Acoustical analysis of English vowels

produced by Chinese, Dutch and

American speakers

Hongyan Wang* and Vincent J. van Heuven

Leiden University Centre for Linguistics
* now at Shenzhen University, Shenzhen, P.R. China

. Introduction

(c) John
The vowel systems of (Mandarin) Benjamins
Chinese (e.g. Wiese 1997), Dutch and Amer-
ican English differ considerably,
Delivered both inby
the number
Ingenta of vowels in the inventory
and in the details of their position within the articulatory vowel space, and pos-
on: Wed, 22 Nov 2006 13:18:49
sibly also in terms of their durational characteristics. When Dutch and Chinese
nationals speak English as to: Guest
a foreign User
language, their pronunciation of English
IP: 192.87.50.3
will deviate from the American native norm for English. As part of a larger
research project, we are interested in a precise characterization of Chinese as
opposed to Dutch-accented English, and in the question how these non-na-
tive varieties of English differ from the native norm. Our description of these
three varieties of English will be based on objective rather than impressionistic
data (as exemplified by textbooks such as Collins & Mees 1981). Specifically,
we used acoustic measurements that are known to have clear correspondences
with articulatory properties of vowels.

. Objective measurement of vowel quality

Vowel quality can be quantified by measuring the centre frequencies of the

lower resonances in the acoustic signal. The lowest resonance of the vocal tract,
called first formant frequency or F1, corresponds closely to the articulatory (and
perceptual) dimension of vowel height (high vs. low vowels, or close vs. open
vowels). For an average male voice, the F1 values range between 200 Hertz (Hz)
for a high vowel /i/ to some 800 Hz for a low vowel /a/. The second formant
frequency (or F2) reflects the place of maximal constriction, i.e., the front vs.

Linguistics in the Netherlands 2006, 237–248.

issn 0929–7332 / e-issn 1569–9919 © Algemene Vereniging voor Taalwetenschap
238 Hongyan Wang and Vincent J. van Heuven

back dimension, such that the F2 values range from roughly 2400 Hz for front
/i/ down to some 600 Hz for back /u/. For female voices the formant frequencies
are 10 to 15% higher due to the fact that the resonance cavities in the female
vocal tract are smaller (shorter) by 10 to 15% than those of a male speaker.
The relationship between the formant frequencies and the corresponding
perceived vowel quality is not linear. For instance, a change in F1 from 200 to
300 Hz brings about a much larger change in perceived vowel quality (height)
than a numerically equal change from 700 to 800 Hz. Experimental phone-
ticians and psycho-physicists have developed an empirical formula that ade-
quately maps the differences in hertz-values onto the perceptual vowel-quality
(or timbre) domain, using the so-called Bark transformation.1
Probably the best known set of formant measurements was produced for
American English by Peterson & Barney (1952) for male and for female speak-
ers separately. These authors used the same stimuli that we used, i.e. vowels
embedded in a /h_d/ consonant frame. A similar vowel set was recorded for 50
male and 25 female speakers of Dutch by Pols and co-workers in the seventies
(Pols, van de Kamp & Plomp 1973 and van Nierop, Pols & Plomp 1973, respec-
(c) John
tively). Formant measurements Benjamins
for the vowels of Mandarin (Beijing dialect)
have become available Delivered byYu,Ingenta
only recently (Li, Chen & Wang 2004). Formant
measurements for Chinese-accented English (aiming at the American pronun-
on: Wed, 22 Nov 2006 13:18:49
ciation norm) were published by Chen, Robb, Gilbert & Lerman (2001). The
authors recorded a subset to:of theGuest
AmericanUser
English vowels (eleven monoph-
thongs) in the same /h_d/ IP: 192.87.50.3
monosyllables that we used ourselves. However,
their speakers (20 male and 20 female adults) had been living in the USA for
at least two years after having received intensive exposure to spoken English in
China in order to qualify for the TOEFL test required to enter a university in
the USA. This is clearly a different type of ESL speaker than we target in our
study, so that it makes every sense that we should measure the formants in
our speaker group separately. No formant data have been published so far for
Dutch-accented English vowels.

