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Correlations between successive segment durations: values in clusters
Journal ofPhonetics (1973) 1, 111-116.
Correlations between successive segment durations: values in clusters Mark Haggard Department of Psychology, Queen's University, Belfast BT7 lNN, Northern Ireland Received 24th August 1972
Abstract:
Correlations between durations of successive segments are reported for six different English clusters, three prevocalic and three postvocalic. The results show predominantly negative correlations for the prevocalic but not the postvocalic clusters, supporting a difference in the organization of serial articulatory gestures between the two positions.
Introduction Workers such as Kozhevnikov & Chistovich (1965) and Lehiste (1970)have used correlations between duration measurements to draw conclusions about timing patterns in speech production. The results of this work and its more general methodological problems have been reviewed by Ohala (I 970). The basic idea is that the durations of two successive acoustical or articulatory segments maximally connected with two successive phonological units will display, over many repeated tokens, a negative correlation. This negative correlation is used to infer a fitting of the articulatory gestures for the units into an overriding plan rather than a low order chaining process where commencement of each gesture awaits termination of the previous one. The presence of a correlation is indeed characteristic of an organized, planned performance while zero correlation is characteristic of a chaining process ; but we cannot identify negative correlation with a process whereby duration of some higher order unit is held constant by "correcting errors" in the duration of the first sub-unit when producing the second. The concept of timing compensation is too teleological to employ for a phenomenon that could be explained by a variety of processes. A sizeable negative correlation requires that the variance of one segment boundary be larger than that of the previous and following boundaries ; it thus depends upon a large variance of the internal boundary as well as the small variance of the outer boundary that is related to the operation of a speech motor plan. Nor can we necessarily identify absence of negative correlation with absence of higher order structuring; for example two consonants very closely integrated into a cluster might jointly undergo duration fluctuations relative to neighbouring phones and be positively correlated as a consequence. Were such a result ever obtained it would tell us something interesting about the hierarchy of units involved in production, but would introduce the embarrasing ambiguity that any non-zero correlation could be the result of opposing positive and negative correlations reflecting " plans" at different levels. For this reason we would not wish to join Lehiste (1970) in asserting against MacNeilage & de Clerk (1969) that VC combinations are more bonded or organized in English than are CV, simply because they display more negative correlation. Correlation values are rather
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to be treated as one source of potentially useful data on articulatory organization whose precise meaning we do not yet know. In a previous paper (Haggard, 1973) we showed abbreviation of oscillographically measured segment durations in clusters as compared with durations of the corresponding single consonants. The results seemed to be partly derivable from the phenomenon of overlapping coarticulation as classically conceived (Ohman, I966) in that most abbreviation was found for clusters involving consonants produced with different articulators. The degree of coarticulation estimated by the obtained amount of abbreviation will not necessarily relate to the amount of joint participation in a fixed articulatory plan that is held to be measured by negative sequential correlations. For example, the earlier investigation showed that in a cluster such as j brj the exclusively bilabial articulation for the stop permits the advancement of onset of the jrj articulation behind the jbj articulation and, within acquired constraints upon intelligibility, its correspondingly early termination. It is reasonable that the amount of such an overlapping, which is optional but limited, should vary over time and possibly from token to token. This will make the total variance large but not necessarily contribute to the variance of the j bj-jr j boundary and hence produce only moderately negative correlations. A sequence such as /dr/ permits less overlapping and less apparent abbreviation of jr j ; but as the two consonants in jdr/ are homorganic it could have a more fixed integrated structure as a pair of gestures and hence display highly negative correlations. Bearing this possible dissociation in mind it seemed desirable to attempt to validate with correlation measurements the concept of homorganicity as a determinant of durations in clusters. Method
