Certain languages like English, Arabic and Russian are called stress-timed or isochronous. In these languages stresses occur at regular intervals within connected speech and the duration of an utterance is more dependent upon the number of stresses than the number of syllables. To achieve the regular stress intervals, unstressed syllables are made shorter, and the vowels often lose their longer quality, with many tending towards the schwa / ə /, and others towards the short / ɪ / and / ʊ /.
We can hear the stress-time or isochronicity in the following examples. We start with a simple sentence; we add syllables to each column on each line, but the time it takes to say the words in the column remains the same.
| they LIVE in an | OLD | HOUSE | |
| they LIVE | in a NICE | OLD | HOUSE |
| they LIVE | in a LOVEly | OLD | HOUSE |
| they’ve been LIVing | in a deLIGHTful | OLD | HOUSE |
| they’ve been LIVing | in a deLIGHTful | OLD | COTTage |
| they’ve been LIVing | in a deLIGHTful | vicTORian | COTTage |
The occurrence of stresses in each column remains regular but the unstressed syllables are compressed in between the stressed ones. Each new unstressed syllable becomes shorter and loses some of the length of the vowel sound. If you simply tap out the rhythm it is easy to be persuaded of the validity of this idea. One can indeed say this sequence of sentences with a regular rhythm, which seems to be preserved as one adds more syllables. There is also a strong contrast between stressed and unstressed syllables.
Consider also the speed at which you are talking by the time you get to the last sentence in the group. From slowly and deliberately in the first sentence, one moves by stages to far more rapid speech in the last line.
It makes sense to imagine English described in terms of a continuum which has tendencies towards stress-timing at one end and syllable-timing at the other. A language like English has more of a tendency
than some other languages to reduce vowel length and quality in unstressed syllables, and so tends towards the stress-timing end of the continuum.
So-called syllable-timed languages also reduce the length of the vowel in an unstressed syllable, though to a lesser extent, but they tend to preserve the quality of the vowel sound.
Paying attention to the rhythm and clear patterns can be very useful for the learner. You will become aware of the importance of stress (and intonation) in English also for highlighting weak forms and other features of connected speech.