A periodic complex waveform is the sum of sine waves that comprise a harmonic series whose fundamental frequency (first harmonic) has the same period as the complex wave. Fourier analysis can be used to determine the amplitudes and phases of the individual sine waves. The pitch that the brain assigns to the complex wave is that of the fundamental frequency, even if the amplitude of the fundamental is zero! This phenomenon is known as virtual pitch since the brain senses a frequency that is not present in the original wave.
This applet requires Java 1.1 and a sound card. The highest frequency that can be produced without aliasing problems is ~3500Hz. This demonstration was created by Dan Boye.