1. The Acoustics and Perception of American English Vowels
Hillenbrand: Vowels

2. Vowel Symbols [i] heed small i [ɪ] hid cap i, or small cap i
[e] hayed, bait small e
[ɛ] head epsilon
[æ] had ash
[ɑ] hod, pod script a (note the difference between [ɑ] and [a]
[ɔ] hawed, caught open o
[o] hoed, boat small o
[ʊ] hood upsilon
[u] who’d, boot small u
[ʌ] hud, but caret or wedge or turned v
[ɚ] heard schwar (you may have learned [ɝ])
[ə] about, mantra schwa

3. Major dimensions of Vowel Articulation
Tongue height [e.g., [i] (“beet”) vs. [æ] (“bat”)]
Frontness or advancement [e.g., [æ] (“pat”) vs. [ɑ] (“pot”)]
Lip rounding (e.g., [u] vs. [ɑ])

4. Vowel Quadrilateral for English

5. If you are not familiar with the vowel quadrilateral – i. e
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.

6. Formant Patterns for the “Non-central” (i. e
Formant Patterns for the “Non-central” (i.e., omitting /ʌ/ and /ɚ/) Monophthongal Vowels of American English (based on Peterson & Barney averages)
Hillenbrand: Vowels

7. Another Way to Visualize Formant
Data for Vowels: The “Standard” F1-F2 Plot
Hillenbrand: Vowels

8. Hillenbrand: Vowels

9. Hillenbrand: Vowels

10. Formant Data for Men
“Standard” F1-F2 Plot
Hillenbrand: Vowels

11. 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

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. Acoustic Vowel Diagram
Conventional F1-F2 Plot
Acoustic Vowel Diagram
Hillenbrand: Vowels

14. 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 formant data in articulatory terms.
Hillenbrand: Vowels

15. 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 (then) recently invented sound spectrograph machine.
1. Recordings
10 vowels (i,ɪ,ɛ,æ,ɑ,ɔ,ʊ,u,ʌ,ɚ) 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

17. Listening Test Results
Simple: The signals were highly intelligible: 94.5%
Error rate varied some across vowels. For example:
Error rate very low for:
[i] (0.1%), [ɚ] (0.4%), [u] (0.8%)
Higher for:
[ɑ] (13.0%, confused with [ɔ])
[ɔ] (12.1%, confused with [ɑ])
______________________________________________________
Details aside, the simple message is that vowel identity was transmitted quite accurately to the listeners.
What information do listeners use to recognize vowels? To answer this, we need to start by looking at the acoustic data.
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Hillenbrand: Vowels

18. English Vowel Formant Data
Peterson & Barney (1952) General American
English Vowel Formant Data
Most striking: Lots of overlap among adjacent vowels
Hillenbrand: Vowels

19. 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 Peterson & Barney, 1952.)
Hillenbrand: Vowels

20. Same Data as Previous Figure, but Plotted on a Single Graph
Hillenbrand: Vowels

21. Michigan (Northern Cities) Vowel Formant Data
Hillenbrand, Getty, Clark & Wheeler (1995)
Michigan (Northern Cities) Vowel Formant Data
1. Lots of overlap among adjacent vowels
2. [æ] and [ɛ] almost on top of one another, and out of order from Peterson & Barney (1952)
Hillenbrand: Vowels

22. Peterson & Barney (Mostly Mid-Atlantic) vs.
Hillenbrand et al. (Upper Midwest/Northern Cities)
1. [æ] is raised and fronted in Northern Cities data
2. Back vowels fronted (e.g., [ɑ,ɔ]) are lower in N. Cities data
3. High vowels ([i ɪ u ʊ]) not quite as high in N. Cities data
Hillenbrand: Vowels

23. This is a much simpler idea than you might be thinking.
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

24. How a Pattern Recognizer Works
Training
Testing
Hillenbrand: Vowels

25. 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.
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Pattern classification results from Hillenbrand-Gayvert (1993)
Automatic Human
Classification Listeners
Peterson & Barney vowels: % %
Hillenbrand et al. vowels: % %
Listeners must be using information to recognize vowels other than F1 and F2. Like what?
Hillenbrand: Vowels

26. F3 and f0: Better still (~89-90%), but still below human listeners.
So, listeners must be using some information to recognize vowels other than F1 and F2. What information?
F3: It helps some (especially for /ɚ/), 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 human listeners.
Hillenbrand: Vowels

27. Patterns of spectral change over time
What does this mean? It appears as though listeners are recognizing vowels based on information other than F0 and F1-F3. What are the possibilities?
Two Candidates:
Duration
Patterns of spectral change over time
Hillenbrand: Vowels

28. Do Listeners Use Duration in Vowel Identification?
___________________________________
American English Vowels Have
Different Typical Durations
/i/ > /I/
/u/ > /U/
/A/ > /‰/
/å/ > /ú/
/Ø/ > /å/
Do Listeners Use Duration in Vowel Identification?
Hillenbrand: Vowels

29. [hæd] 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 same.
Hillenbrand: Vowels

30. Shortened [i] ought to be heard as [ɪ]
Logic: If duration plays no role in vowel recognition, the 4 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 [ɪ]
Lengthened [ɪ] ought to be heard as [i]
Shortened [ɑ] ought to be heard as [ʌ]
Lengthened [ʌ] ought to be heard as [ɑ]
Shortened [u] ought to be heard as [ʊ]
Lengthened [ʊ] ought to be heard as [u]
Shortened [æ] ought to be heard as [ɛ]
Lengthened [ɛ] ought to be heard as [æ]
Hillenbrand: Vowels

31. RESULTS Original Duration: 96.0% Short Duration: 91.4%  
Original Duration: 96.0%
Short Duration: 91.4%
Long Duration: %
Hillenbrand: Vowels

32. 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: [ɑ]-[ɔ]-[ʌ], [æ]-[ɛ]
5. Vowels Not Affected: [i]-[ɪ], [u]-[ʊ]
Hillenbrand: Vowels

33. Effects of Duration on Vowel Perception
Original Duration, Long Duration, Short Duration
Hillenbrand: Vowels

34. The Role of Spectral Change in Vowel Perception
Notice that some vowels – especially [æ] and [ɪ] – show a fair amount of change in formant freq’s throughout the vowel. Is it possible that these formant movements are perceptually significant?
Hillenbrand: Vowels

35. More examples. Note especially the rise in F2 for [ʊ] and [ʌ].
Hillenbrand: Vowels

36. 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., [e,o,ʌ,ʊ,æ,ɪ]).
Hillenbrand: Vowels

37. NAT: Naturally spoken [hæd]
OF: Synthesized, preserving original formant contours
FF: Synthesized with flattened formants
Hillenbrand: Vowels

38. Spectral change patterns do matter – quite a bit.
Key comparison is OF vs. FF: If the formant movements don’t 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

39. 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 [ɚ])
Spectral change patterns
Vowel duration
Hillenbrand: Vowels