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Comparing Identification of Standardizedand Regionally Valid Vowels

Richard Wrighta and Pamela Souzab

Purpose: In perception studies, it is common to use vowel stimulifrom standardized recordings or synthetic stimuli created usingvalues from well-known published research. Although the useof standardized stimuli is convenient, unconsidered dialect andregional accent differences may introduce confounding effects.The goal of this study was to examine the effect of regional accentvariation on vowel identification.Method: The authors analyzed formant values of 8 monophthongvowels produced by 12 talkers from the region where theresearch took place and compared them with standardizedvowels. Fifteen listeners with normal hearing identifiedsynthesized vowels presented in varying levels of noise

and at varying spectral distances from the local-dialectvalues.Results: Acoustically, local vowels differed from standardizedvowels, and distance varied across vowels. Perceptually, therewas a robust effect of accent similarity such that identificationwas reduced for vowels at greater distances from local values.Conclusions: Researchers and clinicians should take care inchoosing stimuli for perception experiments. It is recommendedthat regionally validated vowels be used instead of relying onstandardized vowels in vowel perception tasks.

Key Words: dialect, identification, vowels

T his study consists of two experiments with two re-lated goals: to determine (a) the degree to whichvowels in the Pacific Northwest region matched

General American English vowels and (b) the degreeto which acoustic dissimilarity has an effect on vowelidentification accuracy. Although any number of dimen-sions of the speech signal may be studied to examine dif-ferences across regional dialect, we chose tomake vowelsthe focus of this study for several reasons. They arehigh-intensity quasi-periodic components of the speechsignal; moreover, they occupy lower frequencies (below3 kHz) compared with many consonants that have ape-riodic cues distributed in the higher frequencies (above3 kHz). The frequency distribution and the high inten-sity of vowels make them important perceptual toeholdsfor hearers who are listening under less-than-optimalconditions, such as in noise or with hearing loss. Vowels

are also important because their formants carry infor-mation about not only the vowel’s quality itself butalso the flanking consonants through their formanttransitions. Again, this may be particularly true underconditions of multiple distortions. Recent work byWebster (2002) indicates that as noise increases, lis-teners shift their weighting of cues away from the aperi-odic cues associated with consonants, increasing theirreliance on formant transitions. Accordingly, vowels rep-resent amethodologically important object of study (e.g.,Kewley-Port, Burkle, & Lee, 2007; Nishi &Kewley-Port,2008).

The exigencies of perceptual research and especiallyclinical testing encourage researchers and clinicians touse stimuli that havewell-known properties—in particu-lar, stimuli that have beenwidely normed. These stimuliare sometimes described as General American English.InNorth America, this has led to thewidespread use of arelatively small set of prerecorded or synthetic stimulidrawn from well-known published data. Because thestandard stimuli are taken from a small number of spe-cific geographic regions at specific points in time, thereis often a mismatch between the regional accent in thestimuli and the perceiver’s regional accent. This mis-match introduces a potential problem: Even relativelysubtle differences in accent may introduce a stimulus-goodness effect that varies by laboratory or clinic,

aUniversity of Washington, SeattlebNorthwestern University, Chicago, ILCorrespondence to Pamela Souza: [email protected]: Robert SchlauchAssociate Editor: Mark HedrickReceived October 4, 2010Revision received March 14, 2011Accepted June 3, 2011DOI: 10.1044/1092-4388(2011/10-0278)

Journal of Speech, Language, and Hearing Research • Vol. 55 • 182–193 • February 2012 • D American Speech-Language-Hearing Association182

depending on geographic location, and that may evenvary by listener if the target population is dialectallydiverse.

Regional dialect and accent effects are not uniformacross the different regions in North America; neitherare they uniform across speech sounds. Some accentsare more similar to the standard stimuli than others,and some speech sounds varymore by region than others(e.g., Clopper, Pisoni, & de Jong, 2005; Labov, Ash, &Boberg, 2006). For example, in several dialects spokenin New England and in the northern cities, urban areason the southern shores of the Great Lakes—such asChicago, Illinois; Detroit, Michigan; Cleveland, Ohio;and Buffalo, New York—there is a phonemic contrastbetween an open-mid vowel /� / and a low-central vowel/a/ (as in the words caught and cot, respectively), where-as in much of the West and the South, this contrast isreplaced with a single low-back vowel /A / (e.g., Clopperet al., 2005; Eckert, 1989; Labov et al., 2006). Moreover,different dialects typically have distinct allophonic pro-cesses for otherwise similar phonemes. Take, for exam-ple, a process often referred to as Canadian raising: thediphthong phoneme /aI / (as in “ride” [ɹaId] or “rise”[ɹaIz]) has a “raised” allophone [ÃI] before all voicelesscoda consonants (as in write [ɹÃIt] or rice [ɹÃIs]; e.g.,Vance, 1987). This allophonic variant occurs in severaldialects spoken in the northern and eastern UnitedStates but not in other dialects spoken in the Southand West. Thus, even with identical phonemic invento-ries, there may be context-specific (allophonic) differ-ences in pronunciations of particular words or syllables.

