Acoustic Properties of Vocalic Nuclei Associated with Prosodic Stress in

Spontaneous American English Discourse

By Leah Hitchcock

Introduction and Background

As speech recognition and speech synthesis technology continue to improve, it

becomes increasingly important in the field of linguistics to understand all the details of

speech such as the cues in the sound wave that signal accent in stress-accent languages

like English. Until very recently, it was generally accepted that f0 variation, or pitch

change, was the main factor involved in English stress. However, recent research has

shown that in fact f0 variation is not the only indicator of prosodic stress-accent, or even

the primary cue (Silipo and Greenberg, 1999 & 2000, Beckman 1986). Even as early as

1955 there was evidence that duration and amplitude are cues for perceived stress (Fry,

1955). Stress-accent languages, like English, differ from pitch-accent languages like

Japanese in that they use other features in addition to pitch change to denote phrasal

accent. (Beckman, 1986) While there is much evidence that f0 change is an important

cue of stress, research on automatic stress labeling systems shows that automatic stress

labeling algorithms are most accurate when using a combination of duration and

amplitude rather than pitch change to label stress. (Silipo and Greenberg, 2000 & 1999,

Van Kuijk & Boves, 1999)

This suggests that while stressed syllables do tend to have pitch variation, it may

not be the most important cue of stress in English, and that the most important cue is

actually duration, or a combination of duration and amplitude. (Silipo and Greenberg,

1999 and 2000, Beckman, 1986, Van Kuijk and Boves, 1999). Further, it has been

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suggested that some of the pitch differences exhibited in stressed syllables are an artifact

of duration: longer segments have more time for variation of features like f0 (Silipo and

Greenberg 1999). This is not to say that f0 variation isn’t an important cue of stress-

accent; it is. However, it is clear that vowel duration and amplitude also play a role in

determining which vowels are perceived as stressed, and automatic speech recognition

systems could benefit greatly from an understanding of exactly how different factors

affect the realization of stress-accent in spontaneous speech.

Many studies have looked into the phenomenon of reduction of unstressed vowels

(i.e. Lindblom, 1963, van Bergem, 1993, Koopmans-van Beinum, 1987, Fourakis, 1991,

Engstrand, 1988). Typically, vowels that become reduced in speech are short and

unstressed. Vowel reduction is not necessarily a trend toward centralization of the vowel

(see Lindblom, 1963), as other factors, such as consonantal context also seem to play a

role in the shape of the formant trajectories of unstressed vowels. An increased

knowledge of the acoustics of stressed and unstressed vowels could be very useful in the

study of vowel reduction in spontaneous speech, which in turn should aid the fields of

speech recognition and speech synthesis.

This paper explores the roles of duration and amplitude in determining whether

specific vowels are perceived as stressed in spontaneous American English dialogues.

Based on previous research, we would expect duration to play the clearest role in

determining perceived stress-accent, and that amplitude would play a secondary role for

most vowels (Silipo and Greenberg, 1999 and 2000, Van Kuijk and Boves, 1999,

Beckman, 1986, Fry, 1955). However, most previous research has been done on lab-

speech, so a major purpose of this study is to determine general patterns related to stress-

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accent in spontaneous speech, and to determine to what extent these patterns support the

results of past research on lab speech.

The concept of stress is difficult to define. Many different definitions exist (see

Asher, 1994 and Crystal, 1992), so it is important to clarify what type of stress this paper

deals with. Lexical, or word stress, is the type most commonly referred to. Lexical stress

is canonical, and is what is marked in pronunciation guides in dictionaries. Phrasal stress

refers to which syllables are accented within an utterance. “While this might often be

related to the grammatical structure of the utterance, on cannot predict when the

grammatical structure will be reflected phonetically. The speaker may decide to

recognize a syntactic unit or to overlook it,” (Asher, 1994:4357). Semantics, syntax, and

other factors affect which syllables actually end up as being accented in an utterance (see

Vanderslice and Ladefoged, 1972). This paper deals with phrasal stress-accent rather

than lexical stress. In other words, rather than the dictionary’s markings of lexical stress,

the stress patterns that are realized phonetically in dialogues are what are being studied.

This encompasses many different types of stress; sometimes a syllable that is expected,

according to canonical lexical stress, to be accented is in fact accented in an utterance,

and sometimes a syllable is accented for other reasons, such as emphasis. Because there

are not definitive rules that determine the conditions under which a person will stress a

certain syllable, the data are based on perceived stress.