Measurement considerations Three types of clusters were employed in both prevocalic and postvocalic positions. Experiment I furnished homorganic compatible and heterorganic clusters jsl, Is, fl, If/ and Experiment 2 furnished homorganic incompatible clusters /dr/ and /IJ/. The simultaneous change of manner and voicing in all cases but /dr/ and the considerable differences in the critical constrictions required for the two consonants in each cluster make interpretation of the boundary internal to the cluster relatively simple. The criterion adopted for the fricative-semivowel boundary was the end of discernible friction on the oscillographic trace. This is preferable to voicing onset, but in some prevocalic tokens it still has a disadvantage. The level of friction in the frequency range encompassed by the oscillogram is often comparable to the level of aspiration observed in a following voiceless /1/, and a boundary cannot be drawn between these two acoustical segments. In some subjects there is a type of burst on the change between fsj and /1/ and for some an observable level change in the waveform marks the corresponding change between /f/ and /1/. Accordingly, it was decided for initial clusters in Experiment I to measure on all subjects the duration of initial fricatives, and of semivowels plus vowel according to the above criterion, and then to measure on those, for whom it was possible, three durations: initial fricative; semivowel comprising any aspirated portion and the voiced portion up to termination of the lateral articulation; vowel to voice offset for final consonant to see whether the degree of correlation between the consonants of a cluster was similar to that between semivowel and vowel. Procedure Equipment and conditions were in general similar to those of the previous investigation but only a microphone trace was employed. Measurements were obtained from I2 young
Successive segment durations
s
a Figure 1
113
Oscillogram redrawn schematically from original, illustrating segmentation procedures, in particular the identification of a voiceless portion of /1/.
adult subjects, 7 male, 5 female. The nonsense utterance "Else a slit, elf a flit" was repeated approximately 50 times by 6 speakers. They were instructed to speak as fast as they could without feeling strain, to maintain a constant rate and put no juncture before the "a". Six speakers were given a reversed order of the two phrases in the utterance. As the speech was fluent greater difficulty was experienced than in previous investigations with segment separation of /1/ from the vowel ; so the combined durations /1/+ vowel and vowel +/1/ were taken as the best indices of /1/ duration available for all12 subjects. In this set of measures the fricative included aspirated or fricated /1/. For the three subjects of each order-group whose prevocalic clusters were the most easily measured in the respects outlined above, a set of "refined" measures was also extracted; this involved separation of true fricative, true /1/ with voiceless and voiced parts and then vocalic transition and vowel as depicted in Fig. 1. The remaining six subjects yielded only the first set of measures. In Experiment 2 ten different subjects-8 male and 2 female-repeated the utterance "How Welsh a dream" approximately 50 times. Here an attempt was made to segment between the /c/ and /1/ requiring the discarding of some tokens, but in no subject did the number used for the correlation fall below 39. The measures were punched on tape and analysed by a Pearson partial correlation program. Table I Pearson correlations between successive segment durations for 12 speakers in Experiment 1. Each correlation based on between 40 and 60 pairs of durations
Speaker
/Ell-If/
/f/-/11/
/Elf-/sf
/s/-/11/
1 2 3 4
-0·161 -0·528 -0·142 +0·059 -0·107 -0·012 +0·201 +0·125 +0·031 +0·262 -0·220 +0·103
-0·037 -0·121 -0·164 -0·151 -0·065 -0·248 -0·026 -0·095 -0·129 -0·018 -0·348 -0·067
-0·266 +0·352 +0·039 +0·167 -0·130 0·092 -0·393 -0·150 +0·100 +0·210 -0·115 +0·215
-0·358 -0·061 -0·212 -0·025 -0·515 --{)·593 -0·269 -0·491 -0·487 -0·389 -0·205 -0·343
5
6 7 8 9 10 11 12
