Acoustic and Articulatory Features of Diphthong Production ...homepages.wmich.edu/~stasko/reprints/clarity.pdf · Acoustic and Articulatory Features of Diphthong Production: A Speech

Acoustic and Articulatory Featuresof Diphthong Production: A SpeechClarity Study

Purpose: The purpose of this study was to evaluate how speaking clearly influencesselected acoustic and orofacial kinematic measures associated with diphthongproduction.Method: Forty-nine speakers, drawn from the University of Wisconsin X-RayMicrobeamSpeech ProductionDatabase (J. R.Westbury, 1994), served as participants.Samples of clear and conversational productions of the word combine were extractedfor analysis. Analyses included listener ratings of speech clarity and a number ofacoustic and articulatory kinematic measures associated with production of thediphthong /aI/.Results: Key results indicate that speaking clearly is associated with (a) increasedduration of diphthong-related acoustic and kinematic events, (b) larger F1 and F2excursions and associated tongue and mandible movements, and (c) minimalevidence of change in formant transition rate.Conclusions: Overall, the results suggest that clarity-related changes in diphthongproduction are accomplished through larger, longer, but not necessarily fasterdiphthong-related transitions. The clarity-related adjustments in diphthong productionobserved in this study conform to a simple model that assumes speech clarity arisesout of reduced overlap of articulatory gestures.

KEY WORDS: clear speech, diphthong, acoustics, speech movement,gesture theory

A broad goal of speech research has been to provide a principledaccount of the variation observed in the physical processes as-sociated with sound production. An important source of var-

iation is the adaptation that speakers make to maintain speech claritywhen conversing in noisy environments, over weakmobile phone connec-tions, or with persons who suffer from hearing loss. Beyond the practicalmotivations of making everyday communication easier, studying clarity-related variation can also be used to address a range of basic questionsabout the underlying organization of the speech motor control system.Those acoustic and physiologic features of a specific sound class that sys-tematically varywith speech clarity aremore likely to play a primary rolein the planning, execution, and perception of that sound (e.g., Ferguson&Kewley-Port, 2002). In addition, speech clarity variationmight also serveto reveal more global organizational features of the speech motor system(Bradlow, 2002; Matthies, Perrier, Perkell, & Zandipour, 2001; Perkell,Zandipour, Matthies, & Lane, 2002).

These practical and theoretical issues have led to a growing corpusof speech clarity studies. As a whole, the studies suggest that speaking

Stephen M. TaskoWestern Michigan University

Kristin GreilickSydney, Australia

Journal of Speech, Language, and Hearing Research • Vol. 53 • 84–99 • February 2010 • D American Speech-Language-Hearing Association84

clearly involves adjustment at the segmental and su-prasegmental levels. A ubiquitous finding is that clearspeech is slower than conversational speech, which isdue to a combination of increased articulation and pausetime (Bond & Moore, 1994; Cutler & Butterfield, 1990,1991; Ferguson & Kewley-Port, 2002, 2007; Liu & Zeng,2006; Moon & Lindblom, 1994; Picheny, Durlach, &Braida, 1986). Clear speech has also been reported to beproduced with greater overall intensity levels than con-versational speech, although intensity adjustmentsmaybe greater for sounds such as obstruents (Picheny et al.,1986).

Studies examining general features of clear speechhave noted that vowels tend to have longer durationsand an expanded formant space (Bond & Moore, 1994;Picheny et al., 1986). More recent studies of clarity-related changes in vowels have confirmed and expandedon these basic findings. Bradlow (2002) studied 12mono-lingual English and nine bilingual Spanish–Englishtalkers (speaking English and Spanish) as they producedvarious consonant–vowel (CV) combinations embeddedin words during conversational and clear speaking condi-tions. Both English and Spanish speakers showed simi-lar consonant context effects on vowel formants for bothclear and conversational speech even though the clearlyproduced vowels were much longer in duration. Bradlowsuggested the maintenance of coarticulation in the clearcondition was intentional and served listener-orientedobjectives. Second, there was a clarity-related increase inthe size of the vowel formant space regardless of language.Vowel space expansion in both the crowded English andthe sparser Spanish vowel space was interpreted as sup-port for the notion that speakers apply a global clearspeech strategy that also includes segments unlikely tobe confused.

In a series of studies, Ferguson (Ferguson, 2004;Ferguson&Kewley-Port, 2002, 2007) evaluated acousticand intelligibility features of vowels produced duringclear and conversational conditions. Using data from asingle talker, Ferguson and Kewley-Port (2002) foundthat during clear speech, back and front vowels wereproducedwith lower andhigher second formant (F2) val-ues respectively, whereas all vowels showed an elevatedfirst formant (F1), presumably due to a lower jaw posi-tion. The authors noted that only those vowels in morecrowded regions of the vowel space showed more dy-namic vowel formants in the clear condition, suggestingthat such transformations are not global, but vowel spe-cific. Ferguson (2004) examined the degree of talker var-iability for clear and conversational speech by developinga data set of 41 male and female speakers producingEnglish vowels in a /bVd/ context. Vowel intelligibilityvaried widely across the talker group for both clear andconversational productions. An interesting result was

that not all speakers showed an obvious clear speechimprovement in vowel intelligibility. Intelligibility im-provement ranged from –12% to +33%, indicating thatan instruction to speak clearly does not assure a talkerwill effect an intelligibility improvement, at least forvowels. Ferguson and Kewley-Port (2007) followed upthis work with a study aimed at evaluating the acousticfactors that differentiate speakers with a large, clearspeech benefit from those with no clear speech benefit.Using the Ferguson (2004) database, the authors com-pared six talkers who showed no clear speech benefitwith six talkers who showed a large, clear speech benefiton a number of acoustic measures. Results indicated thatthe large benefit group showed larger increases in vowelduration, a greater expansionof steady-state vowel space,a greater overall increase in F1, and higher F2 valuesfor front vowels. Two measures of vowel formant dy-namics did not differ for the large-benefit and no-benefitgroups.

