Everyone likes fat bottom – at least in their mix, anyway (get your mind out of the gutter). There's no debate that augmenting a system with subwoofers increases the power handling, maximizes SPL and improves clarity in the upper frequency range by reducing intermodulation distortion.
One of the issues that seems to crop up just about every day is whether to route audio to the subwoofers from an aux output or via the main L/R mix bus. These are two distinct options, each with its advantages and disadvantages. Before we discuss the pros and cons of each, let's have a brief review of the connections.
Subwoofing Option 1
Figure 1 is a block diagram showing the first option. All audio is routed via the L/R outs of the mixing console into a crossover or system processor.
Output of the crossover then feeds separate amps for the sub and full-range speakers.
This diagram, by the way, is simplified for clarity. We all know that crossover routing (passive and active) can be more complex.
Subwoofing Option 2
{mosimage}Contrast that with figure 2, where an aux output from the console feeds signal to the subwoofer amp, and the L/R main outs of the mixing console feed the "full- range" speakers via crossover. (Again, it's simplified for clarity.)
The point here is that the subwoofer is being fed by a bus separate from the L/R bus. Setting the crossover frequency is up to you, and the console's aux output should be processed to removed the higher (e.g., above 125Hz) frequencies.
Pros and Cons
Now that your memory is refreshed, let's examine the strengths and weaknesses of each approach.
First, the obvious. Using the connections in figure 1, if you raise the kick drum fader on the mixer, you will feed more kick drum signal to the input of the crossover which, in turn, feeds more kick drum to the subs and the full range speakers in whatever proportions you have set using the crossover.
That's not necessarily so with figure 2. First you have to turn up the aux send on the channel. If the aux send is post-fader, then raising the kick drum fader will increase the signal to the subs and to the full range part of the system. But if the send is set to pre-fader, then raising the kick drum fader does not change the amount of kick drum signal going to the subwoofer.
Remember: a pre-fader send does not change when you move the fader. In fact, with a pre-fader aux send feeding the subs, you can turn the channel fader down completely and you will still hear the low-end rumblings of the subwoofer, because the pre-fader send is still feeding audio to the subs even though the channel fader is off. This is usually not desirable.
When you are setting up your mix, you have to keep in mind that until you turn up the aux send, you are not getting any of the bottom octave, because there is nothing feeding the subwoofers. After 173 years of mixing audio, I still forget this once in a while, raise the kick drum fader and – hearing a thin wimpy sound from the channel – think, "Wow, this system has no bottom end." Then the systems tech reminds me that the subs are on an aux, I turn up the aux, and all is well again in the Emerald City. Since you can dial in the amount of low end you want for each channel, you can adjust the proportion of low end without changing the crossover parameters. Sound companies can take advantage of this when running multi-act festivals, allowing each guest engineer the ability to dial in their preferred proportion of sub without need to change the crossover.
Using an aux to feed the subwoofer also provides the ability to patch a completely separate compressor on the sub out output. You no longer need to worry about the bottom end "sucking out" the remainder of the frequency range if the low end is triggering compression. And, of course, you can add an independent graph or parametric EQ to the subwoofer path.
Social Awareness In Groups
When a set of drum channels is assigned to a pair of audio subgroups, interesting things happen in system where an aux feeds the subwoofer. Many engineers will assign the drum channels to a pair of stereo audio subgroups so that they can make the entire kit louder and lower by moving just two faders – which simplifies mixing. But when you move the group faders, the amount of low end on the kick and floor toms does not change, because the aux sends are "upstream" in the signal pathway before the audio groups. You can even mute the subgroup faders, but your kick drum will still go to the subwoofer. An interesting effect – but keep this in mind between sets when you want all the channels muted: Patching a cable into the kick channel could lunch your subs. Likewise, find out if muting the main L/R bus also mutes the aux sends (It doesn't, I checked). Digital (and analog) consoles with VCA or DCA group masters make it easy to avoid this issue. Assigning the drum channels to a single VCA master allows you to raise and lower the level of all the associated channels with a single fader, while maintaining channel-to-channel balance. When you raise the VCA group fader, you are actually changing the level of the channel fader. So if the aux send for the subwoofer is post-fader, then moving the VCA fader will change the level of the channel faders and thus the subwoofer feed – maintaining that delicate spectral balance you've worked so hard to achieve.
What's the Point?
Having said all that, one of the big advantages of using an aux to feed the subs is that channels which don't require extreme lows (vocals, for example) never get sent to the subwoofer. Think about all the low-frequency garbage that spills into a vocal mic; keep that out of your amplifiers and you increase the low-end headroom. There's no point in amplifying the low-frequency junk that spills into a mic placed on a guitar cabinet or high-hat. Take it out of the subs and you clean up the mix. In this manner, auxing the subwoofer is much more effective that just running a high-pass filter, and you don't get the phase shift that some filters can cause. Sounds such as kick drum, bass, synths, floor toms and large rack toms are all candidates for being auxed to the subwoofer. The tambourine microphone is not. Of course, the flip side of this is that you can create some really cool effects by selectively pumping the subwoofer send on a synth or guitar channel – without disturbing the EQ.
