Vowels and Formants

Vowels and Formants

Source Filter Model

  • The speech signal is the result of the vocal tract functioning as a frequency-selective filter (Textbook p. 197-8).
  • Both source and filter vary continuously during speech.

Source Filter Theory

  • Source: Noise generated by vibrating glottis, which is a complex waveform broken down into frequency x amplitude components (harmonics).
  • Filter: Vocal tract filters this noise, promoting some frequencies and suppressing others.
  • Output: Acoustic output with peaks corresponding to formants (peaks of resonance).
  • Spectrum = Frequency x Amplitude display.
  • Spacing = f0f0
  • Speech synthesis involves creating an artificial speech waveform by combining the source and the filter.
  • Frequencies are generated by the source, and the filter response is generated by the vocal tract.
  • Amplitude = darkness in patterns. Change in f0f0.

Acoustic Analysis

  • Acoustics is the analysis of sound waves and the physical properties of speech.
  • All sound results from vibration, which depends on a source of energy to generate it (CYF 2007, p.205).

Vowel Space

  • Vowel space shows where vowels are located in acoustic space relative to one another, often labeled in Hz.
  • The space is continuous, and vowel production can vary greatly, meaning there are no fixed F1/F2 values.
  • Praat is software used for acoustic phonetics to load sound files (typically .wav) and view waveforms and spectrograms.

Formants

  • Formants are resonating frequencies of the air in the vocal tract; peaks of resonance (L&J Ch.8, p.197).
  • A vowel sound contains a number of different pitches simultaneously.
  • The quality of a vowel depends on its overtone structure, i.e., formants (usually F1 and F2).
  • Vowels are distinguished from one another by differences in overtones (formant differences in F1 and F2, also F3).

Tube Models

  • The vocal tract can be modeled as a series of tubes open at one end.
  • Vocal fold vibration sets air in vibration.
  • Different vowels have different shapes of the vocal tract and different places of constriction.
    • e.g., at the hard palate [i], back/pharyngeal region [ɑ]
    • A = Front cavity, C = Rear Cavity, B = area of maximum constriction

Relationship among Cavity Size, Tongue Position & Formants

  • F1 frequency is closely related to the area of the lower portion of the pharyngeal cavity (C).
  • F1 is also related to Degree of mouth opening at lips (M).
  • F2 is closely related to the length of the front cavity (A).
  • The resonant frequency (formant) is low for close vowels and higher for open vowels.
  • The resonant frequency (Formant) is high for close front vowels and lower for open or back vowels
  • There isn’t an easy way to demonstrate F3 and higher formants

Formant Classification

  • Resonances or Formants numbered from low to high frequency, e.g., F1 F2 F3.
  • Vowels are primarily classified in terms of the first two formants, e.g., F1 and F2, although F3 can be used to determine rounding.
  • Changes in the relative formant values give vowels their quality.

Vowel Formant Patterns

  • F1 equates to opening:
    • Close vowel = low F1
    • Open vowel = high F1 (inverse relationship to aperture/height)
    • F1 of /i/ is lower than F1 of /æ/
  • F2 equates to backing:
    • Backer vowels, lower F2
    • F2 of AusEng /ʉ/ would be higher than F2 of Spanish /u/
  • Rounding lowers both F1 and F2, so [y] has a lower F1 and F2 than /i/
  • F1 value INCREASES as vowel aperture opens from close to open & pharyngeal cavity decreases in volume
  • F2 LOWERS as you move from front to back – lengthening the front cavity by backing the tongue.
  • ROUNDING lowers F1 & F2 - elongates oral cavity (works well with backing!)
  • F3 thought to be important for distinguishing front unrounded [i] from [y]

Mini Quiz

  • Low F1 and high F2: likely to be /i/
  • Low F1 and low F2: likely to be /u/
  • High F1 and high F2: likely to be /a/ or /ɑ/

Vowel Spectrograms

  • Typically measure at the vowel midpoint in AmEng.

Factors Affecting Vowel Formant Values

  • Speaker physiology (larger vocal tract formants typically lower).
  • Language or Dialect (e.g., American English versus Standard Australian English).
  • Number of contrastive vowels (Aus Indigenous languages vs. AusE).
  • Stress & Accent:
    • Unstressed vowels might show formant “undershoot.”
    • Vowels do not achieve their F1/F2 targets.
    • Reduced vowel quality (vowels that are unstressed tend to head towards schwa [ə]).
  • Casual speech or rapidly spoken vowels also show undershoot.
  • Consonant environment – coarticulation.
  • CLEAR SPEECH – more peripheral vowels.

Physiological Differences

  • The length and thickness of the vocal folds affects the Fundamental Frequency.
    • Thicker, bigger vocal folds = lower F0F0 or pitch.
  • The length of the vocal tract changes the resonant frequency of a voice.
    • Longer vocal tracts = lower formants in general.
  • Children have shorter, smaller vocal tract lengths, on average, than adults.
  • Relative F1 F2 is the key.

Kunwinjku (Australian) Vowel System

  • Small vowel systems typically show a lot of variation within a vowel category.
  • Not the same need to keep 15 monophthongs apart (unlike N. Frisian)!

Biological Differences

  • Same shape, different values.
  • German Lax Vowels transcribed in MRPA (machine readable phonetic alphabet – IPA symbols shown in red - (from Harrington 2010)

Australian English Formant Tracks

  • Vowel midpoint
  • /æ / F1 = 800 Hz, F2 = 1614 Hz
  • /iː/ F1 = 300 Hz, F2 = 2420 Hz
  • /ʊ/ F1 = 360 Hz, F2 = 1000 Hz
  • /ɔ/ F1 = 600 Hz, F2 = 980 Hz
  • /ʉ/ F1 = 327 Hz, F2 = 1760 Hz

Additional Activities

  • Video Praat Demonstration to show you how to plot practice.
  • Download sound files and save them in an easily accessible location.
  • Use a printed vowel space or annotate a PDF on screen (or draw in your textbook).