What is the Source-filter Theory?
The generation of a sound source (sound wave), 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 include the nasal cavity (depending upon whether the velum is open or closed).