3. Speech Production and Speech Physiology

ID: 483

AN ARTICULATORY STUDY OF THE WORD-INITIAL /stɹ/ CLUSTER IN
BRITISH ENGLISH
Mitsuhiro Nakamura
Department of English, Aoyama Gakuin University, Tokyo, Japanm_nakamura@cl.aoyama.ac.jp

ABSTRACT

most important factor in modeling /s/-retraction in the/stɹ/ cluster and the rhotic /ɹ/ plays only an indirectrole. According to [2], the retraction of /s/ occurs not
only in the consonant sequence /stɹ/ (eg. street) but
also in /stj/ (phonetically [stʃ] as in student). Since theconsonant sequence /tj/ is affricated, the trigger for/s/-retraction is considered to be the affricate [tʃ]. This
proposal effectively supports Lawrence’s ‘local
assimilation’ [9] as the mechanisms of /s/-retraction,rather than Shapiro’s ‘assimilation at a distance’ [13].
The mechanism proposed by [2] is appealing but
needs further examination. The first point concerns /t/affrication mentioned above. It has been pointed out
by [6, 10, 18] that the /tɹ, dɹ/ and /tj, dj/ clusters are
realized as the affricates [tʃ, dʒ], but this tendency has
not been substantiated fully by the empirical data. The
second point is the coarticulatory effects of the rhotic/ɹ/. A study of the singleton /ɹ/ [5] has shown that the/ɹ/ involves more than one lingual constriction; thearticulators are controlled in the order of the lips > thetongue tip/blade > the tongue root at the syllable onset
position, and in the order of the tongue root-the lips >the tongue tip/blade at the coda position. Furthermore,
long-range coarticulatory effects have been reported
for the /l, ɹ/ in Southern British English [19]: adjacentand non-adjacent vowels preceding /ɹ/ have stronger
lip rounding and tongue dorsum raising and retraction.
It can be asked how the lip gesture is realized in thecontext of consonant clusters.
The current study aims to (i) provide initial
empirical data for the /s/ articulation in the /stɹ/ clusterand (ii) examine the temporal extent of the lip gesturein the production of the /stɹ/ cluster. We will conclude
with future directions of this study.

This paper explores the articulatory characteristics of
the word-initial /stɹ/ cluster in British English. Of
specific interest is the phenomenon called /s/retraction. Using EPG and EMA data collected from
the MOCHA-TIMIT database, variable realizationsof the /stɹ/ cluster were analyzed for four speakers.
The tongue-palate contact pattern of the /s/ in the /stɹ/
cluster (e.g. street) was compared with that of theonset singleton /s/ and /ʃ/ (e.g. seat, sheet). The
temporal extent of anticipatory labial coarticulation
was inspected. The results reveal that (i) for one
speaker, the contact anteriority index of the /s/ in the/stɹ/ cluster is significantly different from that of theonset singleton /s/, (ii) /t/-affrication (e.g.
street>s[tʃ]reet) is found for all the speakers, (iii) the
constriction peak of the lower lip gesture issystematically attained at the release of the /t/ in thecluster. These preliminary findings are discussed interms of speaker-dependent articulatory strategies.
Keywords: /s/-retraction, EPG, EMA, MOCHATIMIT
1. INTRODUCTION
The phenomenon of /s/-retraction in the /stɹ/ cluster(e.g. street > [ʃ]treet) has been investigated in various
accents of English: for instance, [3, 11, 13, 14] for
American English, [1, 2] for British English, [15] forAustralian English, [9, 17] for New Zealand English,[16] for a cross-dialectal analysis. The current study
aims to further investigate the articulatory nature of
the /s/-retraction phenomenon in British English.
Previous studies have identified that the phonetic
basis of /s/-retraction is related to the lower spectral
frequency: the alveolar /s/ in the /stɹ/ sequence has
been judged to become closer to the postalveolar /ʃ/due to its lower fricative noise frequency [2, 3, 11].
This measure successfully captures the systematicacoustic variation of /s/ in various contexts. In thecurrent study, an attempt will be made to explore the‘articulatory’ retraction of /s/ to /ʃ/ in the /stɹ/ cluster
using electropalatographic data from the multichannel articulatory database, MOCHA-TIMIT.
A previous study of /s/-retraction in British
English [2] has proposed that /t/-affrication is the

2. METHOD
2.1. Data collection

The speech materials used in this study were drawn
from the multichannel articulatory database,
MOCHA-TIMIT described by [20, 21]. This database
comprises the articulatory and acoustic data of 460
phonetically balanced sentences read by native
speakers of English. Data acquisition was made with
Carstens Electromagnetic Articulograph (EMA,
500Hz sample rate), Reading Electropalatograph(EPG, 200Hz sample rate), Electrolaryngograph (Lx),997