Although not all regions have been fully documen-ted, existing documentation indicates that regional dia-lects differ not only in the number of vowel contrasts andphonetic realization of vowel allophones but also in theacoustic values that phonemes have even if they are con-ventionally transcribed using the same phonetic symbol.Although this last point may seem obvious to research-ers who are familiar with regional variation and tran-scription conventions, the symbols that are used torepresent vowel categories are often taken as true val-ues by researchers in clinical and psychological settingswho are not trained in regional variation. As a result, itis common to assume that standardized vowels shouldbe used in all regions of the country where the phoneticsymbols match the symbols used in the standarddescriptions. For example, whereas the symbol /u / isused to represent a vowel that is present as a phonemein all dialects of North American English, the acousticrealization of this vowel varies from a high-back variantin Wisconsin to a high-central variant in southern Cali-fornia and parts of the Deep South (e.g., Clopper et al.,2005; Hagiwara, 1997; Jacewicz, Fox, & Salmons, 2007;Labov et al., 2006). In a study of six regional vowel sys-tems, Clopper et al. (2005) found that each region had its

own realization of the vowel system even though thevowels are typically represented with the same symbolsand phonetic descriptors. Their findings led them to con-clude that vowels are best characterized in terms ofregional systems rather than in terms of General Amer-ican English, a finding that echoed Hagiwara’s (1997)observation about southern Californian English andHillenbrand, Getty, Clark, and Wheeler’s (1995) obser-vation about regional effects in their study of southernMichigan speakers.

These findings have potentially serious implicationsfor researchers who use speech stimuli in perceptionexperiments or as comparison points for speech pro-duction. In the absence of acoustic validation throughregional sampling, there is no guarantee that a particu-lar phoneme’s acoustic realization in North AmericanEnglish will be the same from one region to the next. Ig-noring regional dialect and accent variation may resultin regional asymmetries in category goodness of stimuliacross listeners in perception studies.

Phonetic differences in stimuli may seem relativelyunimportant if the task is not tapping into low-level pho-netic perception, but there is evidence that accentedspeech may impose a processing delay or a cognitiveload. For example, Clarke and Garrett (2004) foundthat initial exposure to foreign-accented speech intro-duced a temporary perceptual-processing deficit. Morerelevant to the current research, regional accent differ-ences may have a cascading effect that percolates up tohigher level tasks. Adank and McQueen (2007) con-ducted a noun-animacy (animate vs. inanimate) decisiontask in which listeners were presented with auditorystimuli in a familiar accent and an unfamiliar accent.Subjects’ response times were slower for words pre-sented in the unfamiliar accent. The effect persistedeven when listeners were exposed to 20 min of speechin the unfamiliar accent prior to the animacy decisiontask, indicating a lasting effect for unfamiliar accents.Similarly, Floccia, Butler, Goslin, and Ellis (2009)found that unfamiliar accents in certain tasks imposea processing delay that is long lasting; it continuesafter intelligibility scores reach ceiling. To compoundthe problem, prosody, intonation, syntax, and lexicalchoice vary across regions (e.g. Grabe, 2004); therefore,using sentence-length stimuli does not necessarily solvethe potential problems at the phone level and may evenmake them worse. In a study of cross-dialect intelligi-bility in noise, Clopper and Bradlow (2009) found thatdialects that were more similar to General AmericanEnglish, which included New England, the West, andtheMidland regions, weremore intelligible than regionaldialects that were more distant from General AmericanEnglish, including mid-Atlantic, northern, and southernregions. Although not all dialectal regions were equallyrepresented—so true regional effects were not fully

Wright & Souza: Regionally Valid Vowels 183

probed—on the whole, Clopper and Bradlow’s results in-dicated that there is a negative effect on intelligibilityof mismatch between the listener’s dialect and the dia-lect in the stimuli in noisy listening environments.

How concerned should researchers be about theeffects of regional accents on category goodness, percep-tual processing, and higher level processing in perceptualresearch? After all, there appears to be counterevidencein that listeners seem to be able to adapt to foreign-accented speech (e.g., Clarke & Garrett, 2004; Ferguson,Jongman, Sereno, & Keum, 2010; Munro & Derwing,1995a, 1995b). Moreover, the native dialect effects thathave been observed for speech perception andprocessingfor Dutch (Adank & McQueen, 2007), French (Dufour,Nguyen, & Frauenfelder, 2007), and English (Cutler,Smith, & Cooper, 2005; Evans & Iverson, 2007; Flocciaet al., 2009) involved dialect differences that were largerthan those typically observed within North America.This uncertainty about regional effects has led to a gen-eral trend in North American research to treat a com-mon set of vowel values as “standard” North AmericanEnglish in perception experiments and in clinical stud-ies. No study has directly tested the effect of dialectvowel distance (as measured using first and second for-mant [F1 and F2, respectively] differences between dia-lects) on identification accuracy; therefore, whetherdistance has a negative impact on vowel identificationis unknown.

