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Vocal Tract Filter
Source-Filter Theory
- Acoustic theory of speech production
- Speech results from a combination of sound source(s) and the filtering provided by the vocal tract.
- An example of a forced vibration.
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Acoustic Properties of the Vocal Tract
Uniform Tube Model 1
- Length varies with age and gender: males>females>children
- as vocal tract length increases, resonant frequencies decrease.
- Males: L = 17 cm; F1 = c/4L = 34000/68 = 500 Hz, F2 = 1500 Hz
- Females: L = 14 cm; F1 = c/4L = 34000/56 = 607 Hz, F2 = 1821 Hz
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Damping:
vocal tract is highly damped. Low amplitude of force vibration, but broad range of frequencies resonated.
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Resonance Curve (frequency response)
- Combination of the resonance curves for each natural frequency
- Peaks: regions of resonance
- Valleys: regions of damping
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Vocal Tract Damping Characteristics
- Highly damped:
- low amplitude of forced vibration
- broad range of frequencies resonated.
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Formant:
- region of frequency resonated by the vocal tract due to its shape and corresponding frequency response
- first two most important for vowel recognition
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Relationship between formants (filter) and harmonics (source)
Formant: Contains one or more harmonics from the voice (source)
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Monophthongs:
- vowel sounds produced with a relatively stable placement of the articulators.
- Tongue Height: high, mid, low
- Tongue Advancement: front, central, back
- Lip Configuration: rounded, unrounded, spread
- Tension: tense, lax
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Diphthongs:
vowel sounds produced with changing articulation
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Tongue Height:
- tongue moves from low to high (more constricted)
- F1 decreases
- F2 increases
- compare /i/ and /æ/
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Tongue Advancement:
- tongue moves front-back
- F1 increases
- F2 decreases
- compare /C/ and /æ/
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Lip Configuration:
- lip moves from rounded-unrounded (neutral)-spread
- F1 increases
- F2 increases
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Tension:
- tense-lax
- duration: tense vowels are longer than lax
- f0: higher for tense than lax vowels
- formant frequencies: small differences due to the tongue positioned slightly higher for tense vowels than lax vowels
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Measuring Formants
Wide-band Spectrogram
Place cursor into the middle of each band
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Measuring Formants
Linear Predictive Coding (LPC) Spectrum
- Special mathematical procedure for determining formants
- Place cursor into the middle of each peak
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Measuring Formants
Automatic analysis (LPC)
- Use of software programs to automatically measure formant frequencies.
- Generates a wide-band spectrogram and then automatically draws a tracing of the formants based on the LPC analysis
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