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Quite a few times after recording a vocal or a sample from the outside world, especially when using a low quality microphone, you can end up with some kind of undesirable background noise. We are going to examine a way of reducing these type of noise by using a free Cross-Platform Sound Editor called Audacity. Audacity is free, open source software for recording and editing sounds. It is available for Mac OS X, Microsoft Windows, GNU/Linux, and other operating systems.
You can download the editor here. Once you've installed it load up a sample that contains some background noise. For this example, I have recorded my voice through a microphone that has captured a bit of background noise.
Listen to the sample:
Notice the silence before and after the spoken vocal. That will come in handy in a bit.
Getting the noise profile
Select an area of the waveform that has just the background noise. I selected the area of my waveform just before the spoken work begins.
Select Effect->Noise Removal from the top menu and click on the Get Noise Profile button. Audacity registers this area of the frequency spectrum that needs to be removed. Now hit CTRL(CMD) - A to select the whole file.
Removing the noise
Select Effect->Noise Removal again. Now concentrate on the Step 2 area. Hit preview to audition the changes. Adjust the slider so that you can get the best result possible with as few artifacts as possible. I managed to get a nice result by setting the slider quite low towards the Less value. When you are happy with the result, click Remove Noise, then export your file as desired.
Listen to the sample with noise removed:
Depending on the material, sometimes it's better to apply multiple noise reduction passes with lower values than one pass with more extreme values. Always try and leave a gap at the beginning or the end of the recorded file so that the algorithm can "grab" the noise profile. If not, always choose the part of the sample with maximum audible noise. It seems that there are more advanced noise reduction plugins out there with more option and parameters but as a free alternative the above setup can work quite well for most people.