This section will examine the differences between stressed and unstressed
syllables in terms of three acoustic parameters: fundamental frequency (Section 2.1.1), duration (Section 2.1.2), and amplitude (Section 2.1.3). The idea of viewing stress as a complex of acoustic parameters will also be presented (Section 2.1.4).
2.1.1 Fundamental Frequency
Previous research has shown that English stressed syllables differ from
unstressed ones in terms of fundamental frequency (abbreviated as “F0” hereafter) in both production and perception.
Stressed syllables in English are produced with higher F0s than the unstressed counterparts. Lieberman (1960) investigated English speakers’ production of words that contrasted in the position of stress. Comparing the stressed and unstressed syllable within a word, he found that among 90% of the words, the stressed syllable had a higher peak F0 than the unstressed syllable. Comparing the stressed and unstressed syllable across words with the same spelling but contrasting stress positions (e.g., comparing the first syllable of PERfect3 with the first syllable of
perFECT ), Lieberman found that 72% of the compared cases had the higher peak F0
in the stressed syllable than in the unstressed. On the other hand, Beckman (1986) examined English speakers’ production of words with contrasting stress positions and calculated the difference in peak F0 between the two syllables in a word. The F0 differences in trochaic words were negative values, which resulted from the greater F0 values for the first syllable than for the second. However, the F0 differences in iambic words were positive values because the first syllable had smaller F0 values than the second.
The relatively higher F0 in a stressed syllable enables listeners to identify the position of stress in a word. Fry (1958) conducted a test on English speakers’ stress perception in synthetic disyllabic English words with various level F0s in the target
syllables. A step-up F0 change by at least 5 Hz in a test word (e.g., 97 Hz in the first syllable and 102 Hz in the second syllable) was able to lead to an iambic perception.
On the other hand, a step-down F0 change by at least 5 Hz could induce a trochaic perception. Morton & Jassem (1965) examined English speakers’ stress perception of disyllabic nonsense words. It was found that the words with a higher level or
downward-sloping F0 in the first syllable gave rise to trochaic perception, whereas the words that had an upward-sloping or higher level F0 in the second syllable elicited iambic perception.
To sum up, a stressed syllable is higher in F0 than its unstressed counterpart in both production and perception.
2.1.2 Duration
The durations of stressed syllables are produced to be longer than unstressed syllables. Lieberman (1960) measured the durations of each syllable produced by the English speakers in his study. When the stressed and unstressed syllables from a word were compared, 66% of the words had a longer duration in the stressed syllable than the unstressed one. When the stressed syllable in one word was compared with the unstressed syllable in the other word that had the same spelling but a different stress position, 70% of the compared cases had a longer duration in the stressed syllable
than the unstressed. Beckman (1986) investigated the syllable durations of disyllabic words produced by English speakers and calculated the ratio of the second syllable’s duration to the first syllable’s duration. The ratios in the iambic words were found to be greater than the trochaic words, which was due to the longer durations of stressed syllables.
The comparatively longer duration of a stressed syllable serves as a cue for recognizing where the stress is placed. Fry (1958) looked into the relation between duration ratio and the ratio of noun judgment. The duration ratio was calculated by the first vowel’s duration divided by the second vowel’s duration, and the ratio of noun judgment referred to the percentage of listeners recognizing the stress in the first syllable of a disyllabic word. It was found that the higher the duration ratio was (i.e., the longer the first vowel was), the larger the percentage of noun judgment became.
Morton and Jassem (1965) examined how the variation of syllable duration correlated with the recognition of stress position. When the first syllable in a disyllabic word became longer, the listeners were more likely to perceive the first syllable as the stressed one. When the second syllable got longer, the listeners tended to spot the stress in the second syllable.
In conclusion, stressed syllables had longer durations than the unstressed counterparts, which is not only realized in production but also identifiable in
perception.
