Home Toys Article
- August 2004 -
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The use of powered subwoofers and separate midrange/treble satellites both allows you to be conservative in the amount of power your main amplifier produces.
Since the earliest days of high fidelity, one of the main challenges for the designers of speakers, and of their users, has been management of the lowest frequencies - the deep bass. Many of the most notable developments in speaker design have been made with a view to getting more bass output from smaller boxes.
But although those advances have been important, they have usually not addressed the most important consideration when it comes to bass: the effect the room itself has on the sound.
One consideration is the size of the listening room. The larger the volume of air a speaker must excite, the more acoustic output you will require from it to achieve the sound levels you want. In any environment, sounds attenuate as you move farther away from their source, but in smaller rooms that tends to be offset by reinforcement from wall reflections. The larger the space is, the farther the sound has to travel both to reach the reflecting surfaces and then to get to your ears, which means it has to be louder to begin with.
With traditional full-range speakers, that involves an intricate matching act between amplifier power, speaker sensitivity, impedance and power handling. But the bulk of the power goes to reproducing bass, so the use of powered subwoofers and separate midrange/treble satellites both allows you to be conservative in the amount of power your main amplifier produces and ensures a good match between the low-frequency amplifier and the woofer with which it is paired.
After size, the most important aspect of a listening room is its shape. In any room, sound reflects off the walls, ceiling, and floor. If the distance between two opposite parallel surfaces is a simple fraction of the wavelength of a particular frequency, notes of that frequency will bounce back and forth in perfect phase - an effect called a standing wave or room mode.
At some point in the room, this note will be reinforced substantially; at others it will cancel out almost entirely. If the prime listening seat is placed at either of these locations, the note will boom or will be virtually non-existent. The standing waves are different between floor and ceiling, side walls, and end walls, unless any of these dimensions are the same. An ideal listening room would have no parallel surfaces - an unusual situation, to say the least - so that such waves would not establish themselves. The worst kind of room is a perfect cube.
Almost all rooms are susceptible to some standing waves at low frequencies, but their effects can be minimized by careful positioning of both the speakers and the listening seat. Moving either of these even a few inches is sometimes enough to cure - or create - an intolerable sound. The only way to find out what works best is by experimentation.
With full-range speakers, the range of places you can put the speakers and still get proper imaging may be fairly limited, and some of these positions may result in standing waves that can't be tamed. Things are more controllable through the use of a subwoofer or two. Positioning of the bass speakers has almost no impact on imaging, so a subwoofer can be located with only standing waves in mind.
The best arrangement is a pair of subwoofers in acoustically dissimilar positions. This is in contrast to the rule for positioning full-range speakers, which should be in as similar environments as possible, acoustically. In a normal room, however, there is probably no one location where a subwoofer will excite no standing waves at all, but a second one in a different environment will create different room modes, and the overall acoustic effects will be randomized. Carefully handled, this can result in a much smoother sound.
Bass response of all speakers is affected by their proximity to nearby reflective surfaces. The closer a speaker is to a wall or the floor, the more prominent will be its low-frequency output. So for maximum output, a corner is usually the best place to position a subwoofer, placed at least a foot from back and side walls - the distances should be different. When setting up your system, then, this would be a good place to start. Although corner-positioned speakers tend to excite and couples more efficiently to room modes than units farther from surfaces, that might not be so in your case. If there are obvious standing waves from such an arrangement, it's often reasonable to leave the sub in the corner for maximum output and place a second one in a less reflective area to smooth out the response.
Things may not be so simple, so the best method for positioning a subwoofer, although a rather undignified-looking one, is to begin by putting the subwoofer momentarily in your listening chair, then play something with lots of bass through the system. Move around the room and note where the bass sounds best; if you place the subwoofer there and yourself in your chair, you should get the same bass performance. Bear in mind that the test only works if you have your ears as high off the floor as the subwoofer will be, so don't be afraid to crawl around. A recommended starting point for the actual placement of this subwoofer would be in either of the front corners of the room (on either side of the main speakers).
This technique will optimize performance for one subwoofer, but there may be no combination of speaker and listener position that will be free of obvious acoustic anomalies. The best way to iron out those anomalies is with a second subwoofer. The same "crawl around the room" method as described above should be used for determining the location of the second subwoofer, except in this instance one is listening for the minimum amount of bass output. This is a recommended starting point for determining the best placement for your subwoofer(s).
Once a reasonably smooth response has been achieved by careful positioning of the subwoofers, the overall performance can be fine-tuned by means of the controls found on the speaker. An important one is the low-pass filter, which controls the upper limit of the subwoofer's frequency range. This should be set high enough to overlap the low frequency cutoff of the satellite speakers, but not high enough to localize specific sounds from the sub.
If the frequency response of your satellite speakers is such that the subwoofer's low-pass filter must be set higher than about 80Hz in order to avoid gaps in the overall system response, then you might well be able to localize specific sounds from the sub. This can be very distracting when these sounds appear to come from beside or behind you. One solution is to make sure the subwoofer is in the front of the listening area; another is to use multiple subwoofers to make such sounds more diffuse.
Subwoofers also offer a phase control so the upper frequencies they produce will not cancel out the lower frequencies of the satellites. A judicious tweaking of this control can pay major dividends in spectral smoothness in the crossover area. Phase changes with frequency, however, so these controls should be readjusted every time you vary the cutoff frequency.
Also adjustable is the overall level of the subwoofer's output. Many users tend to set this too high at first, in an effort to achieve truly impressive bass. Again, smooth response is the aim, and it may well be that the two subwoofers end up being set differently - if, for example, one is in a corner and the other is not. It's all part of the overall-balancing act that is bass management in real rooms.
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