.2 The problem of vowel normalization

Unfortunately, formant values measured for the same vowel differ when the to-
kens are produced by different individuals. The larger the differences between
two speakers in shape and size of the cavities in their vocal tracts, the larger
the differences in formant values of perceptually identical vowel tokens are.
Given that the vocal tracts of women are some 15 percent smaller than those
of men, comparing formant values is especially hazardous across speakers
of the opposed sex. In the present study we have opted for a straightforward
Acoustical analysis of English vowels produced by Chinese, Dutch and American speakers 239

vowel normalization procedure, first used by Lobanov (1971), which is simply

a z-normalization of the F1 and F2 frequencies over the vowel set produced
by each individual speaker. In the z-normalization, the F1 and F2 values are
transformed to z-scores by subtracting the individual speaker’s mean F1 and
mean F2 from the raw formant values, and then dividing the difference by the
speaker’s standard deviation. Z-transformed F1 values less than 0 then corre-
spond to relatively close (high) vowels, values larger than 0 refer to rather open
vowels. Similarly, negative z-scores for F2 refer to front vowels, whilst positive
F2 z-values represent back vowels. In our case, we applied the Lobanov nor-
malization after first transforming the hertz values to Bark values.

.3 Vowel duration

The vowels of English and Dutch can be divided into two major groups on
the basis of their phonological behaviour, which largely correspond with pho-
netically short (and lax) versus long (and tense) vowels. Typically, the short/lax
and long/tense vowels are in paired oppositions. In English, examples of such
pairs are /i: ~ i/ and /u:(c)
~ ~/.John Benjamins
Vowel durations for American English were pub-
lished by Peterson & Delivered
Lehiste (1960). Dutch vowel
by Ingenta durations were studied by
Nooteboom (1972). No systematic study of vowel duration exists for Mandarin
on: Wed, 22 Nov 2006 13:18:49
vowels, nor are there systematic data on vowel duration in either Chinese or
Dutch-accented English.2 to: Guest User
Since vowel duration IP: plays192.87.50.3
a potentially important role in marking the
tense ~ lax contrast, next to vowel quality differences, we also measured the
vowel duration in the tokens recorded in our dataset. Since some speakers
speak faster than others, raw vowel duration cannot be used in the comparison.
Rather, durations should be normalized within speakers. Here, too, we used
z-normalization so that negative normalized durations refer to relatively short
vowel tokens, and positive values represent relatively long vowel durations.
Chinese does not exploit length as a vowel feature at the phonological level.
We predict that Chinese speakers of English as a second language (ESL) will
distinguish less adequately between the short (lax) and long (tense) vowels of
English than Dutch ESL speakers, and certainly less than native speakers of
English.

2. Materials

For the present experiment we recorded ten male and ten female speakers for
each of three nationalities: Chinese, Dutch and American. All sixty speakers
240 Hongyan Wang and Vincent J. van Heuven

were students in the Netherlands at the time the recordings were made. Dutch
and Chinese speakers had not studied English after secondary school. Speakers
did not have, or had in the past, regular contact with English-speaking friends
or relatives, nor had they ever lived in an English-speaking country. For a full
description of the methods used in the experiment see Wang & van Heuven
(2003).
A list of words containing 19 full vowels and diphthongs (so excluding
schwa) in identical /hVd/ contexts was recorded: heed, hid, hayed, head, had,
who’d, hood, hoed, hawed, hod, hard, hud, heard, hide, hoyed, how’d, here’d, hoo-
red, haired. The /h_d/ consonant frame is fully productive in English, allowing
all the vowels of English to appear in a word or short phrase.
Speakers were recorded on digital audio tape (DAT) in a sound-insulated
recording booth through a Sennheiser MKH–416 microphone. Materials were
downsampled (16 KHz, 16 bits), and stored on computer disk.
Our recordings contain tokens of 19 vowel types. Given that our speak-
ers, including the Dutch speakers, without having been instructed to do so,
used an American-style pronunciation, with r-coloured (retroflexed) vowels
(c) Johnthere
(instead of centring diphthongs), Benjamins
seemed little point in measuring the
vowels that were followed
Delivered byweIngenta
by /r/. Therefore eliminated the tokens represent-
ing here’d, haired, hard, hoored and heard. Next, we decided not to include any
on: Wed, 22 Nov 2006 13:18:49
full diphthongs, as these would introduce the complication of having to trace
the spectral change over the to:course
Guestof theUser
vowels. This eliminated the types
hide, how’d and hoyed. What IP:remained
192.87.50.3
is precisely the set that was also mea-
sured in Chen et al. (2001). We finally decided also to eliminate the /f:/ type. It
appeared that our speakers (both native and non-native) did not systematically
differentiate between this vowel and /f/. Moreover, quite a few of our L2 speak-
ers pronounced /f:/ in hawed as /haud/, a pronouncing error induced by the
spelling which was not detected at the time of the recording.

