Acoustic Characteristics of Vowels

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