Noise Removal

-- Ed 25 November 2009 on this page we refer to "audio clip" and "waveform" to mean the same thing -- we should choose one and stick with it.

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

Accessed by: Effect > Noise Removal...

The Noise Removal function in Audacity 1.3.x and higher is greatly improved over that included with Audacity 1.2.x but with improvement comes a small amount of added complexity for the user. In order to use Noise Removal, you need an audio clip with two things: audio you want to clean up and a section in that audio clip that contains only the noise.

Step 1 - Get Noise Profile

This first step identifies the type of sound you want to reduce.

• Select a region of the waveform which only contains noise. This doesn't need to be very long, a second is enough. If necessary you can make it up from several places in the project (using copy and paste).
• Click Effect > Noise Reduction
• Click Get Noise Profile.

The Noise Removal effect will analyze the noise, so it knows what you want to remove.

Step 2 - Remove the Noise

It is easiest to set "Noise Reduction" by trial and error, adjusting the sliders and using the Preview button to listen to a few seconds of the audio after having that level of noise removed.

• Select the range of audio you want to remove the noise from.
• Click Effect > Noise Reduction

Noise Reduction: The amount of noise reduction to apply. Small values usually work better. If you set this too large, the audio you want to keep is usually significantly affected too. It's generally better to reduce the background noise a bit than to try to remove it completely.

Frequency Smoothing: 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: How quickly noise removal reacts. Use a larger value if the background noise is pretty constant. Use a smaller value if it varies rapidly.

The technically minded might like to read, on our main Wiki, a description of the noise removal algorithm used.

Tips

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 (i.e. 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 is often 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.
• Make the selection you use to teach audacity about the noise a little quieter before you do "Get Profile", using the amplify effect.
• 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 optional Notch Filter may help. Apply this effect before applying Noise Removal.
  • Download notch.zip from this page. Unzip the archive, drag notchfilter.ny into Audacity's plug-ins folder, and restart Audacity.
  • Select a short portion of the audio where the hum/whistle is clearly audible.
  • Click on Analyze > Plot Spectrum. The hum/whistle should be visible as a spike in the spectrum. For a higher-resolution spectrum, choose Size: 16384 and Axis: Log Frequency. (See the Analyze Menu page for more details about Plot Spectrum).
  • Hover your mouse over the spike. Look for where the text says "Peak" followed by a frequency (in Hz): this is the exact frequency of the hum/whistle.
  • Close the Plot Spectrum window.
  • Select a short range of audio that includes some of the music or speech you want to preserve, as well as a quiet portion where the hum/whistle is audible.
  • Click on Effect > Notch Filter. Set the Notch Frequency to the value you found using Plot Spectrum. Set Notch q to 10. Click "OK" to apply the effect.
  • Listen to the result. If the result is acceptable, click on Edit > Undo, select the entire track, click on Effect > Notch Filter and apply the effect to the entire track.
    • If the effect is unacceptable (degrades the music/speech too much), click on Edit > Undo, then Effect > Notch Filter and set Notch q to 20. Click "OK" to apply the effect and listen again.

-- Ed 25 November 2009 would it be better to suggest setting the q value "a little higher" rather than giving a specific value of "20" and then at the end of this sentence add "if necessary, continue raising the q value and listening until a satisfactory result is obtained"?

ToDo Alistair: Expand a little ???
Gale: I've actually cut it back further, for now, without (I think) losing anything. We want to use the minimum words possible in the Reference consistent with getting a meaning across. We do however want to avoid assuming too much prior user knowledge, even here (a fault with the 1.2 Manual). So is there anything else missing in step 1?

Can we give any guidance on "good settings" for light or heavy hiss, vinyl roar, tape hiss and so on? Should we also mention the optional Notch Filter for hum?

Also to do is to incorporate anything useful from the following old text about Noise Removal. Note we don't want to duplicate less important information from the equivalent Wiki Tutorial

MERGE ANYTHING USEFUL FROM HERE INTO THIS PAGE

The Noise Removal function in Audacity 1.3.x and higher is greatly improved over the Noise Removal included with Audacity 1.2.x. But with improvement comes a small amount of added complexity for the end user.
In order to use Noise Removal, you need an audio clip with two things: Audio you want to clean up. ;A section in that audio clip of "silence" that only contains noise.

Using Noise removal

The procedure for using Noise Removal is as follows:1 Highlight a section of "silence" and click Effects -> Noise Removal. In this dialog box, click "Get Noise Profile." This will teach Audacity what sort of noise it should be looking for when it attempts to remove it. 2Now, highlight the selection that you want to remove the noise from and click Effects > Noise Removal. 3 At this point, we need to find the settings that will work for your signal. Most users can leave the "Frequency Smoothing" and "Attack/decay Time" settings at their default value. It is easiest to set "Noise Reduction" by trial and error, adjusting the slider and using the Preview button to listen to a few seconds of the audio after having that level of noise removed.4 Once all the settings are in place, click OK and Noise Removal will run.

This effect is ideal for removing constant background noise such as fans, tape noise, or hums. It will not work very well for removing talking or music in the background. Removing noise is a two-step process. In the first step you teach the software about the noise, using a portion of your sound which contains all noise and no signal, where the recording is silent except for the noise. In the second you use that information to remove the noise from the rest of the recording.

First off, you need to find a section of recording that contains only noise, and not any of your wanted sound, or people talking and the like. This doesn't need to be very long, a second is enough, and in necessary you can make it up from several places in the project (using copy and paste). Now choose Noise Removal... from the Effect menu and click "Get Profile". Audacity learns from this selection what the noise sounds like, so it knows what to filter out later. Next, select all of the audio where you want the noise removed from and choose Noise Removal... again. In th elower half of the dialog adjust the slider to control how agressive the noise removal is. Most of the time you want a low setting unless the noise is really bad.
Finaly, click the "Remove Noise" button to start processing. This may take a little while because it's quite a complex effect to apply. If too much or not enough noise was removed, you can Undo (from the Edit menu) and try Noise Removal... again with a different noise removal level. You don't have to get a new noise profile again if you think the first one was fine.

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 (i.e. the voice or the music or whatever) is much louder than the noise, this effect works well and there's very little audible distortion. But when the noise is very loud, when the noise is variable, or when the signal is not much louder than the noise, then the result is often too distorted.

If you are still having problems then the following tips may help: Do any click removal, compression or other effects after doing noise removal, not first. It works best as close to the source of the noise as possible. Make the selection you use to teach audacity about the noise a little quieter before you do "Get Profile", using the amplify effect. Duplicate your source track before you apply noise removal, and adjust the relative volume of the two tracks to get the best sound quality.