3. Results: Vowel quality in Chinese, Dutch and American English

Using the Praat speech processing software (Boersma & Weenink 1996), the
beginnings and end points of the target vowels were located in oscillographic
and/or spectrographic displays. Formant tracks for the lowest four formants (F1
through F4) were then computed using the Burg LPC algorithm implemented
in Praat, and visually checked by superimposing the tracks on a wideband spec-
trogram. Whenever a mismatch between a track and the formant band in the
spectrogram was detected, the model order of the LPC-analysis was changed
ad hoc until a proper match was obtained between tracks and spectrogram.
Acoustical analysis of English vowels produced by Chinese, Dutch and American speakers 24

(c) John Benjamins

Delivered by Ingenta
on: Wed, 22 Nov 2006 13:18:49
to: Guest User
IP: 192.87.50.3

e
Ι

Figure 1. F1-by-F2 plots (Bark-transformed and z-normalized axes) for Chinese,

Dutch and American speakers of English. Male and female speakers have been col-
lapsed. Ellipses have been drawn at ±1 SD from the centroids along the two principal
component axes. Vowel tokens are linked to the centroids. Dotted lines connect the five
lax vowels in each graph (shaded inner polygon), dashed lines connect the tense vowels.
242 Hongyan Wang and Vincent J. van Heuven

The values for F1 and F2 were extracted at the temporal midpoint of the target
vowel, and stored together with the vowel duration for statistical processing.
Formant values were then converted to Bark (see § 1.1), z-normalized
within speakers (§ 1.2), and then averaged over the twenty speakers in each
speaker group. These mean F1 and F2 values are plotted in acoustical vowel
diagrams in Figures 1a–c for Chinese, Dutch and American speakers of Eng-
lish. Each plot contains the position of the ten monophthongs selected as ex-
plained in § 1.4.
The vowels of American English are often separated into two length cat-
egories, short and long. Phonetically, the four short vowels, /i, ε, %, ~/ do not
only have short durations, they also take up more centralized positions in the
vowel space. For this reason, this set may be called ‘lax’ as well. The other vow-
els of American English are long and have positions along the outer perimeter
of the vowel space. These are, in the present restricted dataset, the vowels /i:, e:,
æ, f, o:, u:/. Here, the vowel /f/ is classed as a tense vowel on the grounds that it
is a merger of tense /f:/ and lax /f/. Its location in the vowel space (Figure 1c for
the American speakers) motivates this choice quite clearly. Following Strange,
Bohn, Nishi & Trent (2004),(c) John Benjamins
we classified the open front vowel /æ/ as tense.
Although it should beDelivered
phonologically lax (since
by Ingenta it cannot occur at the end of a
word), there are good phonetic reasons to consider American /æ/ a tense vow-
on: Wed, 22 Nov 2006 13:18:49
el: it is clearly longer than any other lax vowel, and it is also peripheral, that is,
on the outer edge of the vowel to:space.
Guest User
In Figure 1 the six tense vowels have been
linked with a dashed line; IP:the lax192.87.50.3
vowels have been linked with a dotted line.
The Chinese ESL speakers’ vowels show little spectral distinction between
intended /i:/ and /i/. Similarly, there is hardly any spectral difference between
intended /ε/ and /æ/ nor between /u:/ and /~/.
Moreover, we observe, in Figure 1a, that the tense and lax vowel polygons
largely overlap, indicating that the Chinese ESL speakers basically fail to dis-
tinguish between the spectrally more peripheral tense set and the spectrally
reduced (centralised) lax set.
The ESL tokens produced by the Dutch speakers show a clear spectral dif-
ference between intended /i:/ and /i/, which is predicted as a case of positive
transfer from Dutch to English. There is also a fair degree of separation between
intended /ε/ and /æ/. Although the separation is not as large as in the native
American speech (see below), the success on the part of the Dutch speakers
is unexpected. The /ε/ ~ /æ/ contrast is typically listed as a learning difficulty
(Collins & Mees 1981), and we are surprised to learn that in our ESL speakers
some notion of the difference has already been established. Interestingly, the
other vowel pair that has traditionally been mentioned as a learning problem,
/u:/ ~ /~/, remains completely undifferentiated by the Dutch ESL speakers.
 Acoustical analysis of English vowels produced by Chinese, Dutch and American speakers 243

