Envelope Tool

In Audacity, every track has an "amplitude envelope" which is controlled with the Envelope Tool on the Tools Toolbar. An amplitude envelope just means that you can control a track's volume changes smoothly over time. People in the recording industry sometimes call this technique volume automation, because in a recording studio you would typically change the volume of tracks by moving volume sliders up and down, and fancy mixing boards had the ability to remember your movements and automate them from then on. Manipulating a track's amplitude envelope in Audacity is similar, except that Envelope Tool is used to create and manipulate "control points" at various points in the track. The control points then determine its volume changes over time.

Envelope Tool

When you select Envelope Tool from the Tools Toolbar, your track, which normally looks like this:

now has a thick blue border at the top and bottom of the waveform, like this:

Here's an example of an amplitude envelope applied to the track. The volume is made to diminish slowly between 9 and 11 seconds, and then to grow again much faster during the next half second (note the much steeper slope of the blue line). The volume is directly proportional to the height of the waveform - the smaller you make the waveform, the quieter it will sound:

An amplitude envelope is manipulated by a number of control points. Each control point is visible by its four handles (the small circles in the image above), by which you can drag the point up or down to control the volume level.

Note that the lines connecting the control points are not straight (unless viewed in Waveform (dB) view). Control points define the end points of an exponential curve. Thus it is not generally possible to use the Envelope tool to fade an audio selection to or from zero amplitude. To achieve this, use one of the fade effects.

Adding and editing Control Points

Just click the mouse button anywhere in a track to create a new control point.

The entire track follows your new control point until you add another one. Click in a different spot to add a new control point.

The audio will always change smoothly between each control point, so you only need to add as many as are necessary. You can click in either the top or bottom half of a track to create a new point. If you have a stereo track, the same envelope will apply to both channels.

If you want to place a new control point very near an existing one, Audacity might get confused and try to move the existing one rather than create a new one. It's sometimes easier to click farther away from the existing one and then drag it closer.

Removing control points

To remove a control point, click on any one of its control points and drag it outside the track. Remember, you can also use Edit > Undo to undo your envelope changes in sequence.

Extra amplification

If you are amplifying a quiet passage, you can amplify outside the original volume envelope of the track by grabbing either of the inner control points. When you do this, Audacity can't display the full shape of the envelope, and changes the border to a dotted line in the affected area:

However you can make the extended amplitude envelope visible by zooming out vertically.

Track gain control

Totally separate from the amplitude envelope, each track has a gain slider in its Track Control Panel (the slider is just underneath the Mute and Solo buttons). This adjusts the volume of the whole track, like Effect > Amplify... can do, but without modifying the waveform itself. Therefore you don't have to use Envelope Tool to adjust the overall volume of a track - use it to specify changes in volume through the course of the track, then you can make fine adjustments to the overall volume with the gain slider.

Rendering envelope changes

A lot of volume changes can make a track more cumbersome to work with, so sometimes when you're sure you've got them right, you may want to commit them all and get rid of the control points. To do this, select the track you're finished with and click Tracks > Mix and Render. This option is sometimes called Bounce in other software. The resulting track will sound the same but will no longer have its amplitude envelope control points. Naturally you can always add further control points later on, or get all the original control points back by undoing the Mix and Render if you're not happy with it.