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Clear speech intelligibility: listener and talker effects

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Rajka Smiljanic at University of Texas at Austin
Rajka Smiljanic
Ann R Bradlow at Northwestern University
Ann R Bradlow
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In this study, we investigated whether the intelligibility-enhancing mode of speech production, known as "clear speech" produced by native and non-native talkers influenced speech intelligibility equally for native and non-native listeners. In a series of three experiments, we explored the effect of clear speech for various native and non-native talker and listener pairs. Combined, the results showed that "native" speech is overall more intelligible than "foreign" accented speech for both native and non-native listeners. Importantly, the proportional intelligibility gain for clear speech produced by both native and non-native talkers was similar across listener groups suggesting common speech processing strategies across all talker-listener groups.
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Clear speech intelligibility: Listener and talker effects
Rajka Smiljanic and Ann Bradlow
Northwestern University
rajka@northwestern.edu, abradlow@northwestern.edu
ABSTRACT
In this study, we investigated whether the
intelligibility-enhancing mode of speech
production, known as “clear speech” produced by
native and non-native talkers influenced speech
intelligibility equally for native and non-native
listeners. In a series of three experiments, we
explored the effect of clear speech for various
native and non-native talker and listener pairs.
Combined, the results showed that “native” speech
is overall more intelligible than “foreign” accented
speech for both native and non-native listeners.
Importantly, the proportional intelligibility gain for
clear speech produced by both native and non-
native talkers was similar across listener groups
suggesting common speech processing strategies
across all talker-listener groups.
1. INTRODUCTION
Speech intelligibility and word recognition depend
on a wide variety of talker-, listener- and signal-
dependent factors. The goal of this paper was to
examine how the native language backgrounds of
listeners and talkers (native vs. non-native)
influence communication for various listener-talker
pairs. Furthermore, we wanted to investigate
whether native and non-native hyperarticulation
articulatory strategies provide similar intelligibility
benefits for both native and non-native listener
groups. To that end, we looked at intelligibility of
plain and “clear” speaking styles in English as
produced by American English (AE) talkers and by
Croatian talkers for AE and Croatian listeners.
Clear speech is a distinct, intelligibility-
enhancing mode of speech production that talkers
naturally and spontaneously adopt under adverse
listening conditions. It is characterized by a wide
range of acoustic/articulatory adjustments,
including a decrease in speaking rate, an expansion
of pitch range and an enhancement of phonological
category contrasts in language-specific ways [1, 2,
3, 4]. These plain-to-clear speech articulatory
modifications enhance intelligibility for normal-
hearing and hearing-impaired adults, children with
and without learning disabilities and non-native
listeners, among others [1, 2, 3, 5].
In their cross-language study, Smiljanic and
Bradlow [3] showed that clear speech produced by
native speakers of English and of Croatian
increased intelligibility by 17 and 15% for English
and Croatian listeners, respectively. Moreover, the
accompanying cross-language acoustic analyses
have shown both similar and different clear speech
production strategies across English and Croatian
[3, 4]. In this paper, we extend these findings by
exploring whether clear speech strategies by native
talkers (in their L1) and non-native talkers (in their
L2) are beneficial to native and non-native listener
groups. We hypothesized that some of the clear
speech enhancement strategies produced by native
talkers are not fully beneficial to listeners who do
not share the same background L1 sound structure
[5]. Similarly, non-native talkers’ clear speech
strategies (in their L2) may include some
enhancement modifications that are specific to
their L1 and may not provide intelligibility benefit
to the native talkers of L2 but may benefit the
listeners who share their background L1.
2. EXPERIMENT 1: NON-NATIVE
LISTENERS AND NATIVE TALKERS
2.1. Method
In order to minimize the beneficial effect of
sentence context on intelligibility, we constructed
semantically anomalous sentences such as in (1):
(1) Your tedious beacon lifted our cab.
Four (3 female, 1 male) native AE talkers were
recorded in a sound-attenuated booth reading the
20 sentences once in plain and once in clear
speaking style. For the plain style, the talkers were
instructed to read as if they were talking to
someone familiar with their voice and speech
patterns. For the clear speaking style, the talkers
were instructed to read as if they were talking to a
listener with a hearing loss or a non-native speaker.
