1. Acoustic Characteristics of Vowels
Robert A. Prosek, Ph.D.
CSD 301

2. F2-F1 Displays Formant frequencies are determined by the articulation
If the articulation changes, the formant frequencies should change
The F2-F1 display shows the second formant frequency on the ordinate and the first formant frequency on the abscissa
The parameter is a particular vowel
Peterson and Barney (1952) and Hillenbrand, Clark, Getty, and Wheeler (1995) are the primary references for vowel spaces

4. Models of Vowels
In addition to the linear source-filter theory, models of vowels attempt to explain the relationship between vowel production and vowel perception
The target model of Lindbloom was one of the early attempts
Vowel articulations are considered invariant or canonical
Because different formant frequencies lead to the perception of the same vowel, normalization is needed
Not simple
Separate normalizations are needed for different areas of the vocal tract
Vowels also differ in duration and formant frequency trajectories
Normalization in terms of mels or Barks
these are non-linear transformations based on audition
not well adopted, but research indicates that Barks have a role

5. Models of Vowels (2) Dynamic specification model
transitions of formant frequencies
duration
F2-F1 chart inadequate

6. Vowel Formant Pattern
The pattern of vowel formant frequencies determines the perception of the vowel
However, static patterns may not be necessary
The standard deviations in Tables 4-1 and 4-1 intersect, for example
But, for any one individual, there must be some separation among the vowels in F2-F1 space
Remember the acoustic-phonetic rules for vowels
F1 varies inversely with tongue height
F2 varies directly with tongue advancement
F2-F1 space can be transformed
Bark scale
ERB scale

8. Short-Time Spectra for Vowels
Spectral variations are important
Spectral tilt
Depth of valleys
Logarithmic changes
Relative position of spectral peaks
Changes in slope near peaks
LTASS
Widely used in Europe, especially for voice disorders
Many similarities across languages

9. Vowel Duration Tense-lax distinction Vowel height Syllable stress
Speaking rate
Voicing of the preceding or following vowel
Place of articulation of surrounding sounds
Word familiarity

10. Vowel Fundamental Frequency
If other factors are controlled, vowels appear to have an intrinsic f0
f0 varies directly with vowel height
Probably not critically important, but it does add naturalness to speech
What is the physiology that causes these f0 changes?

11. Formant Bandwidths and Amplitudes
Formant bandwidth and amplitude interact
Bandwidth is related to damping
Damping is the rate of absorption of sound energy
As damping increases, bandwidth increases and sound waves’ amplitudes decrease quickly
In general, bandwidth increases as formant number increases
Changing bandwidth does not affect vowel identification
Changing bandwidth might influence quality, however
Discrimination among vowels may be enhanced by changes in bandwidth

12. Formant Bandwidths and Amplitudes
An increase in bandwidth often leads to a decrease in amplitude for a particular formant frequency
Formant amplitudes are determined by
Formant frequency
Formant bandwidth
Energy available from the source
Again, all of the acoustic energy for vowels comes from the source (vocal fold vibrations)

13. Diphthongs
For diphthongs, there is no single vocal tract shape that characterizes the articulation
Diphthongs are usually described as having an on-glide and an off-glide
If the articulation changes during the production of a diphthong, then the formant frequencies change as well
The actual formant frequencies realized depends on the speaking rate
The rate of formant frequency change, however, obtains no matter what the speech rate