Change Tempo - Destructive or Non-Destructive?

One thing I really enjoy doing in Audacity is taking several different remixes of one song, slicing them up, moving things around, cutting out parts I don’t like, sometimes superimposing different remixes (sometimes two to four remixes playing simultaneously for a minute or more), all of which requires very precise beat matching. I always match beats at the beginning of the songs and check for synchronicity all the way at the end (or as close as I can get) before I start slicing up tracks and moving things around. So getting the beats so precisely matched with the Change Tempo feature requires trial and error on my part. I’m wondering if Change Tempo is destructive to the track in any way. If I raise and lower the tempo a dozen times, will it affect the track quality? What I’ve been doing is just trying a tempo change, then undoing and trying again over and over, but if these tempo changes are not destructive, I can just continue making adjustments without undoing all the time. What’s the story here?

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(Side note: It would be nice if we could match tempos of multiple tracks by matching the first beats of two different songs, then simply use the mouse to drag the end of the track in or out until the last beat matches. That would save me about 30 minutes of time when I’ve got to match the beats of five different remixes, but I can add that to the suggestion box if there isn’t already a way to do that.)

Yes it is.
The “Change Tempo” effect uses a fast algorithm for “time stretching” which is usually good enough for small changes in tempo.
The “Sliding Time Scale / Pitch Shift” effect uses a more complex algorithm which takes longer to processes but generally gives high quality results.
The “Change Speed” effect changes both tempo and pitch, but provides the least amount of quality loss of these three effects.

Applying “Change Tempo” as multiple incremental steps is likely to be not much different to applying the effect with one large step. In some cases, incremental changes may sound a little better, in others may sound a little worse, but generally not much difference. For high quality tempo changes without affecting the pitch, Sliding Time Scale / Pitch Shift is recommended (

Changing the speed (tempo and pitch) using a Time Track ( is non-destructive.

Yes that would be nice, but is difficult to do. It’s a fairly common feature request, though for some reason I don’t see it listed on our feature request page. I’d suggest that you make a request for it in this part of the forum:
Making a feature request does not guarantee that it will be implemented any time soon, especially for requests that are hard to implement, but it allows us to track what features users want and may encourage one of the (volunteer) developers to tackle it.

Thank you for the reply. I didn’t realize until I looked at the interface for that a minute that I could simply enter the same value in each field and change the tempo without also speeding it up or slowing it down. I’ll probably use the regular tempo change for quick processing to get the proper values via trial and error, then undo and use those values in the sliding time scale. Thanks again!

Wow! Well, I have to say that I just used the sliding time scale to adjust the tempo and the difference in sound quality when playing the two tracks at once (the one I adjusted and the one I was adjusting to) the difference in sound quality is EXTREMELY noticeable. It was worth the five minute wait for the adjustment to process. Sliding time scale is so much better that it makes me want to go back and redo the “mix overlaps” I’ve already done, which is about ten songs, but a heap of work, though only for my own enjoyment of the finished product.

I’m glad I asked this question, and thanks again!