A waveform that appears (on an oscilliscope or in a waveform editor) as a series of rectangles, alternately above and below the horizontal centerline. The relative widths of the above and below ceterline portions influence the harmonic content of the wave, particularly in regard to how much even-harmonic content the wave contains. The relative widths are commonly expressed in terms of "duty cycle", which is stated as the percentage of the time that the waveform is above the centerline. For instance, a 75% duty cycle means that the value of the waveform is above the centerline 3/4 of the time, and below the centerline 1/4 of the time. A 10% duty cycle has a very narrow portion above the centerline and is below the centerline the rest of the time. The 50% duty cycle, where the upper and lower portions are equal width, is usually called a square wave. As the duty cycle moves further from 50% (in either direction), the portion of even-harmonic content increases.
The audio timbre of a pulse wave depends on its duty cycle. The square wave has a nasally sound; as the duty cycle varies farther from 50%, the sound becomes thinner with less bass content. Very high or low duty cycles produce narrow pulses which, at low frequencies, may be heard as individual thumps, clicks, or pops rather than a continuous sound.