Dutch and English both have tense and lax vowel subsets. Inspection of
Figure 1b, however, shows that the tense and lax subsets are not very clearly
separated in Dutch ESL. One reason for the relatively poor separation between
the subsets is the lack of an /u: ~ ~/ contrast in Dutch. The Dutch speakers do
not spectrally distinguish between the two, so that here the two subsystems
merge (interestingly, the two vowels do differ in their duration — see below).
Also, at the lower edge of the vowel space there is little differentiation between
more centralized (half) open lax vowels and peripheral open tense vowels as the
Dutch ESL speakers do not lower /f/ as much as they should for American Eng-
lish, and at the same time observe insufficient contrast between /ε/ and /æ/.
If we now turn to Figure 1c, we notice the American native vowels are
spectrally much more distinct than those produced by the Dutch speakers, and
even more so than the Chinese ESL vowels. There are very large spectral differ-
ences between the members of the pairs /i: ~ i/, /ε ~ æ/ and /u: ~ ~/. Moreover,
the figure illustrates quite convincingly that the tense and lax vowel subsets are
organised in terms of an outer (peripheral) and an inner (more centralised)

(c) John Benjamins

300 Delivered by Ingenta
a. Chinese speakers b. Dutch speakers

on: Wed, 22 Nov 2006 13:18:49

250
to: Guest User
Vowel duration (ms)

200
IP: 192.87.50.3

150

100

300
�    :  : :  :
c. American speakers
Lax/short Tense/long

250
Vowel duration (ms)

200

150

100
�    :  : :  :
Lax/short Tense/long

Figure 2. Duration (ms) of four lax and six tense English monophthongal vowels
spoken by Chinese (a), Dutch (b) and American (c) speakers of English.
244 Hongyan Wang and Vincent J. van Heuven

circle. In this respect, too, the L1 speakers clearly differ from both the Dutch
and (even more) from the Chinese ESL speakers.