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In order to obtain equivalent overall amplitude
levels, all speech files were equated for RMS
amplitude and then mixed with speech-shaped
noise at a +5 dB signal-to-noise ratio (SNR). We
used the results for native listener-native talker
(matched) pairs reported in [3] as a baseline in
deciding the noise levels in the experiments
reported here. We aimed to achieve the same
average intelligibility range of 45-65% across
native and non-native listeners but had to take into
account factors such as using L1 vs. L2 and same
or different background L1, all of which may have
a detrimental effect on intelligibility. In this
experiment, we, therefore, increased SNR for the
miss-matched pairs of native talkers and non-
native listeners from 0 dB SNR to +5 dB SNR.
Each participant in the perception experiment
heard a total of 20 sentences produced by only one
of the talkers. Half of the sentences heard were in
plain style and half in clear style for each talker
condition. The listeners never heard the same
sentence twice. In each talker condition, clear
speech sentences preceded plain sentences so that
any effect of adjusting to the task during the
experiment could not account for the intelligibility
increase in clear speech.
16 native Croatian listeners participated in the
sentence-in-noise listening test. They were either
undergraduate students of English at the University
of Zagreb or had a significant amount of
instruction in English in regular and specialized
language schools. Their English proficiency was
high as determined by a pre-test where they
listened to 16 syntactically simple and meaningful
sentences that included words highly familiar to
non-native speakers mixed with noise at +5 dB
SNR and wrote down what they heard. The
average keyword intelligibility score for these
sentences was 43/50 (range: 33-49). In the test
condition, they were seated in front of a computer
and heard one target sentence at a time over
headphones. They could hear each sentence only
once but could take as much time as needed
between the sentences to record their answer. They
were instructed to write down every word they
heard. Each participant received a keyword correct
score out of 40 for the 10 sentences they heard in
each style (plain vs. clear). All content words were
counted as keywords. All listeners identified the
keywords as highly familiar in a pos-test.
Percentage correct scores were calculated and then
converted to rationalized arcsine transform units
(RAU) for statistical analysis [6].
2.2. Results
The results showed a significant increase in
intelligibility for clear speech when compared with
plain speech for all 4 talkers (Figure 1). The
average intelligibility score was 54% in plain and
70% in clear speech yielding the average clear
speech intelligibility increase of 16%. The result of
a paired-samples t-test showed a significant effect
of style on intelligibility score: t (3) = -9.899, p <
.01.
Figure 1: Average intelligibility scores (percentage
keyword correct) for non-native listeners in plain and
clear speaking styles for each native AE talker.
Target Word Correct
AE Talkers and Croatian Listeners
0
10
20
30
40
50
60
70
80
90
100
AF01 A F02 A F03 AM01
Talker
Percentage keyw ord
correct (%)
plain
clear
The results showed that native talkers were
successful in modifying their speech in a way that
provided more salient acoustic cues for L2
processing by non-native listeners. Compared to
the matched pairs’ results [3], the amount of
intelligibility gain by the non-native listeners in the
current experiment was very similar (17% in [3]
and 16% here). This suggests that native clear
speech strategies are equally beneficial for native
and for fairly fluent non-native listeners.
Combined, the results show that in order to achieve
a similar level of performance by native and non-
native listeners, the level of noise has to be
decreased by 5 dB. In other words, the added
difficulty in speech processing of being a non-
native listener is offset by an increase in SNR of 5
dB.
3. EXPERIMENT 2: NATIVE LISTENERS
AND NON-NATIVE TALKERS
3.1. Method
In this experiment, materials, speech elicitation
methods and the listening task were the same as in
Experiment 1. Talker and listener groups differed
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from those in 1. Here, native AE listeners listened
to non-native speech produced by Croatian talkers.
40 native AE listeners were recruited from the
Northwestern University Linguistics Department
subject pool. The talkers in this experiment were
four (2 female, 2 male) non-native speakers of
English whose first language was Croatian. They
were all undergraduate students at Northwestern
University and came to the US within five years
prior to the recordings to pursue undergraduate
degrees. They were fluent in English as confirmed
by the General Record Examination (GRE) and
Test of English as a Foreign Language (TOEFL)
scores required for admission to a US university.
Since in this experiment native listeners were
listening to their native, albeit “foreign”-accented,
speech, we lowered SNR from +5 dB in
Experiment 1 to 0 dB SNR. The 0 dB SNR
allowed us to make a direct comparison with the
results for matched pairs obtained in [3] and to
estimate how detrimental “foreign”-accented
speech is for speech intelligibility.
