However, the red wave has a frequency that is higher than half the sample rate. We can see that the above set of sample points can fit with two different sine waves. This image helps to explain how aliasing occurs: The frequency range of the sound being generated is limited to less than half the sample rate. That's how the chirp using 'Square, no alias' manages to avoid aliasing. The other approach is to limit the frequency bandwidth of the generated signal. This approach is the only option if you can't limit the frequency range before converting from analog to digital. This works because the very high sample rate is able to handle much higher frequencies so no aliasing there, then downsampling (using anti-aliasing filters) removes the excessively high frequencies before converting. 'Oversampling' (using a much higher sample rate and then converting back down to a 'normal' sample rate) is one way. Note that I described two ways to avoid aliasing. After generating, you can safely convert the sample rate back down to 44100 Hz - the resampler has anti-aliasing filters built in so reducing the sample rate in Audacity will not create aliasing (assuming that Audacity is set to use the default best quality resampling). Notice how the swooshing has almost completely disappeared. Try generating the first type of Chirp ('Square' waveform) but set the sample rate to 192000 first. This avoids most of the aliasing because the high sample rate means there is a high Nyquist frequency (half the sample rate) so most of the generated frequencies can be represented correctly. One way to minimize the amount of aliasing when generating sounds is to generate at a very high sample rate. Now you hear the sweeping tone but without the swooshing. Now try the same again but with 'Waveform' set to 'Square, no alias'. Try as hard as you can and I doubt that you can get rid of it. That swooshing is the aliasing distortion.

For example if you generate a 10 second 'Chirp' from 400 Hz to 1000 Hz with 'Waveform' set to 'Square', then when you listen to it you will hear a swooshing sound in the background. When generating audio you need to avoid aliasing rather than trying to fix it.Īliasing distortion occurs when attempting to represent frequencies that are higher than the Nyquist frequency (half the sample rate).

Audacity aliasing effect