Study Hall

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Cleaning It Up: Methods For Controlling Low-Frequency Energy In The Mix

Considering the implications and merits of high-pass filters and aux-fed subwoofers as the main defensive strategies.

Informed Decisions

This was a nice little reality check for me, as I’ve clocked a lot of hours on this model of console and have always felt like I had to run the HPF up higher than I would on other desks to tame the plosives. This is confirmed by the fact that the filter is a shallower slope than that of the filters on other consoles I work with. Of course, we want to use our ears, not our eyes, when making EQ decisions, so there’s nothing inherently wrong with seeing a high corner frequency onscreen, but I do prefer a steeper slope in general for this task.

The green trace shows another popular digital console’s HPF, also set to 100 Hz. The steeper slope here (fourth order, 24 dB per octave) means more attenuation where it counts, without interfering with the in-band signal.This is certainly not a one-size-fits-all solution – sometimes a steeper HPF slope is a problem-solver, and in other applications it’s not wanted or needed.

(It warrants a mention here that although the phase shift produced by such a steep slope is inaudible in itself, the group delay issues may be problematic when considered in the context of the entire signal chain.)

Figure 2

Some consoles offer two slope options for their channel strip HPF, but barring that, what else can we do if a steeper slope is desired? At the event that prompted this discussion, my colleague was running the channel’s HPF all the way up to 200 Hz and not getting the rejection he needed, so used one of the console’s graphic EQs inserted on the channel (Figure 2) to achieve significantly more attenuation.

The red trace in Figure 3 shows the resulting curve as compared to the previous HPF settings of 100 and 200 Hz.

Figure 3

One the one hand, it solved the problem and gave him the results he needed. On the other hand, you’re not alone if you think this is an inefficient solution to the issue. It might do the job with a single-input show, but it’s not going to scale up for larger productions.

This is the real appeal of aux-fed subs. No filtering required – the LF energy from the mics simply can’t get to the sub, period, unless I decide to route it there. The constant low rumble is effectively banished from the system – Dave Rat once likened the improvement to “turning off an air conditioner in a room.”

The technique seems to have gained some bad PR, probably due to people who use it as an opportunity to run their subs a thousand dB hotter than the rest of the system.

At one outdoor event, I mentioned to a guest engineer that we had the option of patching his desk in with separate drive for the subs if he’d like, and he said “No, I like to hear the whole show.”

For me, aux-fed subs is about cleaner, not louder. I personally choose to run my subs via an aux whenever possible based on the requirements of the show. That being said, it’s not a foolproof solution, and there are still a few things to keep in mind.

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For starters, having the subs on a separate drive can make the system harder to measure and optimize, because what the measurement mic hears won’t match the reference signal sent to the analyzer. I tune the system as a standard crossed-over LR system and then simply route a different drive signal to the sub line once optimization is complete, without changing filters or gains.

Another point: for the same reason that high-pass filters are not a perfect solution, even with aux-fed subs the main system will still be putting out some LF energy below the crossover point (which on larger-format PA systems can be 80 Hz or even lower) and so a channel HPF is still likely to be helpful.

In the end, it’s not an either-or solution. I’ve been able to achieve great results using a combination of both approaches, and understanding the concepts behind both allow me to make the best decision for the show.

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