Spectrum diagrams are useful for seeing the state of a complex wave during a very short period of time. But in speech, sounds are constantly changing. Spectrograms are a convenient way to diagram the changes in a sound's spectrum over time.

In a spectrogram, the horizontal dimension represents time and the vertical dimension represents frequency. Each thin vertical slice of the spectrogram shows the spectrum during a short period of time, using darkness to stand for amplitude. Darker areas show those frequencies where the simple component waves have high amplitude.

narrow-band spectrogram of [haj]

spectrum at the first slice of time spectrum at the second slice of time

For situations where we are more interested in the frequency response curve of the vocal tract than in raw spectra, we can use "wide band" spectrograms. In these, the dark areas are smeared over a wider area. This often hides the individual harmonics, but it makes formants easier to see -- they show up as dark bands.

wide-band spectrogram of [haj]

Spectrograms let us look at, for example, the changing formants in a diphthong.

Here is a spectrogram of an utterance of the sentence We were away a year ago:

Spectrogram of .we were away...

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