Audacity – Changing the pitch/tempo without affecting the otherPosted: February 10, 2012
As our next major project is an audio project, I thought I’d introduce a capability Audacity has that I haven’t seen any other major audio editing software I’ve used offers (admittedly I’ve only tried freeware, but one commercial program I know of, Dexster, doesn’t have it).
I’m sure most people who have had an interest in audio manipulation would have tried at some point or other to reproduce the chipmunk effect made popular by Alvin and the Chipmunks. When I first tried doing that, I realized that the pitch/frequency was highly dependent on the tempo of the sound clip. Which makes sense, as higher pitch requires compressing the data into a shorter amount of time to increase frequency and vice versa.
However, the excellent Audacity filters are capable of changing one property without affecting the other. Open an audio file with Audacity, and you should see a window like the following appear.
Your audio file will be presented as a very nice blue/purple wave. To navigate your file, you can scroll left or right if it is long. You can hold Ctrl + scroll up or down to zoom in and out. In a way, it is similar to Photoshop but it represents sound in a graphical way.
Highlight the portion of your file that you want to modify. Go to Effects > Change pitch… or Change tempo… to change each of those properties respectively. Change speed is the filter that affects both pitch and tempo at the same time.
You will see a dialog like the following pop up.
It will offer you many options to change the pitch. First, you can choose the exact key to change to from the one that the filter has estimated from your clip. I haven’t tried to test the accuracy of the estimate but I still think it’s cool that it can estimate the key from a sound sample.
You can also choose the number of semitones or half-steps to raise or lower, by using a positive or negative number respectively. This is probably the easiest to control without messing up the key if it matters, as semitones are the smallest unit in music.
For the highly technical/physics-inclined people, you can also change the exact frequency in Hertz. I have no idea how such fine control would be used practically, but then I’ve mostly dealt only with music so far, so I didn’t need the feature.
The change tempo dialog looks like the following.
The change tempo filter is much simpler, and allows you to simply change it by a certain percentage. If you know the original BPM of the song, you can specify that and your target BPM instead. The last option for changing tempo is the length of time you want to change that particular portion to.
So those are the two tools available in Audacity that I think are among the more powerful and useful features it provides. There are many other filters in Audacity that can do a lot more, and some of them are quite advanced. It takes a little more knowledge of their function and method in order to use them effectively, but as is the case with most multimedia applications, feel free to experiment all you like. Have fun making chipmunks!