Benefits of a sub
The use of subwoofers in home audio is an issue of contention among audiophiles. While some maintain that it’s too difficult to properly locate a subwoofer so that it blends seamlessly with the main speakers, others argue that not only is this possible, but when done properly, the results can be astounding. Regardless of which side you’re on, I think most listeners agree that there are benefits to adding a subwoofer to a pair of satellite speakers. Perhaps the most important is that a sub allows the sats to be placed where they will image best, while the sub itself can be placed where it can most evenly distribute energy throughout the listening room. Full-range speakers don’t offer such flexibility because their low- and high-frequency drivers can’t be physically separated.
At low frequencies, the room has more of an influence on the sound than does the speaker itself. This isn’t to say that rooms don’t influence higher-frequency sounds -- the reflection of soundwaves off of room walls profoundly affects how we hear and interpret all audible frequencies. However, at frequencies lower than 300Hz, room resonances contribute so much to the speaker’s sound that it is the room that dictates more of what you hear. Improper setup can lead to peaks at frequencies at which soundwaves reinforce each other, and dips, or nulls, at frequencies at which they cancel each other out.
Recently, I spoke with Paul Barton, founder and chief designer of PSB Loudspeakers, about the challenges of subwoofer placement. That Barton is a major proponent of using a dedicated low-frequency speaker for music and movies alike is hardly surprising -- he designed his first sub/satellite system back in 1977. Although he admits that there are issues that need to be considered in finding a subwoofer’s ideal location, Barton believes that the benefits make the investment of time fully worthwhile. As you’re about to learn, experimentation is the key.
According to Paul Barton, three distances must be considered when setting up a sub/satellite speaker system:
- a) from your listening position to the room boundaries
- b) from your listening position to the speakers
- c) from the speakers to the room boundaries?
If your goal is more precise placement of sounds within the soundstage, it’s better to sit farther out in the room. Sitting closer to the walls tends to homogenize the sound.
The listening position and the main speakers should form an isosceles triangle: the two speakers should be equidistant from the listener, with slightly less than that distance between the speakers. Keep in mind that as you move farther away from the speakers, you’ll hear less direct and more reflected sound. This will affect the spatial information and can result in a less well-defined soundstage.
How far the subwoofer should be from the listener is a bit different because, from 150Hz down, the lower the frequency, the harder it is to determine the source of the sound. As the lower limit of human hearing (20Hz) is approached, it becomes nearly impossible to locate a subwoofer by sound alone: the wavelengths are so long that they become nondirectional, or omnidirectional. Not only does this mean the subwoofer doesn’t need to be placed near the main speakers (i.e., at the front of the room), it also means that it can be placed closer to the listener, if that placement gives the most even frequency response.
As mentioned above, one of the major benefits of a sub/satellite system is that the locations of the speakers with respect to the room boundaries can be optimized for both the subwoofer and main speakers. As the speakers are moved closer to the walls, floor, and ceiling, the focusing effect of those surfaces amplifies the bass frequencies. However, this comes at the expense of precise imaging. So to create the most three-dimensional soundstage, position your speakers away from the walls. Moving a subwoofer closer to the walls produces the same amplifying effect as with the main speakers. As the sub is moved away from a corner and along one of the walls, the distribution of bass tends to become more uniform, albeit with a reduction in output. This doesn’t mean that placing a subwoofer in a corner is inherently bad; it’s just one of many possibilities.
Probably the ideal way to set up a subwoofer is to sit at the listening position and have a friend move the sub around the room, until you locate the point where its output sounds most even. Paul Barton suggests an easier alternative: Place the sub in the same spot as the listening chair, then crawl around the room with your ear close to the floor, listening for the spot where the sound is the most even -- that is, where nothing sounds absent or exaggerated. Once you’ve found this spot, move the sub there, return to your listening chair, and listen again. Barton recommends that during this process you play only music with sustained deep bass, not movies or other material that might feature only brief, intermittent periods of bass.
All that said, there will always be some "bad" rooms in which it will be very difficult to position a subwoofer without also getting a noticeable peak or null. In such cases Barton advises the use of a second subwoofer to even out and correct the problems created by the first. The size of the second sub’s driver doesn’t have to be the same as the first’s, nor do its level settings need to be the same. The goal is to place the subs so that they work together to eliminate the standing waves that cause those peaks and/or nulls. (I’ll cover this in a future article.)
Setting the crossover frequency, level, and phase
Like most modern A/V receivers, most subwoofers offer controls for setting crossover frequency, level, and phase. Whether you use the controls on your subwoofer or your receiver will depend on how you connect your sub to your system. In a future article I’ll write in more depth about these controls; in the meantime, here are some general guidelines for their use.
Adjusting the phase will be necessary if bass frequencies from the main speakers are being canceled by the same frequencies as produced by the sub, thus creating a null at that frequency. Switching the phase 180 degrees can eliminate this problem.
Adjusting the output sets the subwoofer’s volume level. Some people want a subwoofer to make the room shake during an explosion in a film, or to feel the bass line in a musical passage. For others, a subwoofer sounds best when it isn’t "heard" at all, but more subtly extends the low-end response of the main speakers without drawing attention to itself. Paul Barton leans toward the latter, and his suggestion is simple: try to keep the sub’s level as low as possible. This is usually the best way to blend its output with that of the main speakers.
The crossover can be thought of as a router for the incoming signal: The subwoofer will reproduce only the frequencies below the crossover setting. Much like placing the subwoofer, setting the crossover requires experimentation. Some people use 80Hz as a rule of thumb, while others favor setting the crossover to only slightly above the lowest frequencies produced by the main speakers. In either case, the integration of sub and satellites is paramount -- the transition between the drivers should be as seamless as possible. The only way to achieve this is through trial and error.
After adding a subwoofer to their systems, many listeners report huge improvements in sonic performance, such as a greater sense of space in which a recording was made, and a greater ease in the sound of their main speakers. The key is not to be afraid to play around with location and settings until you find something wholly satisfying. In fact, proper speaker placement is one of the best and cheapest ways to improve the quality of your system’s sound, because it’s relatively easy to do and it doesn’t cost a thing. If you’ve spent time and money assembling a system you want to enjoy, you owe it to yourself to take a bit more time to optimize its performance. The benefits can be priceless.