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Design Principles For Distributed Systems: Implementing Subwoofers

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

Subwoofers can make the difference between good and great business music systems.

Light background or foreground music might not require subwoofers; however, even in systems where the bass doesn’t need to be a dominant factor, having clean, full low frequencies can make a big difference in the customers’ enjoyment of the music.

The number of subwoofers to use, where to position them, how to set the taps (on 70-volt/100-volt subs) and how loud to run them vary depending on the characteristics of each installation.

Criteria such as loudspeaker placement, boundary loading (are loudspeakers placed close to a wall or in a corner?), size of the room, coupling of multiple loudspeakers/subwoofers, reverberance of the room, the type of music, the type of activity and the expectations of the listeners all come into play.

The following guidelines are given, therefore, in very general terms.

Crossovers
The four ways to cross over to a subwoofer are:

• Passive crossover, which is usually built into the subwoofer
• Acoustic crossover, such as a bandpass box that is acoustically filtered not to reproduce high frequencies
• Active crossover, which may be a separate electronic device or can be built into the subwoofer or a controller
• a combination of these, such as using a bandpass box with an active crossover

In addition, there are two main topographies for crossing over: overlap crossover, where the main speakers are run full range and the subwoofers are just added to them; and full crossover, where the subwoofer covers the subwoofer frequencies and the main speakers are high-passed to cover the rest (Figure 1).

Figure 1.

You need to decide on a system topography — the way you’re going to cross over the system — before you can figure out the quantity of subwoofers needed. Let’s talk for a moment about the options.

Overlap Crossover
In an overlap crossover, the main loudspeakers are run full range, and the subwoofers just add to the bass frequencies. An overlap crossover can be accomplished either with a built-in passive crossover or with an active crossover.

The advantage of using an overlap is that it sometimes allows you to use fewer subwoofers. The BIG downside of this topography is that the main loudspeakers usually only go down to 80 Hz or so, and the subwoofers often have a response as high as 160 or 200 Hz. (Hopefully, the subwoofers are internally low-passed with a passive crossover or they’re limited by being a bandpass design.) Even if the subwoofer only goes up to 120 Hz, you’re often in trouble.

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