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

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

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

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

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

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

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

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?

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

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)

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