Methods and Corpus Data

The data for this paper are from the Switchboard corpus. The Switchboard corpus

contains over 140 hours worth of short telephone conversations on various topics. For

this paper, approximately 54 minutes worth of utterances were transcribed at the phonetic

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segment level by University of California, Berkeley, linguistics students. Level of stress-

accent was also manually labeled by an independent set of transcribers for syllabic nuclei.

Of the 54 minutes of speech, 45.43 minutes was analyzable and the remainder being

filled pauses, stutters, and other non-speech. (The usable 45.43 minutes consisting of

9,922 words, 13,446 syllables, and 33,370 phones, comprising 674 utterances). The

average length of an utterance was 4.76 seconds, the average number of words per

utterance was 18.5 and the average number of syllables was 23.25. (Only utterances

between 2 and 17 seconds were used, and about 60% of these were between 4 and 8

seconds. The number of words ranged from 2 to 64, and the number of syllables from 5

to 81.) The corpus includes 581 speakers, of whom 288 were female and 293 were male.

Most speakers were represented by only a single utterance in the data set; a few had

multiple utterances that were used. Speakers were asked for the region they lived in

during their formative years in order to ascertain what dialect of American English they

speak. The number of speakers for each dialect region are in table 1.

# of speakers by dialect regionNew England 37New York 41North 88North Mid 89South 65South Mid 147West 72Mixed 42

Table 1. Total number of speakers from each of the seven dialect regions, as well as mixed dialect. Mixed speakers are people who moved around a lot during their formative years.

Of the actual speech data, 769 syllables from the utterances were not used because

they had syllabic consonants (such as “el”) for their nucleus rather than a vocalic nucleus.

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631 filled pauses (“um,” “uh,” etc.) were also excluded from analysis because of the

drastically different patterns exhibited in filled pauses compared to all other words in the

data set.

The data were phonetically transcribed by three individuals using a variant of

Arapet (which was originally used to label the TIMIT corpus) (Greenberg 1997). The

interlabeler agreement was approximately 74%. When transcribers disagreed on the

identity of a vocalic segment, it was generally only a slight disagreement, i.e. one level of

frontness or height. It was rare for transcribers to disagree on whether a segment was a

diphthong or a monophthong.

Two different individuals marked the corpus material for stress-accent. I was one

of the transcribers, but I was hired to do the transcription work long before the idea for

this paper came about, so the fact that half of the labeling was done by me should not

create a bias in the data. The material was marked for three levels of accent: fully

accented, completely unaccented, and an intermediate level. The intermediate level

includes all syllables judged to be not primary stress, but not completely lacking stress

either. Fully unaccented vowels were not necessarily reduced, but most of the

occurrences of [ix] and [ax], which are both types of schwa, fell into the fully unaccented

category (see Table 2). Many of these were probably reduced forms of other vowels;

since the corpus was transcribed phonetically rather than canonically the proportion of

reduced vowels is not known. The nuclei were labeled based on perceptually based stress

accent, not based on knowledge of canonical (dictionary based) lexcial stress. The

transcribers and a supervisor met weekly to insure that the proper criteria were being used

for labeling. Other research has relied on modifications of canonical lexical stress

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patterns when studying prosody, (i.e. Van Kuijk & Boves, 1999, Beckman, 1986) but

since spontaneous speech differs greatly from canonical speech, perceived stress-accent

should be more a more accurate representation of the way people speak. When speaking,

people place more emphasis on words or ideas that are most important to the message

they are trying to express. Labeling perceived stress-accent therefore enables someone

reading a transcription to see what the speaker intended to stress in his or her speech,

something that lexical stress does not convey.

All material used was labeled by both transcribers, and the stress-accent markings

were averaged. As with the phonetic transcription, the transcribers generally agreed.

Interlabeler agreement was 85% for unstressed nuclei, 78% for fully stressed nuclei, and

95% for any level of accent (both transcribers attributed some amount of stress to the

nucleus). When the transcribers disagreed, it was usually by only one step (i.e. fully

stressed vs. intermediate, not fully stressed vs. fully unstressed). Generally when the

transcribers disagreed, a third observer attested to the ambiguity of the level of accent of

the syllable in question. Table 2 includes the averaged duration and amplitude data for

each level of stress accent, with 0 referring to completely unaccented nuclei, 0.5 to the

intermediate level, and 1 referring to fully accented nuclei. Levels 0.25 and 0.75 are the

result of the averaging of the two transcriptions.

