The sampling rate is simply the number of samples captured each second, usually measured in kilohertz (kHz), or thousands of samples per second. The audio on a compact disc, for example, is sampled at a rate of 44.1 kHz, so 44,100 samples are taken each second.
Thinking again of our motion picture of the bouncing ball, imagine two movie cameras: one that takes forty-eight photographs each second and another that takes only twelve photographs each second. The first camera, because it samples at a higher frequency, will more faithfully reproduce the continuous motion of the ball's movement than the second camera. The same is true of the sampling rates for sound. The more samples per second, the higher the fidelity of the sound.
When selecting a sampling rate for audio, something known as the Nyquist Theorem comes into play. In 1927, an AT&T physicist named Harry Nyquist determined that the sampling rate must be at least two times the highest frequency to be reproduced.
Because human hearing can perceive frequencies no higher than 20 KHz, according to the Nyquist Theorem, sampling at just over 40 KHz will capture the full spectrum of perceivable sound. Similarly, since the frequencies of human speech typically lie under 3 KHz, spoken word can be sampled effectively at a far lower rate. A sampling rate of 8 KHz is more than sufficient for recorded speech.