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Results Individual correlations upwards of ±0·30 may be considered significant. Table I shows the correlations obtained with the more coarse measures used in Experiment 1; the correlations for final clusters are not significantly different from zero or from one another. In the prevocalic cases the correlations are as a group significantly negative; those for /sf with flf plus vowel are significantly more negative than those involving /f/. The correlations on the "refined" measures in Table II exhibit several interesting trends. The correlations within clusters are more convincingly negative. The /If-vowel correlations are significantly nearer zero than the fricative-/If correlations are (2 exceptions out of I2) while tending nevertheless to be negative, and consistent for a given speaker between the two types of cluster. The /fl / cluster now has a more negative correlation than fsl/ although the two sets of measures only differ markedly where near-zero correlations were obtained with the coarser measures. The change brought about in the relative values of the /fl/ and /sl/ correlations by applying the "refined" measures is itself significant (Wilcoxon T = 0, p < 0·05). Table II Pearson correlations and partial correlations on refined measures of segments for 6 speakers in Experiment 1
Speaker 3 4 6 10 11 12
/fl/
/fl/
/II/
j sl j
/sl/
/II/
- 0-460 - 0·283* - 0·647 - 0·659 - 0-485 - 0-416 - 0·562 -0·555 - 0·607 - 0·635 - 0·303 - 0·311
+ 0·381 + 0·179 +0·113 + 0·233 + 0·184 +0·041 + 0·002 - 0·103 - 0·167 - 0·339 +0·171 + 0·185
- 0-442 -0·278 + 0·101 +0·228 - 0·302 - 0·239 - 0·156 - 0·188 - 0·176 - 0·344 +0·018 +0·424
-0·290 - 0·290 - 0·402 - 0-403 - 0·353 - 0·215 - 0-457 - 0-460 - 0·348 - 0·270 - 0-431 - 0-424
+0·009 +0·016 - 0·058 - 0·062 +0·227 +0·041 - 0·091 - 0·265 -0·130 -0·015 - 0·129 - 0·110
+0·024 +0·027 + 0·002 -0·023 -0·495 - 0-431 -0·352 -0-441 -0-417 - 0·393 +0·070 + 0·016
* The lower row represents the correlation obtained when the effects of shared correlation with the duration of the segment not primarily involved are partially out. There is no overall tendency for the use of partial correlation, holding constant the duration of the segment not under scrutiny, to make values more negative. This suggests the data are desirably free from rate fluctuations. The correlations between the consonants and the vowel have no predominant pattern. The correlations obtained on the clusters /dr/ and /IJ/ in Experiment 2 are given in Table II. The attempt to locate the postvocalic /1/ onset does not appear to have given any more order than the coarser measures in Experiment I gave. There is an overall tendency towards negative values but this is only significant when the coefficients for both clusters are combined . As in Experiment I , there is no tendency for individual subjects to produce characteristically positive or negative correlations; nor is there any difference between the averages for either cluster and the compatible cluster in the corresponding position in Experiment I. The tendency for correlations on /dr/ to be more negative than those for flJ/, while not significant, is at least consistent with the initial-final difference in Experiment I.
Successive segment durations
115
Table ill Pearson correlations between durations of successive segments for 10 subjects in Experiment 2
Speaker
fiJI
/dr/
1 2 3 4 5 6 7 8 9 10
- 0·254 +0·149 - 0·277 -0·295 + 0·156 +0·039 +0·039 - 0·472 -0·289 -0·031
-0·481 -0·030 -0-436 - 0·557 -0·225 +0·040 -0·537 + 0·101 + 0·081 - 0·140
Discussion The differences in magnitudes between the correlations for homorganic /sl/ and heterorganic /fl / is not large enough to permit any firm conclusions about the relevance for plan models of the concepts of compatibility and homorganicity. What is clear is the extent to which the correlation value for /fl / depends upon which set of measures is used. The present data do not say whether this difference depends on the refined measures correctly allocating the voiceless /1/ or upon their removing the contribution of the vowel to the total variance. Subject 11 is one whose correlation values are least dependent on the particular measure used; he is also the only subject lacking a more positive fricative vowel correlation for /fl/ than for jsl j , but the implications of this are obscure. One quite basic problem particularly relevant to incompatible clusters affects our interpretation of these results. Urged to speak fluently, subjects may have abandoned the possibility of achieving the fully lateral /!