High Pass Filter HPF usage question.

[Tiger-Heli]

I searched the forums, but couldn't find an answer to this simple question (I'll update the Wiki Effects page if someone can confirm).

I tried using HPF 75 Hz with some MP3 files and then played them in my car and they sound much better (my doors resonate at around 60 Hz.)

However, I can't tell if the HPF in Audacity is a "Straight-Cut" (all frequencies below 75 Hz are removed), or a sloped filter (Frequencies below 75 Hz are linearly reduced below a certain frequency at a certian db/octave, and if so what is this slope?)?

What I was planning to do was add Bass Blockers or capacitors to the car speakers (I don't have HPF settings on the HU and don't have an external amp) to make the un-edited files match the edited ones when I play them in the car, but as you can see on this page, these follow a fixed sloped frequency that I need to match to make sure I get the right ones, so what I need is a way to mimic this effect in Audacity if that is not what the basic filter is doing.

i.e. - I might need a sloped HPF at 100 Hz to match the reduction in resonance I get with a straight filter at 75 Hz.

UPDATE - BTW, I didn't have them earlier, but I also downloaded the Ladspa Plug-ins for Audacity, and there are some HPF ones in there, but they seem to have even less documentation...

Thanks in advance for all assistance!!!!

[Gale Andrews]

I am no expert in these matters, but High Pass and Low Pass are definitely not blocking filters, they merely attenuate below/above the cut-off frequencies, and more attenuation can be obtained by running further passes. If you use Audacity 1.3.5, it has a more configurable High Pass filter that lets you choose roll-offs from 3 dB to 24 dB per octave. Note you can't just drop this updated plug-in into the Audacity 1.2.x plug-ins folder: Audacity 1.2.x won't see it. But if 60 Hz is your problem, try the Notch Filter instead, setting the Notch Frequency to 60 Hz and experiment with the Q value (a lower value cuts a wider notch).

[Tiger-Heli]

Thanks Gale - someone else replied to this thread a while back and it got deleted for some reason.

Audacity was doing what I needed - I tested with a 70 Hz tone and then HPF'd it at 75 Hz and could hear it.

The notch filter isn't what I would - would sound weird with that much music missing even if it would probably clear up the resonanace.

Car filters (non-polar caps) typically have the same 12db Slope as Audacity, so I could simulate that - I just need to get some Dynamat instead.

[Tiger-Heli]

Thanks Volta-X, but I really didn't mean for you to go through so much trouble.

I listen mostly to FM radio in the car and Audacity can't help with that, but it can let me know what's going on and what I need to do to make things better.

Thank you again though!!!

[Tiger-Heli]

I got the answers I needed.

Thank you again for your efforts!!!