Low-frequency content is an extremely important part of live sound for most modern music. The physical impact created by air movement helps make live music exciting, so even in small clubs, it almost goes without saying that subwoofers are a requirement. There are a few different ways to route audio into subwoofers, each having advantages and disadvantages. Let’s have a look.
Woof! Did You Feed The Dog?
One way of routing audio into subs is shown in Fig. 1, a block diagram of a biamped P.A. system. The main L/R output of the mixing console feeds full-range audio into an active crossover or system processor, and the “divided” outputs feed separate amplifiers for the subs and the full-range boxes; more complex systems might use additional outputs for low-mids and midrange drivers. This tried-and-true approach is a somewhat “old school” but reliable way to run a large P.A. system while providing a tremendous amount of control.
Here, the magic is in the crossover, or system processor. In the old days, the magic was in getting a phone call through to someone at NASA to find out what crossover settings worked best for the particular cabinets used in your system. Just kidding, but analog crossovers will require a bunch of homework regarding the proper settings for your cabinets. Make no mistake here: your ability to correctly set the crossover is critical to the quality of audio that the system will produce. That’s why modern system processors include a programmed database with presets for specific subs and full-range cabinets from a variety of manufacturers. You can keep your nose out of the manual and not have to sweat the fine details.
A variation is shown in Fig. 2, where active cabinets feature onboard DSP. In such a system, we can route audio from the console’s L/R outs to the subwoofer inputs and then patch the (line-level) outputs from the subwoofer to the inputs of full-range cabinets. At the very least, the subwoofer should apply a low-pass filter to its own internal power amp. Some subs offer the ability to route the top cabinet’s output through a high-pass filter, but check the manual to be sure. If not, then you’ll need to confirm that the full-range box has a high-pass filter so it can be free to dedicate its power amp resources to audio above the crossover point (say, above 120 Hz).
This arrangement works best when you use subs designed to complement a particular manufacturer’s own full-range boxes. Their engineering team has done the work of calculating crossover frequencies and filter slopes to ensure a smooth transition between cabinets (let’s face it: they’re smarter than we are). It may not give you the ultimate control, but it will get you 98 percent of the way there in 10 percent of the time. The amount of cabling is reduced, and so is the amount of gear you need to carry. When mixing, it’s crucial that you pay attention to channel EQ and dump the low end out of any channel that doesn’t need it. For example, you don’t want rumble creeping up the stand for the hi-hat mic and getting into the P.A. system (and, trust me, it will), so make sure that the hi-hat channel is high-passed. Ditto for vocal, overhead and possibly guitar microphones (see below). In fact, I’d daresay turn on the HPF for every channel and adjust as you sound check.
Yes, I Fed The Dog
A different approach is using an aux send to feed the subwoofers. We start by patching an aux send output from our console through a low-pass filter (LPF). In the old days, you’d need an outboard device, but these days, many digital consoles feature dedicated (or “patchable”) software EQs and/or filters for the aux outputs (see Fig. 3). The goal here is to remove all audio content above, say, 120 Hz. You’ll need to fine-tune this number based upon manufacturer specs for the crossover frequencies for both the subwoofer and the full-range boxes. In either case, you don’t want low-mid, mid and high frequency audio reaching the subwoofer, because the overlap creates phase issues and frequency response anomalies. Besides, a sub can’t reproduce those frequencies anyway, so why waste amplifier power trying? The output of that aux bus gets patched to the input of a power amplifier, or directly to the input of a powered sub. As in the previous arrangement, powered boxes with onboard DSP will make your life easier, and you’ll have less junk to carry. Note that the main L/R output from the mixer never reaches the subwoofer.
Now we need to address our mix — in this case, by determining which channels get routed to the subwoofer. I think we can safely say that the kazoo mic does not need to be routed to the sub, so at the kazoo channel, we’ll turn down the subwoofer aux send. The kick drum, however, should be sent to the sub(s) or you ain’t gonna get any kick out of it. So we’ll turn up the subwoofer aux on the kick channel. Ditto for the toms, bass and synths. It’s probably safe to say that the hi-hat can also stay out of our subs.
Overhead, vocal and guitar (amp) mics are not so black and white. Whether or not you send the overheads to the subwoofer depends upon your overhead philosophy. Some engineers dump all of the low-end out of the overhead mics and use them just to capture the cymbals — in which case there’s no need to send them to the subs. Other engineers use the overheads full-range to present the entire kit and add the close mics to accent — in which case the overheads should go to the subs. Be aware that, in high-volume situations, this can be an invitation for feedback, particularly if the overhead mics are loud in the house PA.
My personal preference is to not send guitars and overheads to the subs, but your mileage may vary. I haven’t worked with any singers who have significant content to their voice below 100 Hz, so vocal mics stay out of the sub. Guitars, on the other hand, can vary wildly. You might be mixing a metal band with one guitarist. Subs can provide the “chunk” of the guitar for impact, and it’s safe to allow the guitar to take up a lot of room in the mix. But if you are mixing a funk band, keep the guitar out of the subs.
Using an aux send to feed the subs reduces Intermodulation Distortion (IMD) by relieving the full range boxes from trying to reproduce extremely low frequencies (which can actually modulate higher frequencies), thus improving clarity in the mids and highs. It also makes the system more efficient by allowing us to put the power where it’s needed most. In other words, using more powerful amplifiers for low-frequency reproduction.
It’s important to note that there’s an anomaly to feeding subs from an aux send, which is that muting the P.A. is not really muting the P.A. In such cases, when you pull down the L/R master fader, the master for the subwoofer aux remains up — so you’ll still hear the subs rumbling away. Make sure to mute the aux send master for the subs too!