3. Speech Production and Speech Physiology

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and acoustic recordings. In the EMA data recording,the tongue tip (TT) coil was placed at about 5-10mm,
the tongue blade (TB) coil was placed at about 2540mm, and the tongue dorsum (TD) coil was placedat about 50-70mm from the tongue tip, as measured
with the tongue extended. In this study, the EMA data
and the EPG contact patterns were analyzed for theselected utterances spoken by four speakers ofStandard Southern British English referred to as SE(female), JM (female), SA (male), and AP (male).
For collecting the speech materials, the MOCHA
corpus was searched and two sets of data wereprepared. Dataset 1 consisted of words with the wordinitial /stɹ/ cluster: the target words were preceded bya schwa [ə] or non-coronal consonant. Dataset 2
consisted of words with the word-initial (onset) /s/
and words with the word-initial (onset) /ʃ/. Thesetarget words were preceded by a schwa [ə]. All thewords collected are content words. The two datasets
are summarized in Table 1.
Dataset 1street,straightforward,
stray,
stronghold,strongly

The configurational characteristics of the EPG
contact patterns in the alveolar zone were studied. For
the /stɹ/ cluster (dataset 1), three EPG frames wereidentified. The MAX (linguopalatal narrowing) frameof /s/ was determined by the mid-point of thecontinuous noise energy, specified by a spectrogram.
The two other frames were the first frame showing
the complete closure for /t/ and the first frame
showing the release of /t/. For the onset /s/ and /ʃ/(dataset 2), the MAX frame mentioned above wasspecified.
The contact index method [7, 12] was used to
examine the configurational characteristics of the
EPG contact patterns. The contact indices were CAa(alveolar contact anteriority index), CCa (alveolar
contact centrality index), and Qp (quotient of overall
electrode activation at the palatal zone, rows 6-8).
The indices CAa and CCa were calculated using thefollowing formula [12].
(1) CAa=[log[[1(R5/8)+9(R4/8)+81(R3/8)+739(R2/8)+4921(R1/6)+1]]/[log(5741+1)]

Dataset 2see, seeking, same,
save, saw
shipbuilding, shell,shaving, shocked,
shore(2) CCa=[log[[1(C1/8)+11(C2/10)+121(C3/10)+1331(C4/10)]+1]]/[log(1464+1)]

Table 1: Target words in dataset 1 and 2.

The position in the sentence of these target words
varied. The number of tokens in dataset 1 was 20 intotal (5 words x 4 speakers) and that in dataset 2 was
40 in total (10 words x 4 speakers).
2.2. Analysis

The analysis of the EPG and EMA data was
performed using a set of programs called EMA tools[21]. The Reading EPG artificial palate has 62
electrodes arranged in eight rows and eight columns.
The surface of the palate was divided into two majorzones for data analysis: an alveolar zone for rows 1-5
and a palatal zone for rows 6-8. These zones were
subdivided into dentalveolar (row 1), alveolar (row 2),
postalveolar (rows 3-4), prepalatal (row 5), mediopalatal (rows 6-7), and postpalatal (row 8).
C 1 C 2C 3 C4 C 4 C3 C 2C1
R1
R2
R3
R4
R5
R6
R7
R8

dentalveolar
alveolarpostalveolar

Alveolar Zone
(R1- R5)

prepalatal
mediopalatal
postpalatal

Palatal Zone
(R6 - R8)

Figure 1: EPG palatogram and two zones.

998

The CAa index value increases with alveolar contact
fronting. The CCa index was calculated for the foursymmetrical columns (C1 to C4, C4 to C1 in Figure 1)
and the CCa value increases as contact becomes more
central in the alveolar zone. The Qp index was
obtained by averaging all on-electrodes in the palatalzone by the total number of 24 electrodes. This value
is related to the degree of tongue dorsum raising.
In order to explore the timing of the lip gesture on
the /stɹ/ cluster, the EMA data of the lower lip
movement (LLx, LLy) was examined. The timecourse of the lower lip gesture was identified by thefour points (onset, target, release, and release offset)
proposed by [8]. These time points were specified at
20% of the tangential velocity peak associated withmovement towards, or away from, a target
constriction. The total duration was the time interval
between the onset and the release offset of a given
movement and this was used to inspect the temporalextent of the lip gesture in the production of the /stɹ/
cluster.
The question to be explored in this study was
which phase of the /stɹ/ cluster the constriction peakof the lower lip gesture was aligned to. For thispurpose, the times of the tangential velocity minimumof the lower lip vertical movement (LLy) and the
lingual gestures (the tongue tip, blade, and dorsum)
were identified. Based on a qualitative observation,
the interaction between the lip gesture and the lingual
gesture was studied.

