Spectrograms are time-frequency portraits of signals. They are a plot of the intensity of the frequency content of the signal as time progresses. For example, the spectrogram below is for an audio recording of Dr. Walker saying the word "greasy." On the left is a plot of the pressure levels for the microphone as the recording was made (click on the link to play it), while the spectrogram is shown on the right [horizontal units are seconds and vertical units are Hz (cycles per sec)]. In Math 313, we will examine this spectrogram - how to interpret it and how to modify it to create modifications of the original audio signal (see Removing noise from audio for an example of this type of modification).
Audio recording Play Spectrogram