The duration of the segments was computed from the hand-labeled material. About a third of the

material was hand-segmented by transcribers and the remainder was automatically segmented using 72

minutes of hand segmented material to train the automatic labeler (Greenberg, Chang, and Hollenback,

2000). The amplitude, expressed in terms of log base e, of each segment’s pressure waveform was

computed and normalized relative to the mean of the entire utterance (Greenberg, Chang and Hollenback,

2000). Integrated energy has been shown to be the most accurate means of determining the level of stress-

accent for a vocalic segment in past research (Silipo and Greenberg, 1999 and 2000) as it reflects both

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duration and amplitude. For the purposes of this paper, an approximation of integrated energy was

calculated for each vocalic nucleus by multiplying the duration in milliseconds by the normalized

amplitude.

Duration (ms) Amplitude (normalized log) Duration x Amplitude % of total occurrences total

Stress 0 0.25 0.5 0.75 1 all 0 0.25 0.5 0.75 1 all 0 0.25 0.5 0.75 1 all 0 0.25 0.5 0.75 1

[iy] 78 98 114 122 132 100 0.96 0.97 0.99 0.99 1.02 0.98 75 95 111 120 134 97 44.8 14.3 13.4 9.3 18.2 1270[ey] 90 94 122 130 155 129 0.99 1.01 1.03 1.03 1.05 1.03 90 94 126 132 162 132 16.4 9.1 17.3 18.1 39 525[ay] 108 113 126 143 174 141 1 1.02 1.03 1.05 1.08 1.04 108 115 129 149 186 147 16.6 12.8 19.7 14.7 36.2 790[aw] 103 121 150 156 203 168 1.04 1.02 1.05 1.05 1.06 1.05 105 122 157 162 213 175 8 9.6 15.5 23 43.9 187[oy] 98 111 168 154 0.97 1.04 1.06 1.04 94 114 177 161 16.7 4.2 79.2 24[ow] 102 117 126 150 170 136 0.98 1 1.02 1.04 1.07 1.03 100 116 129 155 182 140 22.6 15 17.6 13.8 31 646[uw] 70 101 104 153 152 103 0.95 0.96 0.97 0.98 1.03 0.98 68 98 99 151 156 101 49.4 7.3 10.9 8.6 23.8 478

[ih] 65 78 86 89 95 75 0.96 1 1.01 1.02 1.06 0.99 62 78 86 91 101 74 56.7 13 9.9 7.4 12.9 2126[ix] 49 53 51 50 0.92 0.97 1.01 0.92 45 52 52 46 89.1 7.4 2.3 433[eh] 67 82 79 97 96 82 0.97 1.02 1.03 1.05 1.08 1.02 66 83 81 101 104 85 37 10.8 11.7 12 28.6 1217[ah] 77 89 96 102 115 93 0.98 1.02 1.03 1.05 1.08 1.03 75 90 98 107 124 95 35.6 14.4 15.6 12 22.5 1060[ax] 54 78 76 62 70 56 0.94 1 1.03 1.04 1.09 0.95 51 77 77 65 75 53 89.3 6.7 2.4 0.8 0.8 1729[uh] 61 74 71 70 78 67 0.97 1.02 1.05 1.05 1.09 1.01 59 75 75 73 85 68 54 11.3 11.3 8.8 14.6 328

[ae] 91 113 123 144 165 137 0.98 1.02 1.03 1.04 1.07 1.04 88 113 126 148 175 142 16.3 11.2 15.8 15.3 41.4 823[aa] 86 94 110 116 134 114 1 1.03 1.05 1.07 1.09 1.06 86 96 115 123 144 121 17 12.5 14.5 14.8 41.3 690[ao] 100 79 87 107 143 115 1 1 1.03 1.04 1.08 1.05 102 80 91 112 154 122 13.4 6.8 17.7 21.1 41 351

Table 2. The relationship of stress accent to several acoustic properties of vocalic nuclei. The vowels are grouped into the categories of diphthongs, lax monophthongs, and tense monophthongs, because the vowels within each of these groups tend to behave similarly. The first group of data is the average duration of the vowels, in milliseconds, for each stress level and the intrinsic duration of the vowel. The second group of data is the normalized amplitude for each stress level as well as the intrinsic amplitude. The amplitude for each vocalic segment was normalized (log base e) with respect to the entire utterance within which it occurred. The third group is the simple product of duration times amplitude. The duration and amplitude of each vowel were multiplied together, and then the average of the products was calculated. The final group of data is the percentage of the time each vowel appears at each level of stress accent, along with a column denoting the total number of occurrences of each vowel. Blank cells in the table indicate less than four occurrences of the vowel for that stress level.