To test whether vowel differences that are typical inNorth American English interfere with vowel identifica-tion, we conducted two experiments. In the first, we in-vestigated vowels of the region where the research tookplace, the Pacific Northwest (PNW), to establish whethertheir formant values differ from two sets of formantvalues of vowels that are commonly referred to as Gen-eral American English: (a) Peterson and Barney (1952;PB) and (b) Hillenbrand et al. (1995; HGCW). In the sec-ond experiment, we conducted a perceptual identificationtask using three synthesized vowel sets that differed inspectral distance (F1 and F2 differences in a Euclideanspace) from the vowels of the subjects’ regional dialect(0 = PNW identical, 1 = PNW similar, 2 = PNW dissimi-lar) to test the effect of vowel dissimilarity distance onidentification accuracy.

Experiment 1: Comparing PNWVowels With Standard Vowels

Although all dimensions of language vary across re-gional dialects, vowels are particularly well documentedin English and therefore most easily compared. More-over, there is a widely used comparison metric forvowels: F1 and F2 are typically measured at midpoint

or steady state. This measure was used in Petersonand Barney’s (1952) study as well as Hillenbrandet al.’s (1995) research and other recent vowel studies(e.g., Clopper et al., 2005;Hagiwara, 1997).We predictedthat there would be study effects such that both PB andHGCW vowels would differ from the PNW vowels. Wealso predicted that there would be Vowel × Study inter-actions, such that PB and HGCW vowels would differfrom PNW vowels in study-specific ways.

MethodTalkers

Six adult men and six adult women participated invowel recordings. All of the talkers had grown up in thePNW (Washington, Idaho, Oregon) and were monolin-gual speakers of English.Talkerswhohad lived elsewhereor had significant experience with other languageswere excluded fromparticipation. The PNWwas definedbroadly to include the amount of local variation that islikely to occur in an experimental or clinical population.1

None of the talkers had a history of speech therapy, andall had normal hearing, defined as pure-tone thresholdsof 20 dBHL or better (see AmericanNational StandardsInstitute, 2004) at octave frequencies between 0.25 kHzand 8 kHz, bilaterally. All procedureswere reviewed andapproved by the local institutional review board, andtalkers were reimbursed for their time.

Recording ProcedureWords containing eight monphthong vowels ( / i, I, e,

ɛ, æ, A, ʊ, u/ ) were recorded using randomizedword lists.Themid-back vowel /o/ was excluded because it is a diph-thong in the PNW region (Ingle et al., 2005). The mid-front vowel /e/ was included for qualitative comparisonbut excluded from the statistics because it was absent inthe PB study. Talkers were seated in a double-walledsound-attenuated chamber during the recordings. Allvowels except for /e / and /A / were read in the /hVd /context from a word list following the procedure ofHillenbrand et al. (1995). The vowels /e/ and /A / were spo-ken without the preceding /h/ in the words aid and odddue to unfamiliarity of the words hayed and hawed. Theeight words were read in five randomizations to controlfor list effects on pronunciation, thus minimizing theneed for a carrier phrase. All talkers were instructed toread the list of words at a natural pace and vocal inten-sity. Vocal level wasmonitored with a volume unit meterto ensure sufficient output levels without clipping. Weused a Tucker–Davis Technologies System 2 with an

1Other studies of the PNW accent have defined specific neighborhoodregions of the Northwest and/or specific sounds (e.g., Ingle, Wright, &Wassink, 2005; Wassink, Squizzero, Schirra, & Conn, 2009).

184 Journal of Speech, Language, and Hearing Research • Vol. 55 • 182–193 • February 2012

AP2 sound card and a Shure BG 1.0 omnidirectionalmicrophone for all of the recordings. Four talkers wererecorded direct to disc at a rate of 44.1 kHz. The remain-ing eight talkerswere recorded at a rate of 22.05 kHz.Allrecordings were quantized at 16 bits and down-sampledto 11.025 kHz prior to signal processing and acousticanalysis.

Spectral AnalysisWe selected one representative token of each vowel

for each talker on the basis of two criteria: (a) recordingfidelity and clarity of each subject’s voicewithout hoarse-ness, pitch breaks, or other disfluencies, and (b) visualinspection of the vowel for F1 and F2 steady states andaccompanying pitch steady state. We determined for-mant steady state using a wideband spectrogram andaccompanying linear predictive coding (LPC) formanttrack with 12 coefficients. We determined pitch steadystate using an autocorrelation pitch track with a 25-mswindow overlaid on a narrow-band spectrogram. A for-mant was considered to be steady state if a straightline could be traced through the middle 50 ms of thevowel and the pitch remained constant over the samesection.

Once the vowel was selected and the steady-stateportion identified, the first four formants (F1, F2, F3,F4) were estimated at the center of the steady state.The F1 through F4 values and recordings had beenused in a separate study on hearing aid compression(Bor, Souza, & Wright, 2008). In the current study, weused only F1 and F2. To minimize the risk of error, for-mant measurements were taken from an LPC spectrumoverlaid on a fast Fourier transform power spectrumwith a sample window of 128 points and visually com-pared with its broadband spectrogram. In the eventthat there were LPC errors for a particular talker, thenumber of filter coefficients (poles) was adjusted up ordown for that entire talker’s set of vowel measures.The sample window of the LPCwas 25ms, and the num-ber of coefficients ranged between 10 and 12, dependingon the talker.

