Filters and Cleaning up Audio
Audacity comes pre-packaged with a few third party filters to help you manipulate your audio track. One of these tools is a noise remover filter that can remove sounds like hisses and hums from your audio track.
The noise remover filter cannot remove distinct sounds, like a voice or interruption — rather it is meant to reduce constant sounds that affect large portions of your track.
To use the noise removal filter, you must first sample a portion of your audio track that has only the noise you wish to reduce.
Step 1) Highlight a portion of your track that only has the noise. This is important, or else the noise removal will sample your entire track.
Step 2) Under the Effect menu select Utility --> Noise Removal --> Noise Removal... (location may vary depending on the version of Audacity you are using)
Step 3) The noise removal dialogue will describe the two-step process. In this first step, click Get Noise Profile, which will store a sample of the noise in the system.
After you click Get Noise Profile, the filter will analyze the highlighted portion of your track and learn what the "unwanted" noise sounds like.
Step 4) You should highlight the portions of your track that you want to remove the noise from. Most of the time, this will be your entire track. You can select your entire track by clicking Ctrl-A (pc) or Command-A (mac).
Step 5) Re-open the noise removal tool by clicking Effect --> Utility --> Noise Removal --> Noise Removal...
Play with the sliders at the bottom portion of the dialogue. Remember, despite its name, the noise removal tool only reduces the unwanted noise. Adjust the top slider, Noise Reduction, to specify the amount of reduction you want. The middle slider, Frequency smoothing, will attempt to recover the audio you want at the risk of adding a metalic or hollow sound. The attack/decay setting will also help recover some of the desired audio the noise reduction might remove.
The process is not perfect. Play with the sliders and click the Preview button to sample a few seconds of your audio to see how it is being affected. Once you are happy with the results, click OK to process the track.