Results

The data reveal several patterns about stress-accent in English. First, duration and

amplitude clearly play a large role in determining whether a vowel is perceived as

stressed. Second, high vowels, which are intrinsically shorter and quieter than low

vowels, (see also Lehiste and Peterson, 1959 and 1960, Beckman, 1986, Black 1949) also

tend not to be perceived as stressed nearly as often as their lower counterparts (Hitchcock

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& Greenberg, 2001). Third, the shortest vowels, i.e. the lax monophthongs, tend to

exhibit greater amplitude differences than the intrinsically longer tense monophthongs

and diphthongs. These tendencies are very consistent, with a few minor exceptions that

appear to be the result of an insufficient amount of data (see Table 2).

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Figure 1. (reprinted from Hitchcock and Greenberg, 2001)The graphs above show the relationship between the position of the tongue and

factors associated with prosodic stress-accent. The Y-axis shows the factor (duration, amplitude, or duration x amplitude) being measured for each graph. The Y-axes were inverted in order to show the relationship of vowel height to the factor in question. The graphs in the first column show normalized amplitude, log base e, which was calculated with respect to the average amplitude of the entire utterance in which the vowel occurs. The graphs in the second column show the duration of the vowels in milliseconds. The graphs in the third column show the product of duration times amplitude. The X-axis of all of the graphs shows the vowels, either diphthongs or monophthongs, arranged approximately by the horizontal tongue position of the vowel. The resulting shape of the graphs is strikingly similar to that of a vowel space chart, suggesting that there is a very close relationship between place of articulation and factors such as vowel duration and amplitude.

Figure 2. Reprinted from Hitchcock and Greenberg, 2001.The proportion of fully accented and fully unaccented monophthongs and diphthongs. Along the X-axis are the vowels arranged approximately by horizontal position of the tongue. The Y-axis shows the proportion in terms of percentage of the total number of occurrences of the vowel. The Y-axis is inverted for the graphs showing proportion of fully stressed nuclei to better show the relationship of vowel height to stress-accent.

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There is a clear relationship between vowel height and stress accent. High vowels

tend to be accented much less often than low vowels. This is true of both the diphthongs

and monophthongs. This tendency is consistent with the hypothesis that vowel duration

is one of the main cues of stress accent in English because the low vowels, which tend to

be longer, also tend to be accented more often. Vowel height, duration and stress-accent

appear to be closely tied together. It is clear from looking at the figures and tables that

there is a correlation between these three factors. To a lesser degree, vowel amplitude is

also associated with vowel height and stress-accent, but the correlation between duration

and stress-accent is clearer and more consistent. What is not entirely clear about the

relationship between stress-accent and vowel height, duration and amplitude is whether

the correlation is due to the fact that low vowels tend to be intrinsically longer and louder

than high vowels, and therefore are perceived as stressed more often, or vice versa. We

know that long, loud vowels tend to be perceived as stressed, and that low vowels are

longer and louder than high vowels, and more often stressed than high vowels, but which

tendency is responsible for the other is not clear.

Problems

The patterns exhibited in the data are strikingly clear. However, they deal with

averages: average duration, and average amplitude for a given category. Therefore,

standard deviations were calculated for all of the duration and amplitude data (see Tables

3 and 4).