ResultsVowels from the PNW study were compared with

HGCW and PB vowels using the F1 and F2 values. Be-causewe had a reasonably large sample (sixmen and sixwomen), we were able to make comparisons within gen-der rather than normalizing the data, thus preservingvowel space shape and individual vowel variability. Be-cause there were grossly different sample sizes betweenstudies, we used a random sample of six speakers fromeach of the larger studies (PB, HGCW). To ensure that

there were no sampling artifacts, we plotted the meanof the resulting sample within a 95% confidence intervalellipse representing all data within gender for adults. Inall cases, our sample fell near the center of the F1 × F2ellipse, indicating that our sample was representative ofeach study while still retaining variability within vowel.The results of the within-gender sample were submittedto a series of analyses of variance (ANOVAs) with F1 andF2 as dependent variables, and study (PNW, HGCW, orPB) and vowel (i, I, ɛ, æ, A, ʊ, u) as independent variables.ANOVA results are presented in Table 1 (male vowels)and 2 (female vowels).

The results of the ANOVAs indicated the expectedreliable effect of vowel on F1 and F2 for both men andwomen, indicating that in each study, the vowels werereliably separated on both dimensions. More interestingis that there was an effect of study on F2 for men and aStudy × Vowel interaction for F1 and F2 in the data forboth the men and women. The interactions indicatestudy-specific differences for individual vowels. Toprobe which vowels were contributing to the interac-tions, we submitted the formant values to a series ofBonferroni–Dunn post hoc t tests with an alpha of .05(corrected to .0167). For men’s PNW F1 values, thevowel [æ] was different from its HGCW counterpart,but none were different from their PB counterparts.For men’s PNW F2 values, the vowels [æ], [a], [ʊ], and[u] were all different from their HGCW counterparts,and the vowels [ʊ] and [u] were different from their PBcounterparts. For women’s PNW F1 values, the vowels[i], [æ], [A], and [ʊ] were all different from their HGCWcounterparts, but none were different from their PBcounterparts. For women’s PNW F2 values, the vowels[ɛ], [æ], [A], [ʊ], and [u] were all reliably different fromtheir HGCW counterparts, and the vowels [i], [ɛ], [ʊ], and[u] were all reliably different from their PB counterparts.

To illustrate the Study × Vowel effects, we plottedmen’s and women’s vowel means in Figures 1 and 2, re-spectively, with the PNWvowel plots overlaid onHGCWand PB vowels. Notable differences include a lowered(higher F1) and backed (lower F2) PNW /æ/ relative to

Table 1. Analysis of variance (ANOVA) results for men’s vowels.

Formant df F p

F1Study 2 1.74 .1807Vowel 6 287.23 < .001Study × Vowel 12 3.80 < .001

F2Study 2 15.13 < .001Vowel 6 277.38 < .001Study × Vowel 12 7.07 < .001


the HGCW counterpart, a raised (lower F1) and backed(lower F2) PNW /A / relative to the HGCW counterpart,and a fronted (higher F2) PNW /ʊ/ and /u/ relative to thePB and HGCW counterparts. The PNW /e/ also appearsslightly raised (lower F1) and fronted (higher F2) rela-tive to theHGCWcounterpart. Formant values are sum-marized in Table 3.

To investigate the relative differences further, wecalculated Euclidean distances from each PNW vowel’sF1 × F2 point in the vowel space to its gender-matchedcounterpart from the PB and HGCW values.2 We usedEquation 1 to calculate distance:

s ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðF1p # F1iÞ2 þ ðF2p # F2iÞ2

qð1Þ

Throughout this study, we used this formula to calculatedistance. In the formula, s is the distance between pointsin a two-dimensional Euclidian vowel space defined byF2 on the x axis and F1 on the y axis, and where F1p isan individual PNW speaker’s F1 value for a particularvowel and F1i is a comparison value for the equivalentPB or HGCW vowel F1 mean, and where F2p is an indi-vidual PNW speaker’s F2 value for a particular voweland F2i is a comparison value for the equivalent PB orHGCW vowel F2 mean. Because we were interested inrelating these differences to perceptual effects, we alsocalculated the Euclidean distances in Bark using theformula published in Traunmüller (1990).

The results of the Euclidean distance measures aresummarized in Table 4, which displays the mean dis-tance and SDs by vowel and by study in Hz and inBark, respectively. The distance patterns are similar inHz and Bark because in the frequency region of F1 andF2, there is a fairly linear relationship between Hz andBark. Nevertheless, the transformations are presentedto ease comparisons to other studies for the reader.

