regard for how that position affects the bass response.
  2. The subwoofers can be positioned for the best bass integration in the room without regard for soundstaging.
  3. It is much more cost-effective to build a subwoofer enclosure than it is to build the large enclosure of a full-range loudspeaker (to accommodate large woofers), which may have expensive cabinet construction and wood or paint finishes. The subwoofer is less likely to be a prominent part of the home decor, and thus doesn’t require lavish finish quality.
  4. The subwoofer/satellite system gives you a large measure of control over the bass performance, including the amount of bass and how that bass integrates with the room. A full-range loudspeaker offers no such control.

However, I must add that I hold these things to be true with two big caveats: The first is that the subwoofer must be of exceptional quality and be designed for musical performance, not booming home-theater effects. Only a tiny handful of the hundreds of subwoofers on the market fall into this category. Second, achieving seamless integration between the subwoofer and satellites and realizing smooth bass response requires skill and patience. The subwoofer/satellite system is a better choice for someone who enjoys the technical side of audio rather than for the music lover who just wants good sound without having to ascend the learning curve.

The counterargument suggests that a loudspeaker system should be designed and engineered with a single vision, not with a piecemeal approach by different designers. (For more on this, see the Subwoofer Integration section below.)

 

Subwoofer Placement Tips
Subwoofer placement also has a large effect on how much bass you hear and how well the sub integrates with your main speakers. When a subwoofer is correctly positioned, the bass will be clean, tight, quick, and punchy. A well-located subwoofer will also produce a seamless sound between the sub and the front speakers; you won’t hear the subwoofer as a separate source of sound. A poorly positioned subwoofer will sound boomy, excessively heavy and thick, lacking detail, and slow, with little dynamic impact. In addition, you’ll hear exactly where the front speakers leave off and the subwoofer takes over.

Here are some general guidelines for subwoofer placement. As with full-range speakers, avoid putting the sub the same distance from two walls. For example, if you have a 20′-wide room, don’t put the subwoofer 10′ from each wall. Similarly, don’t put the subwoofer near a corner and equidistant from the side and front walls. Instead, stagger the distances to the walls. Staggering the subwoofer’s distance from each wall smoothes the bass because the frequencies being reinforced by the wall are randomized rather than coincident.

You can also get more dynamic impact and clarity from your subwoofer by placing it close to the listening position. Sitting near the subwoofer causes you to hear more of the sub’s direct sound and less of the sound that has been reflected around the room. You hear—and feel—more of the low-frequency wavelaunch, which adds to visceral impact.

The simplest, most effective way of positioning a subwoofer is to temporarily put it as close to the listening position as feasible. Raise the subwoofer off the floor, if possible, so that it’s close to where the listeners’ ears will normally be. Play a piece of music with an ascending and descending bass line, such as a “walking” bass in straight-ahead jazz. Crawl around the floor on your hands and knees (make sure the neighbors aren’t watching) until you find the spot where the bass sounds smoothest, and where each bass note has about the same volume and clarity. Avoid positions where some notes last longer, and/or sound slower or thicker, than others. When you’ve determined where the bass sounds best, put the subwoofer there permanently. Now, when you’re back in the listening seat, the bass should sound smooth and natural.

Subwoofer Integration Tricks
It’s relatively easy to put a subwoofer into your system and hear more bass. What’s difficult is making the subwoofer’s bass integrate with the sound of your main speakers. Low bass as reproduced by a subwoofer’s big cone(s) can sound different from the bass reproduced by the smaller cones in the left and right speakers. A well-integrated subwoofer produces a seamless sound, no boomy thump, and natural timbre. A poorly integrated sub-woofer will sound thick, heavy, boomy, and unnatural, calling attention to the fact that you have smaller speakers reproducing the frequency spectrum from the lower midrange up, and a big subwoofer putting out low bass.

Integrating a subwoofer into your system is challenging because the subwoofer is optimized for putting out lots of low bass, not for reproducing detail. The main speakers’ upper bass is quick, clean, and articulate. The subwoofer’s bass is often slow and heavy. The word “slow” in this application is not technically correct, but vividly describes how such a subwoofer sounds. More precisely, a “slow” subwoofer is one that has excessive overhang: the cone keeps moving long after the drive signal has stopped, which conveys the impression that the bass is “slow.”

Achieving good integration between small speakers and a subwoofer is easier if you buy a complete system made by one manufacturer. Such systems are engineered to work together to provide a smooth transition between the sub and the main speakers. Specifically, the crossover network removes bass from the left and right speakers, and removes midrange and treble frequencies from the signal driving the subwoofer. If all these details are handled by the same designer, you’re much more likely to get a smooth transition than if the subwoofer is an add-on component from a different manufacturer.

