November 9, 2010

Bass management

Bass management is a tricky and complex subject. Loudspeaker designers spend a considerable amount of time with professional tools and a great deal of experience to design the passive crossover in a 2 way speaker. The challenges involved in the crossover between multiple speakers and a subwoofer are also considerable. One could argue that it is more difficult and more complex than the 2 way passive crossover, yet this task is left to the consumer who tries to do it without the right tools and usually without an understanding of the basic issues.

A textbook solution

Here is a vented subwoofer (red) and a sealed main speaker (blue) along with the same speaker vented (green). They are based on a typical 6.5" driver.

Here we have a 4th order crossover. Electrical filters are 4th order low pass on the sub and 2nd order high pass on the mains. A 4th order Linkwitz Riley crossover sums to a flat response where filters are 6db down. So with the sealed box we get a flat response, but the vented version isn't quite right.

Now what happens if we were to apply 4th order filters to both subwoofer and mains?

This arrangement now suits the vented design better, since it's now close to the 6db down point, while the sealed box will have a dip.

Electrical vs Acoustic
It's important to understand the difference and how they interact. Placing a 2nd order filter on a driver at 80 Hz might not result in what you expect. As shown above, the sealed box may have 2nd order roll off below this point, so the summed response is 4th order. If that filter is applied to a vented version with roll off lower down (green curve above), then the result is more complicated.

Measurements are needed

To ensure that you actually know what is going on, you need to measure the speakers using the nearfield method. In most cases, where you want to aim for a textbook crossover, sealed mains are the simpler solution, ideally with drivers that have an fs one octave below the desired crossover.

Alternative solutions

The textbook crossover isn't necessarily the ideal. Once you start to consider the impact of room modes, things get more complicated. It can be beneficial to actually overlap mains and subs. One example has been shown here:

Subwoofer integration example >

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