Click Removal (under the Effect Menu) is designed to remove individual clicks on audio tracks and is especially suited to declicking recordings made from vinyl records, without damaging the rest of the audio. The tool works by looking for short, abrupt discontinuities (known as "spikes") in the waveform, typical of those produced by a click on a record. Click Removal then interpolates the samples either side of the click to reconstruct the waveform. However, it's common sense not to rely exclusively on digital sound processing to clean up noisy records. Where possible, have the records professionally cleaned, or use your own cleaning methods.
First of all, select the audio to which you want to apply Click Removal. You can move the "threshold" slider to adjust how sensitive the click detection is, and the "spike width" slider to adjust the length of spiked audio to be considered as a click. Then click the "Remove Clicks" button. Softer clicks may require you to move the threshold slider further to left. Broader clicks may require you to move the spike width slider further to right. Very soft and rapid light ticks that sound like static electricity and which are typical of vinyl (even though the pressing is often the cause rather than a static charge) won't be effectively removed with Click Removal. For this type of noise you should select an area of the vinyl that contains only the noise (no music or speech) and use the Noise Removal tool under the same Effect menu - see Noise Removal.
Silencing and Draw Tool
Sometimes an even better result can be obtained by zooming in (CTRL + 1) to near sample level and either silencing the click (Edit > Silence), or using the Draw Tool to smooth out the contours of the samples and so attenuate the click. Remember, the click will be visible as a "spike" in the waveform. Most discrete clicks up to five milliseconds long can actually be simply silenced without leaving an audible gap in the sound, although many spread wider that that. If the click is not suitable for silencing, try using the Draw Tool by clicking the pencil icon top left of the Audacity screen, or press F3 on your keyboard. The mouse cursor will change to a pencil while over the audio track. You may need to zoom in a little further to use the tool. Simply click in the track at the point you wish the sample to be redrawn to, and wait for the samples to be rejoined together, or hold down ALT on your keyboard and click as needed (the cursor changes to a brush) to progressively smooth out the contour of the samples in the area.
Silencing highly zoomed areas or redrawing samples can get tedious even over a relatively short stretch of audio. Here are a few other tricks you can try if Click Removal did not help as much as you hoped.
1) Analyse the area with Analyze > Plot Spectrum to see if any spikes are concentrated in particular frequencies and then use Equalization under the Effect menu to reduce the volume of those frequencies. You can do this more precisely with the Nyquist Notch Filter plugin: http://www.shellworld.net/~davidsky/notch.zip
To install new plugins, unzip them into the Plug-Ins folder inside the Audacity installation folder. On Windows computers, this is usually under "Program Files." On Mac OS X, it is usually under "Applications." The plugins will be available after restarting Audacity.
2) Where the clicks sound equally loud in both left and right channels of a stereo recording (this often isn't the case), but the music information in the channels is very different at that point, you can try to cancel out the click by making the affected section of track mono and inverting one of the channels. First, select the area of track with the click and and hit the downward pointing arrow in the Track Popdown Menu http://audacity.sourceforge.net/onlinehelp-1.2/menu_track.htm
then "Split Stereo Track" followed by Edit > Split which moves the selected area into a new track underneath your original track. Now on the new track, use the same Track Popdown Menu, make both tracks mono and then select one of the channels and Effect > Invert. When you export the result as a stereo track, the area you split out will be effectively mono (in the sense that the previous music signal in the left and right channels will be mixed into both channels), but the click should be sharply attenuated without harming the music too much.
3) Some users find the Hard Limiter an additional way of removing clicks. A Hard Limiter is a particularly strong dynamic compressor to reduce the difference between loud and soft in an area of audio. A Hard Limiter is included in the Audacity 1.2.6 Windows installer under the Effect Menu, or is available in the SWH plugins suite:
The Hard Limiter controls are:
dB limit: the input signal level above which the input signal will be clipped.
Wet level: controls the amount of the clipped signal fed to the output. It thus acts as a volume control applied after the limiter (just as if you applied the effect and then used the Amplify effect).
Residue level: controls the amount of the clipped signal that will be mixed back in. By default the value is set to zero and so the clipped signal is discarded. By raising this level, some of the clipped signal will be restored, making the limit softer and retaining more of the peaks and troughs of the original waveform. Alternatively if you turn the residue full up and the Wet signal full down, you get only the pieces of waveform that are above the dB limit you chose.
The Beta 1.3.2 version of Audacity also has a new and automatic "Repair" effect for sections of very short sections of badly damaged audio up to 128 samples in length.