N. V. Tatsenko introduction to theoretical phonetics of english

On the acoustic level pitch correlates with the fundamental frequency of the vibration of the vocal cords; loudness
correlates with the amplitude of vibrations; tempo is a correlate of time during which a speech unit lasts. Each syllable of the speech chain has a special pitch colouring. Pitch movements are inseparably connected with loudness. Together with the tempo of speech they form an intonation pattern which is the basic unit of intonation. An intonation pattern contains one nucleus and may contain other stressed or unstressed syllables normally preceding or following the nucleus. The boundaries of an intonation pattern may be marked by stops of phonation that are temporal pauses. Intonation patterns serve to actualize syntagms in oral speech. The syntagm is a group of words which is semantically and syntactically complete. In phonetics actualized syntagms are called intonation groups.
It is very important to make the point that we are not interested in all aspects of a speaker’s pitch; the only things that should interest us are those which carry some linguistic information. If a speaker tries to talk while riding fast on a horse, his or her pitch will make a lot of sudden rises and falls as a result of the irregular movement; this is something which is outside the speaker’s control and therefore cannot be linguistically significant. Similarly, if we take two speakers at random we will almost certainly find that one speaker typically speaks with lower pitch than the other; the difference between the two speakers is not linguistically significant because their habitual pitch level is determined by their physical structure. But an individual speaker does have control over his or her own pitch, and may choose to speak with a higher than normal pitch; this is something which is potentially of linguistic significance.
Not all stressed syllables are of equal importance. One of the syllables has the greater prominence than the others and forms the nucleus of an intonation pattern. Formally the nucleus may be described as a strongly stressed syllable which is generally the last strongly accented syllable of an intonation