On the whole, the Hillenbrand et al. (1995) and thePeterson and Barney (1952) studies showed large mean

distances to the PNW vowels: HGCW vowels at 333 Hz(1.67 Bark) and PB vowels at 275 Hz (1.38 Bark). Only aslight difference of 57 Hz (0.29 Bark) is seen betweenthe average HGCW–PNW distance and the averagePB–PNW distance. This accounts for the general lackof effect of study in the ANOVA. However, as indicatedby the Vowel × Study interactions and subsequentpost hoc tests, in some regions of the vowel space, thePB–PNW distances are larger, whereas in others, theHGCW–PNW distances are larger: the back vowels/ʊ, u/ show greater distances to PB, and the low vowels/æ, A / show greater distances to HGCW. For /æ, A /, theHGCW–PNW distances are 357 Hz (1.66 Bark) and257 Hz (1.44 Bark) greater than the equivalent PB–PNW distances. On the other hand, for /ʊ, u /, the PB–PNW distances are 106 Hz (0.67 Bark) and 91 Hz(0.41) greater than the HGCW–PNW distances.

DiscussionThe results of Experiment 1 indicate that PNW re-

gional vowels vary, sometimes substantially, from stan-dardized vowels in terms of their formants. They alsodemonstrate that neither of the standardized vowelsets (HGCWor PB) is optimal in terms of the vowel mis-match because each has vowels that show larger dis-tances than those of the other study. This result shouldbe unsurprising to readers who are familiar with thesociolinguistic literature. After all, nearly all of the sub-jects in the Hillenbrand et al. (1995) study were fromregions that participate in the well-documented “North-ern Cities” vowel shift. As described recently in detail byLabov et al. (2006), Clopper et al. (2005), and Jacewiczet al. (2007), the Northern Cities vowel space is charac-terized by large differences in the low vowels; in partic-ular, it has a raised /æ/ compared with other regions anda relatively fronted /a / vowel compared with otherregions’ back /A /. At the same time, much of the Westis characterized by a fronting relative to other regionsof the high-back vowels [ʊ] and [u], as noted by Eckert(1989) and as documented instrumentally by Hagiwara(1997) and Clopper et al. (2005). However, in agreementwith Ingle et al. (2005), neither of these back vowels inour data show as much fronting as is seen in southernCalifornia.

Given the tolerance to variation in normal speech,whether such differences will lead to errors when non-regional vowels are presented in identification studiesremains to be seen.Moreover, whether dissimilarity dis-tance has an increasingly negative impact on perceptionsuch that at greater distances, recognition accuracydeclines, or whether the negative effect is equivalentacross distances, also remains to be determined.We testedthese questions in Experiment 2 using a forced-choice

Table 2. ANOVA results for women’s vowels.

Formant df F p



2For an alternative approach that incorporates consonant spectra as well,see Heeringa, Johnson, and Gooskens (2009).


vowel identification task in which subjects identifiedsynthetic vowels that varied by distance (as defined inEquation 1) in varying amounts of noise.

Experiment 2: Effect of DissimilarityDistance on Vowel Identification

MethodListeners

There were 15 listeners: 10 women and five men(M age = 24.6 years, age range: 18–38). All were

monolingual English speakers who were native to thePNW. All listeners had bilaterally normal hearing, de-fined as pure-tone thresholds of 20 dB HL or better atoctave frequencies between 0.25 kHz and 8 kHz. Sub-jects from the production study were excluded from theidentification study. All procedures were reviewed andapproved by the local institutional review board, andsubjects were reimbursed for their time.

StimuliThere were three steps to creating the stimuli, each

ofwhichwe describe inmore detail below. First, we chosepairs of vowels for the identification task. Second, wecompared formant values from different parts of North

Figure 1. First (F1) and second (F2) formant values for vowels spoken by female speakers. Circles and dashed lines represent the mean acrosssix speakers for Pacific Northwest vowels. Squares and dotted lines represent published values for PB (Peterson & Barney, 1952) vowels, andtriangles and solid lines represent published values for HGCW (Hillenbrand, Getty, Clark, & Wheeler, 1995) vowels.


America to determine the distance steps and to identifyindividual vowel formant values. Third, we synthesizedstimuli using the identified formant values.

We selected two vowel pairs /æ–ɛ, u–ʊ/ for the taskon the basis of three criteria. First, the vowels /æ–ɛ, u–ʊ/represent near neighbors in the acoustic F1 × F2 vowelspace and, therefore, should be more easily confusedthan more distant pairs such as /A–ʊ, i–u/. Second, thesepairs represent vowels that set the West—and, there-fore, the PNW—vowels apart from other regional vowels.Third, on the basis of our review of regional vowel stud-ies across dialect regions of North America, thesepairs represent sets for which monophthongs can beidentified.

