After Effects Expressions & Audio Tutorial

1. Create a new project and make a new composition. Next, import some audio, it doesn’t matter what it is anything with sound will work fine. Create a circle by double clicking the Ellipse Tool. When you double click, it automatically will make your circle the size of your composition. (This also applies when creating masks.) After you have your circle make the scale 50%.

2. Bring your audio into the composition then right click and go to Keyframe Assistant > Convert Audio to Keyframes.

3. A null called Audio Amplitude will automatically be created with your data. Under the effects you’ll see Left, Right, and Both channels. You can delete Left and Right since we won’t be using them for this tutorial. With scale selected on your shape layer go to Animation > Add Expression.

4. Use the pick whip and choose “Slider”.

5. Do a RAM Preview by pressing the 0 key on the numpad and you’ll notice the animation is rather unpleasant to watch since it’s so quick. Next we’ll change the expression so we can tweak the settings.

Paste this over your expression:
temp = linear(thisComp.layer(“Audio Amplitude”).effect(“Both Channels”)(“Slider”), 0, 40, 50, 100);
[temp, temp]

6. You’ll need to find the max value for your Slider data. Turn on the graph editor and then include the Slider in the graph editor. Move the playhead to where the audio peaks and then look at the corresponding value of the slider, it will probably be around 40-50.

In the expression there was 4 numbers: 0, 40, 50, 100.
To break this down in human terms, when the Slider value is 0 the scale of my circle will be 50%. When the slider value is 40 (my max), the circle will be 100%. Replace “40″ with your max value.

7. We’ll add a outer glow as a finishing touch.

8. Pick whip the Slider after making the Size of your outer glow an expression. RAM Preview your movie again to see the results.

The end result is nothing too interesting, but you can apply the same techniques to particle systems or more advanced effects to make nice graphics driven by your audio.

Also you may want to have a look at Trapcode Soundkeys which will allow you to do what we just did, but with a lot more control. You can target certain frequencies and more. You can see a demo here.