The short version
Audacity's Noise Reduction effect works best on steady background sound such as hum, hiss, or fan noise. First select a short section that contains only the unwanted noise and capture a Noise Profile. Then select the audio to clean, reopen Noise Reduction, preview a modest setting, and apply it only when the wanted voice or music still sounds natural.
Prepare a representative noise sample
Open the recording and find a quiet gap where the background noise is present but the speaker or instrument is not. The sample should represent the noise throughout the section you plan to clean. A sample from a different room, device, or recording moment may describe a different noise floor and produce a poor result.
Audacity's manual specifies a minimum sample length for profile capture; a longer clean sample is generally more useful than the bare minimum. Do not repeat or duplicate a tiny sample to make it longer. Choose a real interval from the original recording.
Capture the Noise Profile
- Drag across the waveform to select only the noise-only interval.
- Open Effect > Noise Removal and Repair > Noise Reduction. Menu grouping can vary slightly by Audacity version.
- Choose Get Noise Profile.
- Wait for the dialog to close; the profile is captured for this session.
The profile is not retained after closing Audacity, so capture it again in a later session.
Apply reduction to the wanted audio
- Select the section that needs cleanup. To process the whole recording, click in the track and select all audio in the project.
- Open Noise Reduction again.
- Start with conservative settings and use Preview to compare a few seconds.
- Listen for a watery, metallic, or thin sound in the voice or music.
- If available, preview Residue to hear what the effect would remove. If recognizable speech or musical notes appear in the residue, the settings are probably removing wanted sound.
- Reduce the strength or sensitivity until the desired signal is mostly preserved, then apply.
Noise Reduction trades background noise against damage to the recording. Raising the reduction amount may suppress more hiss, but it can also remove consonants, room tone, or high-frequency detail. Make small adjustments and compare with the untouched original.
When Noise Reduction is the wrong tool
The effect is designed for relatively constant noise. Individual clicks and pops need a different repair approach. Irregular traffic, crowd noise, or a loud background close to the level of the voice may not separate cleanly; trying to eliminate it completely can make the recording more distracting than the original.
If the sound is a steady electrical hum or narrow whistle, Audacity's manual suggests considering a Notch Filter before Noise Reduction. Save a copy of the project or duplicate the track before major cleanup so you can return to the unprocessed audio.
Check the export
Listen to the cleaned section at normal volume and compare it against the source. Check quiet words and pauses, where artifacts are easiest to notice. Only export after the result sounds clear without obvious loss of voice or music. The goal is a more usable recording, not silence between every word.