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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.

Recently, a colleague and I were working on a show in a highly reverberant space. Although we only had a few vocal microphones on stage, the room was an acoustic challenge, with a really nasty 80 Hz mode that rang like a bell for more than 2 seconds.

I asked my colleague if he was driving the system’s subwoofer via an aux bus, and he replied that he wasn’t, but that he was using high-pass filtering on the mic inputs. This prompted a discussion as to whether the two methods are equally effective at cleaning up the low end of a mix.

First, let’s define the issue.

A quick look with a spectrum analyzer will confirm that there is often a significant amount of sub-100 Hz signal content picked up by vocal mics on stage. This low-frequency energy consists of vocal pops and plosives, wind rumble, stage noise, traffic noise, handling noise, HVAC noise, and other annoyances.

In all but a few specific situations, this LF energy is unwanted clutter, usually manifesting as an annoying rumble from the subwoofers. The “booms” associated with vocal pops are typically the worst offenders – on plosive peaks, the energy between 60 and 80 Hz can actually exceed the level of the desired in-band vocal content if left uncorrected.

Obviously the optimal solution is to use the right microphone, and several models exist that are designed specifically for these types of problems. However, as is often the case, this was a “this is what we have to work with” situation.

Available Options

Aside from mic choice, there are two main defensive strategies: using high-pass filters on all inputs that don’t have useful LF information, and feeding the subwoofers from a post-fader auxiliary send (aux-fed subs or AFS) containing only the channels that do have useful LF information.

As a starting point, let’s look at a system that’s configured as left/right crossed over to the sub and tuned to a flat response.

It would be tempting to think that a 100 Hz high-pass (or “low-cut”) filter effectively removes all energy below 100 Hz, but this is not the case. High-pass filters (HPF) are, by definition, -3 dB at the chosen corner frequency (except for Linkwitz-Riley crossover filters, which are -6 dB since they’re made from two cascaded Butterworth filters).

From there, the steepness of the rolloff depends on the order of the filter, with each filter pole supplying 6 dB per octave rolloff. A second-order filter has a slope of 12 dB per octave, third order a slope of 18 dB per octave, and so on. The HPF does a good job reducing energy at frequencies far below the cutoff, but the critical 60 to 80 Hz region is a different story: it’s often too close to the filter’s cutoff frequency to have an effective amount of attenuation.

To quantify this, I measured a few console high-pass filters (Figure 1). The horizontal yellow line highlights -3 dB to easily compare the corner frequency of each filter. The yellow-shaded vertical rectangle indicates the critical 63 to 80 Hz “pop region” that I’m looking to target with the HPF.

Figure 1

First up is the black trace, which shows the response of the fixed-frequency HPF on a small-format analog mixer. According to the label on the console itself, it’s an 80 Hz filter, but the analyzer reveals the actual -3 dB point around 87 Hz. (This brings up the question of whether the console’s stated EQ parameter values can be trusted. For a look at this, see Bend Me Shape Me, LSI August 2018.)

Only 3 to 6 dB of attenuation in the pop region doesn’t go too far towards a solution. (Note that the LF vocal pop issue can be made worse with systems tuned with a “sub bump” or with the subs gained up higher than the full-range mains. If an HPF is supplying 6 dB of attenuation at a given frequency but the subs are turned up by the same amount, you’re effectively back where you started.)

The blue trace shows an HPF response from a digital console from the same manufacturer. We can see that the slope is the identical second order (12 dB per octave) response, and since we have the flexibility of a sweepable filter, I’ve tuned it to 100 Hz, which nets about 3 dB more rejection in the pop zone without crowding the vocal signal. Being 9 dB down at 60 Hz isn’t as much as I’d like, but it’s an improvement.

As an experiment, I raised the corner frequency of the filter until the vocal pops through the system sounded adequately controlled, and ended up with a corner frequency of 200 Hz (Figure 1, orange curve). The second order slope means that the very effective 15 dB+ of rejection in the “pop zone” comes at the price of intrusion into the vocal range, which is undesirable as my goal is just to control the plosives, not alter the tone of the vocalist’s performance.

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