Stress

In linguistics, stress is the relative emphasis that may be given to certain syllables in a word. The term is also used for similar patterns of phonetic prominence inside syllables. The word accent is sometimes also used with this sense.

The ways stress manifests itself in the speech stream are highly language dependent. In some languages, stressed syllables have a higher or lower pitch than non-stressed syllables — so-called pitch accent (or musical accent). In other languages, they may bear either higher or lower pitch than surrounding syllables (a pitch excursion), depending on the sentence type. There are also dynamic accent (loudness), qualitative accent (full vowels), and quantitative accent (length, known in music theory as agogic accent). Stress may be characterized by more than one of these characteristics. Further, stress may be realized to varying degrees on different words in a sentence; sometimes the difference between the acoustic signals of stressed and unstressed syllables may be minimal.

In English, stress is most dramatically realized on focussed or accented words. For instance, consider the dialogue

"Is it brunch tomorrow?"
"No, it's dinner tomorrow."

In it, the stress-related acoustic differences between among syllables of "tomorrow" would be small compared to the differences between the syllables of "dinner", the emphasized word. In these emphasized words, stressed syllables such as "din" in "dinner" are louder and longer.[1][2][3] They may also have a different fundamental frequency, or other properties. Unstressed syllables typically have a vowel which is closer to a neutral position (the schwa), while stressed vowels are more fully realized. In contrast, stressed and unstressed vowels in Spanish share the same quality—unlike English, the language has no reduced vowels.

(Much literature emphasizes the importance of pitch changes and pitch motions on stressed syllables, but experimental support for this idea is weak. Nevertheless, most experiments do not directly address the pitch of speech, which is a subjective perceived quantity. Experiments typically measure the speech fundamental frequency which is objectively measurable, and strongly correlated with pitch, but not quite the same thing.)

The possibilities for stress in tone languages is an area of ongoing research, but stress-like patterns have been observed in Mandarin Chinese.[4] They are realized as alternations between syllables where the tones are carefully realized with a relatively large swing in fundamental frequency, and syllables where they are realized "sloppily" with typically a small swing.

Stressed syllables are often perceived as being more forceful than non-stressed syllables. Research has shown, however, that although dynamic stress is accompanied by greater respiratory force, it does not mean a more forceful articulation in the vocal tract.