After identifying the two target vowel pairs, we ex-amined published vowel formant values in a large num-ber of sources representing all seven dialect regions:West, North Central, Midland, South, Inland North,Mid-Atlantic, New England (as defined in Clopperet al., 2005; Labov et al., 2006; and Jacewicz et al.,2007). Using the Euclidean distance formula in Equa-tion 1, we calculated a dissimilarity distance from thesix PNW vowels to the vowels as described in Clopperet al. (2005), Labov et al. (2006), Hagiwara (2006), andJacewicz et al. (2007). In calculating distances, weused previously published PNW values (Ingle et al.,2005) to ensure equivalency in the comparisons. Aftercomparing all regions of North America with the PNW

Figure 2. F1 and F2 values for vowels spoken by male speakers. Circles and dashed lines represent the mean across six speakers for PacificNorthwest vowels. Squares and dotted lines represent published values for PB vowels, and triangles and solid lines represent published valuesfor HGCW vowels. To facilitate comparison with Figure 1, we shifted the scales to optimize the plot area, but the frequency spacing is the same asthat in Figure 1.


results, we selected two non-PNW distances that variedby a consistent amount in dissimilarity distance (valuesthat were closest in terms of differences in F1 and F2).These were designated as either 0 (identical to PNWvowels), 1 (similar to PNW vowels; 111–176 Hz, 0.52–0.77 Bark), or 2 (dissimilar to PNW; 276–375 Hz, 1.46–1.70 Bark). We used published values from previousstudies to preserve the greatest similarity to existingregional vowels and to ensure the maximal naturalnessof the subsequent synthetic stimuli, even though thismeant some variation within the Distance 1 and Dis-tance 2 groups. The resulting distances with theirregional (study-based) identifier are summarized inTable 5.

Once formant distances had been established, weused the F1 and F2 values from the published studiesto create a set of synthetic stimuli at each of the threedistance steps (0, 1, and 2). The stimuli were 200 ms

long and had a pitch contour that began at 130 Hz andremained steady state for 50ms, gradually falling there-after to 90 Hz. This created a male-sounding voice. Wetook the F1 and F2 frequencies from the published stud-ies, estimated F3 using published regression formulas(Nearey, 1989), and fixed F4 at 3500 Hz. We calculatedformant bandwidths from the algorithm described byJohnson, Flemming, and Wright (1993). The values usedto synthesize the stimuli are summarized in Table 6. Toensure equivalent signal-to-noise ratios (SNRs) acrossvowels, the stimuli were root-mean-square normalizedfollowing synthesis.

ProcedureThroughout the experiment, listeners were seated in

a double-walled sound booth. They were first trained inthe orthographic decision labels for vowels by associating

Table 4. Mean distance (Dist.) in Hz and in Bark from PNW values (HGCW, PB) by vowel.

Vowel

Dist. Hz

Diff. (Hz)

Dist. Bark

Diff. (Bark)HGCW PB HGCW PB

i 274 (122) 211 (114) 63 1.27 (0.45) 0.76 (0.29) 0.52I 136 (85) 189 (72) 53 0.59 (0.35) 0.92 (0.32) 0.33e 312 (173) 1.47 (0.60)ɛ 159 (106) 209 (127) 50 0.76 (0.47) 0.96 (0.58) 0.20æ 612 (245) 255 (156) 357 2.87 (1.09) 1.21 (0.69) 1.66A 371 (117) 96 (69) 275 2.04 (0.69) 0.60 (0.44) 1.44ʊ 427 (215) 533 (214) 106 2.09 (0.91) 2.76 (0.93) 0.67u 349 (173) 440 (168) 91 2.08 (0.80) 2.49 (0.73) 0.41

M 333 276 57 1.67 1.38 0.29

Note. Numbers in parentheses are SDs. Boldface type indicates that there is a greater distance between the pair.HGCW = Hillenbrand, Getty, Clark, and Wheeler (1995); PB = Peterson and Barney (1952).

Table 5. Region and Euclidean distances of the vowel stimuli.

Vowel Distance—RegionDistance(Hz)

Distance(Bark)

æ 0—PNW 0 0.00æ 1—Western North Carolina 176 0.75æ 2—Northern United States 376 1.46ɛ 0—PNW 0 0.00ɛ 1—Southern California 138 0.56ɛ 2—Northern United States 276 1.52ʊ 0—PNW 0 0.00ʊ 1—Winnipeg (Canada) 111 0.59ʊ 2—Northern United States 326 1.67u 0—PNW 0 0.00u 1—Winnipeg (Canada) 152 0.77u 2—Western North Carolina 323 1.70

Table 3. Formants and SDs of the Pacific Northwest (PNW) vowels(in Hz).

Vowel

F1 F2

Women Men Women Men

M SD M SD M SD M SD

i 327 41 290 10 2,991 131 2,346 188I 502 22 441 34 2,357 77 1,942 167e 405 22 404 42 2,792 143 2,217 192ɛ 687 74 556 34 2,160 113 1,791 133æ 983 69 703 76 1,884 201 1,622 56A 817 78 679 43 1,259 77 1,091 35ʊ 542 41 465 31 1,699 202 1,444 190u 385 29 324 32 1,450 215 1,223 95


visually presented words, such as bet and bat, with theorthographic decision labels. The followingare the labelsused in the experiment preceded by their IPA symbols:/ɛ/ eh, /æ/ ae, /ʊ/ uh, and /u / oo. When subjects achieved88% accuracy in the association of the labels withvisually presented words, they proceeded to the mainexperiment.