3. Speech Production and Speech Physiology/s/

ID: 483

/ʃ/

/s/

SE

SE

JM

JM

SA

SA

AP

AP

/t/ closure/t/ release

Figure 2: Averaged EPG contact patterns for theword-initial /s/ and /ʃ/. (‘black’ 100-80%; ‘grey’
79-60%; ‘dotted’ 59-40%; ‘white’ 39≥)

Figure 3: Averaged EPG contact patterns for theword-initial /stɹ/ cluster: /s/ at the MAX point, /t/closure, and /t/ release. (‘black’ 100-80%; ‘grey’
79-60%; ‘dotted’ 59-40%; ‘white’ 39≥)

3. RESULTS AND DISCUSSION

Figure 2 above shows the EPG palatograms of theword-initial /s/ and /ʃ/ averaged over the productionsof the target words. Figure 3 shows the EPG
palatograms of the word-initial /stɹ/ cluster, which
illustrate the MAX constriction of /s/, the closure of /t/,and the release of /t/, averaged over the productionsof the target words.
The EPG patterns in Figure 2 serve as a basis forcomparison. For the word-initial /s/, a narrow
constriction is formed in the dentalveolar and alveolar
regions (rows 1-2) with accompanying side contact.
There are noticeable differences in the constriction
formation among the speakers: apical (SE) andapicolaminal (JM, SA, AP) realization. For /ʃ/, thenarrow constriction is primarily made in the alveolar
and postalveolar regions (rows 2-4). This suggeststhat the tongue blade is involved in the constriction
formation with the tongue tip directed downwards.

For /s, ʃ/ in the word-initial position and /s/ in the/stɹ/ cluster, the mean values of the contact indices are
summarized: CAa in Table 2, CCa in Table 3 , and
Qp in Table 4 (next page). To examine the differences
between the singleton /s/ and the /s/ in the cluster, a ttest was carried out separately for the four speakers.
An examination of CAa showed a significant
decrease only for speaker SA [t(8)=25.22, p<0.01]:
SE [t(8)=0.92, p=0.38], JM [t(8)=0.74, p=0.47], andAP [t(8)=1.93, p=0.08]. Thus, the constriction
location in the alveolar zone shifted considerably
further back in SA’s production.
In accordance with the results of CAa, a comparison of CCa revealed a significant decrease only
for speaker SA [t(8)=3.78, p<0.01]: SE [t(8)=0.06,
p=0.95], JM [t(8)=0.85, p=0.41], and AP [t(8)=0.68,
p=0.51]. As seen in Figure 3, for SE, JM, AP, there isno substantial divergence between the constriction
width for the /s/ in the cluster and that for the wordinitial /s/. In contrast, for SA, the constriction width
becomes wider at the (post)alveolar region (rows 2-3).

speaker
SE
JM
SA
AP

speaker
SE
JM
SA
AP

3.1. Linguopalatal contact patterns/s/0.96 (0.01)
0.96 (0.02)
0.93 (0.01)
0.95 (0.01)/ʃ/
0.65 (0.04)
0.54 (0.15)
0.41 (0.15)
0.77 (0.09)/s/ in /stɹ/
0.95 (0.02)
0.95 (0.01)
0.71 (0.01)
0.93 (0.02)

Table 2: Mean values of the alveolar contact
anteriority index (CAa) for singleton /s/ and /ʃ/, and/s/ in the /stɹ/ cluster. (s.d. in the bracket)/s/0.61 (0.11)
0.57 (0.08)
0.70 (0.01)
0.48 (0.01)/ʃ/
0.35 (0.08)
0.33 (0.14)
0.36 (0.09)
0.34 (0.05)/s/ in /stɹ/
0.60 (0.12)
0.54 (0.04)
0.49 (0.11)
0.47 (0.04)

Table 3: Mean values of the alveolar contact
centrality index (CCa) for singleton /s/ and /ʃ/, and/s/ in the /stɹ/ cluster. (s.d. in the bracket)999

3. Speech Production and Speech Physiology

speaker
SE
JM
SA
AP

/s/
33.33 (4.56)
36.67 (7.17)
27.50 (3.33)
26.67 (2.04)

ID: 483/ʃ/41.67 (8.74)
45.83 (3.73)
38.33 (4.86)
52.50 (5.65)/s/ in /stɹ/
46.67 (9.28)
39.17 (6.24)
34.17 (5.53)
39.17 (11.67)

