A problem called aliasing occurs when a signal to be
sampled contains energy at frequencies above the sampling Nyquist
frequency. The next figure illustrates how aliasing would occur when
the sampling rate is much too low for the frequency of an input
signal. The solid curve represents the analog signal at a
comparatively high frequency. Circles show where samples were taken at
a relatively low sampling rate. The dotted line illustrates the
apparent frequency of the sampled waveform, completing about two
cycles in the period that the original signal completed 20 cycles.
Obviously, aliasing has the effect of producing sounds of lower frequency from sounds that are higher in frequency than the Nyquist frequency. Once aliasing has occurred, it is absolutely impossible to distinguish a component generated by aliasing from one that was actually present in the input signal. This effect is one of the most common sources of distortion in digitized waveforms. Fortunately, most modern computer hardware for digitizing sound has builtin filters which are tuned to remove sound energy at frequencies beyond the Nyquist frequency for whatever sampling rate is being used.