rolling off everything below X Hz

  • Thread starter Thread starter dobro
  • Start date Start date
dobro

dobro

Well-known member
Here's something I picked up from Paul White that I didn't know before:

"The first thing to notice is that the cutoff point is not actually where the gain reduction starts. This is because the definition of the cutoff point of a filter is that point at which there is 3dB of gain reduction, and this is worth bearing in mind when using any filtering — even though the controls of the filter may indicate that gain reduction is only happening within a certain frequency zone, there will almost always be some gain change outside this zone as well."

Well, okay. So let's say I want to roll off everything below 80 Hz on my guitar. If I set the cutoff point at 80 Hz, then the filter's gonna start attenuating stuff higher than that, right? So how do I calculate the cutoff point so that nothing gets touched above 80 Hz?

I don't trust the dumb little graph on my two digital EQs. I don't think it's accurate.
 
What you first gotta know is what the bandwidth or Q of the EQ is... I'd/ I'll go into it further, but don't have time right now...
 
A highpass filter cornered at 80hz (if it's 3db/octave), will be down 3db at 80hz, 6db at 40hz, and 12db at 20hz. Going the other direction, 160hz is where it will be at unity (no attenuation)...so if you don't want anything touched below 80hz, you'll have to move your HP filter down to 40hz.

That's the technical explaination, but I highly doubt putting a HP@40hz will produce the results you're looking for when applied to a guitar track. Do what sounds right, and typically (for me anyway) I HP a guitar track anywhere from 80hz to 120hz. A HP@40hz is best utilized for things like bass, kick and maybe even the entire mix.
 
I want to add that 3db/octave is probably not typical for software plugin filters (but I can't really confirm that). Take the Waves RenEQ for instance...at it's lowest Q setting, it's about 10db/octave, which would mean that 80hz will be down 10db, 40hz would be down 20db and 20hz is practically non existant (down 30db). At it's highest Q setting it's about 18db/octave, which means thing cutoff much more quickly (I'm sure you can do the math on this one), but it becomes less musical at that point. It's been suggested that keeping the Q around 12db/octave is probably the most musical sounding...but there are certainly cases when you may need to "brickwall" frequencies, or let thing roll off gently. Again, it's all about what sounds right, so don't set anything in stone until you hear what things sound like.
 
corner frequency VS rolloff rate

"which would mean that 80hz will be down 10db,"

This is not the way we spec filters in the electronics world. The frequency stated is always the -3db point regardless of the slope or rolloff rate. This means that in the example stated 80Hz would be 3db down not 10db down and 40Hz would be 13db down.

Note that depending on the specific design of the filter there will be a non linear area on both sides of the corner frequency before the filter starts to approximate the specified rolloff rate.

Software filters, if they are properly designed, have the potential to more closely approximate ideal filters but there are several caveats including a price paid in processing power and diminshing returns in terms of what can actually be perceived with human hearing.

AFAIK software filters should still specified using the same conventions as hardware filters WRT the -3db corner frequency and the rolloff rate. Of course some programmers (and possibly some hardware engineers, for that matter) may not have the engineering background to understand this and may improperly specify thier filters.

SW
 
thevirgin,

You are correct, and thanks for clearing that up... :D
 
"Again, it's all about what sounds right, so don't set anything in stone until you hear what things sound like."

Yup, I always use my ears (for what that's worth), but I like to engage my head too (what's left of it). And yeah, that 80-100 Hz range is where I rolled off my guitars on the last album I did, and it was pretty okay. I've been experimenting with really radical cuts on vocals, using filters with really sharp curves, and so far, I haven't noticed anything nasty.
 
You must log in or register to reply here.
Back
Top