/, particularly preceding /JI, and produced instead only a progressive velarization from vowel through fricative. Listening to the recordings confirms that some speakers do employ such a reduction of postvocalic /1/. Whatever form of articulatory organization may exist in a final cluster such as /If/ the implications for sequential correlations appear different from those in /fl/. There is then at least a superficial difference between prevocalic and postvocalic clusters. Before we can conclude that prevocalic clusters are more coarticulated or "planned" than postvocalic we should compare the results of the correlation method with those of the abbreviation method of assessing coarticulation. In the previous investigation, abbreviation was at least as definite among final clusters as it was among initial ones. This does not necessarily mean that abbreviation has the same articulatory origin in initial and final clusters. Postvocalic consonants are normally oflonger duration than prevocalic and it is reasonable that they should be more subject to reduction. As well as these phonetic reasons, there are linguistic reasons for expecting differences in the organization of prevocalic and postvocalic clusters. In English, most permissible clusters are postvocalic, and word-final, of which many are hi-morphemic. While it would beeconomicalforprevocalic clusters to have a small inventory of coarticulated motor plans, it would be economical for postvocalic clusters to develop a set of more generally applicable tactics for producing a chain of gestures in succession. This implies that we should expect to see more coarticulation on prevocalic consonants and more reduction in postvocalic. The postvocalic tactics could include target-seeking routines mediated by gamma efferent loops and the possibility
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of variable compromise upon target position, affected by fast speech or in clusters. These considerations imply that reduction and coarticulation are complementary, possibly antagonistic, methods for coping with the same problem of fitting many discrete serial gestures into a short time. For the present, however, all we have shown is that the correlation method detects a difference between prevocalic and postvocalic clusters which the measurement of abbreviation tends to submerge. Summary and Conclusions
Some tentative conclusions can be made from the present investigation. (1) Negative correlations are the rule for durations of successive consonants in prevocalic fricative-semivowel clusters; of 34 prevocalic correlations presented only three had positive values. (2) Prevocalic clusters showed more significantly negative correlation than did postvocalic. (3) The lateral semivowel/If was more negatively correlated with a preceding fricative than with a following vowel. (4) Homorganic and heterorganic clusters did not have distinctly different patterns of correlation although there could be a slightly more negative correlation for heterorganic clusters. (5) With speakers asked to maintain a constant rapid repitition of an utterance contributions to positive correlation arising from rate fluctuations appeared to exert minimal effect upon the results. With such precautions, then, removing rate fluctuations by partial correlation may be unnecessary. (6) The correlations obtained with two incompatibly homorganic clusters were somewhat variable and not significantly different from zero nor from those with compatibly homorganic clusters. Different tactics for dealing with incompatibility employed by different speakers may render the concept of incompatibility a difficult one to demonstrate as being relevant to the performance of speakers. (7) There appears to be enough potential in the sequential correlation method for it to be worth refining and extending to a wide range of phoneme sequences, situations and speaker groups. There are cases, however, where the choice of oscillographic segmentation criteria can have specific effects upon the correlation values obtained. This means that obtaining the most suitable measurements along with the numbers of tokens required for reliable correlation studies will, for some classes of phoneme, require articulatory transducers or fast spectrographic analyses. References Haggard, M . P . (1973). Abbreviation of consonants in English pre- and postvocalic clusters. Journal of Phonetics 1, 9-24. Kozhevnikov, V. A. & Chistovich, L.A . (1965). Speech; Articulation and Perception. Joint publications service. Washington DC JPRS 30. 543. Lehiste, I. (1970). Temporal organisation of spoken language. Ohio State University, Working papers in Linguistics, No. 4. MacNeilage, P. F. & de Clerk, J . (1969) . On the motor control of articulation in CVC monosyllables. Journal of the Acoustical Society of America 45, 1217-1233. Ohala, J. (1970). Aspects of the control and production of speech . University of California, L.A. Working Papers in Phonetics, No. 13. Ohman , S. (1966). Coarticulation in VCV utterances: spectrographic measurements. Journal of the Acoustical Society of America 39, 151-168.
