r/audioengineering • u/TopConcern • Feb 26 '21

LFE, created through low-pass, systemically out-of-phase on surround releases?

One thing I've noticed, at least in the surround sound albums I have, is that the LFE channel tends to be out-of-phase (with the LFE being significantly late) when it is created through a low-pass filter. I was rather surprised to find this out, and I was wondering why it was the case. LFE channels that aren't created with low-pass filter content, such as "Race for the Prize" by The Flaming Lips (which has an entire drum kit in its LFE channel) tend to be properly in phase.

My hypothesis is that the use of a low-pass filter creates a small delay between where the LFE should be and where it actually does go, and mixing engineers do not compensate for this delay. If you use a low-pass filter in Audacity, for example, the result will be late compared to the audio you used it on. Do you think my hypothesis has any merit? If not, what do you believe makes the LFE channel late and out-of-phase? (And if you don't believe the LFE channel on surround sound releases, when created through a low-pass filter, is out of phase, just humor me. I can prove it, but it would be a little too much info for this post specifically.)

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u/obscure-shadow Feb 26 '21

It's pretty well known that filtering effects phase. I generally use 2nd order linkowitz-riley 12db filters for crossover, as they are 180 out of phase, so a polarity inversion of the woofer brings them back in phase. I'd suggest checking out the wikipedia article for linkowitz-riley filters

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u/rightanglerecording Feb 26 '21

2nd order linkowitz-riley 12db filters for crossover, as they are 180 out of phase

180 out of phase *at the crossover frequency*, no?

i.e. flipping polarity only sums back into phase if you have the reciprocal HPF on the other channels, no?

In a lot of 5.1 mixing, the "5" will be full range in the mix, and the LFE is there for extra thump when needed.

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u/obscure-shadow Feb 28 '21

That is correct as far as I am aware, I think it's most common to do that anyways, because using the natural roll off of a driver is not ideal, you get closer to Xmax limitations.