Figure 3: The figure shows an example of broadband noise and impulsive noise in a spectrogram of an audio signal extracted from a 78 rpm shellac disc. Broadband noise is due to the granular nature of the material and its degradation over time. Time (seconds) is shown on the -axis; frequency (Hertz) is shown on the -axis. The shades of grey color show the amplitude: broadband noise is visible throughout the excerpt from the grey background (in contrast with the darker harmonic frequencies clearly visible up to 5 kHz); impulsive noises correspond to the vertical lines at seconds 18.08, 18.53, and 18.63.