Move the cursor over the image to highlight the aliased components.

Sounds are pressure density waves that travel through air (or other media). Microphones transduce these waves into fluctuating electrical waves. Originally these signals are continuous, or analog. But for a computer to work with such signals, they must be converted to a string of numbers that describe discrete positions of the wave. This digital representation can be thought of as a connect-the-dots model of the signal. To understand why aliasing occurs, it is important to realize that the dots are acquired as very quick "snap shots" of the signal, separated by the sampling interval.

Aliasing occurs when a signal fluctuates in between the snap shots; in other words, the frequency of the signal is greater than the frequency of the sampling snap shots. This leaves the computer ignorant that any additional fluctuation has occurred, and it thus interprets the signal at a lower frequency, or alias of the real signal waveform.

On a sonogram representaton of such a signal, the alias components fold down from the Nyquist frequency by the same amount that they are above it. Thus for this example with a Nyquist frequency of 150 kHz, a 160 kHz input signal would be represented as a 140 kHz signal; a 200 kHz signal would be represented as a 100 kHz signal and so on.