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More than just the...boom

By Roy Johnson, loudspeaker designer, Green Mountain Audio, Inc.

Subwoofers for home use gained popularity from the demands of home theater. They also add low bass to compact speakers. Like any other speaker, the subwoofer is to do its job with a minimum of resonance and distortion. It must also blend smoothly with the main speakers. Unfortunately, most all subwoofers are designed from the marketing pressures of size and style. In this article, learn more about the design criteria that must be followed for a subwoofer to perform its best and tips for including one in your system.

About specifications
and measurements

The term 'subwoofer' came about as first a reference that it went much lower than regular woofers. It has evolved into mostly a marketing term for a box producing bass. 'Woofer' is a better descriptor for the subwoofer designer to consider, as all woofers follow the same laws of physics, no matter how low they must perform.

Before we touch on some of the details of subwoofer design, it is important to know something about published subwoofer specifications. Most misrepresent what we actually hear. This is because they are measured using test tones that last much longer than the moment-by-moment demands of music and soundtracks. This fools the sound meter because it adds up all the accumulated bass echo in the room. Low-bass tones never last that long on music or soundtracks unless you are a pipe-organ fan.

Yet, incredulously, these exaggerated measurements are okay with marketing, because that subwoofer appears to go much lower than it really does. You will also never see any reference to how loud the subwoofer will play the low bass notes, or at what distance, or in which size of room. There is no indication of how well it will control the beginning and end of low-bass notes. There are no accurate +/- dB specifications that indicate either the smoothness or flatness of its frequency response.

A reviewer may give a subwoofer a big thumbs up, but compared to what? Live sound? Never happens unless you have lived around the explosions simulated on DVDs, or for live music unless one is in the studio right next to the bass player and synthesizer chap. Very low bass is seldom heard in a live concert as it costs too much to reproduce in large venues.

So, with no life experience in real bass, and no accurate measurement techniques, most reviewers can only say "Excellent performance!" when that subwoofer is less boomy than the last one, and goes 'low enough' and 'loud enough' in that room. One thing about poor sound from a subwoofer: at least it does not hurt your ears as does distortion from a tweeter.

Thus, much is forgiven about the sound of subwoofers while many magazines have measured 10-50 percent distortion. They report "That's acceptable because professionals tell us how insensitive we are to distortion on the low tones." Convenient, but those judgments were made while listening to test tones only. When listening to music or dynamic soundtracks, low distortion makes a very audible difference, especially when you play a wide variety of music.

For useful subwoofer measurements, consult the reviews performed by the editor of the Canadian magazine, Audio Ideas Guide. A sidebar explains how he conducts measurements.

Bigger is better

The fluid air in front of any woofer always tries to slide out of the way every time that cone begins to stroke or otherwise changes direction. Thus, one always hears more 'slam' or impact from a larger cone, even when played softly, because the larger cone literally is holding onto more air as it begins and ends any motion.

The measure of this 'grip' is called the woofer's radiation resistance, for which a higher amount is always better. This is not seen as a separate specification. It is increased when a subwoofer is placed in a corner and greatly reduced when that subwoofer is placed out into the middle of the room or outdoors.

With radiation resistance as our guide, we do not consider any design a true subwoofer when the cone is smaller than 10" (25cm). Designers can make a very small woofer go low, or loud, but never both at once. When several small woofers are in one enclosure, they will then play loud, but never go low unless their low-bass is boosted, probably by a built-in amplifier.

That boost (equalization or EQ) adds phase shift and makes the cone(s) stroke much farther. Both of those make the lowest-bass notes sound 'boomy' and/or turn into 'one-note bass,' as with the majority of loud car stereos.

Bigger is better, because one large woofer can always have a far-lower natural resonant frequency than a small woofer or any collection of small woofers. That allows it to go much lower with much less resonance and phase shift (time delay), for more clarity and pitch definition.

The enclosure

The front of the woofer must talk to your room directly, not through a smaller port opening in front of its cone, because that port creates a strong resonance throughout the bass range. That front-port exists to filter off even more highs from the woofer cone, and to look more intriguing for marketing purposes.

If the rear of the woofer is mounted into a sealed enclosure, there will be no very-low bass when the woofer is smaller than 10" (25cm) diameter, unless the subwoofer's built-in amplifier boosts the bass. Again, that added equalization (EQ) adds phase shift and makes the cone stroke much farther. Both make the low bass sound boomier.