3.2. Results
The results showed an increase in intelligibility
that accompanied plain-to-clear speech articulatory
modifications by non-native talkers for native
listeners (Figure 2). The average intelligibility for
plain speech was 31% and for clear speech 41%
resulting in the average intelligibility gain of 10%.
The result of the paired-samples t-test showed a
significant effect of style on the overall
intelligibility score: t (3) = -3.749, p < .05.
Figure 2: Average i
ntelligibility scores for native AE
listeners in two speaking styles for all CRO/AE
bilingual talkers.
Target Word Correct
Bilingual Croatian Talkers & AE Listeners
0
10
20
30
40
50
60
70
80
90
100
CM01 CF01 CF02 CM02
Talker
Percentage keyword
correct (%)
plain
clear
Although there was some variability in how
successful non-native talkers were in modifying
their speech to accommodate native listeners (e.g.,
CM01 vs. CF02), overall their clear speech
strategies benefited native listeners. When
compared with matched pairs, intelligibility for
miss-matched pairs (non-native talkers and native
listeners) is lower. Average intelligibility in plain
and clear speech for AE matched-pairs, as reported
in [3], was 46 and 63%, respectively. There was a
decrease in intelligibility for non-native speech by
15 and 22% in plain and clear speech, respectively.
The overall gain was lower by 7% for non-native
clear speech. Combined, the results show that with
the same noise levels, “foreign” accent is rather
detrimental to speech perception. The effect of
“foreign” accent may be offset by lowering noise
levels, similar to the findings for non-native
listeners listening to L2.
The effect of non-native clear speech was
smaller compared to native clear speech for
matched pairs (10 vs. 17%) and to the non-native
listeners listening to the native speech in
Experiment 1 (10 vs. 16%) although noise levels
differed in Experiments 1 and 2.
4. EXPERIMENT 3: NON-NATIVE
LISTENERS AND NON-NATIVE TALKERS
4.1. Methods
The materials, speech elicitation methods and
listening test procedures were the same as in
Experiments 1 and 2. The talkers were the same as
in Experiment 2: four (2 female, 2 male) non-
native speakers of English whose first language is
Croatian. The listeners were 16 native Croatian
listeners drawn from the same population as in 1
(different individuals). Their fluency in English
was estimated in the same pre-test as in 1. The
average keyword correct score for the pre-test
sentences was 44/50 (range: 38-49). The SNR used
here for mis-matched talker-listener pairs was +5
dB, the same as in Experiment 1.
4.2. Results
The results showed that there was a beneficial
effect of clear speech on intelligibility, i.e., non-
native speakers produced clear speech that
increased intelligibility for listeners listening to
their L2 (Figure 3). The average intelligibility
scores were 49 and 62% for plain and clear speech,
respectively. The intelligibility increase was 13%.
The result of the paired-samples t-test showed a
significant effect of style on the overall
intelligibility score: t (3) = -5.649, p < .05.
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Figure 3: Average intelligibility scores (percentage
keyword correct) for non-native listeners in plain and
clear speaking styles for each CRO/AE bilingual
talker.
Target Word Correct
Bilingual Croatian Talkers & Listeners
0
10
20
30
40
50
60
70
80
90
100
CM01 CF01 CF02 CM02
Talker
Percent keyword correct
(%)
plain
clear
The results showed that there was a slight decrease
in intelligibility for Croatian listeners listening to
L2 by Croatian talkers (Experiment 3) compared
with the results for Croatian listeners listening to
L2 speech by native AE talkers (Experiment 1)
with the same level of noise (49 vs. 54% in plain
speech; 62 vs. 70% in clear speech, 13 vs. 16%
gain). This suggests that sharing the same
background L1 sound structure does not provide an
additional level of benefit when listening to L2
(both plain and clear speech productions).
Similar levels of intelligibility were reported in
[3] for native Croatian matched pairs (50 and 65%
for plain and clear speech; 15% gain). This
suggests that when listening to non-native speech
(L2) the level of noise that allows the same
performance as when listening to native (L1)
speech has to be lower by about 5dB, i.e.,
unfamiliarity with L2 sound structure plus
“foreign” accent in L2 can be offset by increasing
SNR levels by 5 dB for these fluent non-native
groups.
