Example $1\mathrm{.}3\mathrm{.}1$: In a hi$-$fi digital audio application, we wish to digitize a music piece using a sampling rate of $40kHz.$ Thus, the piece must be prefiltered to contain frequencies up to $20kHz.$ After the prefiltering operation, the resulting spectrum of frequencies is more than adequate for this application because the human ear can hear frequencies only up to $20kHz.$