The Pros and Cons of Aux Fed Subs, Pt. 3

Image courtesy of L'Acoustics (SB28)

Image courtesy of L'Acoustics (SB28)

Continuing our series on aux fed subs, today we’ll discuss the cons. Last time, we talked about the pros—and there are some—and before that, I told you how this concept works. 

All good pro/con lists have both. And this concept is no exception. I’ve given you the pros, and now it’s time too look at the drawbacks. 

Constantly Changing Crossover

This is one of the big issues for me. The crossover point is defined as the frequency at which the two drivers are the same level. In our hypothetical example, let’s say that at 100 Hz, the subs and the mains are the same level. Below that, the mains drop off, as do the subs above that. When it comes to setting the timing between the subs and mains, we want them to be reproducing the crossover frequency at the same time. That is, if we send a 100 Hz burst out of both the subs and mains, it will happen at the exact same time. This makes for very clean and tight low end.

The best way to set that timing is by aligning the phase for both boxes. Phase is time, and it’s easier to see time in the phase trace than it is in say an impulse response. So we line up our phase with delay and the system is aligned. 

However, if we’re constantly messing with the level of the subs using that aux master fader, we’re sliding the crossover frequency up and down. As we do that, the timing is going to start to drift. Not a lot, but it can enough that the low end starts to smear and become less clear. Flabby, loose and muddy are all terms we hear when the bass isn’t aligned to the mains. Aux fed subs make it really hard to lock this down. 

Opportunities for Errors

Whenever we add complexity to a system, we increase the opportunity for failure or error. In this case, it’s all too easy to dial way too much of something into the sub aux, thus skewing the mix. Most instruments produce sound over a wider range than just the sub coverage. 

The kick for example has plenty of sub-100Hz content. But it also has a lot of information above that, and to make the kick sound nice and clear and punchy, we need it. If someone dials the sub aux on the kick all the way up to 11, pushing the fader up all the way will make the kick too loud. So it will get turned down. The low end will still be there, but the top end will trail off. A novice engineer might try to compensate with EQ which only further exacerbates our timing issue. Eventually, the low end is mush. Multiply this by 4-5 more channels and you have a recipe for ugly bass.

Added Complexity

This is similar to the above, but I point it out especially for churches that use volunteer, non-professional help behind the console. Aux fed subs are not necessarily hard, but it is harder to get it sounding right, and there is a bigger opportunity for things to go wrong. When setting up systems for volunteers, I like to give them as many opportunities to succeed as possible. Honestly, I don’t believe there are enough pros to outweigh the cons in an aux fed sub situation for most churches, so I go for simple when possible. 

Conclusion

By now, you probably know where I stand. Though what you may not know is that for years I was a staunch aux fed subs guy. I wanted the control and I felt I could do it better that way. However, as my understanding of PA tuning has expanded, I now believe I can get better sound from a full range fed PA. Having mixed on both, I know I can make either sound good. Overall though, I think a system where the subs are part of the main feed will generally result in better mixes for more engineers. Hear me on this; I’m not saying aux fed is categorically wrong. I simply think in most cases, for most churches doing a full range feed is going to produce better results. 

I can make a case that from a very technical standpoint a timed in sub system is going to sound better than an aux fed system (see the previous post). But again, I know some very good engineers who can make an aux fed system sound really good. However, they really know what they’re doing and they effectively treat it like a full range system.

As I write that, it occurs to me that I have one more thing to say on this issue. But that will wait until next time.