Removing background noise

Hey,

I’m using the latest (beta) version of Audacity and I need to hear specific frequencies in audio files. I want to remove every other frequency.

How do I do this?


Thanks in advance,

Sub9

It’s not physically possible to isolate a single frequency as that would require an infinitely steep filter. The closest that you can do is to use a narrow “band-pass filter”.

I’m not sure if there is a ready made plug-in to do this, but it is quite simple to produce a reasonably narrow band-pass filter using the Nyquist Prompt effect.

Select the audio that you want to filter, then open “Nyquist Prompt” from the “Effect” menu.
Copy and paste the following code into the Nyquist Prompt text box:

(setq frequency 1000)

(scale 2 (lowpass8 (highpass8 s frequency) frequency))

In this case, the band-pass frequency is 1000 Hz.
For a different band-pass frequency, change the number in the first line.

Note that this code is for Audacity 1.3.12.
In earlier versions the code may only work on mono tracks.

That worked perfectly! Awesome. Thanks mate!

A comb filter can attenuate (almost remove) every other band of frequencies, in this example a notch every 50Hz is removed …


Where can I find the Comb filter menu? I’m using the Dutch version, so I have some troubles finding the correct menu’s.

Audacity isn’t shipped with a comb fiilter, you have to download a plugin … http://audacityteam.org/download/nyquistplugins

BTW IMO the only noise problem a comb filter could improve is an unwanted tone with harmonics, (a comb filter will add ringing though).
If that is the problem you may be better off using a notch filter and only notching the unwanted tones. A notch filter plug-in is available from the same link as above.

Thanks mate, downloaded the plugins.

There is a better Notch filter plug-in here: Notch Filter (wiki)
Direct link to the plug-in: notch.ny



;control freq “Frequency” real “Hz” 60 0 10000

http://forum.audacityteam.org/download/file.php?id=2565

Why make the upper limit for the notch filter frequency 10000 ?, why not 20000 ? (approximately the limit of human hearing).

[update : Now I get it … Notch Filter (wiki) - #20 by steve (I always type the frequency values in) ]

With the old notch filter (currently on the Audacity web-site) it is important that you do not enter a notch frequency greater than the Nyquist frequency as it can cause data corruption. The new notch filter protects users from this by producing an appropriate error message. Other than error checking (which is absent in the old version) the two effects are functionally identical.