If you do choose a subwoofer made by a different manufacturer, several controls found on most subwoofers help you integrate the sub into your system. One control lets you adjust the crossover frequency—the frequency at which the transition between the subwoofer and the main speakers takes place. Frequencies below the crossover point are reproduced by the subwoofer; frequencies above the crossover point are reproduced by the main speakers. If you have small speakers that don’t go very low in the bass and you set the crossover frequency too low, you’ll get a “hole” in the frequency response. That is, there will be a narrow band of frequencies that aren’t reproduced by the woofer or the main speakers.

Setting the subwoofer’s crossover frequency too high also results in poor integration, but for a different reason. The big cone of a subwoofer is specially designed to reproduce low bass. When it is asked to also reproduce upper-bass frequencies, those upper-bass frequencies are less clear and distinct than if they were reproduced by the smaller main speakers. Finding just the right crossover frequency is the first step in achieving good integration. Most subwoofer owner’s manuals include instructions for setting the crossover frequency. As a rule of thumb, the lower the subwoofer’s crossover is set, the better.

Some subwoofers also provide a knob or switch marked “phase.” Phase essentially is a time difference between two soundwaves. To understand a subwoofer’s phase control, visualize a soundwave being launched from your subwoofer and from your main speakers at the same time. Unless the main speakers and subwoofer are identical distances from your ears, those two soundwaves will arrive at your ears at different times; that is, they will have a phase shift between them. In addition, the electronics inside a subwoofer can create a phase shift in the signal. The sub’s phase control lets you delay the wave generated by the subwoofer so that the subwoofer’s wave is perfectly in-sync with the wave from the main speaker. When the soundwaves are in phase, you hear a more coherent and better-integrated sound. One way of setting the phase control is to sit in the listening position with music playing through the system. Have a friend rotate the phase control (or flip the phase switch) until the bass sounds the smoothest.

But there’s a much more precise way of setting the phase control that guarantees perfect phase alignment between the subwoofer and main speakers. First, reverse the connections on your main loudspeakers so that the black speaker wire goes to the speaker’s red terminal, and the red speaker wire goes to the speaker’s black terminal. Do this with both speakers. Now, from a test CD or device app that offers pure test tones, select the track whose frequency is the same as the subwoofer’s crossover frequency. Sit in the listening position and have a friend rotate the subwoofer’s phase control until you hear the least amount of bass. The subwoofer’s phase control is now set perfectly. Return your speaker connections to their previous (correct) positions: red to red, black to black.

Here’s what’s happening when you follow this procedure: By reversing the polarity of the main speakers, you’re putting them out of phase with the subwoofer. When you play a test signal whose frequency is the same as the subwoofer’s crossover point, both the sub and the main speakers will be reproducing that frequency. You’ll hear minimum bass when the waves from the main speakers and subwoofers are maximally out of phase. That is, when the main speakers’ cones are moving in, the subwoofer’s cone is moving out. The two out-of-phase waves cancel each other, producing very little bass. Now, when you return your main speakers to their proper connection (putting them back in phase with the subwoofer), they will be maximally in-phase with the sub. This is the most accurate way to set a subwoofer’s phase control, because it’s easier to hear and identify the point of maximum cancellation than the point of maximum reinforcement. Unless you later move the subwoofer or main speakers, you need to perform this exercise only once.

A useful modern subwoofer feature is an automatic equalization circuit that measures the frequency response of your system in your room and tailors the subwoofer’s output to provide flatter response. You connect a supplied calibration microphone to the subwoofer, push a button, and the subwoofer emits a series of tones that are picked up by the microphone and analyzed by a circuit in the subwoofer. The subwoofer then equalizes its output, applying a boost to certain frequencies and a cut to others so that the resultant output is as flat as possible.

It’s worth noting that the best integration results from adding two (or more) subwoofers to your system. Two subwoofers drive the air in the room more uniformly, with fewer standing waves. The result is smoother bass throughout the room, and better integration with the main speakers.

Adapted from The Complete Guide to High-End Audio (Fifth Edition). Copyright©1994–2015 by Robert Harley. hifibooks.com. To order call (800) 841-4741.

By Robert Harley

My older brother Stephen introduced me to music when I was about 12 years old. Stephen was a prodigious musical talent (he went on to get a degree in Composition) who generously shared his records and passion for music with his little brother.

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