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Design Principles For Distributed Systems: Implementing Subwoofers

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

The problem is the overlap band. Between 80 Hz and, let’s say, 160 Hz, both the mains and the subwoofers are reproducing, whereas below that range it’s only subs, and above it’s only mains.

Thus, you get much higher sensitivity in this low-to-midrange band. You end up with a big bump in this mid-bass range, which is often perceived as muddiness.

“Don’t mistake loudness for fidelity,” is a good adage here. An overlap-crossover system might get loud but fall far short on the fidelity scale. The customer might comment that the subwoofers don’t seem to get very low because you’re emphasizing the mid-bass range. You can add more subwoofers until the cows come home, but it will only get muddier.

To compensate for this effect, you need to include a good EQ to notch out the bump. A single parametric band can often do it. It’s difficult with a graphic EQ of less than 31 bands.

Even a 15-band graphic can take out too much good stuff along with the bad, unless the frequency and bandwidth happen by chance to match your bump precisely. Certainly, 7-band EQs are of little use with this kind of overlap bump.

While an overlap crossover might allow you to use fewer subwoofers, unless you’re including a really good EQ, an overlap tuning in a business application is probably not advisable.

Another way of dealing with this is to use an active crossover on the subwoofer so you can slide down the subwoofer’s low-pass frequency to reduce the mid-bass bump.

While that will help a lot, it can be difficult to match the electronic low-pass characteristics of the subwoofer band with the acoustic low-frequency roll-off of your main loudspeakers.

Adjusting the low-pass frequency is usually a big improvement from the passive overlap bump, but you can still end up with some abnormalities at and below the crossover point.

Full Crossover
A full crossover high-passes the main loudspeakers and low-passes the subwoofers. The result is a nice, smooth transition.

A full crossover almost always sounds better than an overlap, but since the subwoofers have to cover the bass frequencies all by themselves, you will probably need to use more of them. You can accomplish a full crossover either passively or actively.

Passive systems usually use crossovers built into the subwoofer. The full-range amplified sound goes to the subwoofer, where the lows are sent to the subwoofer driver.

The main loudspeakers are connected to the satellite output, which sends them mids and highs (with the bass removed). This works alright, but the crossover components need to be large (to handle the low frequencies), and they eat up some of your power.

The crossover slope is usually not very steep, around a 12 dB per octave low-pass to the sub and a 6 dB per octave slope to the mains, or satelliteloud speakers. Steeper high-pass slopes are typically avoided because they can self-resonate or cause strange impedances to the amp if a satellite loudspeaker doesn’t get connected to it or if the satellite speaker blows during use.

However, with a first-order (6 dB per octave) passive high-pass crossover (for the satellite speakers), the crossover frequency changes with the impedance that is connected.

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Lessons From A "Seasoned" Church Production Tech

The higher the impedance load, the lower the crossover frequency. An output that works properly with a 4-ohm load — as with two 8-ohm speakers — will be too low in frequency if you only connect a single 8-ohm speaker, leading again to an overlap bump because of misadjusted crossover frequencies.

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