I'm trying to create a quick video in After Effects on audible frequencies.
I'm using this audio clip (20 seconds). The clip starts from 20khz and goes down to 20hz.
As you can see (or hear), we can't hear anything in the beginning at 20khz. Most of us start hearing the frequency at 16khz-15khz.
But when I import this audio clip into AE, it sounds completely different. The sound starts playing from the beginning and it's very loud and sounds nothing like the clip I downloaded.
Here's how the audio clip sounds after export: https://www.mboxdrive.com/soundonly.mp3
What's going on here and how do I fix it?
Two things here.
First off, the sound is exhibiting "aliasing" artifacts. This happens when there it pitch content to the sound that is higher in frequency than the Nyquist frequency for the given sample rate. So, either you are using a low sample rate, or the pitch you are generating has harmonics that are causing the aliasing.
Check to make sure you are using a sine wave (if done correctly there should be no additional harmonic content to the tone besides the fundamental pitch), and that your sample rate is above 40K in order to play a 20K sound without aliasing. The most common sampling rate these days is 44100 fps, but you may be using 8000 fps, which is also still employed and would not work for your application.
Second point, you probably want to change the rate at which you travel through the range of pitches. It sounds like you are going along linearly, but the ear hears things exponentially. The difference in pitch from 100 to 200, for example, is the same (in terms of our perception) as the difference from 1000 to 2000. So you might want to make your rate of descent reflect this if the goal is to spend equal time at every perceived pitch level.