The human voice as a musical instrument combines a non-uniform acoustic tube resonator (the vocal tract) with a forced, ``lip-like'' excitation mechanism (the vocal folds). Unlike the brass player's lips, however, the vocal folds are little affected by acoustic feedback from the vocal tract. A singer trains to control his/her vocal cord mechanism for accurate pitch specification as well as to control his/her vocal tract shape to modify formant structure for better projection.
As singing is primarily concerned with pitch and periodic vibrations, vowels and voiced consonants necessarily take priority.
Trained singers are sometimes able to tune their vowel formants to match one or more harmonics of the sung pitch.
While female voices are generally pitched about an octave higher than male voices, the formants usually differ by less than a musical third (less than 25% in frequency).
The first formant will usually contribute more to timbre because of its greater amplitude and lower frequency, closer to the fundamental.
Bass and baritone singers are rarely able to enhance the fundamental with a formant resonance.
Singers change some vowel sounds to improve musical tone.
The second formant is typically lower in frequency in the sung vowel.
Trained singers show a strong formant around 2500 - 3000 Hz, referred to as the ``singer's formant''. This formant, which seems to be independent of the particular vowel and pitch, adds brilliance and carrying power to the voice.
The singer's formant is attributed to a lowered larynx and widened pharynx, which forms an additional resonance cavity.
The fundamental pitch of a soprano often exceeds the normal frequency of the first formant.
Experienced singers have thus learned how to ``tune'' their formants over a reasonable range in order to make them coincide with the fundamental or one of the overtones of the note being sung.
Formants are usually tuned upwards, though downward tuning is possible.
Voice registers correspond to differences in tone caused by different adjustments of the larynx. Two (``heavy'' and ``light'') or three (``chest'', ``middle'', and ``head'') registers are commonly identified.
In the heavy or chest voice, the thyroarytenoid muscles are active and hence shortened. They thicken the vocal folds, which results in their remaining closed over an appreciable part of each cycle of vibration.
In the light or head voice, the thyroarytenoid muscles are passive. In this state they offer little resistance to the cricothyroids, which apply substantial longitudinal tension to the vocal folds, lengthening and thinning them. The glottis closes only briefly, or not at all, and the resulting sound has fewer harmonics than the chest voice.
