Hi, I just discovered something that I think is really weird. I was comparing EQ plugins and loaded in a drum break that fills out the frequency range. I loaded up an EQ and set one of the bands to high pass, and high-passed it up to ~3khz. To my surprise, this increased the output level by 4db. I'm surprised because I thought that I'm removing energy from the signal by high passing. The break is the only thing I have loaded in the DAW, so it's not revealing some masking that was going on. Clearly I don't understand the effect that an EQ has on a signal. Can someone please explain to me why the output level would go up when I high-pass filter a signal?
Last edited by padillac on Sun Feb 24, 2013 7:09 am; edited 1 time in total

phase shift

too much resonance?

there is no resonance (or at least, no controllable resonance). It's a 24db HPF w/ cutoff set to ~3khz.

I've just done some reading on phase shift and it doesn't help me that much. Hopefully someone is able to go into a bit more detail as to what exactly happens to increase the amplitude of the signal even though I'm supposedly removing most of the energy from it.

I can't go too much into techie stuff, but i'm aware that it is a known and normal behaviour.
Low freq has most energy, so cutting it a bit will let more room for other frequencies to breathe, and therefore you get a small volume increase.

Peak meters don't measure energy as far as I know, just peaks.

My guess is that DC level change, and you might have to remove that to get signal back to normal - or it had DC before since I believe shelving HP filters remove DC.

Maybe the plugin has such an option?

Just guessing....

if this is a normal behavior, does that mean that adding bass to the signal should then lower the amplitude? If not, why? Why does it work only one way around?

hrm. I still don't get it. Low freq has the most energy, so why does cutting most of the energy increase the amplitude of the signal? I would expect the amplitude to be reduced if I remove most of the energy. Obviously I'm *not* removing most of the energy, it's just being shifted elsewhere (phase shift, I guess?).

When I look at the signal on a spectrum analyzer, the high end frequencies peak at about -20db before EQing. After I high pass it, the high end *still* peaks at -20db. Only now I can't even see any low-end energy on the spectrum because it's all been cut out. I guess I could understand the amplitude going up if the high end frequencies also got louder, but they don't. My best guess at this point is that somehow all of the low-end energy has now been shifted into VERY high-end energy beyond my hearing (and beyond my spectrum analyzer's graph).

cutting bass increases the amplitude a little. boosting bass increases the amplitude a lot. beats me why (mostly cutting..)

Very interesting...
which EQ is it - I'd like to see oddity that myself also.
What else is in the chain, just incoming audio & EQ & metering?

I specifically left that info out because I don't think it matters...it happens with Live's built-in EQ as well as three separate commercial plugin EQs. I don't believe it's a bug in the EQs, I believe it to be a side-effect of EQing...I just wish I had any clue as to why.

Nothing else in the chain. No other tracks. One incoming audio signal & EQ & meter.

HPFs always increase the peak amplitude, and yes its phase shift. there's nothing you can do about it. normally dont run your leves too hot and lower the filter slope if possible (lower filter slope = less phase shift). also you can use a linear phase eq.

linear phase mode still increases the amplitude.

I appreciate the folks trying to help, but so far I've seen no explanation of why "phase shift" changes the amplitude. If anyone is able to explain where the "cut" energy *goes* and where the apparent new energy *comes from* I will be a happy camper.

We already had a few threads with that topic.
Here some answers:
http://www.kvraudio.com/forum/viewtopic.php?p=3966518#396651 8

But to answer your question:
There does not come energy, if you cut out bass.
But because of the heavy ringing (from EQing in low frequency areas), your waveform gets another form and (usually) higher peaks.

So your signal peaks higher, but stays at the same (perceived) loudness (or even gets more quiet).

This problem is usually a lot weaker with LP EQs.
Here some more words on it:
http://www.kvraudio.com/forum/viewtopic.php?p=4220396#422039 6
Last edited by Nokenoku on Sun Feb 24, 2013 8:28 am; edited 2 times in total

my english is to bad to explain but i found a nice explaination from a producer called Lagerfeldt:

It's the phase relations that change.

The original waveform (your mix) consists of a variety of frequencies that interact and constitute a whole.

When you use an equalizer you're not only changing the amount of gain in a particular frequency area, but you're also changing the phase and therefore the timing between the various frequencies.

This causes the various frequencies to interact in a different way that may or may not lead to a positive change (peak) as the frequencies shift - even though you're cutting. Remove enough though and you'll see the peak level go down again, but that's not the solution of course.

Most equalizers are so-called minimum phase and have this natural and normally desirable behaviour. Quite often when you're equalizing you're not just changing the amount of gain in a particular area, but you're trying to obtain the right phase response.

However, you can use a phase linear equalizer to offset the relative phase changes. In your case you'll observe almost no peak change, but still get the low cut you're looking for. Linear phase is no magic bullet though, and you'll probably hear some transient blurring caused by pre-filter ringing, which is an unwanted side-effect of linear phase technology. This is one of the reasons why linear phase is not the first choice for many engineers.

Logic Pro has a bundled linear phase equalizer, but you can also get the DMG Audio EQuality and switch between modes on the fly.