4. Results: Vowel duration in Chinese, Dutch and American English

Figure 2 plots mean duration for each of the ten vowel types, separately for
lax and tense categories in for Chinese ESL speakers (panel a), for the Dutch
speakers (panel b) and for the American L1 speakers (panel c).
Taking the native speakers as our starting point, Figure 2c clearly shows
that the four lax/short vowels have a much shorter duration (with means be-
tween 169 and 185 ms) than the six long/tense vowels (with means between
225 and 266 ms). As a result of this, vowels that are spectrally close to each
other, such as /e:/ (266 ms) and /i/ (184 ms), are yet acoustically distinct. Note
also that when the vowels are ordered from short to long, as has been done in
Figure 2c, the increment between adjacent vowels in the figure is never more
than 14 ms (which is the difference in mean vowel duration between /o:/ and
(c) John
/æ/). However, the discrepancy Benjamins
between the longest of the short vowels (/ε/,
185 ms) and the shortest of the long vowels
Delivered by Ingenta (/u:/, 225 ms) is 40 ms. These re-
sults can be taken in evidence of the phonetic correctness of the subdivision of
on: Wed, 22 Nov 2006 13:18:49
the American English vowels into the short and long categories made here.
to:durations
Turning now to the vowel Guestproduced
Userby the Chinese ESL speak-
IP:the192.87.50.3
ers (Figure 2a), we note that short vowels are roughly within the duration
range of the American L1 speakers. Also, the long vowels are generally within
the native range for long vowels, with the exception of the vowels /æ/ and /f/.
Interestingly, these are precisely the vowels that distributionally pattern with
the short vowels, as they cannot occur at the end of a word in English.
The Dutch ESL vowel durations are rather similar to the Chinese realisa-
tions. Again, there are two gross duration categories, one for short vowels with
durations less than 200 ms, and one for long vowels with durations in excess of
240 ms. As in the Chinese ESL tokens, the Dutch speakers make the long vow-
els /æ/ (208 ms) and /f/ (172 ms) too short by American-English standards.
Moreover, the Dutch speakers, who did not differentiate between /u:/ and /~/
in spectral terms (see Figure 1b), also have a tendency to make the short /~/
too long (202 ms) — even though this still is still some 40 ms shorter than
their mean duration for long /u:/. Unexpectedly, then, it seems as if the Dutch
ESL speakers are not more successful in keeping the American-English lax and
tense vowels distinct than the Chinese speakers, even though Dutch — unlike
Mandarin — is a language with a tense ~ lax subdivision.
Acoustical analysis of English vowels produced by Chinese, Dutch and American speakers 245

Figure 3. Correctly classified vowel tokens (%) by Linear Discriminant Analysis for
Chinese, Dutch and American male and female speakers of English. LDA functions
were derived on the basis of F1 and F2 with (squares) and without (circles) duration.

(c) John Benjamins

5. Results: Automatic vowel classification
Delivered by Ingenta
on: Wed, 22 Nov 2006 13:18:49
So far we have only been considering the means of the realisations of the vow-
to: Guest
els — in terms of vowel quality (F1 and F2)User
and of duration — averaged over
groups of twenty speakers.IP: The 192.87.50.3
means do not tell us anything about how well
the individual speakers keep the vowels distinct in their pronunciation of Eng-
lish. Figures 1a-c also plot the individual realisation of the vowels in the F1 by
F2 plane as scatter clouds, enclosed by spreading ellipses. These were drawn
along the principal component axes optimally capturing the directionality of
the scatter of the vowel tokens within one vowel type. The ellipses have been
plotted at ±1 SD away from the F1–F2 centroids and therefore enclose the most
typical 45 per cent (two thirds squared) of the vowel tokens in the category.
The figures show that, generally, the Chinese speakers (Figure 1a) have
more overlap between the ellipses of neighbouring vowels than is the case in
the Dutch ESL realizations (Figure 1b). The native American L1 speakers have
the smallest degree of overlap (Figure 1c), indicating that these speakers keep
the ten monophthongs optimally distinct.
We will now attempt to quantify the difference between the three speaker
groups in terms of the degree of success in keeping the ten vowels distinct. We
have used Linear Discriminant Analysis (LDA, Klecka 1980, Weenink 2006)
for this purpose. LDA is an algorithm that computes an optimal set of param-
eters (called discriminant functions) and automatically classifies objects in
246 Hongyan Wang and Vincent J. van Heuven

