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Design Principles For Distributed Systems: Implementing Subwoofers

A defining factor between good and great business music systems...

Achieving Even Coverage
In most installations, there are a lot more satellite loudspeakers than there are subwoofers.

Because there are often so few subwoofers (maybe only one), you can have a problem getting even coverage of the space.

People sitting or standing very close to the subwoofer are going to get blown away with lows while people who are father away might not be getting enough. How do you make the subwoofer coverage as even as possible?

As you move farther from the sub, the volume drops off, typically at 6 dB per doubling of distance.

Then, when you reach a certain distance, the subwoofer level stops dropping off at such a high rate.

This is called the critical distance, which is where the reverberant field within the room equals the direct sound from the subwoofer (Figure 2).

The critical distance depends on how reverberant the room is. As you get farther past critical distance, even though the level of the subwoofer doesn’t drop off nearly as quickly, the quality of the subwoofer sound might not be as good. Even though this may happen, it’s sometimes acceptable for subwoofers in business music applications.

Figure 2.

One way to make the subwoofer coverage as even as possible is to use more than one. It’s a myth that all you ever need is one subwoofer. In many places, it’s a good idea to add a second subwoofer, or more.

Even if you don’t need additional subwoofers for volume reasons, you might want to consider them just for evenness of subwoofer coverage. If I absolutely have to use just one subwoofer, my personal preference is to sacrifice the sensitivity increase and place the subwoofer for most even coverage, as long as I can achieve the SPL goals.

Placing Two Subs
In systems with two subs, it is often best to place them asymmetrically within the room. In other words, if one sub is in the middle of a wall, try to avoid placing the second sub in the middle of the opposite wall.

Small-room acoustics can cause interactions between subwoofers to create places where the bass builds up and other places where the bass cancels out and disappears. The topic of room modes is a paper in itself, but for now it’s important to realize that, while there is not much you can do about room modes, you can minimize their effect with conscious subwoofer placement.

If you place both subs symmetrically (on opposite walls), they will excite the same room modes in the same way, making disparities worse. If you place the second sub in a different position, it will tend to excite the room modes in a different way, and this is usually better.

It’s also good to know that placing the sub in the corner, while it does excite room modes, usually results in fewer mode bumps as compared to mid-wall placement. My experience when using two subs has been that one goes in the corner and the other goes close to, but not in, the opposite corner, about 10 feet out along one of the walls.

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