Tomeasure vowel identification, we presented stim-uli monaurally to the right ear via an insert earphone.Each vowel was presented in a background of a maskernoise with frequency spectrum matched to the long-term spectrum across all vowels. The masker con-sisted of a white noise that was low-pass filtered witha 300-Hz cutoff frequency and a 5-dB-per-octave spectralslope. Vowels were presented at SNRs of +2 dB SNR,+6 dB SNR, and +10 dB SNR. In each case, the level ofthe vowel was fixed at 65 dB SPL, and the level of thenoise was adjusted to the desired SNR.

Stimuli were presented blocked by vowel condition(front pairs, back pairs) and SNR, creating six blockstotal. The order of the blocks was randomized for eachsubject. A block consisted of 18 randomly ordered trials(two vowels, three distances, and three repetitions). Tomimic clinical (i.e., untrained) presentation, each blockwas presented once. Subjects responded to each trial in atwo-alternative forced-choice paradigm using a touchscreen. The choices were presented on the screen as but-tons labeled eh or ae for the front vowel block and uh oru for the back pair block. The location of response but-tons on the touch screen was randomized on each trialto minimize response bias.

ResultsWe analyzed the results with a three-way ANOVA

with the factors distance (0, 1, or 2), SNR (+2 dB, +6 dB,or +10 dB), and vowel. The results of the ANOVA arereported in Table 7.

The effect of distance for each SNR is shown in Fig-ure 3. In general, scores were similar between distances0 and 1 and dropped significantly for distance 2 for allSNRs. As suggested by Figure 3, there was no effect ofSNR. There was an effect of distance and a significantinteraction between distance and SNR. To investigatefurther, we collapsed the data across vowels and con-ducted a post hoc analysis by comparing the distancescoreswithin each SNR. TheDistance×SNR interactionwas due to a small difference in the magnitude of theeffect whereby the difference between distances 0 and1 approached significance for SNR 10 (p = .087) butnot for SNR 6 and 2 (ps = .475 and .384, respectively).Note that for SNR 10, there was a slightly higher scoreat distance 1 compared with distance 0, but given thenonsignificant p value, we considered this as reflect-ing measurement variability. At all SNRs, there was asignificant decrease in score between distances 1 and 2(p < .005 in each case). We used post hoc means compar-isons to examine the Distance × Vowel interaction (seeFigure 4). Post hoc analyses indicated that three of thevowels showed a significant effect of distance (p < .005for ɛ, æ, and u). In each case, the difference between dis-tances 0 and 1 was nonsignificant (p > .050), and thedifference between distances 1 and 2 was significant(p < .005). For the remaining vowel (ʊ), the effect ofdistance was not significant (p = .107).

DiscussionAlthoughwe focused on a specific region of theUnited

States (PNW), we anticipate that the results seen herecan be generalized to other regions of the country. In Ex-periment 2, the effect of distance 2 was robust, whereasdistance 1 did not prove reliable. This may indicate thatwhen making comparisons across dialects, small differ-ences have little or no effect on identification, whereaslarger differences have large negative effects. This find-ing needs to be testedmore thoroughly by examining theperceptual effects of subregional or sociolectal variationwithin dialect-specific vowel systems.

Table 6. Formants (Fs) and bandwidths (Bs) of the vowel stimuli.

Vowel F1 F2 F3 F4 B1 B2 B3 B4

æ-0 635 1,579 2,279 3,500 71 50 170 200æ-1 675 1,750 2,504 3,500 73 69 230 200æ-2 588 1,952 2,700 3,500 66 101 284 200ɛ-0 483 1,834 2,504 3,500 59 92 232 200ɛ-1 458 1,698 2,329 3,500 58 76 185 200ɛ-2 630 1,600 2,301 3,500 71 53 176 200ʊ-0 423 1,445 2,146 3,500 59 60 120 200ʊ-1 459 1,340 2,213 3,500 65 63 117 200ʊ-2 469 1,122 2,300 3,500 74 72 99 200u-0 313 1,176 2,158 3,500 60 74 77 200u-1 313 1,328 2,102 3,500 55 68 91 200u-2 390 1,490 2,104 3,500 55 60 118 200

Table 7. ANOVA results for the effect of Distance × Signal-to-NoiseRatio (SNR) × Vowel.

Variable df F p

Vowel 3 6.26 < .005SNR 2 0.33 .719Distance 2 180.76 < .005Vowel × SNR 6 1.04 .398Vowel × distance 6 12.90 < .005SNR × distance 4 2.91 .021Vowel × SNR × distance 12 1.34 .190Error 504


Our production results show that grouping all ofthe regional accents into any one of the dialect regions(e.g., the South or West) based on overall vowel similar-ity creates vowel-specific mismatches: Whereas somePNW vowels are typical of other West coast varieties ofEnglish, such as the one spoken in southern California,other PNW vowels differ quite dramatically. For exam-ple, PNW /æ/ shows a Euclidean distance of 157 Hz(1.06 Bark) compared with descriptions of the West in

general as defined by Clopper et al. (2005) and Labovet al. (2006), or compared with regionally specific valuesreported for southern California (e.g., Hagiwara, 1997;Johnson et al., 1993). PNW vowels also show lessextreme fronting of /u/ than seen in the generic West orin the specific Southern Californian descriptions. If thedistance effects in perception found in Experiment 2 ex-tend to other vowels as predicted, then the treatment ofthe entire West as a single dialect group may be too

Figure 3. Mean proportion correct vowel identification for three signal-to-noise ratios (+2, +6, and+10 dB) as a function of vowel distance. Error bars show ±1 SE of the mean.

