STRESS VERSUS PROMINENCE
When a syllable receives stress, it becomes prominent. It would have been logically possible for every syllable to have exactly the same loudness, pitch, and so on. In fact, some early attempts at speech synthesizers sounded like this. However, human languages have ways to make some syllables more prominent than others. A syllable might be more prominent by differing from the surrounding syllables in terms of (a) loudness, (b) pitch, or (c) length.
Pitch is an auditory sensation that places sounds on a scale from high to low. Every syllable has pitch, however, any syllable that is articulated with a noticeably different pitch will be deemed to carry stress. This can go either way: if all the syllables are said in a low pitch except one, then that higher pitch syllable will be deemed to carry the stress of the word. Pitch also plays a central role in intonation. Length, too, seems to play a role in stress. Generally, if one syllable has a longer length than the others in the word then it is deemed to be the one carrying stress. Length is one of the more important determiners of stress. In addition to length and pitch, loudness can result in prominence. It seems obvious that if one syllable is articulated louder than the others then it will have achieved some prominence from the other syllables. This prominence would then make that syllable the stressed syllable. However, it is very difficult to make a sound louder without affecting the length, pitch or quality of that syllable. If you could only change the loudness of a sound then the perceptual change is not as great as you would expect.
Prominence is relative to the surrounding syllables, not absolute. A stressed syllable that is nearly whispered will be quieter than an unstressed syllable that is shouted. In English, the three ways to make a syllable more prominent are to make it: (a) louder, (b) longer, or (c) higher pitched. The last one (i.e., change of pitch) is, in fact, the most usual way of achieving prominence in English. However, English typically uses all three kinds of prominence simultaneously. Other languages might use only one or two of them. The cues can also be used differently in other languages. In Swedish, stressed syllables are usually lower in pitch (which is, in effect, one of the most noticeable features of a Swedish accent).
Even in English, stress does not always mean higher pitch. In one of the intonation contours used to convey surprised disbelief, the most strongly stressed syllable of the utterance has the lowest pitch. Take the following example:
You're taking phonetics!
One of the most interesting points about stress in English is that vowels in unstressed syllables are systematically reduced. English speakers will not try to control the position of the tongue body during the vowel of an unstressed syllable. Instead, the tongue body will reach whatever point is convenient in getting from the preceding consonant to the following consonant. The average position reached is mid-central schwa /ə/. Failing to reduce unstressed vowels is one of the major contributors to an accent in non-native speakers of English. Reducing vowels inappropriately is one of the major contributors to an English accent in other languages. In general, the differences between stressed and unstressed syllables are more extreme in English than in most languages.
