Change Tempo

Use Change Tempo to change the tempo and thus the length (duration) of a selection, without changing its pitch.

To change tempo and pitch simultaneously, use Effect > Change Speed and Pitch.

Note carefully that when you apply an effect to a time-stretched clip the changed speed of the clip will be automatically rendered. If you apply an effect to a selection within a time-stretched clip then Audacity will split the original clip so that the selection can be rendered as part of applying the effect.

If you apply this effect with an envelope present then there is a bug whereby the envelope is removed and the audio reverts to its full amplitude state prior to the effect being applied.

Accessed by: Effect > Pitch and Tempo > Change Tempo

The input boxes are linked. Therefore changing the value in one box will change the values in other boxes as appropriate.

Percent Change

If you know the amount you want to change the tempo of the selection, enter that value in the input box. You can also drag the slider to select a value: the input box will update as you drag the slider.

Beats per minute (BPM)

If you know the BPM of the selection and the BPM you want to change it to, enter those values here. The BPM "to" value will then update the Percent Change and Length "to" values.

Length

If you know what you would like the new length of the selection to be, enter that value in the to input box. The from box shows the current length of the selection and cannot be changed. This is useful if you have a piece of narration that is just a little bit too long or too short to fit where you want it.

Use high quality stretching (slow)

If this checkbox is enabled, the same SBSMS high quality algorithm is used that is used in the Sliding Stretch effect. Change Tempo will process much more slowly if this checkbox is enabled, but will retain the complete content. For small to moderate tempo changes, the resulting quality will usually be higher, especially with percussive music (such as piano music).

Limitations

Change Tempo is a time-stretching effect, because it defies the normal expectation that reducing the speed of the audio (thus increasing its length) will reduce the pitch and vice versa. As with any time-stretching effect, some audible distortions will be expected at more extreme settings.

When using the default (fast) algorithm, Change Tempo may remove some audio from the start or end of the selection, or not stretch the end of the content to the end of the selection when slowing down. Both of these symptoms may leave a short silence at the end of the resultant selection. If the exact length of the audio is important, select the Use high quality stretching (slow) option.

The default (fast) algorithm may sometimes sound echoey, especially when slowing down percussive music. For small to moderate tempo changes, use the "high quality" option to avoid this problem.

The "high quality" algorithm is only suitable for small to moderate tempo changes and the sound quality will deteriorate very badly for extreme changes. For extreme slowing down, consider using the Paulstretch effect.

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