From Audacity Manual
Use Change Tempo to change the tempo of the selection without changing its pitch.
- Accessed by:
The input boxes are linked. Therefore changing the value in one box will change the values in other boxes as appropriate.
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.
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.
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.
Due to weaknesses in its algorithm, Change Tempo can remove a very short length of content 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. Therefore while fractional values up to three decimal places are supported for BPM changes and up to two decimal places for length changes, the resultant stretch length may have lesser accuracy.
To avoid this problem, you can useinstead, setting initial and final tempo change to the same amount.
Clicking on the command buttons give the following results:
- Manage presets. gives a dropdown menu enabling you to manage presets for the tool and to see some detail about the tool. For details see
- plays a short preview of what the audio would sound like if the effect is applied with the current settings, without making actual changes to the audio. The length of preview is determined by your setting in , the default setting is 6 seconds.
- applies the effect to the selected audio with the current effect settings.
- aborts the effect and leaves the audio unchanged.