The Source-Filter Theory of Vowels
Although voice production does not specifically deal with speech articulation, all human vocalizations require some configuration of the vocal tract. Unless a vocalist is deliberately humming or hissing, the configuration is usually an open tract, as in a vowel. Vowel formation is therefore an integral part of the phonation process and needs to be discussed in detail.
Overview
The acoustic properties of vowels have traditionally been described on the basis of a source-filter theory (Fant, 1960). In this theory, the sound source is the time-varying glottal airflow and the filter is the vocal tract. Whereas the glottis produces a sound of many frequencies, the vocal tract selects (filters) a subset of these frequencies for radiation from the mouth.
This chapter introduces the source-filter theory with tools of analysis developed in previous chapters. We revisit the principle of resonance, introduced in Chapter 4 with regard to vocal fold vibration and aerodynamic driving forces. This time resonance is applied to the interference of acoustic waves traveling in opposite directions.
What You’ll Learn
Acoustic Fundamentals
- Sound propagation in tubes: How acoustic waves behave in confined spaces
- Acoustic impedance: The relationship between pressure and flow in tubes
- Wave reflection: How area changes create reflections and standing waves
- Resonance frequencies: The natural frequencies of the vocal tract
Vowel Acoustics
- Formants: The resonance frequencies that characterize vowels
- F₁-F₂ vowel space: How the first two formants define vowel categories
- Articulatory-acoustic relationships: How tongue position affects formant frequencies
- Tube models: Simplified approximations of vocal tract shapes
Analysis Tools
- Fourier transformation: Converting time-domain signals to frequency spectra
- Filters and filtering: How the vocal tract selectively amplifies frequencies
- Spectrographic analysis: Visual representation of vowel spectra
- Formant bandwidth: The effect of energy loss on resonance peaks
Practical Applications
- Vowel modification: Rules for changing formant frequencies through articulation
- Vocal tract adjustments: Effects of larynx height, lip rounding, and jaw position
- Vowel focus: The relationship between acoustic pressure patterns and sensations
- Clinical and pedagogical implications: Applications for voice training and therapy
Theoretical Foundation
The source-filter theory provides a powerful framework for understanding how the same sound source (the vibrating vocal folds) can produce many different vowel qualities simply by changing the shape of the vocal tract. This theory explains:
- Why vowels have characteristic formant patterns
- How singers and speakers modify vowels for different acoustic effects
- Why certain vowel modifications increase vocal power or change tone quality
- How spectral analysis can reveal information about vocal tract configuration
Connections to Previous Material
This chapter builds directly on concepts from earlier chapters. The acoustic wave propagation discussed in Chapter 5 is now applied to confined spaces (tubes). The principle of resonance from Chapter 4, previously used to understand vocal fold vibration, is now applied to the air column of the vocal tract. The glottal airflow waveform and its spectrum, introduced in Chapter 5, becomes the “source” in the source-filter framework.
This chapter provides essential knowledge for understanding how vocal tract shape determines vowel quality, a fundamental concept in voice pedagogy, speech therapy, and vocal performance.