Correlations between successive segment durations: values in clusters Mark Haggard Department of Psychology, Queen's University, Belfast BT7 lNN, Northern Ireland Received 24th August 1972
Abstract:
Correlations between durations of successive segments are reported for six different English clusters, three prevocalic and three postvocalic. The results show predominantly negative correlations for the prevocalic but not the postvocalic clusters, supporting a difference in the organization of serial articulatory gestures between the two positions.
Introduction Workers such as Kozhevnikov & Chistovich (1965) and Lehiste (1970)have used correlations between duration measurements to draw conclusions about timing patterns in speech production. The results of this work and its more general methodological problems have been reviewed by Ohala (I 970). The basic idea is that the durations of two successive acoustical or articulatory segments maximally connected with two successive phonological units will display, over many repeated tokens, a negative correlation. This negative correlation is used to infer a fitting of the articulatory gestures for the units into an overriding plan rather than a low order chaining process where commencement of each gesture awaits termination of the previous one. The presence of a correlation is indeed characteristic of an organized, planned performance while zero correlation is characteristic of a chaining process ; but we cannot identify negative correlation with a process whereby duration of some higher order unit is held constant by "correcting errors" in the duration of the first sub-unit when producing the second. The concept of timing compensation is too teleological to employ for a phenomenon that could be explained by a variety of processes. A sizeable negative correlation requires that the variance of one segment boundary be larger than that of the previous and following boundaries ; it thus depends upon a large variance of the internal boundary as well as the small variance of the outer boundary that is related to the operation of a speech motor plan. Nor can we necessarily identify absence of negative correlation with absence of higher order structuring; for example two consonants very closely integrated into a cluster might jointly undergo duration fluctuations relative to neighbouring phones and be positively correlated as a consequence. Were such a result ever obtained it would tell us something interesting about the hierarchy of units involved in production, but would introduce the embarrasing ambiguity that any non-zero correlation could be the result of opposing positive and negative correlations reflecting " plans" at different levels. For this reason we would not wish to join Lehiste (1970) in asserting against MacNeilage & de Clerk (1969) that VC combinations are more bonded or organized in English than are CV, simply because they display more negative correlation. Correlation values are rather
112
Mark Haggard
to be treated as one source of potentially useful data on articulatory organization whose precise meaning we do not yet know. In a previous paper (Haggard, 1973) we showed abbreviation of oscillographically measured segment durations in clusters as compared with durations of the corresponding single consonants. The results seemed to be partly derivable from the phenomenon of overlapping coarticulation as classically conceived (Ohman, I966) in that most abbreviation was found for clusters involving consonants produced with different articulators. The degree of coarticulation estimated by the obtained amount of abbreviation will not necessarily relate to the amount of joint participation in a fixed articulatory plan that is held to be measured by negative sequential correlations. For example, the earlier investigation showed that in a cluster such as j brj the exclusively bilabial articulation for the stop permits the advancement of onset of the jrj articulation behind the jbj articulation and, within acquired constraints upon intelligibility, its correspondingly early termination. It is reasonable that the amount of such an overlapping, which is optional but limited, should vary over time and possibly from token to token. This will make the total variance large but not necessarily contribute to the variance of the j bj-jr j boundary and hence produce only moderately negative correlations. A sequence such as /dr/ permits less overlapping and less apparent abbreviation of jr j ; but as the two consonants in jdr/ are homorganic it could have a more fixed integrated structure as a pair of gestures and hence display highly negative correlations. Bearing this possible dissociation in mind it seemed desirable to attempt to validate with correlation measurements the concept of homorganicity as a determinant of durations in clusters. Method