If the rear of the woofer is mounted into a ported enclosure, the port must be large to generate loud-enough very-low bass. If a 10" (25cm) woofer is relying only one one port, that port must have a diameter of at least 3" (75mm). That port would also be aerodynamically flared at its intake and exhaust to minimize air turbulence. The large diameter and the flares lower harmonic distortion by a large amount. You hear more impact and pitch definition.

The subwoofer cone can face the floor or it can fire straight ahead. The former arrangement looks nice and can lower the natural resonant frequency because the air trapped underneath literally sticks to the cone, making it heavier. The latter delivers a quicker, livelier dynamic response, and is more easily blended with the main speakers.

Finally, the enclosure behind the woofer cone must be large enough, on the order of 3,000 cubic inches (50 liters) for a 10" (25cm) woofer in a sealed box, and at least 2,000 cubic inches (33 liters) for a ported box. A single 12" (30cm) woofer needs about 1.7x those volumes. Marketing finds these sizes 'too large' so almost everyone listens to subwoofers that are nowhere as nimble and clear as those could be, because their enclosures are far too small.

When a too-small box is used, a designer only has two choices to create low bass: make the cone much heavier, or boost the low-bass via the built-in amplifier. The former means applying more than twice the power for the same loudness, resulting in less peak loudness and possibly less reliability. The latter adds phase shift and makes the cone stroke much farther, both of which which make the low bass sound boomy.

The cabinet construction must be solid and well braced, to prevent vibration. Nothing should move but the cone, which is why spikes are used under the cabinet to penetrate carpet. Soft rubber feet should be avoided. The feet should all touch down equally, so there can be no corner-to-corner rocking motion.

Home theater systems

When a subwoofer is used for a home theater, an owner faces two choices: to use the subwoofer for only the LFE (Low-Frequency Effects) channel (the 0.1 channel) of the DVDs, and/or allow the subwoofer to extend the bass of at least the main speakers, to also benefit two-channel music. The control center may or may not have the ability to switch easily between those two modes.

When the subwoofer is to receive only the LFE channel, the subwoofer's built-in amplifier, if there is one, should not be 'crossed over' at all. Any crossover circuit built into that amplifier should thus be bypassed or that crossover-frequency knob set as high up the frequency scale as possible because the studio has already properly filtered out the 0.1 LFE channel's highs.

Any LFE-channel crossover inside the home-theater control center should also be turned off/bypassed/set as high as possible for the same reason. The goal is to send an unaltered, 'direct' signal to the subwoofer from the LFE channel of the DVD. Anything less than a direct 0.1-channel signal sounds muddier and boomier because the added crossover-circuit filtration is creating more phase shifts. In this setup, that subwoofer would be silent on two-channel music.

When that subwoofer is to aid the low-bass of the main speakers, look first to your home-theater control center's settings. Follow its Owner's Guide for the recommended adjustments to its built-in crossover circuit, while bypassing/setting as high as possible any crossover circuit that is part of the subwoofer's built-in amplifier. The control center will still allow the 0.1 LFE channel to go to the subwoofer for movies, along with the Left and Right low bass of DVDs and CDs.

Two-channel music systems

One can use the speaker-wire inputs on the back of the subwoofer's built-in amplifier to take some signal from the main speakers (which does not change their sound). This allows the subwoofer to be faded in, to aid the main speakers without changing the signal to them or passing their signals through extra circuits.

For small bookshelf speakers, assume you need to fade in the subwoofer below 70-80Hz. For medium-size stand-mount speakers, try augmenting their response from 50Hz on down. For floor-standing speakers, try fading in the subwoofer from 40-45Hz on down. The above technique is also good for a home theater. In that case, tell the home theater control center that you have large main speakers and no subwoofer.

Location

For the most natural sound on music, place your subwoofer next to one of the main speakers and not far off in a corner. This matters less for the 0.1 LFE channel of DVDs.

Blending by ear

For two channel music, listen to the sound of a steady kick drum from a well-produced rock or jazz album. Adjust the crossover point and the subwoofer's front-to-rear position for the best definition of the drum beat. Then listen to an acoustic bass from a jazz album run up and down the scale to determine if the subwoofer is loud enough.

Trust your ears more than a sound meter. The sound meter responds to all the standing bass-waves in the room. Your ears do not.

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