The rate at which an analog to digital converter measures the instantaneous amplitude of a source signal. Measured in samples per second, the higher the sample rate, the higher the frequency content of the corresponding digital data. The default rate for CD production is 44.1kHz, or put another way, the source material is sampled at 44,100 times a second. Since the highest frequency that can be captured is one half the sampling rate, a CD will reproduce audio up to about 21kHz, the actual perceived range of human hearing. Arguments rage as to whether signals outside the range of human hearing have any bearing on the sounds humans do hear, and if they do, then filtering out everything above the Nyquist threshold, damages the accurate reproduction of the source.