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[M. Erik Matlock]

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.
By Michael Lawrence • December 4, 2018

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...

Continue reading here: https://www.prosoundweb.com/channels/study_hall/cleaning-it-up-methods-for-controlling-low-frequency-energy-in-the-mix/

[Paul Rennick]

Great write-up.  Can someone please clarify what the author means by the following in regards to a steep Rolloff curve?:
 "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"

Comment, or point me in the right direction.  How can a steep curve create phase-shift or delay issues?

[Art Welter]

Great write-up.  Can someone please clarify what the author means by the following in regards to a steep Rolloff curve?:
 "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"

Comment, or point me in the right direction.  How can a steep curve create phase-shift or delay issues?

Each "pole" (capacitor or inductor) of an filter causes a 90 degree phase shift and a 6 dB per octave slope, so a four pole, 24 dB per octave "steep" slope imparts 360 degrees of phase shift, while a single pole "shallow" slope imparts only 90 degrees.  These textbook analog filters when recreated digitally (digital IIR filters) result in the same phase shifts.

Not sure what the "problematic" group delay issues of an individual channel HP filter the author is contextualizing, though there are possibilities of combining "with" and "without" filters (and their delay) in the signal chain that could cause comb filtering.
That said, if the additional group delay of a steep filter causes a problem, the group delay of a shallow slope would also cause a problem, just time (and frequency) shifted.

Richard's post #21763 in

https://www.minidsp.com/forum/rephase/9790-phase-shift-time-delay-group-delay

has much good information, I'll quote a bit:
"Group delay is just another way of plotting phase response as millisecond delay vs frequency, which is perhaps an easier way to visualize it, because delay is viewed in real-world units like milliseconds, rather than “phase degrees” which requires more abstract thought, compounded by wrap-arounds, relativity and logarithmic graphs.
Impulse response is another way of viewing this information which is even more abstract to visualise how you’re deviating from the target."

This article might also help you understand some phase concepts:
https://www.prosoundweb.com/topics/education/a_meaningful_loudspeaker_phase_response/7/

[Michael Lawrence]

Great write-up.  Can someone please clarify what the author means by the following in regards to a steep Rolloff curve?:
 "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"

Comment, or point me in the right direction.  How can a steep curve create phase-shift or delay issues?

Hi Paul -
I did not realize this had been posted here until just now, so sorry to 'zombie' this thread, but I believe it is relevant to your question.

The group delay bit was added to the article (by me) after a tech review by Pat Brown. He pointed out the fact that, although the phase shift itself is of course inaudible, the group delay from many points in the signal chain adds up and at some point becomes "too much."

"[P]hase shift produces group delay, which can get pretty long with decreasing frequency. See attached for the GD caused by a 4th order HPF at LF. GD is a bit like latency - we know it matters, but how much is too much? Every component in the chain is a HPF and those responses superimpose to become a very steep HPF (two 12 dB/oct make a 24 dB/oct, etc.). Now stir in an additional HPF or two (artistic purposes) and the GD can really get out there. Audibility? Debatable, but I hear a lot of complaints that the "bass isn't tight" and it can nearly always be improved by simplifying the system. Of course they are complaining about the transient response of a system that has massive GD at LF, played into a room that rings like a bell in that frequency range."

I've attached the graphic that Pat included with his email (or at least attempted to. We shall see if it shows up). So I would agree with Pat that it's not necessarily audible in any given circumstance but it's something to be aware of just for the sake of having a more complete discussion of the issue. These are what I call "minority statements" - things that pass over the head of many readers but tend to prompt "Dear Sir" emails from the more technically astute.

I'm certainly interested in your thoughts.

Best,

M

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