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.

How does the source-filter model work?

Visualizing the vocal tract position as a filter

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).

State University of New York at Stony Brook

San Jose State University

Articulatory Phonetics is an area of study that focuses on how throat, mouth and nose work together to modify the airflow from lung to produce phonetics,

Articulatory Phonetics

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

Speech and Language Processing (3rd ed. draft) 

1) Trachea

2) Larynx

3) Vocal cord

4) Pharynx

5) Epiglottis

6) Tongue

7) Soft palette

8) Hard palatte

9) Nasal cavity

Vocal Organs

There are several ways of classifying sounds or phones in language:

1)	Voiced or unvoiced

2)	nasal sound or non-nasal sound

3)	Consonant, vowels, or semivowels


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