Audio Tools is a set of open source tools including an Audio Signal Generator, two Audio Level Measuring Sets and, what I was after, an Audio Oscilloscope. The depiction above shows a simple pulse wave where the pusewidth is set to 50%, why this one is also called a square wave.
The oscillators of the AL-1 analog modeling engine of the Korg Kronos features a couple of interesting parameters. One of them is named »Edge«, which adds partial tones to the sound. In the oscilloscope, those appear as spikes at the edges:
A nice tool for some late saturday night synth programming session. I do not really need it, but it’s fun to visualize the sounds anyway :) .