In a spectrogram, the vertical axis of the picture represents the frequency scale: the lowest frequencies are shown at the bottom. From left to right is the time axis, with the beginning of the sound analyzed shown at the left. The degree of blackness of the markings shows the amount of energy present in the signal at a particular point in time. Some spectrographic displays now show levels of energy with different colours instead, but although these look pretty and are nice to pin on your wall, most people find they are harder to interpret than the grey-scale spectrograms that have been around since the 1940ís.
 
                         

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