Question

Summarize the Source-Filter Theory. Be sure to incorporate glottal spectrum, resonance, bandpass filter, variable resonator, and the three functions.

Answer 1

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. For this filtration, a bandpass filter is generally used. A bandpass filter is an electronic device or circuit that allows signals between two specific frequencies to pass, but that discriminates against signals at other frequencies. Noise can be eliminated using this filter.

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.

To enhance the amplitude of the signal, the variable resonator is used. This allows getting the maximum amplitude of the signal by the interference of the signal.

This model has 3 parameters that help determine resonance: the location of the main tongue constriction, the amount of lip protrusion, and the cross-sectional area of the vocal tract; these are three functions of the source-filter theory.