Comparatively little is known about the effect ofspeech clarity on diphthong production. Given that diph-thongs involve a marked articulatory and acoustictransition (Gay, 1970), it is unclear how findings ob-tained formonophthongswould generalize to diphthongs.Wouters and Macon (2002) is one of the only studies weare aware of that specifically evaluated clarity-relatedadjustments to diphthongproduction. The authors used aspectral rate of change measure based on the first threeformants to compare liquid-vowel, vowel-liquid, anddiph-thong transitionsproduced bya single talker across rangeof speech conditions, including clear speech. The authorsfound that for liquid transitions, spectral rate of changewas greater for clear speech. This result was similar toMoon and Lindblom’s (1994) finding for clearly producedglides, although a different approach for measuring tran-sition rate was used. In contrast to the liquid transition,Wouters and Macon found that clearly produced diph-thongs showed a reduced spectral rate of change. Thesefindings suggest that clarity-relatedadjustments inacous-tic transitions may be phoneme specific.

Given that one of the most common findings in clearspeech studies is a reduced speaking rate, itmaybe help-ful to review how speaking rate influences diphthongproduction. Gay (1968) examined the formant charac-teristics of a range of diphthongs produced by five adultmen in a variety of CVC contexts at fast, conversational,and slow speaking rates. He found that for a given diph-thong, the F1 and F2 values at transition onset and theF2 transition rate were minimally affected by speakingrate. However, the formant offset values were clearlyrate dependent. Slow speech rate was associated withmore extreme F1 and F2 offset values, whereas for thefast rate, there appeared to be a truncation of the for-mant transitions.

Tasko & Greilick: Speech Clarity and Diphthongs 85

Weismer (1991) also examined rate-induced varia-tions in diphthong production and provided clear evi-dence that the duration and extent of diphthong-relatedformant transitions were not independent. On the basisof the analysis of three healthy adult talkers speaking atslow, habitual, and fast rates, Weismer found a strongpositive relationship between F2 transition durationand extent for the diphthong /aI/. However, the best fitline for the duration-extent relationship was quadratic,suggesting that shifting from fast to slow ratemaynot bea strictly linear process.

Tjaden and Weismer (1998) tested the hypothesisthat rate modification may involve a relatively simpleprocess of varying the degree of overlap in the core ar-ticulatory gestures used to form the utterance. Fast-speaking rates would involve more gestural overlap,whereas slow-speaking rates would be achieved byreducing the amount of overlap. With such a strategy,scaling adjustments to individual gestures would beunnecessary because the overlapping of gestures wouldserve to truncate or expand the amount of gesture real-ized in production. Using this framework, Tjaden andWeismer argued that for CV contexts, formant values atvowel onset would be closer to theoretical locus valuesfor the consonant at slow rates (due to less overlap) andfarther from locus values for fast rates (due to greateroverlap). Results from eight speakers across a range ofCV conditions (including the diphthong /aI/) producedalong a rate continuum provided only partial support forthe hypothesis. A wide range of speaker variation wasobserved, making generalization difficult. In particular,the success of the model for predicting the formantfeatures of diphthongs was highly variable.

Weismer and Berry (2003) evaluated second formantpatterns for a number of vowels, including the diphthong/oI /, as six healthy talkers recited /bVC/ words in a car-rier phrase across a continuum of speaking rates. Tworesults relevant to the present study were found. First,the F2 frequency of the schwa vowel preceding the targetword had a significant negative relation to target vowelduration formonophthongs but not for the diphthong /oI/.This relation between preconsonantal vowel formant val-ues and postconsonantal vowel duration was interpretedas evidence for rate-induced variations in anticipatorycoarticulation. More specifically, slowed speech was as-sociated with reduced anticipatory coarticulation. Second,attempts to use relatively simple temporal and spectralrescaling approaches failed to account for rate-inducedvariations in /oI/ formant patterns. In otherwords, theredoes not appear to be a simple rate transform to whichall speakers adhere. For example, some speakers variedthe duration of the initial formant steady-state, whereasothers varied themaximum rate of change of the formanttransition.

Overall, the results of speaking rate studies thathave included diphthongs indicate that changes in dura-tional aspects of the sound are typically accompanied bychanges in the duration and extent of the acoustic tran-sition, and, although the form of this transition may besystematic (Weismer, 1991), it does not necessarily con-form to simple speaker-wide rules (Weismer & Berry,2003).

The goals of this study were threefold. The first goalwas to describe how speech clarity impacts the acousticand articulatory kinematic features of diphthong pro-duction in a large group of healthy speakers. A secondgoal was to compare these clarity results with publishedspeech rate data to determine whether the two speechmodes operate in a similar way for diphthong produc-tion. A third goal was to attempt to account for claritychanges for diphthongs using a relatively simple modelof gestural overlap.

MethodSpeakers

Speakers were selected from the University of Wis-consin X-ray Microbeam Speech Production Database(XRMB-SPD; Westbury, 1994). This publicly availabledatabase includes the acoustic signal and synchronousmidsagittal-plane motions of eight fleshpoints recordedfrom 57 neurologically and communicatively healthy,native English-speaking young adults performing arange of speech and nonspeech oral activities. Not allspeakers could be used in this study. For technical rea-sons, most speakers in the data set have some missingdata. In some cases, a particular record was not re-corded. In other cases, a record exists, but a portion ofthe data was not usable. Each speaker ’s data set wasinspected to establish that the records containing therelevant speech samples exist and to ensure that therecords of interest were relatively complete. Only thosespeakers whose records of interest were either absent orcontained large amounts of missing data were excludedfrom the study. This process yielded 49 speakers. Theparticipant pool was weighted toward female speakers(29 women and 20 men). The median speaker age was20.9 years (range = 18.3–37.0 years). The vast majorityof speakers spent their early years in the midwesternstates, so it might be reasonably assumed that mostspeak an upper midwest dialect of American English.However, dialect was not formally assessed. Additionaldetails of chosen speakers’ demographic and physicalcharacteristics can be found in the XRMB-SPD Hand-book (Westbury, 1994).

