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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.

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.

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.

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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