Last month, we looked at the differences between using aux sends and subgroups to manage your mix. This month, we’ll look specifically at what happens when using those outputs to route audio to subwoofers.
Rewind
Before we jump down a rabbit hole, let’s briefly review the signal flow for a multi-amp PA that utilizes a system processor with a built-in crossover. Fig. 1 shows a biamped P.A. system with passive full-range and subwoofer cabinets.
Audio from the mixing console is sent into the system processer for filtering, EQ, compression, limiting, and crossover. The crossover applies a high-pass filter to the full-range output, and a low-pass filter to the subwoofer output (“high-pass” is relative — probably around 100 to 125 Hz). Typically, the system processor is either designed specifically for the speakers, or has onboard presets to ensure the proper crossover parameters. You can usually tweak those settings if you feel it necessary. Easy peasy. This configuration uses only the two main outputs from the mixing console.
Aux to Subwoofer
Next, let’s see what happens if we want to use an aux send to feed the subs, which is a popular way of managing low-end. In fact, some engineers feel that aux-driven subs provide maximum control over the low-frequency spectrum.
Using passive speakers, the routing is a bit complex, as shown in Fig. 2. This setup assumes the system processor has sufficient I/O and processing power to process the signal from the aux independently from the L/R mix. The aux send for the subs is mono, which is usually fine because there’s little point in panning bottom end to one side of the P.A. or the other.
Life becomes infinitely easier if the speakers are active. (See Fig. 3.)
Being an astute reader of FRONT of HOUSE, you ask “where’s the crossover?” It’s built into the powered speakers. Most manufacturers provide onboard DSP or at least basic crossover functions for their active speakers, usually with settings created to ensure smooth integration of their full-range boxes and subwoofers. You simply adjust the rear-panel controls for the correct settings, as in Fig. 4.
(full-range) and DXS18 (subwoofer) powered speakers
This arrangement still gives you the advantages of aux-driven subs, but no headache regarding processor settings. The aux out can be patched directly to the first subwoofer, and an output jack feeds audio from the first sub to the second.
Don’t Be Tempted by The Dark Side
If you’re using passive speakers, you might be tempted to forgo the crossover. Why not let the passive crossovers in the respective cabinets do their thing and hope for the best? (See Fig. 5.)
This is not a good strategy for several reasons:
- Audio going to the full-range speakers needs to be high-pass filtered to ensure smooth overlap with the subs.
- The lower and upper ranges of the subwoofer need to be filtered. Filter the lower range with an HPF set very low — say, 10 to 20 Hz — to prevent DC from potentially reaching the drivers. Filter the upper range to achieve a smooth transition between the upper extreme of the sub and the lower extreme of the full-range cabs. If the subs and full-range cabinets overlap too much, you’ll have a bump in the frequency response and there may be phase issues in the transition range. If the subs and full-range speakers don’t overlap at all, there will be a hole in the frequency response.
- There’s no easy way to flip the polarity of the subs should they be in reverse relative to the full-range cabs.
The Digital Advantage
Using a digital console with outboard amps and passive speakers, you can probably create a crossover by using the EQ and/or filters that are typically available on all output buses.
The L/R bus will need an HPF, while the aux output will need a low-pass filter set to a complementary frequency and slope to prevent the problems previously mentioned. A trip to the speaker manuals will help you sniff out appropriate settings (a starting point would be somewhere around 100 to 120 Hz, with filter slopes of 12 to 18 dB/octave). After you’ve set the filter parameters, check the results by running pink noise through the system and using an RTA to see if there are bumps or holes in the frequency response.
Mixing Tips
When creating your mix, dial up the subwoofer send only on the channels that need bottom end (kick drum, for example). On channels that have no business feeding the subwoofers (high-hat and vocals for example), the subwoofer aux remains at -∞. A benefit of this is that the HPF settings on each channel are less critical so if (for example) you forget to turn on the HPF for the high-hat microphone, you won’t have to worry about rumble from the stage transmitting through the mic stand into the microphone and making its way to the subwoofer. The aux send should be post-fader so that when you raise or lower a channel fader, the feed to the sub stays proportional.
The aux-driven sub approach also allows the option of throwing frequency response to the wind and dialing the “thump” up or down simply by tweaking the aux send master (no need to access a system processor). And, you’ll be able to EQ the subs separately from the L/R mix bus, which comes in handy if you’re trying to tame a boomy frequency that’s related to a room mode.
Bus to Subwoofer
When you’re mixing monitors from front of house and don’t have a lot of sends, you can use a bus or subgroup output to feed the subwoofer, with limited results. You’ll run into the same issues regarding crossover as you would if you were using an aux to feed the subs. Also, you lose some control compared with aux-driven subs because routing a channel to the subwoofers relies upon the assignment switches, not variable pots.
Other Options
Some mixing consoles offer other ways of routing audio to subs such as a dedicated mono output or dedicated subwoofer out, sometimes with a built-in LPF. The mono bus output is a pretty good bet because many consoles with a mono bus provide a separate mono fader or pot on each channel, making it easy to dial in as much sub as you need. You’ll still, however, be on the hook for setting the crossover to ensure a smooth transition between full-range and subs.
Reach Steve La Cerra, FOH engineer for Blue Öyster Cult, at woody@fohonline.com.
