San Jose State University

State University of New York at Stony Brook

The source filter model explains the acoustics of a sound by modeling how the pulses produced by the glottis (the source) are shaped by the vocal tract (the filter).

The Source-Filter model

An on-going but a helpful book resource about NLP
https://web.stanford.edu/~jurafsky/slp3/

Speech and Language Processing (3rd ed. draft) 

A harmonic is another wave whose frequency is a multiple of the fundamental wave.
Vocal tract acts as a kind of filter or amplifier, in which any cavity causes waves of certain frequencies to be amplified and others to be damped. 
Thus, by changing the shape of the cavity, we can cause different frequencies to be amplified.
Likewise, when we produce particular vowels, we essentially change the shape of the vocal tract cavity by placing the tongue and the other articulators in particular positions.
Thus, a wave of the same fundamental frequency passed through different vocal tract positions will result in different harmonics being amplified.

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