Data acquisition and processing. Briefly, the x-raymicrobeam system records articulator motion by directing

86 Journal of Speech, Language, and Hearing Research • Vol. 53 • 84–99 • February 2010

a narrow high-energy x-ray beam to track the midsag-ittal position of a number of 2- to 3-mm diameter goldpellets glued to various structures within and aroundthe oral cavity. A total of eight pellets were recorded.Four pellets were positioned on the midline surface ofthe tongue. Two pellets were positioned on themandibleat the gum line between the central incisors and betweenthe first and second molar on the left side. The final twopellets were positioned midline at the vermilion borderof the upper and lower lips. Pellets were sampled atrates ranging from 40 to 160 Hz, and the simultaneoussound pressure level signal was recorded at 22 kHz.Following acquisition, the position histories for eachpellet underwent a series of processing steps. One ofthe processing steps involved reexpressing the data inan anatomically based Cartesian coordinate system inwhich the horizontal (or x) axis is located along themaxillary occlusal plane, and the vertical (or y) axis isorthogonal to the x-axis where the central maxillaryincisor meets the maxillary occlusal plane. The positionhistories of the pellets were low-pass filtered at 10 Hzand resampled at 145 Hz. For a complete description ofthe data acquisition and processing steps, see Westbury(1994). Pelletmotion reflectsmovement of discrete flesh-pointswithin and around the vocal tract. For the presentstudy, analysis was limited to motion of two fleshpointson the tongue (hereafter T2 and T3) and a single flesh-point on the mandibular incisor (hereafter MI).

Speech tasks. Speakers performed an oral reading ofthe sentence “combine all the ingredients in a large bowl”under clear and conversational conditions. To elicit theclear condition, speakers were directed to “repeat thissentence five times, very distinctly and clearly, as if youare trying to make someone understand you in a noisyenvironment. Do not pause between words.” Unlesstechnically unfeasible, the first complete production ofthe test sentence was selected for analysis. Otherwise,the second complete production was used. The conver-sational production of the test sentencewas not collectedin isolation. Rather, the sentencewas part of a long list ofsentence-level stimuli that compose the task set. There-fore, for the conversational condition, the speaker wasprompted to refer to the general instruction “I to recitethe speech stimuli at a comfortable rate and loudnessI ”

The word combine, which contains the diphthong/aI/, was extracted from the sentence for analysis. Be-cause it was the initial word in the sentence, it wasrelatively easy to extract and retain acceptable auditoryquality for use in a perceptual study. For each speaker, asingle token of the clear and conversational productionsof the word was selected for inclusion in the analysis.Using only a single replicate for analysis raises the pos-sibility that it is not representative of the subject’stypical performance. However, this choice was made inorder to maximize the number of participants and still

limit the duration of the listening study (described below)to a reasonable length.

Acoustic, Articulatory Kinematic,and Auditory Perceptual Analysis

Acoustic analysis focused primarily on the diph-thong /aI/ contained in the test word combine. First, theacoustic durations of the entire word (DURword) and thediphthong (DURdiph) were measured using a combinedwaveform-spectrogram display of TF32, an acoustic soft-ware analysis package (Milenkovic, 2000). An example isshown in Figure 1A.Word onset andword offsetwere de-fined by the /k/-related release burst and the final glottalpulse of /n/, respectively.Diphthong onsetwasdefined asthe time of the first glottal pulse following plosive release.Diphthong offsetwas defined as the time of a sudden dropin signal amplitude associated with nasal production.Durationmeasures weremade on two separate occasionsby two different graduate students trained in acousticphonetic segmentation. Interrater reliability was assessedby calculating a Pearson correlation between the twojudges’ measures. For both duration measures, the cor-relation coefficient was greater than .99, suggesting agood correspondence between the judges’ measures.

Formant Estimation and MeasurementOnce the temporal extent of the diphthongwas iden-

tified, the first (F1) and second (F2) formanthistorieswereestimated using a multistep process. SpeechTool, a locallydeveloped software package (Gayvert & Hillenbrand,2000), was used to generate a linear predictive coding(LPC)–based spectrum at 10-ms intervals for the entireduration of the diphthong. At each interval, all spectralpeaks were automatically identified by the softwareprogram. At this point, no attempt was made to identifythe peaks as corresponding to a particular formant. Fol-lowing the peak-picking algorithm, the time histories ofthe spectral peaks were overlaid on an LPC-based spec-trogram, and each token was examined visually and, ifnecessary, edited by Kristin Greilick. Those peaks thatcorresponded to the F1 and F2 regions in the spectro-gram were extracted for further analysis. For example,Figure 1B shows the F1 and F2 values extracted fromthe signal in Figure 1A. These F1-F2 histories were usedto determine the onset and offset frequencies, duration,extent, and rate of F1 andF2 transitions within the diph-thong. There are a variety of ways to measure formanttransitions. For example, Weismer and colleagues (e.g.,Weismer, Kent, Hodge, & Martin, 1988) have popular-ized a method in which transition onset or offset are op-erationally defined as the start or end of a 20-Hz formantchange over a 20-ms window. F1 and F2 are measuredseparately and, therefore, may have different onset and


offset times. This approach has been used with such reg-ularity that it could be considered a measurement con-vention. We were interested in deriving single onset andoffset time for thediphthong transition,while at the sametime recognizing that both F1 and F2 frequencies exhibitsubstantial change during /aI/ production. To accomplishthis, instead of measuring F1 and F2 change separately,

the difference betweenF1 andF2was plotted, generatinga formant separation history (see Figure 1C).Because thetransition in /aI/ is characterized by an increase in F1-F2spectral separation, this provides a single time historythat takes into account both F1 and F2 frequency change.A custom written MATLAB routine was used to generatethe formant separation history and its first-order time

