The Acoustics and Perception of American English Vowels
The Acoustics and Perception of American English Vowels Hillenbrand: Vowels 1 Vowel Symbols [i] heed  hid small i cap i, or small cap i [e] hayed, bait small e  head epsilon  had ash
 hod, pod  hawed, caught script a (note the difference between  and [a] open o [o] hoed, boat small o  hood upsilon [u] whod, boot small u  hud, but caret or wedge or turned v  heard schwar (you may have learned )  about, mantra
schwa Major dimensions of Vowel Articulation 1.Tongue height [e.g., [i] (beet) vs.  (bat)] 2. Frontness or advancement [e.g.,  (pat) vs.  (pot)] 3. Lip rounding (e.g., [u] vs. ) (There are many secondary dimensions as well.) Vowel Quadrilateral for English If you are not familiar with the vowel quadrilateral i.e., which vowels are high, low & mid, which are front, back & central, which are rounded and which are retracted you will need to review. If you need help finding material, let me know. Formant Patterns for the Non-central (i.e., omitting / / and //) Monophthongal Vowels of American English (based on Peterson & Barney averages) Hillenbrand: Vowels
6 Another Way to Visualize Formant Data for Vowels: The Standard F1-F2 Plot Hillenbrand: Vowels 7 Hillenbrand: Vowels 8 Hillenbrand: Vowels 9 Formant Data for Men Standard F1-F2 Plot Hillenbrand: Vowels 10 Notice that the formant values for women for a given vowel are shifted up and to the right,
indicating higher values for both F1 and F2. This is due to the shorter vocal tracts of women vs. men. The same is true of the relationship between the formant values of children relative to women and for the same reason; i.e., children have shorter vocal tracts than women. Hillenbrand: Vowels 11 Q: Are the upward shifts in formants (M vs. W vs. C) also due to differences in the length and mass of the vocal folds across the three talker groups? Hillenbrand: Vowels 12 One More (apparently screwy) Way to Visualize
Vowel Formant Data: The Acoustic Vowel Diagram Note that in the Acoustic Vowel Diagram: (1) the axes are reversed, (2) the numbers go backwards. Why would anyone do such a screwy thing? Hillenbrand: Vowels 13 Conventional F1-F2 Plot Acoustic Vowel Diagram Hillenbrand: Vowels 14 Peterson & Barney Averages (for men only) Plotted on an Acoustic Vowel Diagram Formant data are being plotted, but the result strongly resembles an articulatory vowel diagram, with the x axis corresponding to tongue advancement (i.e., front vs. back) and the y axis corresponding to tongue height. This gives us a convenient way to interpret 15 formant data in articulatory terms. What is the
articulatory explanation for the differences in formant frequencies? What effect might this have on the intelligibility of the vowels spoken by the deaf talker? Data shown above are hypothetical, but this is exactly the sort of thing that has been observed in the speech of deaf talkers. For example, Monsen (1978) showed that: (a) the formant values of deaf talkers tend to be centralized relative to NH talkers, and (b) the degree of centralization is a good predictor of speech intelligibility. 16 Peterson & Barney (1952) Study conducted at Bell Labs. The 1st big acoustic study carried out with the (at the time) recently invented sound spectrograph machine. The great Gordon Peterson Bell Labs at the time of the study, later the Univ. of Michigan Hillenbrand: Vowels 17
Peterson & Barney (1952) This is one of the best known studies in our field, yet the design of the study is quite simple. 1. Recordings 10 vowels (i,,,,,,,u,,) in /hVd/ context (heed, hid, head, had, etc.); 76 talkers (33 men, 28 women, 15 children) 2. Measurements: f0, F1-F3 3. Listening Study 70 listeners asked to identify each test signal as one of ten words (heed, hid, head, had, etc.) Hillenbrand: Vowels 18 Listening Test Results Simple: The signals were highly intelligible: 94.5% Error rate varied some across vowels; e.g.: Very low error rate for: [i] (0.1%),  (0.4%), [u] (0.8%) Higher for:  (13.0%) &  (12.1%), confused mainly with one another _____________________________________________________ _ Details aside, the simple message is that the
vowels were highly intelligible. Question: What information do listeners use to recognize vowels? To answer this, we need to19start Peterson & Barney (1952) General American English Vowel Formant Data Most striking: Lots of overlap among adjacent 20 vowels Hillenbrand: Vowels It is mostly the case that the men occupy the lower left portion of each ellipse, the children occupy the upper right portion, and the women cluster toward the center. This is mainly due to differences in vocal- tract length. There is quite a bit of variability across individual talkers, though. (Data from 21 Peterson & Barney, 1952.) Same Data as Previous Figure, but Plotted on a Single Graph Hillenbrand: Vowels
