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Design Principles For Distributed Systems: Implementing Subwoofers

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

SPL And Frequency Response
Let’s delve a little more into the SPL requirements for subwoofers.

Typically the goal for subwoofers is to be somewhere between 2 and 10 dB louder than the main loudspeakers in the system.

Low- and medium-level music requires subs to be louder than the main loudspeakers because at low sound levels the human ear needs more bass for a perception of well-balanced sound.

Given the same music, higher levels of music can sound well balanced with less relative bass increase.

There are also some music types and applications, like dance music in upscale fashion retailers, that may require more than +10 dB of bass. Setting the ratio of subs to main speakers somewhere between +2 dB and +10 dB, as measured by a flat SPL meter, is usually a good starting point.

As for subwoofer frequency response, especially for business applications, you probably don’t want the subwoofer to go much below 40 Hz. Below that, you end up with bothersome rumble, which can build up in corners or at room-mode nodes (function of a room’s dimensions).

Rumble can annoy customers. They might not be conscious of it, but when they’re standing near the corner of the room, looking at clothing on a rack, low-frequency rumbling can make them uncomfortable enough to chase them out of the store.

If a customer says they don’t want subwoofers because they heard subs at the XYZ Store where the subs were bothersome, then there is a good chance that XYZ Store’s system installer made the mistake of putting in too much rumble below 40 Hz, or left in a mid-bass bump because of an overlap crossover, or used a passive crossover that doesn’t work correctly with the selected product mix.

The list of pitfalls goes on, but if it’s well-implemented by a skilled contractor, a business music system with subwoofers can sound absolutely wonderful!

Subwoofer Positioning
So how many will you use, where will you put them, how will you arrange them, and how will they relate to the mains?

Effect on SPL
Hanging a subwoofer in the middle of a room results in the lowest possible output from the subwoofer. Placing a subwoofer at the ceiling, wall or floor increases its output. Placing it within a few feet of a within 3 feet of a corner increases its output still more.

In these cases, there is both an increase in sensitivity (output per watt of input) and in maximum total SPL capability. This can help in getting as much sound as possible from a few subwoofers.

However, there is a potential pitfall in placing a subwoofer in a corner: You can wind up with uneven bass coverage in the room.

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