LINGUISTICS 330 Lecture #9

THE VOCAL TRACT: AN ACOUSTIC FILTER

VOCAL TRACT: an air-filled tube which is open at one end (the mouth is open for the most part during speech) and closed at the other end (larynx).

Reflection occurs where the air pressure is maximally different from normal air-pressure (= at the open end of the tube): this will be at the crest or trough, i.e. halfway between nodes ↓ one quarter of a wavelength

STUDY Figure 10-19; READ p. 261.

Only 1/4 of the wave can fit into the tube (=vocal tract) at any one pass.

The lowest resonance frequency at which such a tube resonates will have the wavelength four times the length of the tube.

FORMANT: VOCAL TRACT RESONANCE ↓ A band of resonant energy that in combination with others acoustically characterizes a particular vowel sound.

(see below)

SOURCE FUNCTION:

• the glottal source with its many harmonics; • it is filtered according to the frequency response of the vocal tract (= transfer function)

The harmonics of the glottal sound wave which correspond to, or are near, the resonant frequencies of the vocal tract are amplified; those harmonics that are distant from the resonant frequencies of the vocal tract lose energy. The length of the male vocal tract is about 17 cm.

The lowest resonance frequency at which the air in such a tube would vibrate will have the wavelength

4 x 17 cm = 68 cm

1 c 34,000 cm/sec f = = = about 506 Hz → lowest resonance frequency! λ 68 cm

The tube will also resonate at odd multiples of that frequency.

WHY ODD MULTIPLES??

The compression waves and rarefaction waves coincide in direction at the zero crossing, but frequencies which are even multiples of the principal resonant frequency neutralize (=cancel) one another. ODD MULTIPLES ARE COMPATIBLE!

see APPENDIX pp. 9,10

The sound which emerges at the end of the tube (=lips) has the same harmonics as the sound at the source (=glottis).

But the amplitude of the harmonics has been modified, ALTERING THE QUALITY OF THE SOUND.

Study Figure 10-21.

The acoustic resonator (=vocal tract) will pass energy at frequencies corresponding to spectral peaks ↓ POLES it does not pass energy at spectral valleys ↓ ZEROS

Constrictions at points of maximum velocity lower the resonance frequency, while constrictions at points of maximum pressure raise the frequency of the resonances.

SUMMARY OF THE SOURCE-FILTER THEORY FOR VOWELS

• Vibration of the vocal folds produces the energy source

2 • This energy activates the RESONANCES ↓ FORMANTS (=poles)

• The resonances act as a FILTER ↓ the energy in the various harmonics of the source is not transmitted equally

• Although theoretically there is an infinite number of formants, the FIRST THREE are sufficient for distinguishing between different vowels: F1, F2, F3

Predictions of the effects of constriction in the vocal tract on the formant frequencies:

1. All three formants frequencies are lowered by labial constriction or by lowering the larynx.

2. All three formant frequencies are raised by constriction near the larynx.

3. F1 decreases in frequency as lip opening and pharyngeal enlargement accompanies tongue elevation.

4. F1 increases in frequency when the constriction moves back in the vocal tract.

5. F2 is high in frequency when the oral cavity is constricted, and low in frequency when it is more open or elongated.

REMEMBERING FORMANT FREQUENCIES:

1. Line up the vowels in this order:

high front → low front → low back → high back

2. Apply the following:

a. F1 starts low and rises until the vowel [a], then F1 falls b. F2 starts high and falls c. F3 is similar to F2, but is higher.

WE CANNOT DETERMINE ABSOLUTE FREQUENCIES THIS WAY, BUT WE HAVE AN IDEA OF FORMANT PATTERNS!

Study Appendix p.17

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