# Speed change error

Audacity has a program error in the way it does speed change. Say I want to change from 100 sec to 110s. That’s a 10% change. -10%. But Aud increases it by 11 sec. It figures 10% of 110s is 10%.
Basic arithmetic logic. Or maybe you are using a different valid logic.
All versions including 2.1.1.

The maths behind the Audacity speed change are correct.
The time expands inverse proportionally:
100 s * 100 % / 90 % = 100 s * 1/0.9 = 100 * 1.1111… = 111.111 s.

This is why the change speed effect has additional fields, it facilitates those calculations because you can choose between factor, target duration, Rpm and per cent.

Sounds like confusion between change BY% versus change TO%.
If I want to change 100 by 50% that gives 150.
But you change it TO 50% which gives it 200s.
Does everyone feel Audacity is clear enuf on this for users?

I’m thinking that when we say we want to decrease speed by 20%, that is not the same as saying to make it 80%.

When we give you a negative percent you must ADD that to 100 to get your factor.

We were aware that some users found the “Percent change” control too difficult, which is why I added the “Speed multiplier”.
Do you find the “Speed multiplier” easy and clear enough?

The speed change percent is inverse because increasing length reduces speed and vice versa.

The speed change percent is what Audacity acts on, whichever measure you change in the effect.

Gale