Click Removal

How does it work?
The tool looks 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.

The Click Removal algorithm consists of two parts:

The first part, click detection, works by measuring the amplitude of the sound in each small piece of the selected sound, and comparing it to the average amplitude in surrounding pieces. Short pieces of sound that are much louder than the region before or after might be spikes. (The threshold slider bar sets the square of the required ratio between the amplitudes inside and outside the loud section). Pieces of extra-loud sound that are shorter than the max spike width (measured in milliseconds) are marked for removal. This technique works well for vinyl record pops and clicks because it catches oscillations (not just single spikes). Electronic spikes typically drive the speaker in just one direction, but LP pops are harder to find because they often contain oscillation as the stylus and cartridge settle back down after the initial hit of the dust mote, groove gouge, or whatever.

The second part click removal, replaces the marked segment of sound. This part of the algorithm is pretty simple-minded -- it tries to make the audio samples move smoothly across from the position before the pop to the position after the pop. This may have the effect of creating a very short, hopefully not-very-noticeable silence in the track where before there was a loud click.

Click Removal steps
See Click Removal in the Manual.

Silencing and Draw Tool
Sometimes an even better result can be obtained by zooming in to near sample level and either silencing the click, or using 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 10 milliseconds long can actually be simply silenced or deleted without leaving an audible gap in the sound, although many clicks spread wider that that.

If the click is not suitable for silencing or deletion, enable Draw Tool by clicking the pencil icon center top of the Audacity screen, or press on your keyboard. The mouse pointer will change to a pencil while over the audio track. You must zoom in until you can see the individual sample dots before you can use Draw Tool.

Click in the track at the point you wish a sample to be redrawn to, and wait for the samples to be rejoined together. Alternatively, click in the area of track where the line of samples is not smooth and hold down on your keyboard. The pointer will now change to a brush. Each mouse click will then progressively smooth out the samples in the area. See Draw Tool in the current Manual for more help.

Use the Spectrogram view to identify clicks more easily
Click removal using the Spectrogram view is a workflow Tutorial giving steps to remove hard-to-spot clicks using Audacity's Spectrogram view. In the default Waveform view, loud clicks often show up as easily seen spikes, but smaller, lower amplitude clicks can be very hard to find without zooming in to near sample level then scrolling the waveform to identify the exact location of the clicks. Considerable time can be saved by first using Spectrogram view which identifies clicky regions more readily.

Other techniques
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.

Repair of very short sections
Audacity has a Repair effect for very short sections of badly damaged audio up to 128 samples in length.

Spectrum analysis
Analyse the area with 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 Notch Filter plug-in.

To install new plug-ins, unzip them into the Plug-Ins folder inside the Audacity installation folder. On Windows computers, this is usually under. On Mac OS X, it is usually under. The plug-ins will be available after restarting Audacity.

Subtraction
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 that has the click, then click in the track name by the downward-pointing arrow to open the Track Drop-Down Menu (right). Choose "Split Stereo Track". Then in the Audacity menus at the top, choose . This moves the selected area into a new track underneath your original track.


 * Now on the new track, use the same Track Drop-Down Menu, make both channels mono and then select one of the channels by clicking where it says "Mono". In the menus at the top, choose.

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.

Hard limiting
Some users find the Hard Limiter an additional way of removing or attenuating 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 Mac and Windows versions of current Audacity, underneath the divider in the Effect Menu. For Linux or for legacy versions of Audacity, a Hard Limiter is available in the SWH plug-ins suite as follows:


 * Windows: LADSPA 0.4.15
 * Mac: LADSPA 0.4.15
 * Linux: http://plugin.org.uk/download.php.

The Hard Limiter controls are are described in the Manual.

To try and remove the clicks, set the "dB limit" in Hard Limiter to the level of the bottom of the click spike, where it protrudes above the level of the music. If some of the click remains, set the dB limit a little lower to see if it is worth losing a little of the real audio signal to attenuate the click further.

Repair of broader clicks
Click Removal may not work well with broader clicks of 10 milliseconds or longer. Here are two Nyquist plug-ins you can try. Installation instructions for Nyquist plug-ins are here.


 * PopMute heavily attenuates loud clicks and pops (and even hand claps or small bangs) to make them less obtrusive.
 * Really loud and wide glitches may still sound too bad even if moderated with "Pop Mute". In that case, try EZ-Patch. This lets you repair the damage by selecting the glitch together with some undamaged audio on one side of the glitch, then smoothly replaces the glitch with the undamaged part of the selection.

Other software
It's also possible to use other software than Audacity for click removal, while still using Audacity for the other editing tasks and to produce the final master. Export your audio track from Audacity as a lossless WAV or AIFF file, and also save the click-removed audio as a WAV or AIFF for import back into Audacity.