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Standard Deviations of Vowel Durations0 0.25 0.5 0.75 1 all

dur (ms) sd dur (ms) sd dur (ms) sd dur (ms) sd dur (ms) sd dur (ms) sd[iy] 78 0.035 98 0.05 114 0.057 122 0.068 132 0.066 100 0.055[ey] 90 0.049 94 0.038 122 0.054 130 0.057 155 0.074 129 0.066[ay] 108 0.059 113 0.048 126 0.049 143 0.066 174 0.075 141 0.069[aw] 103 0.077 121 0.043 150 0.051 156 0.057 203 0.074 168 0.073[oy] 98 0.013 111 168 0.075 154 0.072[ow] 102 0.043 117 0.052 126 0.053 150 0.076 170 0.084 136 0.071[uw] 70 0.046 101 0.063 104 0.052 153 0.089 152 0.099 103 0.077

[ih] 65 0.034 78 0.036 86 0.041 89 0.039 95 0.055 75 0.041[ix] 49 0.023 53 0.019 51 0.016 0.025 0.028 50 0.023[eh] 67 0.038 82 0.043 79 0.035 97 0.047 96 0.049 82 0.044[ah] 77 0.049 89 0.043 96 0.064 102 0.063 115 0.068 93 0.059[ax] 54 0.026 78 0.055 76 0.056 62 0.027 70 0.04 56 0.031[uh] 61 0.036 74 0.053 71 0.036 70 0.05 78 0.049 67 0.042

[ae] 91 0.055 113 0.062 123 0.059 144 0.073 165 0.071 137 0.072[aa] 86 0.043 94 0.038 110 0.047 116 0.044 134 0.059 114 0.054[ao] 100 0.058 79 0.047 87 0.05 107 0.057 143 0.072 115 0.066

Table 3. Standard deviations of vowel durations. Each stress-accent level has two columns, the first being the average duration of the vowel in milliseconds, and the second being the standard deviation (in seconds) of the mean duration. The vowels are grouped into the categories of diphthongs, lax monophthongs, and tense monophthongs.

Standard Deviations of Vowel Amplitudes0 0.25 0.5 0.75 1 all

amp sd amp sd amp sd amp sd amp sd amp sd[iy] 0.96 0.08 0.97 0.064 0.99 0.074 0.99 0.066 1.02 0.064 0.98 0.077[ey] 0.99 0.081 1.01 0.059 1.03 0.056 1.03 0.064 1.05 0.061 1.03 0.068[ay] 1 0.085 1.02 0.063 1.03 0.057 1.05 0.059 1.08 0.057 1.04 0.07[aw] 1.04 0.061 1.02 0.084 1.05 0.058 1.05 0.057 1.06 0.058 1.05 0.061[oy] 0.97 0.054 1.04 1.06 0.049 1.04 0.058[ow] 0.98 0.085 1 0.074 1.02 0.057 1.04 0.067 1.07 0.056 1.03 0.076[uw] 0.95 0.08 0.96 0.055 0.97 0.064 0.98 0.084 1.03 0.064 0.98 0.08

[ih] 0.96 0.083 1 0.067 1.01 0.073 1.02 0.073 1.06 0.069 0.99 0.086[ix] 0.92 0.104 0.97 0.089 1.01 0.095 0.035 0.035 0.92 0.105[eh] 0.97 0.097 1.02 0.058 1.03 0.083 1.05 0.063 1.08 0.059 1.02 0.091[ah] 0.98 0.079 1.02 0.068 1.03 0.072 1.05 0.058 1.08 0.055 1.03 0.079[ax] 0.94 0.097 1 0.075 1.03 0.069 1.04 0.048 1.09 0.055 0.95 0.097

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[uh] 0.97 0.079 1.02 0.076 1.05 0.062 1.05 0.061 1.09 0.075 1.01 0.086

[ae] 0.98 0.083 1.02 0.076 1.03 0.065 1.04 0.065 1.07 0.061 1.04 0.076[aa] 1 0.086 1.03 0.071 1.05 0.061 1.07 0.057 1.09 0.062 1.06 0.074[ao] 1 0.068 1 0.071 1.03 0.067 1.04 0.071 1.08 0.058 1.05 0.071

Table 4. Standard deviations of vowel amplitudes. Each stress-accent level has two columns, the first being the normalized average amplitude and the second being the standard deviation of the mean amplitude. The vowels are grouped into the categories of diphthongs, lax monophthongs, and tense monophthongs.

These standard deviations show that there is a lot of variation in the durations and

amplitudes of the vowels; however the standard deviations are fairly consistent across

categories, and vowels with higher intrinsic durations and amplitudes tended to have

higher standard deviations. There are many possible reasons for the amount of variance

in the data. An obvious contribution to the variance is the fact that the speakers of the

corpus were of both genders and were from all over the United States, and therefore their

speech can be expected to exhibit gender differences as well as dialect differences.