Measurement considerations Three types of clusters were employed in both prevocalic and postvocalic positions. Experiment I furnished homorganic compatible and heterorganic clusters jsl, Is, fl, If/ and Experiment 2 furnished homorganic incompatible clusters /dr/ and /IJ/. The simultaneous change of manner and voicing in all cases but /dr/ and the considerable differences in the critical constrictions required for the two consonants in each cluster make interpretation of the boundary internal to the cluster relatively simple. The criterion adopted for the fricative-semivowel boundary was the end of discernible friction on the oscillographic trace. This is preferable to voicing onset, but in some prevocalic tokens it still has a disadvantage. The level of friction in the frequency range encompassed by the oscillogram is often comparable to the level of aspiration observed in a following voiceless /1/, and a boundary cannot be drawn between these two acoustical segments. In some subjects there is a type of burst on the change between fsj and /1/ and for some an observable level change in the waveform marks the corresponding change between /f/ and /1/. Accordingly, it was decided for initial clusters in Experiment I to measure on all subjects the duration of initial fricatives, and of semivowels plus vowel according to the above criterion, and then to measure on those, for whom it was possible, three durations: initial fricative; semivowel comprising any aspirated portion and the voiced portion up to termination of the lateral articulation; vowel to voice offset for final consonant to see whether the degree of correlation between the consonants of a cluster was similar to that between semivowel and vowel. Procedure Equipment and conditions were in general similar to those of the previous investigation but only a microphone trace was employed. Measurements were obtained from I2 young
Successive segment durations
s
a Figure 1
113
Oscillogram redrawn schematically from original, illustrating segmentation procedures, in particular the identification of a voiceless portion of /1/.
adult subjects, 7 male, 5 female. The nonsense utterance "Else a slit, elf a flit" was repeated approximately 50 times by 6 speakers. They were instructed to speak as fast as they could without feeling strain, to maintain a constant rate and put no juncture before the "a". Six speakers were given a reversed order of the two phrases in the utterance. As the speech was fluent greater difficulty was experienced than in previous investigations with segment separation of /1/ from the vowel ; so the combined durations /1/+ vowel and vowel +/1/ were taken as the best indices of /1/ duration available for all12 subjects. In this set of measures the fricative included aspirated or fricated /1/. For the three subjects of each order-group whose prevocalic clusters were the most easily measured in the respects outlined above, a set of "refined" measures was also extracted; this involved separation of true fricative, true /1/ with voiceless and voiced parts and then vocalic transition and vowel as depicted in Fig. 1. The remaining six subjects yielded only the first set of measures. In Experiment 2 ten different subjects-8 male and 2 female-repeated the utterance "How Welsh a dream" approximately 50 times. Here an attempt was made to segment between the /c/ and /1/ requiring the discarding of some tokens, but in no subject did the number used for the correlation fall below 39. The measures were punched on tape and analysed by a Pearson partial correlation program. Table I Pearson correlations between successive segment durations for 12 speakers in Experiment 1. Each correlation based on between 40 and 60 pairs of durations
Speaker
/Ell-If/
/f/-/11/
/Elf-/sf
/s/-/11/
1 2 3 4
-0·161 -0·528 -0·142 +0·059 -0·107 -0·012 +0·201 +0·125 +0·031 +0·262 -0·220 +0·103
-0·037 -0·121 -0·164 -0·151 -0·065 -0·248 -0·026 -0·095 -0·129 -0·018 -0·348 -0·067
-0·266 +0·352 +0·039 +0·167 -0·130 0·092 -0·393 -0·150 +0·100 +0·210 -0·115 +0·215
-0·358 -0·061 -0·212 -0·025 -0·515 --{)·593 -0·269 -0·491 -0·487 -0·389 -0·205 -0·343
5
6 7 8 9 10 11 12
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Mark Haggard
