Noise Reduction

Paul 27Jan15: Screenshot of dialog is outdated. Units for frequency smoothing are now bands, not Hz. Default Reduction has changed to 12.

• Peter 28Jan15: Updated image with defaults as set for 2.1.0 in r13893 28Jan15 nightly alpha
• Peter 28Jan15: @Paul: I removed your ednote on the functionality that is "hidden" for 2.1.0 - it broke the page formatting - and we have it already safely (a full copy of the previous page) on the Talk page. And anyway it is easily retrievable from the page history stored in the Manual wiki (try clicking on the blue "history" tab above.
• Peter 28Jan15: ToDo-2 Page almost ready for 2.1.0 release, subject to final editorial reviews.

Noise Reduction can reduce constant background sounds such as hum, whistle, whine, buzz, and "hiss". It is not suitable for individual clicks and pops, or irregular background noise such as from traffic or an audience.

To use Noise Reduction, you need a region in the waveform that contains only the noise you want to remove.

Be aware that it may be impossible to get a satisfactory removal when the noise is very loud, when the noise is variable, when the music or speech is not much louder than the noise or when the noise frequencies are very similar to those of the music or speech.

If your problem is mains hum or a high-pitched whistle, the use of a Notch Filter... may help. This page on the Audacity wiki outlines the steps. Apply this effect before applying Noise Reduction.

Peter 09Aug14: ToDo James wrote by email yesterday "When the effect is fixed the tip in the manual about using a notch filter (for mains hum or whistle) first won't be needed as the noise reduction will/should behave perfectly when used as a notch filter. It's quite clear from my tests that it not doing so is the bug Paul talks about."

• Bill 13Dec2014: I disagree with James. For mains hum the frequency discrimination of the noise reduction effect is too coarse (about 22 Hz per band). Notch filters work much better on mains hum.

The Noise Reduction effect supports Spectral Selection in Step 2 - Reduce the Noise below. If a spectral selection exists noise will be treated only in the frequency range specified by the spectral selection. For example, if you wanted to leave the frequencies below 150 Hz untreated you could make a spectral selection from 150 Hz to the Nyquist frequency of the track then click Effect > Noise Reduction. When you click OK to reduce the noise the frequencies below 150 Hz will not be treated. Note that during Step 1 - Get Noise Profile below, any spectral selection is ignored. That is, the noise profile always includes the entire frequency range. Steve 09Jan15: Assuming that Spectral Selection for Noise Reduction is to be released in Audacity 2.1, I think we need more about it here. It is a major departure from how noise reduction has previously worked in Audacity (it just caught me out because I was unaware of the feature). I think there should probably be an image showing Noise Reduction being used in the Spectrogram or Spectrogram (log f) view. This section should probably go higher up the page so that it is less easy to miss. Peter 28Jan15: I agree with Steve that this should come high up the page as it is likely to catch folk out as there is no indication in the GUI dialog for Noise Reduction that a Spectral Selection is active or not - perhaps there should be. But probably not in 2.1.0

Accessed by: Effect > Noise Reduction...

Step 1 - Get Noise Profile

This first step teaches Audacity about the noise you want to remove by identifying the noise floor of the different frequencies comprising the noise.

1. Select a region of the waveform which contains only noise. A minimum of 2048 samples (0.05 sec. at 44100 Hz sample rate) is needed. Longer is better. If there are very different types of noise in different places in the track, they are best dealt with by grabbing the profile for the first type, reducing the noise for it, then grabbing the profile of the next type of noise and reducing that.
2. Click Effect > Noise Reduction
3. Click Get Noise Profile

Duplicating a very short sample of noise in order to get a longer sample for the noise profile will not help. The effect is calculating the statistics of the noise. Examining repeated samples of the same noise will not change the statistics.

