The spectrum of the speech sound shows the frequency components of the vibration of the radiated speech wave (at a given moment) and the amplitude of each component. In the case of vowels, the formants form the final waveform of the actual speech sound from the simple spectrum of the fundamental voice. The formant is an overtone beam amplified by the resonant frequencies of the articulation channel. Formants shape the sound of speech. Each vowel has a different formant values.
The image of the continuous speech spectrum is called sound spectrogram. It shows the spectral change in time, so it is a 3-dimensional image. The horizontal axis is time, the vertical is frequency, and the degree of darkness is the third dimension. The sound spectrographs of Hungarian vowels (left) and their spectral contents (in the middle of the sound) are shown below.
The sound spectrogram of the word ‘vajaskenyér’ (bread and butter) is shown to demonstrate the continuous change of the spectral content of speech.
