But they still interact with frequencies lower than 20khz right?
Think about it like this - I have a single steady saw wave at 10khz. If I started playing waves above 20khz, would I be able to detect any disturbance in that 10khz wave?
> But they still interact with frequencies lower than 20khz right?
No. Different frequencies don't generally “interact”, that's much of the point of frequency analysis.
> Think about it like this - I have a single steady saw wave at 10khz. If I started playing waves above 20khz, would I be able to detect any disturbance in that 10khz wave?
No.
If you have a green laser and put its beam under an ultraviolet light source, does the green laser change? It does not. It's the same thing.
Of course, if your amplifier is badly nonlinear-and-non-time-invariant in a very weird way, it _might_. But usually, even nonlinearities go the other way (creates overtones upwards in frequency, not undertones).
Hmm… then what causes the obvious beating you hear when you have two saw waves close in frequency but not exact.
I guess when the anti-nodes match its silence and so it’s obvious to your ear, but nothing magical is happening to the signals themselves. Interesting, thanks for answering!
In that case filtering out everything over 20khz makes total sense!