A complex signal such as the voice is actually made up of the sum of sound waves of many different frequencies. As described in the web page on sound production of the larynx , the vibration of the vocal folds produces a series of harmonics. In addition to the sounds produced by the larynx, we can also produce sounds in other parts of the vocal tract. These sounds are usually made my forcing air to flow through narrow openings. The turbulence created with this air flow produces the desired sound.
Vowels are typically in the low frequency, and consonants are in the high frequencies.
In a spectrogram, time is plotted along the horizontal axis, and frequency is plotted along the vertical axis. The intensity of the sound at each particular time represented by the amount of shading in the graph. There also are harmonics, which show up as the parallel horizontal bands. Beneath the spectrogram is a plot of the actual speech signal which is used to calculate the spectrogram. Below is another example of a spectrogram of a short sentence.