http://auditoryneuroscience.com/topics/speech-modulated-signal
take the spectrogram of an audio recording of speech, (being sure to plot frequency on a linear, not logarithmic scale) and take the 2d fourier transform of that, considered as an image. Because you plotted frequency on a linear scale, harmonics of a fundamental frequency show up as oscillations, in the spectrogram, in the y-direction! Low-frequency oscillations in time in the original signal still show up as oscillations in the spectrogram in the x-direction.