Step 2 - Reduce the Noise

1. Select the region of waveform which you want to reduce the noise in, then set the Noise Reduction parameters. This is often best done by trial and error, adjusting the sliders and using the Preview button to listen to a few seconds of audio after noise reduction. Listening to the Residue can also be useful in determining how much damage is being done to the desired (non-noise) sound.
2. Click Effect > Noise Reduction:
   • Noise Reduction (dB): Controls the amount of volume reduction to be applied to the noise. Use the lowest value that reduces the noise to an acceptable level. Higher values than necessary may make the noise even quieter, but will result in damage to the audio that remains.
   • Sensitivity: Controls how much of the audio will be considered as noise. Greater sensitivity means that more noise will be removed, possibly at the expense of some damage to the desired signal. Set this to the lowest value that achieves effective noise removal without the introduction of artifacts.
   • Frequency Smoothing (Hz): This control should be left a 0 except in extraordinary circumstances. If you are still hearing artifacts in the noise-reduced audio, and you have set the Sensitivity as high as possible without damaging the desired audio, try setting this control to a value between 25 and 100.
   • Noise:
     • Reduce: Select this option to filter out the noise from the selection.
     • Residue: Select this option to hear the sound that is filtered out. This is useful for finding the optimum settings that do not damage the audio. If you can hear recognizable bits of the desired sound in the residue, it is likely that you have set Noise Reduction too high or Sensitivity too low.

Artifacts: These typically consist of random bursts of very short tones at random frequencies, sometimes called "bird song" or "tinkly-bells". This can happen if the noise floor is higher than was estimated, either because Sensitivity was set too low, or because the noise profile was not representative of the noise throughout the track. Although most of the noise will be suppressed most of the time, random distortion will remain, which can be even more objectionable that the original noise. For a more detailed explanation of artifacts and how to deal with them see this pagein the Wiki.

Tips

After creating a noise profile, CTRL + R or Effect > Repeat Noise Reduction will apply Noise Reduction.

Reducing noise usually results in some distortion. This is normal and there is nothing you can do about it. When there's only a little bit of noise, and the signal (that is, the voice or the music or whatever) is much louder than the noise, this effect works well and there's very little audible distortion. Unfortunately, when the noise is very loud, variable, or when the signal is not much louder than the noise the result may be too distorted.

If you are still having problems the following tips may help:

• Select  Residue from the Noise: options, click Preview, listening for traces of the desired signal. You can usually reduce the amount of desired sound in the residue by decreasing Sensitivity or decreasing Noise Reduction.
  • Decreasing Sensitivity increases the chance of their being musical noise in the result.
  • Decreasing Noise Reduction will increase the amount of noise in the result.
• When you have found settings of Sensitivity and Noise Reduction that appear to do the least damage to the desired sound (based on there being little of that sound in the residue), click the  Reduce radio button then click OK to apply the effect.
• Do any Notch Filtering or Click Removal before doing noise reduction
• Do any amplification, compression or other effects after doing noise reduction, not before. It works best as close to the source of the noise as possible.

Bill 13Dec2014: I disagree slightly - click removal should be done first, as the frequency content of the clicks may confuse the noise reduction algorithm. I agree that notching should be done before noise reduction.

• Steve by email 28Jan15: I agree with Bill quite strongly. After Noise Reduction, any clicks during quiet sections ("silence") tend to gain a harmonic tone which may be more conspicuous than the original click.
  • Peter 28Jan15: fair enough Steve, but does this mean you are supporting Bill's advocating of doing Click Removal before Noise Reduction? Or, even with the additional harmonics, are the clicks easily discernible and dealable with (including by software)?

When using Noise Reduction in Chains the profile is captured as below. If a Noise Profile exists, that Noise Profile will be used. It is often best to capture a suitable Noise Profile before running a Chain. If a Noise Profile does not exist: If the Chain is applied to the current project, the current selection is used to create the Noise Profile. Therefore, other effect commands in the Chain will also only apply to that selection. If an export command is added, the entire file will be exported. If the Chain is applied to files, the first file (all of it) is used to create the Noise Profile. It may be useful to prepare a file containing a suitable Noise Profile and name it so that it is alphabetically the first file of those to be run in the Chain.

Advanced Tips

• For a more technical explanation of the algorithm used in Noise Reduction, see Noise Reduction algorithm in the Wiki.
• For a more detailed explanation of artifacts and how to deal with them see Noise Reduction artifacts on the same page in the Wiki.
• Also see Sample workflow for LP digitization which lists some alternative noise reduction software that you can use as part of your audio restoration routine.

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