Figure 1. Panel A shows the sound pressure waveform and wide-band spectrogram of the test word combine. Theduration of the test word (DURword) and diphthong (DURdiph) are marked. Panels B and C plot the F1-F2 history andthe F1-F2 separation history for the diphthong. F1 and F2 transition onset was determined by at least a 20-Hz changein the formant separation history over 20 ms. F1 and F2 transition offset was identified when the transition in theseparation history dropped below the 20-Hz change over 20-ms threshold. These time values were used to segment theformant history into a diphthong onset (DURon), diphthong transition (DURtran), and diphthong offset (DURoff). F1 andF2 frequencies at transition onset and offset were also extracted. These values were used to determine the F1 andF2 transition extents (F1extent, F2extent) and rates (F1rate, F2rate).


derivative. Diphthong transition onset was defined asthe onset of a sustained (threshold: 20 Hz over 20 ms)increase in F2-F1 velocity, whereas the transition offsetwas defined as the pointwhen the velocity drops below thisthreshold.1 Temporal measures included the durationof the diphthong-related formant transition (DURtran) andthe duration of the diphthong preceding (DURon) and fol-lowing (DURoff) the primary formant transition. F1 andF2values were extracted at transition onset (F1on and F2on)and offset (F1off and F2off). These variables were thenused to derive the transition extent of F1 (|F1off–F1on|)and F2 (|F2off – F2on|) and rate of transition of F1(F1extent/DURtran) and F2 (F2extent/DURtran).

Articulatory Kinematic MeasuresArticulatorykinematic analysis focused ondiphthong-

relatedmotion of theT2,T3, andMI fleshpoints. Figure 2Aand 2B, respectively, plot the spectrogram and fleshpointspeeds associated with target word production. The speedhistory for each fleshpoint is the magnitude of the “x”and “y” velocity vectors ( [(dx/dt)2+ (dy/dt)2]1/2), whichweredeterminedusinga3-point central differencemethodof differentiation. Figure 2C plots the mid-sagittal, two-dimensional motion paths for each of the fleshpoints forthe duration of the diphthong transition. Note that duringthe diphthong transition, the speed for each fleshpoint isgenerally characterized by an acceleration-decelerationsequence that is unimodal and bell-shaped. These speedhistories were used to define the onset and offset ofmove-ment. Movements were identified as the period boundedby two successive minima in the speed history (Tasko &Westbury, 2002). Minima correspond to the moment-of-sign change in the first-order time derivative of the speedhistory (not shown). The filled and unfilled symbolsin each speed history mark the onset and offset of thetransition-relatedmovement, respectively. These samesymbols are used to mark the onset and offset in thefleshpoint motion paths in Figure 2C. Note that duringthe diphthong transition, the fleshpoints move in a su-periorly oriented direction toward the speaker ’s palate.Once themovement boundarieswere determined, a num-ber of kinematic measures were selected for analysis.The duration (DURkin), peak speed (SPDpeak), and pathdistance (DIS) were determined for each movement. Inaddition, the “x” and “y” position and fleshpoint speedweremeasured at movement onset and offset. The pathdistance covered by eachmovement was extracted fromthe time integral of the speed history.

Auditory Perceptual Scalingof Speech Clarity

Ferguson (2004) provided evidence that individualtalkersmay be quite variable in the degree to which theyattempt to produce clear speech. Therefore, an auditory-perceptual evaluation was conducted to attain a listenerpanel rating of the clarity difference between each speak-er ’s clear and conversational production of the test word.The listener panel consisted of 30 undergraduate andgraduate students from theWesternMichiganUniversityDepartment of Speech Pathology andAudiology. All werenative English speakers with normal hearing. Each lis-tener performed the evaluation alone while seated in aquiet room in front of a PC computer, which randomly pre-sented the speech stimuli through an amplifier (Realis-tic MPA-30) and loudspeaker (Paradigm Titan v.3). ThePC computer used a 16-bit sound card to convert thedigitized stimuli to analog waveforms. The loudspeakerwas located about 100 cm from the listener, and signalswere presented at a sound pressure level of 70–75 dBA.Stimulus presentation and response recordingwere con-trolled by Alvin (Hillenbrand & Gayvert, 2005), an opensource, windows-based program for controlling listeningexperiments. Alvin allows the listener to control the rateof stimulus presentation and response. Each listenerwas presented a series of trials. Each trial contained apair of clear and conversational productions of the testword produced by one of the speakers. The test wordswere separated by a 1,000-ms pause.While the stimuluspair was presented, the computer screen displayed ahorizontally oriented “slider scale,” with the slider but-ton positioned at the scale’s midpoint. Each end of theslider scale was labeled Sample 1 ( left side) and Sample2 (right side). The following instructionswere read to thelisteners prior to commencing the experiment.

You will be presented with a series of paired wordsread by a number of different speakers. For eachpresentation, the words will have differing degrees ofclarity. Your task is (1) to indicatewhether sample oneor sample two is clearer, and (2) by howmuch. Youwilluse the mouse to move the marker from the middle ofthe scale towards the presentation that is clearest.You will indicate the degree of clarity difference byhow close you move the marker to the speech sample,the closer to either end, the larger the clarity dif-ference. Try to make each choice match the clarity, asyou perceive it.

Listenerswere allowed to play each stimulus pair asmanytimes as they wished. No practice trials were offered. Atotal of 196 signal pairs were presented (49 speakerspresented four times). Both the trial order and the wordorder within each trial were randomized for each judge.The entire listening experiment lasted approximately20–30 min.

1Analysis was also performed using the more conventional method ofidentifying F1 and F2 formant transitions separately, popularized byWeismer et al. (1988). The results using this method were nearly identicalto the data reported here except that when using the approach of Weismerand colleagues, the F2 transition rate was significantly reduced for theclear speech condition.


