ICEinLAVA's Domain

Another Holy Grail many sound persons worship is the concept of an auxiliary send feeding subwoofers (Aux Fed Subs). Traditionally a tactic of big arena shows and sometimes used as a performance “special effect,” aux fed subs can make sense for smaller show soundcos.

Why, Why Not?

First, we need to grapple with the question of change. One of the reasons we should not use aux fed subs is that simplicity is wonderful. We got top-boxes and subwoofers, and a crossover in between; why change? For small, budget-conscious sound providers, the thought of just going “conventional” and hooking up normally is comforting. Another reason not to go there is that aux feed subs now require an extra electronic crossover/speaker processor just to block the high frequencies and pass the lows, an extra expense. The last reason is that some sort of “calibration” procedure is required to bring the aux/sub gain back with the main mix. And if your subs are not co-located with your tops, getting the phasing to match up requires a measurement tool like Smaart or equivalent time-domain audio tools. Sometimes you can run a test tone at the crossover point, and tune by ear the low/mid delays for maximum coupling.

The reasons why to go with aux fed subs mostly revolve around clarity. By picking and choosing the mixing console channel strips to route through the subwoofer auxiliary send, the subwoofers sound remarkably clean compared to a conventional top/sub mains split. The “why” in the clarity equation is that channel sources (mics) that do not have low frequency sources still collect bleed from other parts of the performance area and create additional low frequency mud. Another “why” is that switched or swept high-pass filters are not brick walls, as we like to imagine them. Most of these filters are two- or three-pole (12dB/octave or 18dB/octave), but even with -3dB at the typical frequency of around 100Hz, plenty of 70, 80 and 90Hz get by the filters. When many channels are summed in groups and on to the mains, all that partially attenuated rumble is still audible. And don’t get me started on stage floors or platforms with subwoofer resonances.

How To

Now that you have had fair warning, here are the steps to get into aux fed subs. First, another speaker processor is going to be required. If you already have a stereo processor, you may be able to get by on a mono mix by feeding the main mix on one side and getting your top-box feeds from that. Then take your chosen aux send channel and feed the other side for a subwoofer-only mix. When deciding on another speaker processor for the aux fed subs, you may be able to take a chance on a more bargain model/brand since the low frequencies tend to be less critical than the highs on audio fidelity. Just make sure to get reasonable control on the gains and phase (delay) of the outputs.

When wiring to the power amps, the subwoofer amplifiers now get re-wired from the mains crossover/processor to the new speaker processor. This usually means another drive line used up in your snake, unless you have the crossover/processors located at Front of House. For simplicity, I recommend keeping the processors co-located in the mains amp racks. Dial the processors initially like they are identical setups. In other words, if you have been using a top (mid)/sub crossover point of 100Hz and a Linkwitz-Riley filter of 24dB/octave, initially stay with those settings until you need to adjust the aux processor gains and phase response (delay).

In Practice

When using aux feed subs, typically the selected channels needing subwoofer feeds have a post-EQ, post-fader assigned auxiliary send control at the unity gain setting. Depending on your console, this may be a setting from half-up to three-quarters of the way to full travel. The reason for this is that all the sends will maintain the balance as set by the faders. Of course, this advice can be ignored under the “feels good, do it” mantra. Setting or “calibrating” the master aux send control or fader should follow around your group and master gain structure practices. On non-VCA boards, if you typically back off 10dB on the groups and run your masters at zero dB, try a nominal -10dB setting on the aux send master for starters. But just like setting sub gains on main crossovers/processors, you may either goose that setting or back off depending on music style or subwoofer locations.

Doing aux fed subs on VCA consoles is much more a walk in the park, as the channel VCAs get gain cues from the VCA groups. This means less of a math problem and more of an autopilot function as the groups are fiddled with. On non-VCA consoles, you may have to periodically goose the aux master control feeding the subs, if you are in the habit of bumping the masters or groups during the show. In this case, remember a cardinal rule of mixing in that making something “louder” can also be done by making everything else “softer.” A rule I wish more of us would exercise more frequently.

The oft-asked question is, “What signal sources are prime candidates for aux fed subs?” The quick and easy answer is kick drums, floor toms, bass guitar, some keyboards, some seven-string guitars and your break music decks. Again, the “feels good, do it” mantra can be enacted, but keep in mind why you are weeding out channels in the first place. Aux fed subs may force some practices that smaller consoles do not have the luxury of handling. One example is that my stereo playback deck now has to reside on the channel strips to gain access to the subwoofer aux send, and eats up precious and limited performance channels. This may be resolved if you can swap an effects return up to the console’s master section if possible. Most effects returns such as common reverbs and delays can neglect the subwoofer feed. Of course, an exception applies when I want to select my favorite “Devil Voice” pitch shift effect. Darth always sounds better with subwoofer support.

A last question is the crossover frequency of choice when choosing aux fed subs. The best choices tend to be in the 90 to 120Hz region, and mostly a trade-off on your top-box low-mid frequency capability and from which signal types you will chop off the low frequencies. A good choice is 100 Hertz, especially with 15-inch drivers in the top-box cabinets. My first rationale comes from the six-string guitars that have a fundamental low E-string frequency of 82Hz. Losing the fundamental harmonic on the first couple notes on a guitar is usually not noticeable in a rock ‘n’ roll performance because the harmonics tend to fool your ears into phantom fundamental notes. Pushing your crossover to 120Hz and beyond starts getting into the zone were you and the listeners begin missing depth in vocals and some non-sub instruments.

My crystal ball foretells the future in which power amplifiers will have DSP-based preamps with digital remote control. These features will usher out the extra speaker processor requirement, as the amplifier will be programmed with its own audio pass-band. If each amp has remote control via a computer at FOH, then it will be possible to optimize the gain and phase delays for the performance venue using the measurement tools.