Spectrum Plot for Pure Sine Wave

Audacity Help Forum Windows
1

Hi, why is it that when I plot the spectrum of a generated pure sine wave tone (say 440 Hz), I got a spectrum from 80+ Hz to about 1600 Hz? What is the technical/theoretical explanation for this? Is this the so-called “Uncertainty Principle”?


Thanks,
Dave

2

Analyze > Plot Spectrum: Size 16384
Koz

3

Ok, that and re-reading the manual explained it.


Thanks,
Dave

4

Dave,
Koz didn’t fully answer your question and it’s one I’ve had for a while. Can you explain why a pure sine wave in Audacity shows more frequencies then the one it is theoretically oscillating at? Thank you!

5

That is rather complicated to explain fully. Basically it is as a result of the way that it is measured. As an analogy, if you filter white light through an orange filter, then you will mostly get light frequencies coming through that correspond to the colour orange, but in practice you will also get a certain amount of red and a certain amount of yellow, and even a tiny bit of other colour frequencies coming through.

For a detailed explanation, it is due to the window function in the FFT analysis (see: http://en.wikipedia.org/wiki/Window_function)