A spectrogram is a visualization tool extending the one-dimensional representation of a spectrum (usually vibration or noise magnitude as a function of frequency) to a second dimension which can be time or speed. In abscissa (x-axis), the frequency of the acoustic noise or vibration of the electrical machine is represented, while time or speed is represented in coordinate axis (y-axis). The colour (z-axis) represents the magnitude of the acoustic noise or vibration harmonic.
Using the rotating speed in y-axis makes all excitations proportional to speed appear as straight lines crossing the origin of the graph. When using time as y-axis, vibration and noise lines might not be straight lines if the speed is not increasing linearly.
Electromagnetic forces, electromagnetic vibrations and electromagnetically-excited noise harmonics are all proportional to speed in electrical machines, except when coming from asynchronous switching harmonics due to Pulse-Width Modulation. In synchronous machines they are more specifically all proportional to twice the fundamental electrical frequency.