2.1.3 Amplitude
Stressed syllables are produced with higher amplitudes than the unstressed counterparts. Lieberman (1960) tracked the peak amplitudes of the syllables recorded from his English-speaking participants. When inspecting the syllables from the same word, among 87% of the words, the stressed syllables were found to have higher amplitudes than the unstressed ones. When a stressed syllable was compared with its unstressed version in another word, it was found that 90% of the compared cases had a higher amplitude in the stressed syllable. Beckman (1986) noted down the peak amplitudes of every English syllable in her study. The difference of peak amplitude between the two syllables in the disyllabic words was calculated by subtracting the first syllable’s peak amplitude from that of the second syllable. It turned out that when the stress was on the first syllable, the peak amplitude difference was negative, which suggested that the first syllable had a higher amplitude. When the second syllable was stressed, the peak amplitude difference was positive, revealing that the second
syllable had a higher amplitude.
The relatively higher amplitude in a stressed syllable is a cue for listeners to tell a stressed syllable from an unstressed syllable in perception. Fry (1958) found in his
perception test of disyllabic words that with the increase of the first-to-second-vowel amplitude ratio, the percentage of noun judgment rose. In other words, the higher the first vowel’s amplitude was, the more likely it was to perceive the first syllable as stressed. Morton and Jassem (1965) had a similar observation in their disyllabic words as well. The listeners were prone to mark the second syllable as stressed when the amplitude of the first syllable was lowered. When the amplitude of the second syllable went down, the majority of the listeners preferred to mark the stress in the first syllable.
Generally speaking, the amplitude of a stressed syllable is higher than an unstressed syllable. This generalization was seen in the measurements of syllable production, and it was also observed from the recognition of stress in perception.
2.1.4 Viewing English Lexical Stress as a Complex of Parameters
Sections 2.1.1 to 2.1.3 have demonstrated how F0, duration and amplitude each contribute to the formation of stress individually. However, the three acoustic
parameters have also been found to interact with each other, so this section is going to present how the parameters influence each other or how they collectively realize English lexical stress.
As Fry (1958) put in his introduction to stress, the acoustic parameters of stress
depend on each other and build up a complex, which was evidenced in the results of his experiments and also the ones in other studies. Earlier in the sections on duration and amplitude, it has mentioned that Fry (1958) found that the longer the first syllable was, the higher the percentage of noun judgment was; in addition, the percentage of noun judgment also increased with the rising of the first syllable’s amplitude. Besides these two findings, Fry (1958) also reported that when the first syllable became longer in duration and higher in amplitude at the same time, the increase of noun judgment was amplified. However, when duration and amplitude went in opposite directions (e.g., when a syllable’s duration was lengthened but its amplitude was lowered), the percentage of noun perception dropped. These observations indicated that duration and amplitude were able to strengthen or weaken each other.
Lieberman (1960) examined the relation between F0 and peak amplitude in production. He discovered that when the F0 of the stressed syllable was lower than the unstressed counterpart, which is not a typical F0 realization for stress, the amplitude of the stressed syllable was definitely higher than the unstressed one.
Following the same logic, when the amplitude of the stressed syllable was lower than the unstressed, which is not how stress is typically realized in amplitude, the stressed syllable absolutely had a higher F0 than the unstressed. In other words, there is a trade-off between F0 and peak amplitude in realizing English lexical stress. In
addition to peak amplitude, Beckman (1986) also calculated the total amplitude of each syllable measured in her study. The total amplitude could be seen as the contribution from two parameters, amplitude and duration, because it was the
amplitudes accumulated throughout the entire duration of a syllable. The difference of total amplitude was obtained by calculating the difference between the second
syllable’s total amplitude and the first syllable’s total amplitude. It was found that iambic words had positive total amplitude differences, whereas trochaic words had negative values. The pattern of amplitude and duration working together was similar to the pattern of them working alone.
2.1.5 Summary
At this point, it has been manifested that F0, duration and amplitude all are the parameters contributing to the formation of English stress. A stressed syllable is higher in F0, longer in duration and higher in amplitude than an unstressed syllable, and these parameters are related to each other, so stress should be seen as a complex of parameters.