From Audacity Manual
To use Noise Removal, you need a region in the waveform that contains only the noise you want to remove.
Step 1 - Get Noise Profile
- Select a region of the waveform which contains only noise. This doesn't need to be very long, a second is enough. 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, removing the noise for it, then grabbing the profile of the next type of noise and removing that.
- Click Get Noise Profile
Step 2 - Remove the Noise
- Select the region of waveform which you want to remove noise from, then set the Noise Removal 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.
- 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 distortion of the audio that remains.
- Sensitivity (dB): Controls how much of the audio will be considered as noise, by applying a gain to the noise thresholds obtained in Step 1. Moving the slider rightwards from zero will treat more audio as noise, which will then be reduced by the level set on the Noise Reduction slider. With light noise or noise that is very different to the audio, you may not need to adjust Sensitivity. For heavier noise or noise that is blended into the audio, move the Sensitivity slider rightwards from zero, and experiment by setting both this and the Noise Reduction slider to the lowest levels conducive with effective noise reduction. If you still get a little distortion, try adjusting the following two sliders.
- Frequency Smoothing (Hz): The larger this value the more that the effect considers different frequencies as "the same". If your noise is a single frequency such as mains hum or a high-pitched whistle, then keep this value small. If your noise is more like a hiss, then a larger value will generally be better.
- Attack/decay time (secs): How quickly noise removal reacts. Use a larger value if the background noise is pretty constant. Use a smaller value if it varies rapidly.
- Remove: Select this option to remove the noise from the selection
- Isolate: Select this option to leave just the noise - useful if you want to hear exactly what the Noise Removal effect is removing.
After creating a noise profile, CTRL + R or will apply Noise Removal.
Removing noise usually results in some distortion. This is normal and there's virtually 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:
- Do any click removal, compression or other effects after doing noise removal, not before. It works best as close to the source of the noise as possible. One exception to this rule is applying the optional Notch Filter... to remove hum or whistle before tackling the hiss.
- Duplicate your source track before you apply noise removal, and adjust the relative volume of the two tracks to get the best sound quality.
- 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 Removal.
- Steve 22Oct13: Fwiw, I agree that personals like "I" and "my" are inappropriate for the manual, but I don't agree that the comment is being (negatively) critical of Audacity.
- Peter 23Oct13: I basically agree with the de-personalization and have implemented that re-organizing this note considerably in the process. I removed the image as I felt that added little.
This note came about from a side discussion (re tinkly artifcats in NR) on a current thread of Paul L's on the forum, it led to Steve suggesting some alternative settings for me (I know Steve and I know his work so basically I trust his judgment. Accordingly I was prepared to invest some time testing Steve's versus default settings. On the three use cases which are my typical workflow, Steve's settings gave superior results:
- removal of FM hiss
- removal of web-stream carrier noise
- hum removal - a small amount of hum has crept into my TT/arm on one channel which I can't seem to fix
- Since these three are all very common use cases I would prefer this note to stand as-is.
- Gale 23Oct13: I have depersonalised your note further so that it looks less like a criticism of Audacity. It can stand from that point of view as far as I am concerned. I tend to use Goldwave Noise Removal as IMO it is far easier to get a good result with minimal artifacts.
Are the Audacity defaults wrong? Should they be e.g. Noise Reduction of -18 and Sensitivity of +5? It's normal for me to tell users to turn Sensitivity up to give them a better result. I assume they have heavy noise on poor equipment. Also higher up the page we suggest Frequency Smoothing be kept down for hum, so that contradicts what you say about the higher smoothing solving your hum. Are you sure that smoothing of 500 is better for hum than the default?
Also I recall some Forum discussion of the best way to deal with noise that is in only one one channel of a stereo recording. What is our advice here? It may be worth giving that advice as one of the tips above.
- Steve 23Oct13: As seen on the forum, many users have very noisy recordings and will need to make the effect more aggressive to have an appreciable effect. On the other hand, many users have pretty good recordings and just need a touch of noise reduction. Overall I think the defaults are a good compromise - perhaps a bit too aggressive for some, but not quite enough for others. Importantly the default settings are likely to be in the right ball park for most users.
For heavier noise, the default settings above are recommended, increasing the "Sensitivity" slider if necessary.
For light noise (for example FM hiss or web-stream carrier noise) the default settings in Noise Removal may be a little too aggressive, possibly introducing tinkly artifacts.
- Noise Reduction (dB): Try setting this to about -12. It will still have an appreciable effect on low level noise, but will reduce the risk of tinkly artifacts.
- Frequency Smoothing (Hz): Try increasing this a little, typically to around 500 Hz.
- Sensitivity control (dB): Reducing this can further help to make the effect less aggressive, but may require a lot of experimentation for only slightly better results. For a quick result, this slider is usually best left at the default 0.0 dB.
For critical work it is best to experiment for optimum settings; see this page in the Wiki for further advice on using the settings for this effect.
- Steve 22Oct13: Fwiw, I agree that personals like "I" and "my" are inappropriate for the manual.
- Peter 23Oct13: Can we use Steve's fuller anecdotal style note as a note on the Wiki page?
- Gale 08Mar14: Certainly the above div or something like it should go on the Wiki page. However the only personals there are about the Notch Filter plug-in which is now shipped with Audacity. I don't see how personals help. It is agreed this is a good tip for better recordings, so why assign it only to an unknown specific person, which would seem to give it less weight? I would not want the image on the Wiki page unless you are also going to show the default settings.
A skilled user of Audacity offers this advice on the use of Noise Removal:
The default settings in Noise Removal are a bit too aggressive for my liking, but many users require the settings to be quite aggressive so as to have some impact on bad recordings. Setting the "Noise Reduction (dB)" to about -12 will still have an appreciable effect on low level noise, but will produce much lower tinkly artefacts than the default settings. I generally like to increase the "Frequency Smoothing" a bit too - typically around 500 Hz, but for critical work it's best to experiment for optimum settings.
The "Sensitivity" control is interesting, but a bit tricky to use effectively. Usually I leave this at the default 0.0 dB. Some sounds "mask" noise better than others. Sounds that have a significant amount of "breathyness" will cover up (mask) low level hiss much better than say low piano notes.
When the noise is not masked, it will be audible "through" the retained sound. This can be countered by increasing the Sensitivity (dB) control, but note that increasing the sensitivity makes the effect more aggressive so it is also necessary to reduce the "Noise reduction (dB)" setting and possibly also increase the "Frequency Smoothing" and reduce the Attack/decay to zero. For this type of situation I may increase the sensitivity up to about +8 dB and reduce the reduction to around 8 dB.
Reducing the sensitivity control can help to make the effect less aggressive, though personally I find that reducing the "Noise reduction (dB)" control does this better.