Digital Signal Processing Reference
In-Depth Information
Nasenhöhle
Nasal cavity
Pharyngeal cavity
Nasaler Rachenraum
Oral cavity
Mundraum
Unterer Rachenraum
Bottom pharyngeal
cavity
Kehlkopf
Larynx
Stimmritze und Stimmbänder
Vocal cord and glottis
Luftröhre
Speiseröhre
Windpipe
Esophagus
Illustration 77:
The human speech-tract as a deformable cavity resonator
The human vocal organs consists of an energy store - the lungs - , an oscillator - the vocal chords - , and
an adjustable cavity resonator - the mouth and throat cavity. The lungs mainly serve to generate an air
stream with sufficient pressure. The air flows through the glottis, the space between the two vocal chords at
the lower end of the larynx. If pressure builds up the vocal chords are pressed apart in a fraction of a
second. As a result the pressure is suddenly released, the vocal chords close again, and in the case of a
vowel or in singing this is near-periodic. If the vocal chords vibrate near-periodically the vocal chord
spectrum is then practically a line spectrum.
Illustration 77 explains the role of the cavity resonator mouth and throat. The vocal chord
spectrum - as described for the guitar - is influenced as far as frequency is concerned by
the cavity resonator. The resonance chamber acts like a "filter bank" with several
parallel band pass filters on the near periodic signal produced by the vocal chords. In this
way all the frequencies which lie near formant frequencies are ultimately emphasised. To
put it another way - a frequency evaluation/weighting of the vocal chord signal takes
place. A vowel belongs to each combination of the four to five formants. Vowels must
therefore be harmonic, i.e. near-periodic because the resonance chamber carries out a
controlled near-periodic signal, in contrast to the free oscillation (natural frequencies) of
a drum membrane. The frequencies of the vocal chord spectrum which lie in these
resonance areas are clearly "emphasised" or amplified. All the others are subdued.
