Hi,

I have noticed this several times before and I have never been able to work out why this should happen. If I apply a HPF to a sound the output after the EQ is sometimes noticeably higher that it was going into the EQ.

Today I have been working on a sound that peaks at 0dB FS (not one I recorded), it has a lot of low frequency content. I am applying a HPF at around 100Hz, with a 24dB per Oct slope. The output from the EQ is peaking at around +4dB FS on the meters within Logic. I am working quickly and so using the standard Logic EQ< I haven't tried this with my PSP EQ's to see if they exhibit the same behaviour, may try to experiment when I have more time.

Can anyone tell me if the same phenomenon occurs with analogue EQ? And most importantly, why should this happen, as most of the power in this sound is in the low frequencies, and it has little going on above 2KHz, so surely with the HPF, most ofthe energy from the sound has gone!

Thank you

Quote LimitedHeadroom:

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Can anyone tell me if the same phenomenon occurs with analogue EQ?

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That's a good example actually. If you're putting something through an analogue EQ you're putting it through different gain stages. Same can be said for some plugs. It's not cutting the lows that's increasing the gain. It's putting it through said EQ in the first place.

Just use the gain utility in Logic to attenuate the level before it gets to the EQ.

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http://www.loverslanestudios.co.uk

Yes, it is a very common effect.

There are several possible reasons for it.

It can happen because the filtering process changes the shape of the waveform, so although there may be less total energy in the signal, the peak amplitude may well increase.

If you think about a bunch of different frequency tones all playing at the same time, their phase relationships vary continuously and add or cancel each other to create the total waveform. Remove some of the tones and some of those cancellations won't occur anymore...

It can also happen because some equalisers actually boost the region just above the turnover point below which they are attenuating.

Also, most equalisers introduce significant phase shifts and that again, will change the way different frquencies combine and cancel.

Hugh


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Technical Editor, Sound On Sound

Great, thanks for the answers. I does make sense when put like that.

Quote Hugh Robjohns:

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It can happen because the filtering process changes the shape of the waveform, so although there may be less total energy in the signal, the peak amplitude may well increase.

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Which is why one should also be mindful of LF noise during recording.

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