Originally Posted by electricpete
Hi Ali - I definitely respect your opinion on vibration matters.
I would add to number 1 - Other examples include periodic non-sinusoidal response to a forcing frequency at 1x.
I think I have a semantic disagreement with the first sentence of 2. If you have a non-sinusoidal signal periodic at 1x, there are harmonics present, and I would call them "real", even if they are difficult or impossible for us to visually pick out in the time waveform (we can demonstrate the equivalence by adding the sinusoids together with appropriate magnitudes and phases to recreate the original waveform). I would agree the 2nd sentence in #2 - the FFT tells us what set of sinusoids is necessary to reproduce a periodic signal. Some people say the FFT "creates" the harmonics, I would prefer to say that it "reveals" the harmonics that are present in the signal.
In the end again it may be semantics.