Using Audacity Voice recorder To Remove AC Line Mains Hum

The Notch Filter reduces narrow frequency noise like AC Line Hum without changing anything else, within the limits of width set by Q. Great for low hum on voiceovers.

Notch Filter (notch.ny) Download  Copy to Programs-Audacity- Nyquist Plugin Folder.
Activate using Audacity Effect tab-add/remove plugins
 
Frequency
Enter any frequency into the frequency text box up to half the sample rate of the track. Filter frequencies above 10000 Hz may be entered by typing the value but are only valid up to half of the sample rate of the audio being processed

Q factor
This determines the width of the notch cut from your audio (default value is 1). Values above 1 create a narrower notch, and values below 1 create a wider notch. The Q Factor must be at least 0.1).

Don't worry about Manage Presets and Debug.
Preview plays a short preview of what the audio would sound like if the effect is applied with the current settings, from the beginning of the selected area without making actual changes to the audio.
OK applies the effect to the selected audio with the current effect settings.
Cancel aborts the effect and leaves the audio unchanged.

Example
Choose 60 Hz as FREQUENCY which is correct for North America. The frequency will be 60 or 50 HZ no matter where you are. Probably. That is your starting point,
 
Recorded hum often contains noisy harmonic frequencies above the fundamental frequency of the hum. A good approach is thus to apply Notch Filter to the fundamental hum frequency (for example, 50 Hz or 60 Hz), then use Analyze > Plot Spectrum... to identify other broad noise spikes (for example, at 120 Hz and 240 Hz). You may want to increase the spectrum Size to 2894 and use the "Log" Axis to see more clearly. Run Notch Filter again at each of the other noise frequencies
 
It is recommended to increase the Q factor when removing higher harmonics so as to lessen possible damage to audio. Generally, a Q of between 2 and 10 works well for mains hum removal. Choosing appropriate Frequency and Q factor settings from the noise spikes in Plot Spectrum is an approximation. Always check and go back to Edit-Undo Notch Filter if necessary.
 
Depending on the audio, you might find in this case that the spectrum has been notched too deeply, thus affecting the content, for example the quality of the voice on a voiceover, and that a higher Q factor would have been preferred. You might want to notch the visible spikes at about 100 and 350 Hz as well. As always with Audacity, you can click on  Edit - Undo and experiment with different notch filter settings to remove AC Line Hum.

 
 
 

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