Source-Filter Theory of Speech Production
The source-filter theory describes speech production as a two stage process involving the generation of a sound source, with its own spectral shape and spectral fine structure, which is then shaped or filtered by the resonant properties of the vocal tract.
Most of the filtering of a source spectrum is carried out by that part of the vocal tract anterior to the sound source. In the case of a glottal source, the filter is the entire supra-glottal vocal tract. The vocal tract filter always includes some part of the oral cavity and can also, optionally, include the nasal cavity (depending upon whether the velum is open or closed).
Sound sources can be either periodic or aperiodic. Glottal sound sources can be periodic (voiced), aperiodic (whisper and /h/) or mixed (eg. breathy voice). Supra-glottal sound sources that are used contrastively in speech are aperiodic (ie. random noise) although some trill sounds can resemble periodic sources to some extent.
A voiced glottal source has its own spectrum which includes spectral fine structure (harmonics and some noise) and a characteristic spectral slope (sloping downwards at approximately -12dB/octave).
An aperiodic source (glottal or supra-glottal) has its own spectrum which includes spectral fine structure (random spectral components) and a characteristic spectral slope.
Periodic and aperiodic sources can be generated simultaneously to produce mixed voiced and aperiodic speech typical of sounds such as voiced fricatives.
In voiced speech the fundamental frequency (perceived as vocal pitch) is a characteristic of the glottal source acoustics whilst features such as vowel formants are characteristics of the vocal tract filter (resonances).
What is a filter?
A filter is anything that can selectively permit some things to pass through and block other things. For example, a piece of filter paper used in chemistry blocks the passage of solid particles larger than a certain size and permits smaller particles and liquids to pass through unhindered. An acoustic filter selectively attenuates (reduces in intensity) certain frequencies and allows other frequencies to pass through relatively unattenuated.
- Clark and Yallop, section 7.10
- Harrington and Cassidy, chapter 3.