Subwoofer Basics

Subwoofer Basics

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.)

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