Hello, I am using the new download of audacity, and I am trying to make a negative frequency or sound wave. Can someone please give input on how to do this? This is better than protools 
Frequency measures something that happens in time, so you can’t have a negative frequency, but sound is vibrations in air, and you can make Audacity produce a sound track with the vibrations backwards.
Select a sound track and Effect > Invert
Koz