I have recordings of several sentences in foreign language. I want to slow down the speaking to hear it more clearly so I adjusted the tempo. This works but I get a echo/ripple. How can I remove that?
By the way, I adjusted the pitch as well and that works but then it is either way too low sounding or way too high.
Try ACID Xpress, it's basically designed around time-stretching and has probably the most refined time-stretching algorithms in any consumer DAW.
The ripple is an effect of the time stretching algorithm. There are several different algorithms that are good at different kinds of sound. One for a drum loop would be very bad for an audio book. You will just have to experiment with different software and different algorithms and different settings.
The one I used the most is Ableton Live, which have three different algorithms, one which might work well. But that would be a very costly way of stretching just one recording. :-)
You can't. The best you can do is try and find time compression/expansion software that has better algorithms/math behind them than other programs.
The only alternative I can think of is to chop the audio into segments and time expand each one with different values based on how sensitive each one is to the effect.Then cross fade them.This will sound inconsistent but it might be good enough for whatever application you are doing this for.
You might be able to mitigate the effect using something similar to Logic Pro's "Flex time" feature. .
Hope that helps
sox input.out output.wav tempo -s 0.7<--- that '-s' flag is very important