Finally, the performance of native AE listeners
listening to “foreign-accented” speech (Experiment
2) was overall lower compared with non-native
listeners listening to non-native speech with shared
background L1 (Experiment 3). This difference
could be in part due to a lower SNR in Experiment
2.
5. DISSCUSSION AND CONCLUSIONS
This study investigated how native language
background interacts with clear speech strategies in
determining levels of speech intelligibility. The
results showed that “native” speech is preferred
over “foreign” accented speech by both native and
non-native listeners. Furthermore, listening to
“foreign” accented speech affects both native and
non-native listeners regardless of whether they
share the same background L1 or not. We also
demonstrated that various talker-listener native
language mismatches (which affect intelligibility
negatively) can be offset by varying signal-to-noise
ratio levels.
Finally, the results of this study revealed that
clear speech is a beneficial articulatory
modification regardless of the listener and talker
L1 backgrounds. Moreover, if we examine
proportional clear speech increase relative to the
plain speech intelligibility (clear minus plain
divided by plain intelligibility score), there is a
remarkable similarity in intelligibility gain
regardless of the native language background of
either talkers or listeners. The average proportional
intelligibility increase for native AE talkers and
non-native Croatian listeners is 30% (Experiment
1), for non-native Croatian talkers and native AE
listeners is 32% (Experiment 2) and for non-native
Croatian listeners and non-native Croatian talkers
is 27% (Experiment 3). These results are fairly
close to the results for native-native AE and
Croatian pairs: 39% and 31%, respectively [1].
These data provide strong evidence that clear
speech as a listener-oriented and intelligibility-
enhancing mode of speech production is helpful
even when the overall intelligibility levels vary for
various listener and talker groups.
Ultimately, we would like to develop a detailed
understanding of how all of these factors interact in
real-world listening situations and how we can aid
listeners in unfavorable listening conditions.
6. REFERENCES
[1] Uchanski, R. M. 2000. Clear speech. In D.B. Pisoni and
R.E. Remez (Eds.), The Handbook of Speech Perception.
Blackwell Publishing. 207-235.
[2] Bradlow, A., Kraus, N., Hayes, E. 2003. Speaking clearly
for learning-impaired children: Sentence perception in
noise. J. Speech Hear. Res. 46, 80-97.
[3] Smiljanic, R., Bradlow, A. 2005. Production and
perception of clear speech in Croatian and English. J.
Acoustical Society of America, 118(3), 1677-1688.
[4] Smiljanic, R., Bradlow, A. In press. Stability of temporal
contrasts across speaking styles in English and Croatian.
J. Phon.
[5] Ferguson, S., Kewley-Port, D. 2002. Vowel intelligibility
in clear and conversational speech for normal-hearing
and hearing-impaired listeners. JASA, 112, 259-271.
[6] Bradlow, A., Bent, T., 2002. The clear speech effect for
non-native listeners. J. Acoustical Society of America,
112, 272-284.
[7] Studebaker, G. 1985. A ‘rationalized’ arcsine transform.
J. Speech Hear. Res. 28, 455-462.
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Chapter
  • Jan 2021
This book focuses on contemporary sociolinguistic approaches to Spanish dialectology. Each of the authors draws on key issues of contemporary sociolinguistics, combining theoretical approaches with empirical data collection. Overall, these chapters address topics concerning language variation and change, sound production and perception, contact linguistics, language teaching, language policy, and ideologies. The authors urge us, as linguists, to take a stand on important issues and to continue applying theory to praxis so as to advance the frontiers of research in the field. This edited volume in honor of Professor Terrell A. Morgan is a means of celebrating an amazing friend, advisor, and human being, who has dedicated his career to teaching graduate and undergraduate students, performed key research in the field, and helped to further pedagogy in the classroom through his textbooks, seminars and websites.
