The sequence of syllables in the word is not pronounced identically. The syllable or syllables which are uttered with more prominence than the other syllables of the word are said to be stressed or accented. Stress in the isolated word is termed word stress; stress in connected speech is termed sentence stress.
Stress is defined differently by different authors. B.A. Bogoroditsky, for instance, defined stress as an increase of energy, accompanied by an increase of expiratory and articulatory activity. D. Jones defined stress as the degree of force, which is accompanied by a strong force of exhalation and gives an impression of loudness. H. Sweet also stated that stress, is connected with the force of breath. According to A.C. Gimson, the effect of prominence is achieved by any or all of four factors: force, tone, length and vowel colour.
If we compare stressed and unstressed syllables in the words contract ['kσntrækt], to contract [kən'trækt], we may note that in the stressed syllable:
(a) the force is greater, which is connected with more energetic articulation;
(b) the pitch of voice is higher, which is connected with stronger tenseness of the vocal cords and the walls of the resonance chamber;
(c) the quantity of the vowel [æ] in [kən'trækt] is greater, the vowel becomes longer;
(d) the quality of the vowel [æ] in the stressed syllable is different from the quality of this vowel in the unstressed position, in which it is more narrow than ['æ].
On the auditory level a stressed syllable is the part of the word which has a special prominence. It is produced by a greater loudness and length, modifications in the pitch and quality. The physical correlates are: intensity, duration, frequency and the formant structure. All these features can be analyzed on the acoustic level. Word stress can be defined as the singling out of one or more syllables in a word, which is accompanied by the change of the force of utterance, pitch of the voice, qualitative and quantitative characteristics of the sound, which is usually a vowel. In different languages one of the factors constituting word stress is usually more significant than the others. According to the most important feature different types, of word stress are distinguished in different languages.
1) If special prominence in a stressed syllable or syllables is achieved mainly through the intensity of articulation, such type of stress is called dynamic, or force stress.
2) If special prominence in a stressed syllable is achieved mainly through the change of pitch, or musical tone, such accent is called musical, or tonic. It is characteristic of the Japanese, Korean and other oriental languages.
3) If special prominence in a stressed syllable is achieved through the changes in the quantity of the vowels, which are longer in the stressed syllables than in the unstressed ones, such type of stress is called quantitative.
4) Qualitative type of stress is achieved through the changes in the quality of the vowel under stress.
English word stress is traditionally defined as dynamic, but in fact, the special prominence of the stressed syllables is manifested in the English language not only through the increase of intensity, but also through the changes in the vowel quantity, consonant and vowel quality and pitch of the voice.
Russian word stress is not only dynamic but mostly quantitative and qualitative. The length of Russian vowels always depends on the position in a word.
Now we should like to distinguish the notions of word stress and sentence stress. They are first of all different in their sphere of application as they are applied to different language units: word stress is naturally applied to a word, as a linguistic unit, sentence stress is applied to a phrase. Secondly, the distinction of the rhythmic structure of a word and a phrase is clearly observed in the cases when the word stress in notional words is omitted in a phrase, e.g. I 'don't think he is 'right or when the rhythmic structure of the isolated word does not coincide with that of a phrase, e.g. 'Fifteen. 'Room Fifteen. 'Fifteen 'pages.
So in a speech chain the phonetic structure of a word obtains additional characteristics connected with rhythm, melody, and tempo. Though the sentence stress falls on the syllable marked by the word stress it is not realized in the stressed syllable of an isolated word but in a word within speech continuum. Since the spheres of word stress and sentence stress fall apart their functions are actually different. Sentence stress organizes a sentence into a linguistic unit, helps to form its rhythmic and intonation pattern, and performs its distinctive function on the level of a phrase.
Stress difficulties peculiar to the accentual structure of the English language are connected with the vowel special and inherent prominence. In identical positions the intensity of English vowels is different. The highest in intensity is /a:/, then go /î:, ç:, i:, u:, æ, σ, e, υ, i/.
All English vowels may occur in accented syllables, the only exception is /ə/, which is never stressed. English vowels /i, è, ə υ/ tend to occur in unstressed syllables. Syllables with the syllabic /1, m, n/ are never stressed. Unstressed diphthongs may partially lose their glide quality. In stressed syllables English stops have complete closure, fricatives have full friction, and features of fortis/lenis distinction are clearly defined.