Hello - I need to slow down audio by 25% and want to verify the math, after reading the wiki article, I was hoping you someone could validate that I am doing it correctly: http://wiki.audacityteam.org/wiki/Change_Speed

If the original duration is 100 minutes and the “target” (final) length is 75 minutes, then it must be playing faster!

This is the tricky part. “Percent” does not mean anything unless you say what it is a percentage of.
If I say that I want 25% of the money in your pocket, that is (probably) very different to saying that I want 25% of the money in Bill Gates’ bank account.

If I have a recording and say that it is “slowed 25%”, that is ambiguous because I have not specified whether the “25%” is “25% of the original” or 25% of the new track.

The way that it works in Audacity (and I don’t think that this is very intuitive) is that it is 25% of the new track.

If your original file 100 minutes duration, and you slow it down by 25% in the Audacity “Change Speed” effect, (Percent Change = -25), then the equation is:

There’s that “tricky part” again. “Percent” does not mean anything unless you say what it is a percentage of.

If the original length is 100 seconds and the final length is 125 seconds,
then the final length is ( ((125 -100) / 100) * 100) = 25 percent longer than the original
which is also ( ((125 -100) / 125) * 100) = 20 percent of the final length.

The same goes for tempo and frequency.
As it speeds up, the pitch goes up and the duration becomes less.
As it slows down, the pitch goes down and it goes on longer.