Additionally, there may have been a slight dialect-bias among the phonetic segment

transcribers for segments such as [ao], which differ according to dialect regions of the

US.

Analysis of the data for the different genders shows only very slight differences in

the vowels of female versus male speakers. The vowels of female speakers were on

average 9 milliseconds longer than the vowels of male speakers, with intrinsically short

vowels exhibiting a smaller gender difference and intrinsically long vowels exhibiting a

greater gender difference. Male speakers tended to have a slightly larger dynamic range

of amplitude between accented and unaccented occurrences of each vowel than female

speakers. These two tendencies suggest that female speakers may speak at a slightly

slower rate than male speakers, and that male speakers might utilize amplitude

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differences a little more to convey accent. However, since these differences are very

slight, more research would need to be done to substantiate these findings. Overall, the

patterns of female and male speakers did not differ significantly from the patterns for all

speakers (see figures 1 and 3-6).

Figure 3. Duration in seconds, male vs. female speakers, diphthongs. The Y-axis is the average duration, in seconds. The vowels are arranged along the X-axis approximately according to horizontal tongue position. The Y-axis was inverted to show the relationship between vowel height and duration.

Figure 4. Duration in seconds, male vs. female speakers, monophthongs. The Y-axis is the average duration, in seconds. The vowels are arranged along the X-axis approximately according to horizontal tongue position. The Y-axis was inverted to show the relationship between vowel height and duration.

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Figure 5. Amplitude range for male and female speakers for the diphthongs. Range is defined as the normalized average energy of the stressed occurrences of the vowel minus the normalized average energy of the unstressed occurrences. The Y-axis shows the range, and the vowels are arranged approximately according to horizontal tongue position along the X-axis.

Figure 6. Amplitude range for male and female speakers for the monophthongs. Amplitude range is defined as the normalized average amplitude of the stressed occurrences of the vowel minus the normalized average amplitude of the unstressed occurrences. The Y-axis shows the range, and the vowels are arranged approximately according to horizontal tongue position along the X-axis.

Unfortunately, there was an insufficient amount of data from each dialect region

to obtain information about the general patterns associated with each region. The

expectation, had there been more data, is that speakers from certain regions would tend

to, on average, have longer vowels than speakers from other dialect regions, and that

some regions would exhibit a more pronounced amplitude difference between accented

and unaccented vowels than other regions. Theoretically, this would account for some of

the variance.

Most likely, however, the variance among the duration data is due to different

speaking rates among different speakers, and the variance in amplitude data, similarly, is

due to that fact that individual speakers vary the volume of their voices differently. What

is important in this research is not that there is variance among the data; in fact, with the

high number of factors, such as different speakers, genders, and dialects, which

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contribute to variance in spontaneous speech, a relatively large amount of variance in the

data is expected. What is interesting about the data is that despite the variance, the

patterns came out so clearly. The study of spontaneous speech is a fairly new area of

linguistics, made possible by extensive improvements to computer technology in recent

years, and the fact that such clear patterns exist is very encouraging to the field of speech

technology, particularly for speech recognition. Understanding the patterns that exist in

actual speech should make possible much more accurate recognition systems than those

that currently exist.

Future research projects could attempt to correct for factors like gender, dialect

and speaking rate in order to understand even better how stress-accent is exhibited in

spontaneous speech, but it is doubtful that the results of such research would contradict

the general patterns shown in this paper. However, an understanding of the factors that

contribute variety to spontaneous speech should have a profound affect on the field of

speech recognition, and therefore deserve attention. This paper is a first look at general

patterns in spontaneous speech; hopefully these patterns will be studied much more

closely in the future.

Conclusions

The data in this paper clearly show a relationship between stress-accent and vowel

height, duration and amplitude in American English. This supports the results of much

recent research on stress accent (Beckman, 1986, Van Kuijk and Boves, 1999, Silipo and

Greenberg, 1999 and 2000) which has shown this relationship in both lab speech and

spontaneous speech to a certain extent. The data on vowel duration and amplitude

support Lehiste and Peterson’s research (1959 &1960) on intrinsic duration and

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amplitude of vowels. While their intrinsic durations, which were based on lab speech, are

much longer than what was shown in spontaneous speech, the overall patterns are the

same. Low vowels and diphthongs tend to be louder and longer than high vowels and

monophthongs. The prosodic data clearly show a close relationship between vowel

duration and stress-accent, as well as amplitude and stress-accent. Additionally, duration

and amplitude are clearly related to vowel height, which is also related to stress-accent.

