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Design Principles For Distributed Systems: Implementing Subwoofers

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

Sensitivity Balance Issues
The biggest potential problem of a full passive crossover is that you’re at the mercy of the sensitivities of the subwoofer loudspeakers versus that of the satellites and the need to properly balance the volume of each.

The subwoofer might have a sensitivity of 89 dB, while the full-range loudspeaker might have a sensitivity of 92 dB.

In business applications, because of the low volumes, the bass often needs to be between 6 dB and 10 dB louder—not quieter nor even equal in volume—than the satellites in order for the system to sound balanced.

In passively crossed systems, the subs are often softer than the mains, and that’s a problem.

The fix? Well, one fix is our friend the high-resolution EQ that can pinpoint and boost the exact frequency where the volume drops. A standard bass control (shelving type) is usually not a good solution. The chance that any standard bass control will match with the exact frequency, slope and shelving characteristics that are needed by any particular system are slim.

It is difficult enough for engineers to design a good passive crossover when they know the exact characteristics of every component in a single cabinet. With a business music system, you’ve got so many variables — sensitivities, roll-off characteristics, number of loudspeakers, placement, boundary loading effects, etc. — that it is difficult to get a passive crossover to work well.

Although a well-done full passive crossover can sound quite good, it’s much easier to wind up with one that sounds pretty bad.

Full active crossover is the most reliable way to get a good subwoofer sound. This means using an active crossover and a separate power amplifier for the subwoofer(s).

The subwoofer gets low-passed using a steep slope, usually 24 dB per octave, and the mains get high-passed with a steep slope. They interact in predictable ways. There is virtually no overlap between the subs and the main loudspeakers. There is no booming overlap bump like you get with the overlap crossover.

In addition, you get independent control over the bass volume so you can easily balance it with the mains by ear, or via an SPL meter. If the customer doesn’t like the balance, you can easily adjust it.

There are business music controllers that include a subwoofer crossover built right into them, like JBL Soundzone controllers, dbx ZonePro controllers, Crown USM, BSS ProSys and others that are on the market.

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