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Neighborhood-conditioned phonetic enhancement of an allophonic vowel split
Article
  • Jun 2019
Vowels are enhanced via vowel-space expansion in perceptually difficult contexts, including in words subject to greater lexical competition. Yet, vowel hyperarticulation often covaries with other acoustic adjustments, such as increased nasal coarticulation, suggesting that the goals of phonetic enhancement are not strictly to produce canonical phoneme realizations. This study explores phonetic enhancement by examining how speakers realize an allophonic vowel split in lexically challenging conditions. Specifically, in US English, /æ/ is raising before nasal codas, such that pre-nasal and pre-oral /æ/ are moving apart. Speakers produced monosyllabic words varying in phonological neighborhood density (ND), a measure of lexical difficulty, with CæN or CæC structure to a real listener interlocutor in an interactive task. Acoustic analyses reveal that speakers enhance pre-oral /æ/ by lowering it in Hi ND words; meanwhile, pre-nasal /æ/ Hi ND words are produced with greater degrees of nasalization and increased diphthongization. These patterns indicate that ND-conditioned phonetic enhancement is realized in targeted ways for distinct allophones of /æ/. Results support views of hyperarticulation in which the goal is to make words, that is, segments in their contexts, as distinct as possible.
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Stability of Temporal Contrasts across Speaking Styles in English and Croatian
Article
Full-text available
  • Feb 2008
In this paper we investigate the effect of clear speech, a distinct, listener-oriented, intelligibility-enhancing mode of speech production, on vowel and stop consonant contrasts along the temporal dimension in English and Croatian. Our previous work has shown that, in addition to enhancing the overall acoustic salience of the speech signal through a decrease in speaking rate and expansion of pitch range, clear speech modifications increased the spectral distances between vowel categories in both languages despite the different sizes of their vowel inventories (+10 in English, 5 in Croatian). Here, we examine how clear speech affects the duration of English tense ('long') vs. lax ('short') vowels, English vowels preceding voiced ('long') vs. voiceless ('short') coda stops, Croatian long vs. short vowels and Croatian and English VOT duration for voiced and voiceless stops. Overall, the results showed that the proportional distance between the 'short' and 'long' vowel categories and between the voiced and voiceless stop categories was remarkably stable across the two speaking styles in both languages. These results suggest that, in combination with the spectral enhancement of vowel contrasts, language-specific pronunciation norms along the temporal dimension are maintained in clear and conversational speech.
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A "Rationalized" Arcsine Transform
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Full-text available
Arcsine or angular transformations have been used for many years to transform proportions to make them more suitable for statistical analysis. A problem with such transformations is that the arcsines do not bear any obvious relationship to the original proportions. For this reason, results expressed in arcsine units are difficult to interpret. In this paper a simple linear transformation of the arcsine transform is suggested. This transformation produces values that are numerically close to the original percentage values over most of the percentage range while retaining all of the desirable statistical properties of the arcsine transform.
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Speaking Clearly for Children With Learning Disabilities
Article
Full-text available
This study compared the speech-in-noise perception abilities of children with and without diagnosed learning disabilities (LDs) and investigated whether naturally produced clear speech yields perception benefits for these children. A group of children with LDs (n=63) and a control group of children without LDs (n=36) were presented with simple English sentences embedded in noise. Factors that varied within participants were speaking style (conversational vs. clear) and signal-to-noise ratio (–4 dB vs. –8 dB); talker (male vs. female) varied between participants. Results indicated that the group of children with LDs had poorer overall sentence-in-noise perception than the control group. Furthermore, both groups had poorer speech perception with decreasing signal-to-noise ratio; however, the children with LDs were more adversely affected by a decreasing signal-to-noise ratio than the control group. Both groups benefited substantially from naturally produced clear speech, and for both groups, the female talker evoked a larger clear speech benefit than the male talker. The clear speech benefit was consistent across groups; required no listener training; and, for a large proportion of the children with LDs, was sufficient to bring their performance within the range of the control group with conversational speech. Moreover, an acoustic comparison of conversational-to-clear speech modifications across the two talkers provided insight into the acoustic-phonetic features of naturally produced clear speech that are most important for promoting intelligibility for this population.
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Production and perception of clear speech in Croatian and English
Article
Full-text available
  • Oct 2005
Previous research has established that naturally produced English clear speech is more intelligible than English conversational speech. The major goal of this paper was to establish the presence of the clear speech effect in production and perception of a language other than English, namely Croatian. A systematic investigation of the conversational-to-clear speech transformations across languages with different phonological properties (e.g., large versus small vowel inventory) can provide a window into the interaction of general auditory-perceptual and phonological, structural factors that contribute to the high intelligibility of clear speech. The results of this study showed that naturally produced clear speech is a distinct, listener-oriented, intelligibility-enhancing mode of speech production in both languages. Furthermore, the acoustic-phonetic features of the conversational-to-clear speech transformation revealed cross-language similarities in clear speech production strategies. In both languages, talkers exhibited a decrease in speaking rate and an increase in pitch range, as well as an expansion of the vowel space. Notably, the findings of this study showed equivalent vowel space expansion in English and Croatian clear speech, despite the difference in vowel inventory size across the two languages, suggesting that the extent of vowel contrast enhancement in hyperarticulated clear speech is independent of vowel inventory size.
