Réduction du bruit

Réduction du bruit... peut réduire des sons de fond constants comme des bourdonnements, sifflements ou des buzzs, et modérer la quantité de "grésillements". Il n'est pas adapté pour des clics et des pops individuels, ou des bruits de fond irréguliers comme du trafic ou du public.

Pour utiliser Réduction du bruit..., vous avez besoin d'un région de la forme d'onde qui contient uniquement le bruit que vous voulez retirer.

Accessible par : Effets > Réduction de bruit...

La fenêtre de Réduction de bruit... montrant les deux étapes et les contrôles associés

Étape 1 - Prendre le profil du bruit

La première étape montre à Audacity le bruit que vous voulez retirer en identifiant le niveau de bruit des différentes fréquences comprises dans le bruit.

1. Sélectionner une région de la forme d'onde qui contienne uniquement le bruit. Ceci ne doit pas être forcement très long, une seconde suffit. S'il y a des types de bruits vraiment différents à différents endroits de la piste, il vaut mieux attraper le profile du premier type, retirer ce bruit, puis attraper le profile du type de bruit suivant et de le retirer.
2. Cliquez sur Effets > Réduction de bruit...
3. Cliquez sur Prendre le profile du bruit

Étape 2 - Retirer le bruit

1. 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.
2. Click Effect > Noise Removal :
   • 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.

1. • 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.
   • Noise :
     • Remove : Select this option to remove the noise from the selection
     • Isolate : Select this option to leave just the noise - useful is you want to hear exactly what the Noise Removal effect is removing.

Astuces

After creating a noise profile, CTRL + R or Effect > Repeat Noise Removal 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 (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 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. Cette page (en anglais) on the Audacity wiki outlines the steps. Apply this effect before applying Noise Removal.

For a more technical explanation of the algorithm used in Noise Removal, see this page on our main Wiki (en anglais).