Subwoofer Basics

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Subwoofer Basics

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.

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