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Speaking clearly for learning-disabled children: Sentence perception in noise
Article
  • Nov 2000
This study compared the speech-in-noise perception abilities of children with and without diagnosed learning disabilities (LDs) and investigated whether naturally produced clear speech yields perception benefits for these children. A group of children with LDs (n = 63) and a control group of children without LDs (n = 36) were presented with simple English sentences embedded in noise. Factors that varied within participants were speaking style (conversational vs. clear) and signal-to-noise ratio (-4 dB vs. -8 dB); talker (male vs. female) varied between participants. Results indicated that the group of children with LDs had poorer overall sentence-in-noise perception than the control group. Furthermore, both groups had poorer speech perception with decreasing signal-to-noise ratio; however the children with LDs were more adversely affected by a decreasing signal-to-noise ratio than the control group. Both groups benefited substantially from naturally produced clear speech, and for both groups, the female talker evoked a larger clear speech benefit than the male talker. The clear speech benefit was consistent across groups; required no listener training; and, for a large proportion of the children with LDs, was sufficient to bring their performance within the range of the control group with conversational speech. Moreover, an acoustic comparison of conversational-to-clear speech modifications across the two talkers provided insight into the acoustic-phonetic features of naturally produced clear speech that are most important for promoting intelligibility for this population.
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The Handbook of Speech Perception
Article
  • Feb 2006
  • Ref Rev
This article has no abstract
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The Clear Speech Effect for Non-Native Listeners
Article
  • Aug 2002
Previous work has established that naturally produced clear speech is more intelligible than conversational speech for adult hearing-impaired listeners and normal-hearing listeners under degraded listening conditions. The major goal of the present study was to investigate the extent to which naturally produced clear speech is an effective intelligibility enhancement strategy for non-native listeners. Thirty-two non-native and 32 native listeners were presented with naturally produced English sentences. Factors that varied were speaking style (conversational versus clear), signal-to-noise ratio (-4 versus -8 dB) and talker (one male versus one female). Results showed that while native listeners derived a substantial benefit from naturally produced clear speech (an improvement of about 16 rau units on a keyword-correct count), non-native listeners exhibited only a small clear speech effect (an improvement of only 5 rau units). This relatively small clear speech effect for non-native listeners is interpreted as a consequence of the fact that clear speech is essentially native-listener oriented, and therefore is only beneficial to listeners with extensive experience with the sound structure of the target language.
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Vowel intelligibility in clear and conversational speech for normal-hearing and hearing-impaired listeners
Article
  • Aug 2002
Several studies have demonstrated that when talkers are instructed to speak clearly, the resulting speech is significantly more intelligible than speech produced in ordinary conversation. These speech intelligibility improvements are accompanied by a wide variety of acoustic changes. The current study explored the relationship between acoustic properties of vowels and their identification in clear and conversational speech, for young normal-hearing (YNH) and elderly hearing-impaired (EHI) listeners. Monosyllabic words excised from sentences spoken either clearly or conversationally by a male talker were presented in 12-talker babble for vowel identification. While vowel intelligibility was significantly higher in clear speech than in conversational speech for the YNH listeners, no clear speech advantage was found for the EHI group. Regression analyses were used to assess the relative importance of spectral target, dynamic formant movement, and duration information for perception of individual vowels. For both listener groups, all three types of information emerged as primary cues to vowel identity. However, the relative importance of the three cues for individual vowels differed greatly for the YNH and EHI listeners. This suggests that hearing loss alters the way acoustic cues are used for identifying vowels.
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Clear speech The Handbook of Speech Perception
  • Jan 2000
  • 207-235
  • R M Uchanski
Uchanski, R. M. 2000. Clear speech. In D.B. Pisoni and R.E. Remez (Eds.), The Handbook of Speech Perception. Blackwell Publishing. 207-235.