Hands on: Improve noisy recordings

In this column we’re going to investigate some of the ways in which you can use noise-removal tools to clean up your recordings. Our examples will be based on the noise-removal tools available in V1.3.5 of Audacity, which can be downloaded free.

However, the methods we’re looking at can be easily transferred to other packages, such as Adobe’s Audition and Sony’s Sound Forge.

Before using the noise-reduction tools we first need to make sure our audio file is in its best possible state. There are two processes you should always perform on audio files before starting any processing. The first is to remove any DC offset and the second is to Normalise the file.

If a file is suffering from DC offset, the mean amplitude of its waveform will not be set to zero. As a result, when you go to normalise the file it will not increase to its maximum amplitude. Normalising increases the amplitude of the whole audio file to its maximum level without introducing any distortion.

Your file may initially sound worse, because normalisation increases the whole amplitude of the file, including the background noise. However, using normalisation at the end of the noise-reduction process may pull out background noise you had previously missed.

Load your file in Audacity and then click on the Effect menu and select Nomalize. Make sure both the ‘Remove Any DC Offset’ and ‘Normalize Maximum Amplitude’ boxes are ticked and then click OK.

The golden rule of noise reduction is to try to apply as little processing as possible to the signal, because the more aggressive you are with the noise-reduction controls, the more artefacts from the noise-reduction process itself will come through in the final results.

Using Audacity
Audacity’s Noise Removal tool can be confusing for beginners, as it doesn’t come with any instructions. The main hurdle is knowing how to tell the tool to lift a noise profile for your audio file.

First you need to ignore the noise-reduction tool completely. Instead, scan through your file looking for a place where only the background noise can be heard. Then you need to use the selection tool to highlight a section of it that’s less than half a second in length.