Death to... test what you think you know

[Jay Barracato]

I had been turning over the possible effects of stacking a fully cut graphic eq on top of a crossover in my mind, I was was coming up against 2 apparently conflicting interpretations as to whether this would make a difference so I decided to measure it.

By "death to..." I mean the practice of cutting frequency bands on an eq that are then further cut by a crossover.

Best professional practices say "Don't do it" but I was wondering if I could determine why.

What I think I know:
1. The frequency range over which an Geq filter makes a change in phase basically matches the same range in which it makes a magnitude change.
2. Adjacent frequency ranges on a Geq overlap so they also create summed changes in response in both magnitude and phase.
3. Depending on the type and slope, crossover filters make changes in magnitude and phase well beyond their "label" frequency.
4. Stacking filters, i.e. Geq followed by crossover, should give a response in phase and magnitude based on the summation of the filters which includes their relative sizes.

What I am not as sure of that I would like to be:
1. Do multiple filters create a change in phase response BEYOND the region where they make a magnitude change?

The two possible interpretations that I came up with:

1. Cutting all the frequency bands by 12 db will make an audible difference because the phase is affected by all those cuts.
2. Cutting all the frequency bands by 12 db will not make an audible difference because even though the phase is affected, the magnitude is so low that it doesn't matter.

Now the kicker that confused things:

Would having multiple cuts on a Geq below the crossover point affect the phase enough that it would mess up the phase AT the crossover point?

So today I was not in the position to measure the acoustic crossover of a sub/top combination, I was in the position to measure the electronic crossover of a sub top combination, both with and without the "death to..." Geq.

The null hypothesis: There is no difference in the phase response at the crossover between the "No Geq" test and the "Death to..." test.

The alternative hypothesis: There is a difference in the phase response at the crossover between the "No Geq" test and the "Death to..." test.

The experimental setup: Two channel measurement of phase and magnitude using SMAART of a pink noise signal passed through a KT DN360 Geq and a Sabine navigator DSP.

Controls: The KT DN360 and the Sabine Navigator were measured as flat in phase and magnitude both individually and in series with no filters in place prior to setting up the crossovers.

The crossover were set up using LR filters with both 12 db/octave and 48 db/octave slopes. The results were measured both with and without the "Death to.." Geq filters in place.

The results:

LR 12 db/octave with no geq (with polarity flip to match the phases)


LR 12d/octave with "Death to..." cuts above and below the crossover


The phase traces no longer lie on top each other above and below the crossover, and the summed magnitude response at the crossover is far below the trace with no eq

LR 48 db/octave with no Geq (and the appropriate polarity flipped)


Once again the phase traces lie on top of each other nicely over the entire crossover range.


LR 48 db/octave with "death to..." Geq added



This is the one I really wondered about because the magnitude from the steep slope is already so great that the extremes of the phase and magnitude wasn't even measured (note the coherence in the region away from the crossover). However; once again, there is a fairly large change in the timing of the phase that has been introduced.

So my conclusion is that the "Death to... " Geq technique does introduce significant changes in the behavior of the electronic crossover without any added benefit. (The null hypothesis is rejected and the alternative hypothesis accepted). The phase response of the 48 db/octave crossover with no eq is better than the phase response of the 12 db/octave crossover with the added 12 db cuts on the Geq.

Entering the realm of speculation, I would surmise that these negative effects the Geq has on the electronic crossover would be audible in the acoustic crossover.

 

[Bennett Prescott]

Hey Jay, interesting measurements. The phase in the 12dB/8ve version is actually not so bad, you can find worse offset by walking up into a balcony. Did you record what the summed versions look like?

I see this sort of GEQ nonsense mostly on wedges, where a proper HPF is either unavailable or poorly understood. Often the "death to 100" will be applied to one source or the other, but not necessarily both - more likely just the wedge to protect it. That is going to create some weirdness for sure, at least by applying to both sources you have some hope of coherence.

 

[Jay Barracato]

I did not try to measure the summed version at once, because that would have introduced a second eq unit and differences between them as well as another device to sum the signals. Also, I thought it was interesting because you can see the coherence drop off as the level passing the filter gets lower and lower.

I have to admit to using it occassionally on wedges, especially one one stage that is a corner stage that acts like a bass trap, and the only other processing on the wedges is a 12 db/oct high pass built into the eq. Like all things, it is about compromise, and I think I would argue that a 20 foot wide stage with 4 monitors going full blast has plenty of other phase problems going on as well.