pre-established categories. The more distinct the categories are in the dataset,
the fewer the number of classification errors yielded by the algorithm. In the
case at hand, the discriminant functions are based on linear combinations of
weighted acoustic parameters F1, F2 and duration. Again, before running the
LDA, speaker normalization was carried out using the z-transformation on the
durations and on the formant frequencies (after Bark conversion).
We ran the LDA algorithm twice. The first time we just included the two
spectral parameters as possible predictors of vowel identity, i.e. F1 and F2 (con-
verted to Bark and z-normalized within individual speakers). The second time
we extended the set of predictors by also including vowel duration. Figure 3
presents the results of the LDA. The figure shows at a glance that the vowels as
produced by the native speakers afford the best automatic identification, those
spoken by the Dutch learners can be less successfully identified, and the Chi-
nese ESL tokens are poorest. Adding duration to the set of predictors boosts
the correct identification by some 10 percentage points (a little less for the
American L1 vowel tokens, possibly due to a ceiling effect). Finally, the vowel
tokens produced by the female speakers tend to be more distinct, and therefore
better identified, than (c)
thoseJohn Benjamins
spoken by the males. However, there is no such
gender effect in the Dutch vowel set.
Delivered by Ingenta
on: Wed, 22 Nov 2006 13:18:49
6. Conclusions to: Guest User
IP: 192.87.50.3
No comprehensive studies are available on the acoustic realisation of English
vowels produced by Chinese and Dutch learners, covering both the spectral
characteristics of the vowels (in terms of formants) and the duration, and ex-
amining the interaction between the two types of parameters in keeping the
vowels in the English system distinct. The present study aimed to fill in this gap
in our knowledge.
We contrasted learners of English who speak a native language that has
a relatively small vowel inventory (Mandarin) and no tense~lax subsets with
Dutch learners of English, whose native language has a richer inventory (com-
parable in size to the English set) and tense versus lax vowels subsets. Chinese
and Dutch learners were comparable in the sense that both groups represent
non-specialized academic users of English as a foreign language.
Our results shown that the Chinese learners have a rather distorted
conception of the American-English vowel system — at least where the ten
monophthongs are concerned. The mean positions of the ten vowels in Chi-
nese-accented English are all situated along the outer perimeter of the vowel
space, whilst the split of the English system in an outer circle with six tense
Acoustical analysis of English vowels produced by Chinese, Dutch and American speakers 247

vowels and an inner circle with four lax vowels is not observed. As a result,
there is insufficient spectral separation between the tense and lax vowel pairs.
Interestingly, and unexpectedly, the Chinese learners observe a clear length
difference between the four lax vowels of English and the long tense vowels,
with the proviso that the two phonologically lax vowels within the tense set,
are pronounced short (as would be the case in British English). As a result,
the members of the pairs /i: ~ i/ and /u: ~ ~/ but not in /ε ~ æ/ are acoustically
distinct in Chinese-accented English. Consequently, the Chinese vowel tokens
are relatively poorly classified by Linear Discriminant Analysis, with roughly
75% correct. This patterning of the results cannot be predicted by a classical
type of contrastive analysis of Chinese and English; such an analysis would in
fact predict failure of contrast in each of the three pairs.
The Dutch speakers have better acoustical separation of their English vow-
el tokens, with an average 85% correct classification. Unexpectedly, the Dutch
learners observed a reasonably good separation between the members of the /ε
~ æ/ pair but predictably failed to keep /u: ~ ~/ apart.
The American native speakers have a very clear separation between the
tense and lax subsystems,(c)which
John split Benjamins
is fully supported by a systematic differ-
ence in vowel duration. As a result,
Delivered by the American
Ingenta vowel tokens are classified
correctly by the LDA in more than 90%.
on: Wed, 22 Nov 2006 13:18:49
This acoustical analysis would predict, finally, that Chinese-accented Eng-
to: Guest
lish vowels will be more difficult to identifyUser
correctly by human listeners than
IP: 192.87.50.3
Dutch-accented English vowels. This prediction is borne out by data of two
separate series of experiments which we published in earlier articles (Wang &
van Heuven 2003, 2005).

Notes

. We used the Bark formula as advocated by Traunmüller (1990): Bark = [(26.81 × F) /

(1960 + F)] − 0.53, where F represents the measured formant frequency in hertz.

2. However, Elsendoorn (1984) measured vowel durations for six vowel types /i:, i, e:, æ, o:,
u:/ in Dutch-accented English spoken by pupils between 12 and 17 years of age at secondary
schools.

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Report of the Institute of Phonetic Sciences Amsterdam, 132.
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Elsendoorn, Ben A.G. 1984. Tolerances of durational properties in British English vowels.
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