Forming the Vowel Sounds

The vocal resonances are altered by the articulators to form distinguishable vowel sounds. The peaks in the vowel spectra are called vocal formants. Note the prominent role of the tongue in this process. The jaw position and lips also play a major part.

The sketches at left above are adapted from Gunnar Fant's "Acoustic theory of speech production" and are reportedly sketches taken from x-rays of the head during the production of these sounds. These are the vowels classified as IPA [a], [i], and [u] and roughly correlate with the vowels represented in the spectra from Benade. The emphasis should be on "roughly" since I don't know how close the correlation is. The intent here is to illustrate the role of the articulators and to point to the fact that their action has a major influence on the harmonic content of the voiced sounds. The normal ear is able to clearly distinguish those differences.

Vocal Formants

The term formant refers to peaks in the harmonic spectrum of a complex sound. They are usually associated with, but not necessarily equal to some sort of resonance of the source. Because of their resonant origin, they tend to stay essentially the same when the frequency of the fundamental is changed. Formants in the sound of the human voice are particularly important because they are essential components in the intelligibility of speech. For example, the distinguishability of the vowel sounds can be attributed to the differences in their first three formant frequencies. Producing different vowel sounds amounts to retuning these formants within a general range of frequencies. Benade suggests the following ranges of frequencies for the formants of a male voice:

1st formant 150-850 Hz 2nd formant 500-2500 Hz 3rd formant 1500-3500 Hz 4th formant 2500-4800 Hz

The process of articulation determines the frequencies of the vocal formants. Sundberg has identified portions of the vocal anatomy which he associates with the formant frequencies. The jaw opening, which constricts the vocal tract toward the glottal end and expands it toward the lip end, is the deciding factor for the first formant. This formant frequency rises as the jaw is opened wider. The second formant is most sensitive to the shape of the body of the tongue, and the third formant is most sensitive to the tip of the tongue.

Vowel Formants The recognizable quality of the sound of different vowel sounds is attributed to the existence of formant regions: frequency ranges where the sound is enhanced by the cavity resonances of the human vocal mechanism. The plots at left show that the formant frequencies are distinctly different for the three vowel sounds indicated. The inner ear acts as an analyzer and can clearly distinguish these vowel sounds. The formants are quite similar for the same vowel at different frequencies, enabling it to be recognized regardless of the pitch. Sketches of vocal mechanism during vowel formation How does the ear distinguish the vowel sounds?

This is another example of vowel sounds produced at a frequency of 325 Hz by a female voice. It shows the display vs time on the left to show the waveforms, and the Fast Fourier Transform on the right to show the distinctive harmonic content of the vowels. There is considerable variation, but the ear acts as a harmonic analyzer and can easily distinguish and recognize these sounds.

Representative Vowel Formant Frequencies Ladefoged lists representative vowel formant frequencies, averages from several U. S. speakers. The symbols used represent standard English phonemes. First three vowel formant frequencies Vowel I II III /i/ 280 Hz 2250 Hz 2890 Hz /I/ 400 Hz 1920 Hz 2650 Hz /a/ 710 Hz 1100 Hz 2450 Hz

The formant frequencies are keys to the distinguishablility of the vowel sounds.