22 Hillenbrand, Getty, Clark & Wheeler (1995) Michigan (Northern Cities) Vowel Formant Data 1. Lots of overlap among adjacent vowels 2.  and  almost on top of one another, and out of order from Peterson & Barney (1952) Hillenbrand: Vowels 23 Peterson & Barney (Mostly Mid-Atlantic) vs. Hillenbrand et al. (Upper Midwest/Northern Cities) 1.  is raised and fronted in Northern Cities data 2. Back vowels fronted (e.g., [,]) are lower in N. Cities data 3. High vowels ([i u ]) not quite as high in N. Cities data 24 Hillenbrand: Vowels Question: How well can vowels be separated based on F1 and F2 alone? This is the kind of question that can be answered with a statistical pattern recognition algorithm. This is a much simpler idea than you
might be thinking. Hillenbrand: Vowels 25 How a Pattern Recognizer Works Training Testing Hillenbrand: Vowels 26 Q: So, how well can vowels be separated based on F1 and F2 alone? A: Pretty well, but not nearly well enough to explain human listener data. ____________________________________________________________ Pattern classification results from Hillenbrand-Gayvert (1993) Automatic Classification Peterson & Barney vowels: Hillenbrand et al. vowels: Human Listeners
74.9% 68.2% 94.4% 95.4% ____________________________________________________________ Listeners must be using information to recognize vowels other than F1 and F2. Like what? 27 Hillenbrand: Vowels So, listeners must be using some information to recognize vowels other than F1 and F2. What information? F3: It helps some (especially for //), but not enough: Automatic classification improves to about 80-85% better, but still well below human listeners. f0: Ditto: It helps some, but not enough: Automatic classification improves to about 80-85% better, but still well below human listeners. F3 and f0: Better still (~89-90%), but still below 28 human listeners. Hillenbrand: Vowels What does this mean? It appears as
though listeners are recognizing vowels based on information other than F0 and F1F3. What are the possibilities? Two Candidates: Duration Patterns of spectral change over time Hillenbrand: Vowels 29 7. Length/Duration/Quantity ___________________________________ ___________________________________ American English Vowels Have Basic idea is simple: Any vowel ___________________________________ can be spoken at [i] >  any duration, but some vowels are [u] >  typically longer >  than others. This is called the vowels [e] >  inherent duration or  > 
typical duration.  >  Different Typical Durations Hillenbrand: Vowels ________________________________ ____ 30 [hd] Original Duration Short Duration Long Duration Utterances were presented at their original durations, or they were artificially shorted or lengthened but keeping everything else the Hillenbrand: Vowels 31 same. Logic: If duration plays no role in vowel recognition, the three signal types ought to be equally intelligible; i.e., artificially modifying duration will not affect what vowel is heard. On the other hand, if duration plays a role in vowel perception, the OD signals ought to be more intelligible
than any of the duration-modified signals. Also, there are specific kinds of changes in vowel identity that we would expect. For example: Shortened [i] ought to be heard as  Lengthened  ought to be heard as [i] Shortened  ought to be heard as  Lengthened  ought to be heard as  Shortened [u] ought to be heard as  Lengthened  ought to be heard as [u] Shortened  ought to be heard as  Lengthened  ought to be heard as  Hillenbrand: Vowels 32 RESULTS Original Duration: 96.0% Short Duration: 91.4% Long Duration: 90.9% Hillenbrand: Vowels 33 Effects of Duration on Vowel Perception
Original Duration, Long Duration, Short Duration Hillenbrand: Vowels 34 CONCLUSIONS 1. Duration has a measurable but fairly small overall effect on vowel perception. 2. Vowel Shortening (-2 SDs): ~5% drop in overall intelligibility 3. Vowel Lengthening (+2 SDs): ~5% drop in overall intelligibility 4. Vowels Most Affected: --, - 5. Vowels Not Affected: [i]-, [u]- Hillenbrand: Vowels 35 The Role of Spectral Change in Vowel Perception Notice that some vowels especially  and  show a fair amount of change in formant freqs throughout the vowel. Is it possible that these formant 36 movements are perceptually significant? More examples.
Note especially the rise in F2 for  and . Hillenbrand: Vowels 37 Another way to visualize patterns of formant frequency change in vowels: This figure shows formant frequencies measured at the beginning of the vowel and a 2nd time at the end of the vowel. (The phonetic symbol is plotted at the 2nd measurement). Note that some vowels (e.g., [i] & [u]) are pretty steady over time, but others have formants that change quite a bit throughout the course of the vowel (e.g., Hillenbrand: Vowels 38 [e,o,,,,]).
