This week’s blog post summarizes an article I read on Pro Sound Web about aux-fed subwoofer applications for common system configurations. While this sounds like a concept that might be employed only in large scale events, like concerts-in-the-round or the like, this is actually a technique that should be observed by anyone who takes the brave responsibility for building a system (from scratch). With it, however, is a necessity for a skilled operator who understands why such a setup is being used, the possible negative consequences that could occur if used improperly, and how to effectively use such a system.
Now that seems like I’m talking about something mega complex that needs licensed doctorate operators, but it’s quite a simple concept. If you, like me, can’t quite wrap your head around the way a crossover separates a full spectrum of signal and breaks it into frequency bands, and you believe that such voodoo probably does quite a wicked number on the quality of sound, then hey, we are of the like mind here. It’s important to understand that crossovers cannot simply drop an axe into the spectrum and cleanly cut out high frequencies from your subs and low frequencies from your upper end drivers. They work by attenuating frequencies on either side of the cross over point – that is, the frequency at which 3dB of attenuation of both bands overlap. It’s also important to know that low frequencies have extremely large wavelengths, and when they mesh together by phase issues or by noise issues, the result is mud. With that being said, it is easy to see that by leaving the frequency separation magic up to a crossover will still let some (quiet) mid frequencies into the subwoofers. This isn’t quite a problem, except that the subs are usually not placed directly in-line with the loudspeakers; more phase issues. Ok, I think you are starting to grasp the entirety of the problem here.
Enter the auxes.
By using post-fader, post-EQ aux sends, we can circumvent trusting a crossover to produce a clean low-end signal. Think of it like simply sending a special mix that contains no frequencies, say, above 120 Hz. There would be no mid frequencies to worry about muddying things up. The fundament to this solution is you literally send only low end channels from the console to that aux; things like kick, bass, floor tom, keys, etc. You still need a crossover to protect your subs, but this one is dedicated to the low end only since there is very little mid-range to compete with.
It’s not necessarily just the crossover that’s the problem. In fact, that’s really the least of our “trust issues”. In actuality, the problem is microphones (using the articles example). Most subs have a response, again say, from 20 (ideally) to 100 Hz. Microphones were made for humans, and I really can’t think of anyone who speaks below 100 Hz, or more specifically why that range (if it does exist in someone’s voice) contributes to intelligibility in any fashion. So this is obviously not something we want in the subwoofers. Normally what we do is use the high-pass filter on the console just as a “safety button”. But like Pro Sound tells us, most consoles only use a 12dB/octave roll-off filter. Remember that microphones do have the ability to transduce sound down to 20 Hz. By rolling out at 100 Hz, with two microphones, that means that there is 0dB of attenuation, and -12dB at 50 Hz. If you have four microphones, that’s +3dB at 100 Hz, -9dB at 50 Hz. And that’s only after four microphones! If this is a large enough venue to have a need for subwoofers, I would think that you have more than four active microphones at any given time.
Again, the aux-fed sub solution.
By lifting out channels that do not need to be in the subs, you are effectively being more effective than high-pass filters. You are cleaning up the low end and preventing phase issues. But it takes knowledge; a skilled operator. After calibration (gaining, timing, crossover points, etc), any alterations to the aux sends or, especially, the aux send master could have a very negative consequence to the phase relationship and coherency between the subs and the loudspeaker arrays.
While this solution is definitely a great concept to employ to clean up the mud and protect your subs, it does take quite a bit of skill and knowledge to keep it running efficiently.