Figure 4. Mean proportion correct vowel identification for four vowels as a function of vowel distance.Data are collapsed across signal-to-noise ratio. Error bars show ±1 SE of the mean.


broad for many purposes, and this underscores the im-portance of considering regional-specific vowels ratherthan broad areas of the United States (e.g., the West,Midwest, South) as is typically done.

The perception results have both methodologicaland theoretical implications for studies of speech percep-tion. They indicate that vowel category distances relatedto regional variation have a reliable negative impact onvowel identification. Stated another way, greater dis-similarity increases the risk that a vowel may be identi-fied poorly in perception experiments; however, slightdissimilarities appear to have a negligible effect. Itmust be noted that in creating our stimuli, we chose arelatively modest distance for the distance 2 conditionbecausewewanted to avoid vowels thatwere so differentthat no researcher would use them as stimuli. Accord-ingly, these results should represent the range of dia-lects that could be encountered by an individual in arealistic situation.

It is important to note that these were vowels pre-sented in isolation, and therefore the task is not directlyrepresentative of a listener’s everyday experience withaccent variation. Whether larger stretches of speechwith context effectswill show the same reliability remainsto be seen; moreover, the negative impact of dialect differ-ences on speech perception may be mitigated by a varietyof factors, such as the experience of the listenerwith otheraccents. For example, Evans and Iverson (2004) foundthat listeners were able to shift their perceptual targetstomatch that of the input accent over time.However, ad-aptation to an accent may require long-term exposureand may not occur for all individuals (Evans & Iverson,2007). Sumner and Samuel’s (2009) study providedevidence that there are dialect effects in both the im-mediate perceptual processing and long-term recall ofspeech stimuli. They concluded that there are individualexperienced-based differences that affect one’s ability toprocess stimuli presented in a nonnative dialect.

Research on highly proficient nonnative and bilin-gual listeners suggests that noise interacts with lan-guage background (e.g., Mayo, Florentine, & Buus,1997; Meador, Flege, & MacKay, 2000; Rogers, Lister,Febo, Besing, & Abrams, 2006). In these studies, listen-ers appear native-like under ideal listening conditionsbut experience amore extremedecline in perceptual per-formance in noise than native listeners do. Age of acqui-sition also plays a role in these studies; the earlier thesecond language was acquired, the less noise seemedto affect their perceptual performance. The lack of inter-action with noise shown in the present studymay be dueto an ability of native speakers to dynamically adapt tomoderate levels of noise; the more extensive the experi-ence with variation, the more robust the perceptual re-sponse in the face of noise. This is consistent with recentfindings that the combined effects of dialect variation

and noise on speech processing are lessened when thelistener has extensive experience with a similar dialect(Clopper & Bradlow, 2009; Clopper, Pierrehumbert, &Tamati, 2010, Sumner & Samuel, 2009).

We believe that these findings also suggest thatearly and extensive experience with relevant variationcreates a robust representation that listeners with nor-mal hearing can draw on under difficult listening condi-tions, such as in background noise. Individuals withhearing loss may experience an increased difficultyadapting to dialect variability in noise because of thereduced redundancy in the received signal. There is adearth of research on how listeners with hearing lossrespond to dialect variation and other types of phono-logical distortion. Such work continues to be a focus ofinterest in our laboratories.

ConclusionThe results of the production study indicate that

even when vowels are labeled with the same phoneticsymbol (by convention), there can be large acoustic dif-ferences between one region’s vowels and another’s;moreover, there are vowel-specific effects such thatsome vowels are quite similar across studies and othersvary widely. On the whole, the PNW vowels were moresimilar to the PB vowels than to the HGCW vowels, butfor each of the standard vowel sets, some PNW vowelsshowed larger distances. When presented as stimuli innoise, vowel distance had a reliable (negative) impacton the listener’s ability to correctly identify the targetvowel. These findings indicate that researchers and clini-cians should take care in choosing stimuli for perceptionexperiments. We recommend that researchers use re-gionally validated vowels instead of relying on standard-ized vowels in tasks that use vowel perception.

AcknowledgmentsThis work was supported by Grant R01 DC60014 from

the National Institutes of Health. The authors are grateful toStephanie Bor, Star Reed, Daniel McCloy, and KerryWitherellfor their assistance with stimulus preparation, data collection,and analysis.

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DOI: 10.1044/1092-4388(2011/10-0278) 2012;55;182-193; originally published online Dec 22, 2011; J Speech Lang Hear Res Richard Wright, and Pamela Souza

Comparing Identification of Standardized and Regionally Valid Vowels

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