NAT: Naturally spoken [hd] OF: Synthesized, preserving original formant contours FF: Synthesized with flattened formants Hillenbrand: Vowels 39 Key comparison is OF vs. FF: If the formant movements dont matter, the recognition rates for OF and FF should be very similar. On the other hand, if the formant movements are important, the FF signals will be less intelligible than the OF signals. Conclusion Spectral change patterns do matter quite a bit. Hillenbrand: Vowels What can we conclude from all this about how listeners recognize which vowel was spoken? 1. Primary Cues: F1 and F2 Relationships among the formants matter, not absolute formant frequencies 2. Cues that are of secondary importance, but
definitely play a role in vowel perception: f0 F3 (especially for ) Spectral change patterns Vowel duration Hillenbrand: Vowels 41 Implications for 2nd Language Learning Is any of this information e.g., the role played by vowel duration and spectral change in vowel perception useful? These findings we just reviewed are not universal facts about vowels; they are facts about English vowels. Other languages will behave the same way only by accident. Hillenbrand: Vowels 42 English, for example, has a pretty large (and therefore crowded) vowel system. Only 12 vowels are plotted below, and its pretty crowded. Including
diphthongs, English has 15 vowel phonemes. This almost certainly explains why duration and spectral change are important these features give speakers two more ways to differentiate on vowel from another. Second Formant (Hz) 3400 3000 2600 2200 1800 1400 1000 600 Vowel Formant Frequencies for Men, Women & Children i i ii ii i i i (from Hillenbrand et al., 1995) i ii i i i i i i i
i i i i i i i i i iii i A i ii i i ii i i II i ii i i i i A i ii I I I I i i i ii i A i iI I A I I A AA A A A i i i I I I iI I I
A IAII A A A A I II I A AA i I i i A AA i I II I i i i i A A AAA A A A I I I I A A A A A I A
I I A AA A I A ii ii i i i A I I I A A A A A I II I I A AA i ii i i I I I
AA A A i A A I A i I I i ii I I A A A A I A A i I II III AI A A A I I I A
i A A ii i I I II II I I A A i i I I I II A AA A AA A A A I I I I I I II I
A A A I AU A I I u A U A
uA U U A u u u U u U U U U U
U U U U u U u UU U U U u U U U
uU uU U uu u U U u u U u U Uu
u U uuU u uuU U U u u UU Uu UU UU UUUU u u U U u
U u UU u uu uuuu uU UU UU UU U u u u u uU U u u u u uuU u uuu Uu u u u u u u u u u u u uu u
u u i i 300 i i 450 600 750 900 First Formant (Hz) 1050 1200
43 Example: Notice how close  and  are to one another. How do speakers distinguish these two vowels? How do listeners figure out which is which? (The same question posed from two points of view.) Second Formant (Hz) 3400 3000 2600 2200 1800 1400 1000 600 Vowel Formant Frequencies for Men, Women & Children i i ii ii i
i i (from Hillenbrand et al., 1995) ii i i i iii i i i ii i i i i i i iii i A i ii i i ii i i II i i i A ii I i i Ii iii I I i ii i A i ii I iI I A I I A AA A A A i i I I iI I I A
IAII A A A A I II I A AA i I i i AA i I II I i i i i AA A AAA A A I I I I A A A A A I A A AAA A
ii ii i i i A I I I I I I II A I AA A A A II I i ii i i I I AA A A A AA A AA i i ii i I I I I I II A A A
A i I I A III AI A A A I I I A i A A ii i I I II II I I A A i i I I I II A A A
A A A AA A I I I I I I I I II A A A A I U I I u
A U A A A U U A u u u U
u u U U U U U U U U U uu UUU U UU u U
uU uU uu UU U U uU uU UU u u u uU u
U U u U u uu U U u u UU U UUUU uu uUUu UU U U
U u U U U U U u uu uuuu U u U U u Uu u u u uU U u u u u uU
u uuu Uu u u u u u u u u u u u uu u u u i i i i   300 450 600
750 900 First Formant (Hz) 1050 1200 44 Why does any of this matter? Many languages have much smaller vowel systems than English. Examples: Spanish (5), Italian (7), Japanese (5), Spanish vowels 45
The simple point is that a speaker of a language like Spanish has some work to do as a speaker (learning many brand-new vowels) AND as a listener learning what native English speakers learned as children: e.g., learning that features like duration and spectral change now matter. Spanish vowels Well be talking about some closely related aspects of 2nd-language learning a little later. 46
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