what does the phenomenon of 'folding back' in frequency spectrum means?

Answer 1

"Frequency foldback" is a technique used in some power converters to reduce the output current as the load impedance falls toward a fault condition. Basically the converter's switching frequency is reduced as the output is sensed to have fallen below a user-programmed minimum value. See the ref.
EDIT: I think mikeburns's answer is the one you are looking for. Google "Nyquist frequency" and "aliasing" for a full explanation. One interesting aspect of folding is that it not only folds the spectrum, such that the apparent frequency = sampling frequency - actual frequency, but it also apparently "folds" time backwards, similarly to the "wheel-turning-backwards" effect seen in old newsreels. Successive samples of the 10 Hz wave, sampled at 15 Hz in mb's example, appear to be taken 1/3 cycle earlier (but actually are 2/3 cycle later as the frequency ratio indicates). This effect is more convincing when the sample rate is only slightly higher than the actual frequency, as often happened in the newsreels (e.g., with a 16/sec framing rate and wheel spokes passing at 15/sec, the wheel seems to run smoothly backwards at 1 spoke/sec).

Answer 2

A frequency spectrum shows the amplitude of the countless frequencies that are modern as a function of frequency. No sound is a organic sine wave so there are continuously some harmonics modern. The amplitude of the harmonics is under that of the classic.

Answer 3

If you sample a voltage at less than 2 times the frequency, then it will appear to have a lower frequency than it really does.

Example: if you have a sine wave at 10Hz and you sample it at 15Hz, it will appear to be at 5Hz (i.e. 15-10). You need to sample slightly faster than 20Hz (i.e. double 10) to accurately measure the 10Hz sine wave.

So in this case, the 10Hz has "folded back" and appeard to be 5Hz. Similarly, 11Hz would appear to be 4Hz, and so on.