Sharper waveform corners require more high-frequency components

Evidence

In general, as the rise time gets faster and/or the corners get sharper, more high-frequency sine waves are required to represent the waveform accurately. In other words, we can look at a square wave as a waveform that is formed by summing together sine waves that are odd multiples of its fundamental frequency and whose amplitudes are inversely proportional to frequency. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 449

Categorization

• XREF: This is the perceptual basis of timbre: sharp transients carry the high-frequency energy that distinguishes instruments and clicks.