Results Individual correlations upwards of ±0·30 may be considered significant. Table I shows the correlations obtained with the more coarse measures used in Experiment 1; the correlations for final clusters are not significantly different from zero or from one another. In the prevocalic cases the correlations are as a group significantly negative; those for /sf with flf plus vowel are significantly more negative than those involving /f/. The correlations on the "refined" measures in Table II exhibit several interesting trends. The correlations within clusters are more convincingly negative. The /If-vowel correlations are significantly nearer zero than the fricative-/If correlations are (2 exceptions out of I2) while tending nevertheless to be negative, and consistent for a given speaker between the two types of cluster. The /fl / cluster now has a more negative correlation than fsl/ although the two sets of measures only differ markedly where near-zero correlations were obtained with the coarser measures. The change brought about in the relative values of the /fl/ and /sl/ correlations by applying the "refined" measures is itself significant (Wilcoxon T = 0, p < 0·05). Table II Pearson correlations and partial correlations on refined measures of segments for 6 speakers in Experiment 1
Speaker 3 4 6 10 11 12
/fl/
/fl/
/II/
j sl j
/sl/
/II/
- 0-460 - 0·283* - 0·647 - 0·659 - 0-485 - 0-416 - 0·562 -0·555 - 0·607 - 0·635 - 0·303 - 0·311
+ 0·381 + 0·179 +0·113 + 0·233 + 0·184 +0·041 + 0·002 - 0·103 - 0·167 - 0·339 +0·171 + 0·185
- 0-442 -0·278 + 0·101 +0·228 - 0·302 - 0·239 - 0·156 - 0·188 - 0·176 - 0·344 +0·018 +0·424
-0·290 - 0·290 - 0·402 - 0-403 - 0·353 - 0·215 - 0-457 - 0-460 - 0·348 - 0·270 - 0-431 - 0-424
+0·009 +0·016 - 0·058 - 0·062 +0·227 +0·041 - 0·091 - 0·265 -0·130 -0·015 - 0·129 - 0·110
+0·024 +0·027 + 0·002 -0·023 -0·495 - 0-431 -0·352 -0-441 -0-417 - 0·393 +0·070 + 0·016
* The lower row represents the correlation obtained when the effects of shared correlation with the duration of the segment not primarily involved are partially out. There is no overall tendency for the use of partial correlation, holding constant the duration of the segment not under scrutiny, to make values more negative. This suggests the data are desirably free from rate fluctuations. The correlations between the consonants and the vowel have no predominant pattern. The correlations obtained on the clusters /dr/ and /IJ/ in Experiment 2 are given in Table II. The attempt to locate the postvocalic /1/ onset does not appear to have given any more order than the coarser measures in Experiment I gave. There is an overall tendency towards negative values but this is only significant when the coefficients for both clusters are combined . As in Experiment I , there is no tendency for individual subjects to produce characteristically positive or negative correlations; nor is there any difference between the averages for either cluster and the compatible cluster in the corresponding position in Experiment I. The tendency for correlations on /dr/ to be more negative than those for flJ/, while not significant, is at least consistent with the initial-final difference in Experiment I.
Successive segment durations
115
Table ill Pearson correlations between durations of successive segments for 10 subjects in Experiment 2
Speaker
fiJI
/dr/
1 2 3 4 5 6 7 8 9 10
- 0·254 +0·149 - 0·277 -0·295 + 0·156 +0·039 +0·039 - 0·472 -0·289 -0·031
-0·481 -0·030 -0-436 - 0·557 -0·225 +0·040 -0·537 + 0·101 + 0·081 - 0·140
Discussion The differences in magnitudes between the correlations for homorganic /sl/ and heterorganic /fl / is not large enough to permit any firm conclusions about the relevance for plan models of the concepts of compatibility and homorganicity. What is clear is the extent to which the correlation value for /fl / depends upon which set of measures is used. The present data do not say whether this difference depends on the refined measures correctly allocating the voiceless /1/ or upon their removing the contribution of the vowel to the total variance. Subject 11 is one whose correlation values are least dependent on the particular measure used; he is also the only subject lacking a more positive fricative vowel correlation for /fl/ than for jsl j , but the implications of this are obscure. One quite basic problem particularly relevant to incompatible clusters affects our interpretation of these results. Urged to speak fluently, subjects may have abandoned the possibility of achieving the fully lateral /!