Gabor transform
Fig.1 shows the waveform of a piano music audio file with 44100 Hz sampling frequency. And Fig.2 shows the result of short-time Fourier transform (we use Gabor transform here) of the audio file. We can see from the time–frequency plot, from "t" = 0 to 0.5 second, there is a chord with three notes, and the chord changed at "t" = 0.5, and then changed again at "t" = 1. The fundamental frequency of each note in each chord is shown in the time–frequency plot.