The Alvin program recorded the listener response toeach stimulus pair as an integer ranging from –500 to+500. For example, a score of –500 would indicate thatthe listener perceived the first sample to bemuch clearerthan the second sample, in which a rating of +500 would

indicate that the listener perceived the second sample tobe much clearer than the first sample. A rating of zerowould indicate that the listener could not distinguish theclarity of the two samples. Given the random stimu-lus order, responses were rescaled so that ratings with

Figure 2. Panel A shows the wide-band spectrogram of the test word. Panel B shows the T2, T3, and MIfleshpoint speed histories. Panel C shows the midsagittal two-dimensional motion paths of the tongue andMI fleshpoints. The horizontal axis represents the maxillary occlusal plane, and the vertical axis represents aline that is orthogonal to the horizontal axis at the central maxillary incisor. The solid and open symbolsin the speed histories respectively mark the speed minima that were used to define movement onset andoffset. The same symbols are used in panel C to mark the spatial location of the fleshpoints at movement onsetand offset. Articulatory kinematic variables selected for this study are shown for the T3 fleshpoint.


positive integers would always be associated with thestimulus in the clear conditions. For each stimulus pair(i.e., speaker), a listener rating was derived on the basisof themean of that listener ’s four ratings. A panel ratingwas then derived on the basis of the mean rating acrossthe 30 listeners. Interrater reliability was assessed us-ing the intraclass correlation coefficient or ICC (2, k)(Shrout & Fleiss, 1979). The ICC (2, k) for the listenergroup was 0.97.

Statistical AnalysisComparisons between clear and conversational con-

ditions were assessed using a series of individual re-peated measures analyses of variance (ANOVAs). Foracousticmeasures, speech claritywas considered awithin-subjects factor, and gender was treated as a between-subjects factor. For the articulatory kinematic measures,a more complex ANOVAmodel was used in which clarityand articulator fleshpoint identity served as within-subjects factors, and gender was treated as a between-subjects factor. Because the articulator factor containsmore than two levels, aHuynh-Feldt correctionwasusedto adjust the degrees of freedom. Gender was considereda factor in the statistical model because it is well knownthat males and females differ with regard to absoluteformant values. In addition, Simpson (2001) has reportedthatmales and females exhibit differences in articulatorykinematic measures during diphthong production. Whenall measures were tallied, a total of 22 separate ANOVAswere performed. In order to hold the experimentwideType I error rate to less than 5%, the p values wereadjusted for multiple comparisons (.05/22 = p < .002).

ResultsAuditory Perceptual Ratingsof Speech Clarity

Recall that the listener’s task in the auditory per-ceptual experiment was to rate the difference in claritybetween the conversationally and clearly produced testwords for each of the speakers. Therefore, the listenermay judge the clear test word to have greater clarity,equivalent clarity, or less clarity than the conversation-ally produced test word. Figure 3 plots a frequency his-togram of themean panel ratings for the speaker pool. Itmay be seen in the figure that the listener panel showeda distribution of mean ratings that are typically greaterthan zero, indicating that the clear productions werejudged to have greater clarity than the conversationalproductions. The mean panel rating for the speakergroup is 150 with a standard deviation of 85. We per-formeda one-sample t test to evaluate thehypothesis thatthe panel ratings of the speaker pool were statistically

different from zero (i.e., no judged clarity difference). Re-sults were significant, t(48) = 12.29, p < .001. However, itmust be noted that the panel ratings for individual speak-ers varied widely, spanning from –10 to +299. Panel rat-ings for a number of speakers were near zero, suggestingminimal differences in perceived clarity of the clear andconversational productions.

Acoustic ResultsThe top panel of Figure 4 is a bar plot that compares

the duration of test word, the diphthong within the testword, as well as the diphthong components for the clearand conversational speech conditions. The error barsreflect standard error. It can be observed that, with theexception of the diphthong offset duration, all segmentsare longer in the clear speech condition. Table 1 sum-marizes the results of a series of repeated measuresANOVAs performed on these data. The results supportthe observations in the top panel of Figure 4. Therewas neither a significant gender effect nor a significantClarity × Gender interaction. Both the word durationand thediphthongduration increasedduring clear speech.Did they increase to a similar degree? We completed afurther analysis in which we compared clear and casualdiphthong duration as a proportion of theword duration.During conversational speech, the diphthong occupied45% of the total word duration, whereas during clearspeech, it occupied 46% of the word. This similarity

Figure 3. Frequency histogram of mean panel rating for speakerpool. The ratings reflect the perceived difference in clarity between theconversational and clear test words. Positive mean ratings indicatethe listener panel considered the clear token to sound clearer than theconversational token. Negative mean ratings indicate that the listenerpanel considered the conversational token to sound clearer thanthe clear token. Values near zero suggest no difference between thetwo tokens.


suggests that the speaker group rescaled the word anddiphthong duration at a similar amount.

Table 2 summarizes the ANOVA results for thetransition-relatedmeasures of F1 and F2. First, the gen-der main effect was significant for many of the analyses,which is not surprising given the gender-related differ-ences in the vocal tract size.More important for the goalsof this study, there were no significant Clarity × Genderinteractions. Therefore, the results reported below arefor the speaker group as awhole. The left side of the tablesummarizes the F1 and F2 frequencies at transition on-set and offset. Significant clarity main effects were onlyobserved for F1 onset and F2 offset. These results aredisplayed graphically in the bottom panel of Figure 4.Here, it can be observed that F1 is significantly higherin the clear condition at transition onset, but there is noclarity-related difference at offset. F2, however, showsno clarity-related difference at transition onset but issignificantly higher in the clear condition at transitionoffset. The right side of Table 2 highlights the analyses ofthe features of the formant transition. There was a sig-nificant claritymain effect for F1 and F2 extent. Both F1and F2 showed significantly larger transition extentsin the clear versus conversational speech condition. Fi-nally, there were no clarity-related differences in therate of F1 and F2 transitions, suggesting that the extentand duration of the transition scaled to roughly similardegrees.