/, particularly preceding /JI, and produced instead only a progressive velarization from vowel through fricative. Listening to the recordings confirms that some speakers do employ such a reduction of postvocalic /1/. Whatever form of articulatory organization may exist in a final cluster such as /If/ the implications for sequential correlations appear different from those in /fl/. There is then at least a superficial difference between prevocalic and postvocalic clusters. Before we can conclude that prevocalic clusters are more coarticulated or "planned" than postvocalic we should compare the results of the correlation method with those of the abbreviation method of assessing coarticulation. In the previous investigation, abbreviation was at least as definite among final clusters as it was among initial ones. This does not necessarily mean that abbreviation has the same articulatory origin in initial and final clusters. Postvocalic consonants are normally oflonger duration than prevocalic and it is reasonable that they should be more subject to reduction. As well as these phonetic reasons, there are linguistic reasons for expecting differences in the organization of prevocalic and postvocalic clusters. In English, most permissible clusters are postvocalic, and word-final, of which many are hi-morphemic. While it would beeconomicalforprevocalic clusters to have a small inventory of coarticulated motor plans, it would be economical for postvocalic clusters to develop a set of more generally applicable tactics for producing a chain of gestures in succession. This implies that we should expect to see more coarticulation on prevocalic consonants and more reduction in postvocalic. The postvocalic tactics could include target-seeking routines mediated by gamma efferent loops and the possibility
116
Mark Haggard
of variable compromise upon target position, affected by fast speech or in clusters. These considerations imply that reduction and coarticulation are complementary, possibly antagonistic, methods for coping with the same problem of fitting many discrete serial gestures into a short time. For the present, however, all we have shown is that the correlation method detects a difference between prevocalic and postvocalic clusters which the measurement of abbreviation tends to submerge. Summary and Conclusions
Some tentative conclusions can be made from the present investigation. (1) Negative correlations are the rule for durations of successive consonants in prevocalic fricative-semivowel clusters; of 34 prevocalic correlations presented only three had positive values. (2) Prevocalic clusters showed more significantly negative correlation than did postvocalic. (3) The lateral semivowel/If was more negatively correlated with a preceding fricative than with a following vowel. (4) Homorganic and heterorganic clusters did not have distinctly different patterns of correlation although there could be a slightly more negative correlation for heterorganic clusters. (5) With speakers asked to maintain a constant rapid repitition of an utterance contributions to positive correlation arising from rate fluctuations appeared to exert minimal effect upon the results. With such precautions, then, removing rate fluctuations by partial correlation may be unnecessary. (6) The correlations obtained with two incompatibly homorganic clusters were somewhat variable and not significantly different from zero nor from those with compatibly homorganic clusters. Different tactics for dealing with incompatibility employed by different speakers may render the concept of incompatibility a difficult one to demonstrate as being relevant to the performance of speakers. (7) There appears to be enough potential in the sequential correlation method for it to be worth refining and extending to a wide range of phoneme sequences, situations and speaker groups. There are cases, however, where the choice of oscillographic segmentation criteria can have specific effects upon the correlation values obtained. This means that obtaining the most suitable measurements along with the numbers of tokens required for reliable correlation studies will, for some classes of phoneme, require articulatory transducers or fast spectrographic analyses. References Haggard, M . P . (1973). Abbreviation of consonants in English pre- and postvocalic clusters. Journal of Phonetics 1, 9-24. Kozhevnikov, V. A. & Chistovich, L.A . (1965). Speech; Articulation and Perception. Joint publications service. Washington DC JPRS 30. 543. Lehiste, I. (1970). Temporal organisation of spoken language. Ohio State University, Working papers in Linguistics, No. 4. MacNeilage, P. F. & de Clerk, J . (1969) . On the motor control of articulation in CVC monosyllables. Journal of the Acoustical Society of America 45, 1217-1233. Ohala, J. (1970). Aspects of the control and production of speech . University of California, L.A. Working Papers in Phonetics, No. 13. Ohman , S. (1966). Coarticulation in VCV utterances: spectrographic measurements. Journal of the Acoustical Society of America 39, 151-168.