help me understand filters?

[dobro]

There's some stuff I don't understand about filters. Can you shed some light on this for me?

Let's say I want to get rid of some lower frequencies, like on guitar, I'm not interested in anything below the range of the guitar, so I want to cut all that noise from the mix.

1 Okay, so what's the difference between using a highpass filter and a low shelf?

2 Let's say I set the filter at 80 dB. How does that get translated into action? The gain *starts* to drop off at that frequency? (In other words, there's noise below 80 dB, but it gets less and less loud the further you go below 80 dB? Is that it?)

3 The rate at which the gain drops off is controlled by Q, right? So, a narrow Q means it drops off *really* fast, right?

[crosstudio]

1) the difference is that a low shelf only cuts the low frequency by -18db whereas a high pass drops the frequencies below the cut point altogether.

i'm not sure if its -18db or if it is more.

2) yup

3) yup

[dobro]

Well, if the highpass drops off into the noise floor altogether, that would provide the quietest option. Why would anyone use a low shelf then?

[littledog]

Originally posted by dobro
There's some stuff I don't understand about filters. Can you shed some light on this for me?

Let's say I want to get rid of some lower frequencies, like on guitar, I'm not interested in anything below the range of the guitar, so I want to cut all that noise from the mix.

1 Okay, so what's the difference between using a highpass filter and a low shelf?

2 Let's say I set the filter at 80 dB. How does that get translated into action? The gain *starts* to drop off at that frequency? (In other words, there's noise below 80 dB, but it gets less and less loud the further you go below 80 dB? Is that it?)

3 The rate at which the gain drops off is controlled by Q, right? So, a narrow Q means it drops off *really* fast, right?

Whoa! Whoa! Whoa!

Let's start over before this conversation gets too bizarre.

1) A shelf does not lower by -18dB (unless that's how you set it). A shelf lowers OR RAISES by whatever you set it to do. Everything below (or above in other cases) the pivot frequency is raised or lowered by the same amount. So it might look something like this: (ignore the dots - they're just there to make the image line up)

................____________
_________/


Note that the above picture could represent a high shelf with a boost or a low shelf with a cut, (or both) depending on where the zero axis is. Certain vintage EQ's actually put a slight boost just above the shelving point (forgive the feeble graphic attempt):

................^__________
_________/

2) Unlike a shelving curve, a HPF starts dropping at a set frequency and then KEEPS DROPPING - it doesn't level out. Think of one leg of an upside-down parabola. (although that probably isn't the actual math of the curve).

3) I don't think you meant 80dB. Clearly you meant 80hz.

4) In choosing between shelving and a HPF, the best way to choose is to listen to the results both ways and judge for yourself. Will it sound better to get rid of SOME or ALL of the low end? Will that little boost above the shelving point (if your EQ creates that) sound good or bad? Sometimes a particular brand of EQ will do some curves better than others, which might also influence your decision.

[dobro]

Crosstudio, littledog - thanks.

What I'm concerned to do is get rid of as much low-end noise as I can from most tracks. So I'm thinking the way to go is the highpass filter.

That's interesting about that bump at the shelving point. My software EQ does it, depending on the settings, and I always wondered why.

And yes, 80 Hz, not 80 dB.

[TexRoadkill]

A little tech note- When you see filter specs and they talk about 'poles' that determines how steep of a cut they can do. More poles = more db per octave.

I think it is around 12db per pole but I'm not sure if it is that cut and dried, probably not.

[knightfly]

Hey Tex, it's actually 6 dB per pole - a 4-pole filter gives 24 dB/octave -

NBTW, how you likin' yer Radar so far? Steve

[TexRoadkill]

I knew it was some multiple of 6

The radar is pretty sweet so far. I finally finished up building the console for the Ghost and am getting setup to where I can have some bands come over. Then I can really put it through the paces.