Articulatory Kinematic ResultsTable 3 summarizes theANOVA results for the range

of articulatory measures. The effects of most interest tothe study questions are the speech clarity main effectsand interactions between speech clarity and articulatortype and gender. Although all of the articulator maineffects were significant, this is a wholly expected findinggiven that each articulator fleshpoint has a unique posi-tionwithin the vocal tract and a different operating range(Tasko & Westbury, 2002) and therefore is not discussedfurther. There were no significant gender main effects orClarity × Gender interactions. Therefore we collapsedmen and women into one group for graphical display.Figure 5 summarizes the mean duration, distance, andpeak speed of the transition-related movement for theconversational and clear speech conditions. The speakergroup produced larger and longer articulatory move-ments in the clear speech condition. This is true for bothtongue and mandible fleshpoints, although a significantClarity × Articulator interaction for movement distancesuggests that the clarity-related increase in movementdistance was not equivalent for all fleshpoints. For peakfleshpoint speed, the clarity main effect was not signif-icant, but there was a significant Clarity × Articulatorinteraction. Only the T3 fleshpoint showed a significant

Figure 4. The top panel is a bar plot comparing mean acousticsegment durations for clear and conversational conditions. Thebottom panel is a bar plot comparing F1 and F2 onset and offsetfrequencies for clear and conversational conditions. Error bars reflectthe standard error about the mean. *p < .002.

Table 1. Summary of repeated measures analyses of varianceperformed on temporal measures.

Factor df

F values

DURon DURtran DURoff DURdiph DURword

Clarity 1 33.2* 70.2* 5.5 118.9* 152.1*Gender 1 1.7 2.9 0.0 0.6 1.5Clarity × Gender 1 0.0 3.2 2.1 0.8 3.9error 47

Note. DURon = duration of diphthong onset; DURtran = duration ofdiphthong transition; DURoff = duration of diphthong offset; DURdiph =duration of diphthong; DURword = duration of test word.

*p < .002.


increase in peak articulatory speed. There was not asignificant clarity main effect for the speed at move-ment onset, but a significant clarity main effect andClarity ×Articulator interaction for speed atmovementoffset. The lower right panel of Figure 4 shows theseresults. Pairwise comparisons revealed that the speedof T2 at movement offset was significantly slower forclear speech.

Figure 6 plots the mean midsagittal positions of thethree fleshpoints at the onset and offset of movement forthe conversational and clear speech conditions. A lineconnects the onset and offset. This is not the actual tra-jectory path, but represents the Euclidean distancebetween the two points. The broken line marks the con-versational speech condition, and the solid line marks

the clear speech condition. The ellipses that surroundthe onset and offset points reflect the standard error ineach dimension. This plot combined with the results ofTable 3 reveal some interesting patterns in the data.First, at movement onset, there was not a significantclarity main effect for horizontal fleshpoint position. Forvertical fleshpoint position, the clarity main effect wassignificant. In Figure 6, it can be seen that all threefleshpoints begin the articulatory transition at a lowerposition in the clear speech condition. At transition off-set, the clarity main effect and the Clarity × Articulatorinteraction was significant for both the horizontal andvertical fleshpoint positions. Figure 6 reveals that bothtongue fleshpoints were higher and more anterior in theclear speech condition. MI did not show this pattern.

Table 2. Summary of repeated measures analyses of variance performed on the formant transition measures.

Factor df

F values

F1on F2on F1off F2off F1extent F2extent F1rate F2rate

Clarity 1 11.6* 5.5 7.0 72.2* 13.9* 39.3* 0.5 8.0Gender 1 76.2* 103.3* 23.6* 66.7* 15.1* 1.5 17.1* 5.7Clarity × Gender 1 0.3 1.6 0.2 0.0 0.4 0.6 0.3 4.2error 47

Note. FIon and F2on = F1 and F2 onset; F1off and F2off = F1 and F2 offset; F1extent and F2extent = F1 and F2 transition extents; F1rateand F2rate = F1 and F2 transition rates.

*p < .002.

Table 3. Summary of repeated measures analyses of variance performed on the articulatory kinematic measures.

Factor df

F values

Position atmovement onset

Position atmovement offset Movement characteristics

xon yon xoff yoff DURkin DIS SPDpeak SPDon SPDoff

Clarity (C) 1 1.7 13.7* 19.3* 26.3* 62.7* 42.1* 2.5 0.51 16.2*C × G 1 0.51 0.01 3.5 0.04 0.03 0.66 1.0 0.15 5.5error (C) 37

Articulator (A) 2 2387.7* 334.8* 2589.2* 1223.4* 29.6* 230.9* 243.4* 16.8* 57.8*A × G 2 5.0 2.2 2.6 2.1 0.86 8.4* 7.5* 3.5 3.2error (A) 74

C × A 2 1.35 0.52 15.3* 19.9* 1.4 11.3* 10.1* 1.3 13.2*C × A × G 2 0.22 0.97 6.3 1.1 0.12 2.7 4.8 0.2 2.4error (C × A) 74

Gender (G) 1 0.06 1.7 0.37 0.28 1.6 3.1 1.3 3.9 2.8error (G) 37

Note. x : position with respect to the central maxillary incisor; y : position with respect to maxillary occlusal plane. DURkin = durationof kinematic measure; DIS = distance; SPD = speed.

*p < .002.


Subgroup analysis based on clarity judgments. Oneconcern is that some speakers in the group who were notperceived by the listener panel to have made a sub-stantive speech clarity adjustment may serve to obscureclarity-related differences in the data. To address thisissue, a subset of 34 speakers was drawn from the largerpool. The inclusion criterion for the subset was a min-imum of a 100-point conversational clear difference inthe mean listener rating. We submitted this subset to astatistical analysis that was identical to that used on thefull group. The results were identical to the full groupwith the exception of two comparisons. In the subgroup,there was a significant clarity main effect for the F1 off-set frequency, F(1, 32) = 16.70, p < .002, and the maineffect for speed at transition offset was near but did notreach significance, F(1, 25) = 10.5, p = .003. However,the Clarity × Articulator interaction remained significant,F(2, 50) = 9.9, p < .002. In the next section, we examinea smaller subgroup of speakers.

