Subwoofer Basics

Subwoofer Basics

Excerpted and adapted from The Complete Guide to High-End Audio (Fifth Edition). Copyright © 1994–2015 by Robert Harley. 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.