Transcript Acoustic Characteris..

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
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Spectral variations are important
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Spectral tilt
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Depth of valleys
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Logarithmic changes
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Relative position of spectral peaks
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Changes in slope near peaks
LTASS
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Widely used in Europe, especially for voice disorders
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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
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Formant bandwidth and amplitude interact
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Bandwidth is related to damping
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Damping is the rate of absorption of sound energy
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As damping increases, bandwidth increases and sound waves’
amplitudes decrease quickly
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In general, bandwidth increases as formant number increases
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Changing bandwidth does not affect vowel identification
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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