Longer, louder vowels tend to be perceived as accented; low vowels tend to be longer and

louder than high vowels; therefore low vowels tend to be perceived as accented more

often than high vowels.

There are a couple of theories as to why low vowels are longer and louder than

high vowels (there is a good summary of some of these in Beckman, 1986). One

explanation for the durational differences is that physically it takes longer to transition

from a low vowel to a surrounding consonantal environment because the constrictions

made to produce consonants are high in the mouth, and the tongue is low in the mouth

when producing low vowels. The traditional explanation for the amplitude differences is

also based on physics. Basically, to produce a high vowel that is equal in loudness to a

low vowel, a different amount of effort is required of the speaker depending on the

constriction in the vocal tract, which has a different shape for low vowels than high

vowels. (see Beckman, 1986, Lehiste and Peterson, 1959 and 1960).

For the purposes of this study, the physical reason for the different intrinsic

amplitudes and durations of vowels is not as important as the effect that these differences

have on stress-accent. A very short vowel, like [ix], probably will be perceived as

stressed at a shorter duration than a very long vowel, like [aw]. It has been shown in

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several languages that intrinsic duration differences affect phonemic length contrasts (see

Beckman, 1986, chapter 5). This is consistent with the findings of the current study.

Table 2 shows the varying average length and amplitude of vowels, and clearly some

vowels are perceived as stressed at much shorter durations than others.

However, despite different length requirements for different vowels, it is also true

that vowels that are intrinsically longer and louder tend to be perceived as stressed much

more frequently than their shorter, quieter counterparts. Apparently, the actual duration

and amplitude of a vowel relative to other vowels affect perceived stress-accent, as do the

proportional duration and amplitude of a vowel to its intrinsic duration and amplitude.

Vowels with high intrinsic durations and amplitudes (i.e. the low vowels) are perceived

as stressed much more frequently than those with lower intrinsic durations and

amplitudes. However, even those vowels that are typically very long and loud are

sometimes unstressed, generally when the duration and amplitude of the vowel are much

lower than usual.

This finding is consistent with the theory of prosody proposed by Vanderslice and

Ladefoged (1972) that separates syllables into heavy versus light and accented versus

unaccented. According to this theory, light syllables are typically reduced, or in any case

unstressed, while heavy syllables tend to not be reduced, even when they are unaccented.

Light syllables can only “one of the vowels , I [], o (the monophthongal reduction of

ou) or u [] or a syllabic consonant. To these we would add [r] and, occasionally in final

open syllables, [i] as in northern US city, and [u] as in Hindu,” (p.823). This list

corresponds very closely with the high vowels, both monophthongs and diphthongs,

which the data from this paper clearly show are the vowels most likely to appear in

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unaccented syllables. Vanderslice and Ladefoged’s paper suggests that all other vowels

(i.e. the low vowels) appear in heavy, (i.e. likely to be accented and unreduced) syllables.

This paper makes a high versus low distinction between the two classes of vowels, but

the result is similar: some vowels are much more likely to be accented than others are.

The difference is that their theory is a theory on how to label prosodic features, while this

study shows concretely the patterns that exist in spontaneous discourse.

The extent to which the data collected for this study support various research

projects done on lab speech is encouraging. The finding that low vowels are intrinsically

longer and louder than high vowels has been shown by Lehiste and Peterson (1959 and

1960, Lehiste, 1996) in studies done on lab speech. Fox (1955) first showed with

synthesized speech that duration and amplitude are important cues of perceived stress-

accent in English. Now that technology has made it possible to conduct in-depth research

on spontaneous speech, many studies done previously on lab speech will be questioned;

the results of this study show that some patterns exhibited in lab speech are very similar

to spontaneous speech.

Acknowledgements

I would like to thank Steve Greenberg and John Ohala for their advice and

assistance, Joy Hollenback and Shawn Chang for their help with the research and

compiling data, Jeff Good for prosodically transcribing the data, and Candace Cardinal,

Rachel Coulston and Colleen Richey for phonetically transcribing the corpus.

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