F2 and T3 “Patterns” DuringDiphthong Production

In summary, the main results suggest that clarity-related adjustments in diphthongproduction are generallycharacterized by a “scaling up” of a number of discretetemporal and spatialmeasures.We also wanted to eval-uate for clarity-related differences in the continuous time-varying patterns of the diphthong production. Becausethis would involve examining individual speaker data,instead of using the full data set, we focused attention onthe 10 speakers the listener panel judged to exhibit thelargest difference in clear and conversational produc-tions. Furthermore, we limited analysis to theF2historyand the T3 fleshpoint motion and speed over the entireduration of the diphthong. Because the speakers arequite variable in the absolute timing of the diphthonggestures, we performed a simple linear time normali-zation so all speaker formant/movement histories had a

Figure 5. These four bar plots compare clear and conversation speech conditions for movement duration, movement distance, peak movementspeed, and speed at movement offset. *p < .002.


commonoverall duration. Similarly, individual speakersshowed substantial differences in overall magnitude ofspeech events. Therefore, formant /movement historiesfor a given speaker underwent a z transformation so thatall traces had a commonmean (zero) and standard devia-tion (one). The goal was tominimize variation captured inthe previous analyses and highlight clarity-related var-iations in formant/movement history patterns. Figure 7shows the results of this analysis. Solid fine lines rep-resent traces for individual speakers. The heavy grayline marks the average trace for the group. The top twopanels shows the F2 histories, the middle two panelsshow the vertical position histories for T3, and the bottomtwo panels plot the T3 speed histories. The left columnplots the conversational productions, and the right col-umn plots the clear productions. There are a number ofobservations worth noting. First, the individual speakersshowed remarkable similarity in the F2 and T3 verticalposition traces for both conditions (although there is onespeaker who showed a unique F2 pattern for the con-versational condition). Second, the pattern of the F2history is very similar to the pattern of the T3 verticalposition. The conversational condition is characterized bylack of a clear initial steady-state and a more constantslope over the course of the diphthong. In contrast, theclear condition showsa relatively “flat” region for the firstthird of the diphthong followed by a large increase inslope for the remaining two thirds of the diphthong ’s du-ration. Finally, the T3 speed history (bottom panel) alsoshows some noteworthy clarity-related differences. First,

the conversational condition shows more cross-speakervariation as compared with the clear condition. Second,for many speakers, a steady acceleration of the flesh-point has already begun at the onset of the diphthong inthe conversational condition. In the clear condition, theprominent acceleration associated with the transitionoccurs much later. Individual speakers typically showminimal speed change, indicating relatively little move-ment during this period. This finding supports the ob-servations in the F2 and T3 vertical position plots. Forthis speaker group, the clearly produced diphthong ischaracterized by a prolonged initial steady-state inwhichthere is either little movement or movement unrelatedto the diphthong transition.

DiscussionComparison of Present Resultsto Other Clear Speech Studies

Consistent with previous studies, we also found thatclear speech was characterized by an increased durationof acoustic and articulatory kinematic events. Specifically,all measured intervals, with the exception of the diph-thong offset, were longer in the clear speech condition.However, it is important to point out that the offset is themost variable part of the diphthong and is frequentlyabsent in natural speech (Lehiste & Peterson, 1961). Itshould be noted that although the diphthong is abso-lutely longer when spoken clearly, speakers spend an

Figure 6. This x–y plot shows the spatial positions of T2, T3, and MI at onset and offset of diphthong-related articulatory transitions for conversational and clear speech conditions. Lines connecting the meanonset and offset positions that mark the conversational (broken) and clear (solid) speech conditions arebased on Euclidean distance and not on the actual path of the fleshpoints. The size of the ellipses around themean positions reflects the standard error in each spatial dimension.


Figure 7. This figure plots selected diphthong-related acoustic and kinematic traces for the 10 speakers who demonstrated the largest clarity ratingdifferences. Each solid line represents a single token produced by a single speaker. The heavy gray traces represent the average trace for thegroup. The traces were submitted to a linear time normalization procedure to eliminate individual speaker differences in overall duration. Also,each trace underwent a z transformation to reduce individual speaker differences in overall scaling. The top row plots F2 histories across the twospeaking conditions. The plots in the middle and bottom rows compare the vertical (“y”) position and speed of the T3 fleshpoint, respectively.


equivalent proportion of the total word duration in theclear (46%) and conversational conditions (45%). In otherwords, clarity-related adjustments in diphthong durationcould simply be arising out of word-level timing changes.In addition to the temporal adjustments, speakers madea number of clarity-related changes to the spectral andkinematic characteristics of thediphthong transition.Ki-nematic analysis revealed that during clear speech, thetongue (T2 and T3)-mandible (MI) complex was moreinferiorly oriented at transition onset andmoved fartherduring the transition. T2 and T3 fleshpoint movementsended at a more superior and anterior position, whereasthe MI ended movement at a position similar to conver-sational speech. Results from the formant analysis weresimilar to the kinematic results. During clear speech, F1was higher at transition onset, both F1 and F2 had largerexcursions during the transition, and F2 was higher attransition offset. The lower spatial position/higher F1position at transition onset is consistent with clarity-related adjustments observed in vowels (Ferguson &Kewley-Port, 2002, 2007). We did not find a significantclarity-related change in formant transition rate. This re-sult is at oddswithWouters andMacon (2002), who foundthat, compared with conversational mode, clearly pro-duced diphthongs exhibited a lower spectral rate ofchange. However, the large methodological differencesbetween the two studies make such a direct comparisondifficult. Peak movement speed did not systematicallyvary with speech clarity. Only T3 showed greater peakmovement speed in the clear speech condition. Theseequivocal findings are consistent with previous studiesin which the articulatory kinematic features of clearspeech have been evaluated, albeit in a different pho-netic context (Matthies et al., 2001; Perkell et al., 2002).Finally, we failed to find any gender-related interactionsin the acoustic and kinematic measures. This was a littlesurprising since Simpson (2001) reportedmarked genderdifferences in diphthong-related tongue movements usingthe same x-raymicrobeam data set (but different speechsamples) used in the our study. Althoughmethodologicaldetails are different, it is difficult to reconcile our resultswith those of Simpson.

