Hello Everyone,
 
noob question here.   So I was reading that I should use a HPF and cut out 30 Hz and below, I notice though using the built in EQ in the prochannel that when I dial in 30 Hz on the HPF the slope starts out at around 90 Hz and goes down from there.   

If I am seeing this correctly, frequencies from 90 Hz and below are being reduced, correct?   Should there even be a slope, or should there just be a hard cut at 30 Hz?
 
This applies to LPF as well.
 
Thank you in advance, sorry if this was a noobie question.
 
Adam

Any sort of hard cut will introduce distortion; a gentle (or not so gentle) slope will prevent this from happening.
 
It would be very difficult to implement a brickwall cut of this nature in the analog domain, I imagine it would be easy enough with digital, but the same caveat about distortion still applies.

The slope may start higher,,,,, but notice that at the top of the slope the freqs are reduced less. As the slope or graph goes down, the amount of gain reduction at a given freq increases.
 
It can be a straight line slope or a curve...linear or non-linear...
 
You always want a slope, never a hard cut....although you can adjust the slope to become nearly a hard cut. Let your ears determine the slope and where it starts and ends. 
 
Remember that you're talking about the bottom.....same things apply up top.

The target frequency is the mathematical center of the slope based on a simulated analog curve.  Not the beginning or the end.  A steeper filter will narrow range of both high and low frequencies affected.  

it's a unique issue with every individual track.
 
judge each track for it's 'requirement'
 
low end rolloff does not hurt tracks with very little low end energy, or tracks with much below 80hz, but there are always tracks that have information in that reqion that doesn't seem apparent, those are the tracks you have to be careful with.
 
plus, just because a track has a peak at 60 hz, does not mean you should get rid of any of it.
 
low end rumble at 25, yea, probably so.

There are times when you use a very steep slope and other times you might use a much more relaxed slope. eg 6 db/oct slope is gentle. This type of slope is good when you have got quite a lot of excess low end energy on a track. You might set it up reasonably high eg 200 Hz. At 100 Hz you are only 6 db down and at 50 Hz 12 dB down.. This type of slope will sound good and it will just clean up excess low end without changing the sound too much.
 
There are other times when you want all the bass energy to stay right in up to a point and then everything below that point you want to remove. This is when you might select a steeper slope eg 48 db/oct. And set this to say 40 Hz. Now everything will remain intact right down to 40 Hz but below that everything is removed fast. This is good for removing rumble or very low end (rumble) in a track without changing its low end content hardly at all. Sometimes this is needed on a track or in mastering too.
 
You just have to listen to the material and decide what needs to be done. That will determine the slope required. Steeper slopes leave the sound basically alone and clean up sharply below the cutoff frequency. Gentle slopes do effect the sound in a nice way and are good for just thinning out a sound that is a little bass heavy. With gentle slopes you have to move the cutoff freq higher so you can hear the effect of them more so. The position of the cutoff freq with a gentle slope will determine how much low end is removed. It is a nice way to tame excess low end without effecting the sound too much.
 
The same applies to the high end except you tend to use more gentle slopes up the high end to perform more subtle rolling off of high frequencies.

I think what the OP is asking is, why, when you set the cut at 30, does it appear on the graph to begin its slope at 90.

different software eq's are written different ways.
 
one size does not fit all.
 
that's why people pay big bucks for well written software EQ's, and argue endlessly about which ones sound the best.
 
math is math, right?
 
not always
 
there's a thing called 'fuzzy math', beauty is in the eye of the beholder.

Keep in mind that you can do low cut or low shelf. Low shelf cuts the dB below a frequency but then levels off. Low cut goes down in an exponential curve.

Cutoff frequency for LPF and HPF is usually defined as the point at which there is 3dB reduction.  So if you set your cutoff on a HPF to 30Hz, the signal will start being attenuated at a somewhat higher frequency.