The picture above is a display of the parameters used by the Haskins SineWave Synthesizer (SWS) in experiments that study the temporal aspects of speech.
The horizontal axis shows time in milliseconds; the vertical axis shows frequency in Hz. The pattern is a graph of frequency and amplitude variations of three sinusoids. Height in the plane indicates frequency; the thickness of each tracing indicates amplitude.
The properties of tonal analogs of speech vary over time. Accordingly, the tones rise and fall in frequency and amplitude in imitation of the frequency and amplitude variations of vocal resonances over the course of an utterance.