Table 4: Mean values of the quotient of overall
electrode activation at the palatal zone, rows 6-8(Qp) for singleton /s/ and /ʃ/, and /s/ in the /stɹ/
cluster. (s.d. in the bracket)

The results of Qp indicated a significant increase
only for SE [t(8)=-2.57, p=0.03]: JM [t(8)=-0.52,
p=0.61], SA [t(8)=-2.06, p=0.07], and AP [t(8)=-2.11,
p=0.06]. As seen in Table 4, the increase in the palatalcontact (rows 6-8) was found for all the speakers, andthe significance of SE might be related to the
production characteristic specific to the speaker: theapical articulation of /s/.
Based on the results of comparing the EPG indices,/s/-retraction was found for SA (characterized as aretractor) but not for SE, JM, and AP (characterized
as non-retractors). Note that all the four speakers
pronounced the /t/ in the /stɹ/ cluster as affricated.
It is worth noting that, while the non-retractors
showed the EPG contact configuration of the /s/ in thecluster similar to the word-initial singleton /s/, they
revealed idiosyncratic patterns at the /t/ closure andrelease phase. For JM, the contact at the frontmostrow (row 1) tended to be lost: the complete closure
for /t/ shifted backward (row 2); the constrictive
approximation at the release phase of /t/ became
similar to that of /ʃ/. For AP, the contact at both sides
increased at the closure and release phase of /t/, andthe contact configuration at the release phase wasalmost the same as the word-initial /ʃ/. For SE, theEPG contact at the closure and release phase of /t/
showed no substantial changes to /ʃ/: SE’s productionof the /stɹ/ cluster can be transcribed as [stswɹ]. It isevident from our observation that when the‘articulatory’ retraction of /s/ occurs in the /stɹ/ cluster,
the affrication of /t/ also takes place, not vice versa.
(a) SA

/ s

t

ɹ

iː

t /

3.2. Timing of the lower lip movement

Figure 4 (a, b) below shows the EMA recording of the
tongue tip (TT), the tongue blade (TB), the tongue
dorsum (TD), and the lower lip (LL) in the productionof ‘street’ in the MOCHA sentence, ‘Spring Street isstraight ahead.’ For speaker SA (a retractor) and SE(a non-retractor), the raising gesture of the lower lip
extends over the whole /stɹ/ cluster (see the box in the
LLy panel). The constriction peak of the lip gesture is
attained at the release phase of the /t/. After the
release of /t/, the lingual gestures (TTx, TBx, TDx)
move smoothly to prepare the following /ɹ/. (For SE,
no clear TD velocity minimum was observed.)
Such a coarticulatory accommodation of the lower
lip was systematically observed across speakers andthe /stɹ/ cluster in the target words. This could beconsidered as the effects of the rhotic /ɹ/. Anotherpossibility is that, if the /stɹ/ sequence is realized
phonetically as [stʃɹ], a postalveolar affricate might
be accompanied by the lip rounding [4]. To
understand these patterns fully, we need to analyze
the rhotic gesture in various consonant sequences (e.g.
/st/, /tɹ/, /spɹ, skɹ/), and to investigate the acoustic
consequences of the lip gesture in the production of
the cluster involving the /ɹ/.
4. CONCLUSION
The current study has explored the articulatory
characteristics of the /stɹ/ cluster in British English.
We have shown that (i) while /t/-affrication isobserved for all the speakers, the ‘articulatory’ retraction of /s/ in the /stɹ/ cluster is found for one speaker
only; (ii) the constriction peak of the lower lip gesture
is systematically attained at the release phase of the/t/ in the sequence; and (iii) the non-retractors reveal
idiosyncratic EPG contact patterns at the closure andrelease phase of /t/. These results provide evidence forspeaker-dependent strategies and variability in therealization of /s/-retraction. Articulatory coordinationin various clusters and its acoustic consequences will
be substantiated by subsequent research.
(b) SE

/

s t

TTx
TBx
TDx

LLx

LLy

Figure 4: EMA trajectories of TTx, TBx, TDx, LLx, and LLy for (a) speaker SA and (b) speaker SE.

(red line=the constriction peak of LL; blue line=the constriction peak of TTx, TBx,TDx)

1000

ɹ

iː

t /

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ID: 483

5. ACKNOWLEDGEMENTS
I would like to thank anonymous ICPhS reviewers for theircritical and constructive comments. All remaining errors
are of course mine. This research is supported in part by agrant from Japan Society for the Promotion of Science,
Grant-in-Aid for Scientific Research (C) 2023, Grant no.
21K00487.

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