Comparing Present Results With SpeakingRate Studies of Diphthongs

One of the strongest findings in the present studywas that durational measures of diphthong productionwere increased for the clear speech condition. In otherwords, clear speech is slower than conversational speech.This observation begs the question: Are clearly produceddiphthongs the same as slowly produced diphthongs? Insome key respects, the answer seems to be yes. Theclarity-related increases in formant extent and durationwith minimal change in formant transition rate are

similar to the results of Gay (1968) andWeismer (1991).However,wedid observe reliable clarity-baseddifferences inF1-onset frequency, which is inconsistent with Gay’s find-ings. It shouldbenoted thatenvisioning cleardiphthongs asslow diphthongs does not necessarily bring us closer to anexplanation for how the clarity transformation is achieved.Rate transformation is not a simple matter, and thereappears to be a variety of strategies that speakers mayuse (Tjaden &Weismer, 1998; Weismer & Berry, 2003).

A Proposed Mechanism for DiphthongClarity Change

It may be possible to interpret the present resultsusing the gestural overlap model outlined in the intro-duction (Tjaden & Weismer, 1998; Weismer & Berry,2003). In this model, shorter overall durations can beachieved by increasing the degree of gestural overlap,whereas increasing duration would be achieved by re-ducing the degree of gestural overlap. In other words,durational changes may be accomplished by varying thephasing of gestureswithout any rescaling of the gesturesthemselves. Recall that the diphthong /aI / was drawnfrom the word combine that was contained in the phrase“combine all the ingredients in a large bowl.” Figure 8

Figure 8. This figure outlines how a gestural sliding model might helpexplain the clarity-related variations in acoustic and articulatorykinematic measures. The thick sigmoid-shaped traces representhypothetical vocal tract constriction and opening gestures associatedwith utterance. The gray box represents the time interval duringwhich bilabial closing and opening gestures occur.


provides a simple illustration of how a gesture overlapmodelmight help explain the present data. The sigmoid-shaped traces in the figure represent hypothetical tongueblade gestures. Downwardmoving traces represent vocaltract opening gestures, and upward moving traces repre-sent vocal tract constriction gestures. The top plot repre-sents a conversational mode of speech. In this plot, thefirst trace in the sequence is a vocal tract-opening gestureassociated with the following series of events: releasefrom the velar plosive, production of the mid-centralschwa vowel, and continued motion toward the openvocal tract needed for the onset of the diphthong. Fol-lowing this opening gesture is a vocal tract constrictiongesture associated with the diphthong transition andending with a constricted position associated with thediphthong offglide (and tongue-tip elevation for thealveolar nasal). Following this constriction gesture isanother vocal tract-opening gesture that is associatedwith transition toward the low, back vowel in the wordall. The gray box in each plot marks the hypotheticaltime interval associatedwith lip closing-opening gesturesequence for the /m/-/b/ combination. In this example ofconversational speech, the tongue gestures overlap oneanother. The result is that each gesture is truncated intime and distance by the onset of the next gesture. Thisresults in a shorter overall duration of the sequence andreduced articulatory excursions. The bottom plot illus-trates how the model operates in a clear speech mode.Note that the size, duration, and sequence of the tonguegestures are identical to the conversational speakingmode. The only difference in the two plots is the amountof gestural overlap. For the clear speech mode, thereis very little gestural overlap. This change in gesturalphasing predicts some differences in diphthong featuresfor clear and conversational productions. First, the re-duced gestural overlap will result in an increase in thesize and extent of the diphthong transition without anychange in the rate of the transition. These predictionsmatch both the majority of acoustic and articulatorykinematic results (see Figures 4 and 5), although themodel does not predict the clarity-related increase in thepeak speed of the T3 fleshpoint (see Figure 5). Second,because the reduced overlap reveals more of the onsetand offset of the constriction gesture, the tongue willstart the transition in a lower position and end it in ahigher position. Furthermore, movement speed at tran-sition onset and offset should be lower because as theoverlap is minimized, more of the low-speed portion ofthe gesture is revealed. Our data provide partial supportfor these predictions. There is acoustic and kinematicevidence that the transition begins and ends in moreextreme positions (see Figures 4 and 6). However, move-ment speed was only smaller in magnitude for move-ment offset in the clear speech condition (see Figure 5).Finally, the reduction in gestural overlap also allows thebilabial constriction-release gesture sequence to occur

earlier in the tongue constriction gesture. As a result, agreater proportion of the early component of the con-striction gesture follows the bilabial release gesture,which would predict a longer formant steady-state. Thiswas observed in the plots of Figure 7.

This interpretation does come with a number ofcaveats. First, the gestural overlapmodel predicts groupdata. It is probable that there are individual speakerswho show patterns that are distinct from the group.However, Figure 7 suggests that those speakers showingthe greatest perceptually based clarity effect exhibit arelatively consistent pattern of change in diphthong pro-duction. Second, this model requires that clear speechshould also be slow speech. There is evidence that speak-ers can speak quickly and clearly (Krause & Braida,1995). Although this does not invalidate our interpreta-tion, it does imply that there may be multiple strategiesto achieve speech clarity. Third, the gestural overlapaccount predicts that clear speech should be associatedwith reduced coarticulation. This view is at odds withsome previous clear speech studies of vowels (Bradlow,2002;Matthies et al., 2001) aswell aswith rate studies ofdiphthongs (Tjaden &Weismer, 1998; Weismer & Berry,2003). Finally, our results are based on a single phoneticcontext. It is not immediately apparent how our resultsgeneralize to other diphthongs and other contexts, whichis not to suggest that findings should be identical to thepresent results. Predictions based on the gestural over-lap model would differ in their details due to a variety ofcontextual factors. The key point is that within such amodel, such predictions can be made and tested.

AcknowledgmentsPortions of this work were part of a master ’s thesis com-

pleted by the second author at Western Michigan University.Some of these results were presented at the 2004 AmericanSpeech-Language-Hearing Association Convention inPhiladelphia, PA.

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Received June 15, 2008

Accepted July 1, 2009

DOI: 10.1044/1092-4388(2009/08-0124)

Contact author: Stephen M. Tasko, Department of SpeechPathology and Audiology, Western Michigan University,Kalamazoo, MI 49008. E-mail: [email protected].


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