Theory of phonetics

1) The placement of the pitch change marks the seat of the stress. It should be noted here that the very type of pitch change, its direction, doesn’t influence the word stress. For example,
\ Export . / Export ? –– ˙
The pitch direction is changed but the stress remains unchanged.
2) Certain distinctions of the vowel length and colour are reduced or lacking in unstressed syllables. The vowel of the stressed syllable is perceived as never reduced or obscure and longer than the same vowel in the unstressed syllables. Thus, the word “stress” or “accent” is also defined as qualitative where the vowel colour or quality is a means of stress and quantitative with relatively increased length of the stressed vowel. Compare the quality (colour) and quantity (length) of the same vowel in the words:
'Madam ['mxdqm], 'car-park ['kRpQ·k] or
úли , огó.
On the acoustic level the counterpart of force is the intensity of the vibrations of the vocal cords of the speaker which is perceived by the listener as loudness. Thus the greater energy with which the speaker articulates the stressed syllable in the word is associated by the listener with greater loudness. The acoustic counterpart of voice pitch and length are frequency and duration respectively.
3) In English the quantitative component of word stress is not of primary importance because of the non-reduced vowels in the unstressed syllables which sometimes occur
in English words. For example,
'Architect ['RkItekt], 'transport ['trxnspLt], partake ['pRteik], can'teen [kxn'tJn]. The sounds are not reduced; they seem to be stressed, but they are not marked. If a vowel is not reduced and not marked as a stressed vowel it doesn’t mean it’s unstressed. The degree of stress can be of different nature:

1. 
   the main stress / the primary stress (red [red]);
   
2. 
   a strong secondary stress ("consti'tution ["kPnstI'tjHSn];
   
3. 
   a weak secondary stress / tertiary stress ("edu'cation ["edju'keISn]);
   
4. 
   a zero stress / absence of stress. ('never ['nevq]).
   

The criteria of the difference of a strong secondary stress and a weak one are rather vague.
The stressed syllables are often said to be the most prominent syllables in the word. It shouldn’t be mixed with the notion “stressed”. It is common knowledge that sounds of speech have different degrees of sonority. Vowels are more sonorous than consonants. Open vowels are more sonorous than closed ones. Sorority is the inner quality of vowels which is not directly connected with the accentual structure of words
but with other articulatory characteristics, it contributes to the effect of prominence.
Another characteristic of a vowel is historical (traditional) length of vowels. Naturally the historical length of vowels is the vowel inner quality which shouldn’t be mixed with the quantitative characteristics of word stress. A traditionally long sound is present in the stressed syllable and a traditionally short sound is present in the unstressed syllable. For example,