Removing background noise is core to most audio cleanup operations. Audacity has several tools to aid in this.
Best Practice: While it's possible to rescue an otherwise unusable file with noise reduction techniques, your recording will sound best if you take steps to ensure a noise-free environment before recording.
The Noise Reduction effect works best to remove a constant source of noise, like the hiss of fans, the hum of fridges, or whines, whistles and buzzes.
To use it, use the following steps:
Locate a section of your recording that's just your background noise, preferably a few seconds long, and select it.
Go to Effects > Noise Reduction and press the "Get noise profile" button.
Select all the audio for which you want to reduce the noise.
The Noise Gate effect attenuates any sound quieter than a certain threshold while leaving sounds louder completely unaffected. To use it
Select a part of the audio that's just background noise.
Go to Effects > Noise Gate... to open the effect.
Click Select Function: Analyse Noise Level and press OK. Audacity will now tell you where your noise level lies and recommending a threshold.
Best practice: Use the noise gate after applying noise reduction. This way, you can use less aggressive noise reduction settings, which may grant you a cleaner end result.
The notch filter removes a hum or whistle at a specific frequency. To use it:
Select the audio you want to apply the effect on
Go to Effects > Notch Filter to open the effect.
Enter the frequency you want to reduce, together with the Q-value (how many frequencies around the main one you want it to affect - the higher the number, the less frequencies).
Press OK to apply the effect.
Once you're satisfied with the result, click OK to apply it to the selected audio. Note: If you used the Residue toggle before, make sure to switch it back to Reduce before pressing OK.
Cmd+RGo back into Effects > Noise Gate..., set it back to Select Function: Gate, and enter the threshold level from earlier.
Tweak the other parameters as sound best to you.
Press OK to apply the noise gate.
Sounds often have harmonics or overtones. They are at a multiple of the main frequency, so for a 50Hz sound, you may need to apply the notch filter as well at 100Hz, 150Hz, and so on to remove it fully.