The conditions that define the attributes of the human voice are in essentials similar to those of musical instruments in general. The source of energy is the lungs. By them the air is forced under pressure through the glottis causing the vocal cords to move rhythmically, thereby producing a musical note. This musical note is a tone-complex of simple harmonic vibrations some of which are modified by the supraglottal cavities acting as resonators. It is the train of sound waves thus modified, issuing from the mouth, which gives rise to those elements of speech termed vowels and voiced consonants. Such sound waves can be, and sometimes are produced to a limited extent by an in-drawn current of air actuat ing the vocal cords.
To what extent the infraglottal cavities, e.g., the trachea and chest cavity, influence the quality of the glottal note has not been determined, but they certainly do affect it. Investigators are however agreed that among the supraglottal cavities the effect of the sinuses, e.g., the maxillary sinus, the ethmoidal cells, sphenoidal sinus and frontal sinus, is negligible owing to their small size, un favorable positions, and minute openings. The expression "sinus tone production" would thus appear to be devoid of justification. Nor can there be any question of directing the voice to a definite point in the buccal cavity, as the dimensions of the mouth cavity, in comparison with the wave-length of sound, are too small for reflexion to be possible.
Duration.—The duration of voice in vowels varies with the speed of utterance. In words of two or more syllables, results show that for English spoken at an average speed, the duration of so called short vowels may vary from .047 to .095 of a second, while that of so-called long vowels may vary from .12 to .255 of a second.
Pitch.—The pitch of the glottal note, i.e., the lowest tone of the complex, is determined by the frequency, or number of cycles the vocal cords execute in one second, frequency and pitch being physical and sensory aspects of the same thing. The conditions determining pitch are the mass, length and elasticity of the cords, and the pressure actuating them. Increase of pressure in the expiratory current leads to a raising of pitch, but the pitch of the resonance cavities appears to have no effect on the pitch of the glottal note.
The vocal cords are tightened if the arytenoid cartilages be braced back by contraction of the crico-thyroid, and posterior thyro-arytenoids will then give elasticity to the margins of the glottis so that they will recoil after being blown apart. The greater the degree of contraction the higher will this elasticity become. All the muscles except the thyro-cricoid (which is inervated by the superior laryngeal) receive nerve filaments from the inferior laryngeal branch of the vagus, the fibres being derived from the ac cessory roots. Both the abductor and adductor nerves come there fore from the inferior laryngeal.
In men, by the development of the larynx the cords become more elongated than in women, in the ratio of 3 to 2, so that the male voice is of lower pitch and is usually stronger. At the age of puberty the larynx grows rapidly, and the voice of a boy breaks in consequence of the lengthening of the cords, generally falling an octave or so in pitch. A similar change but less in amount occurs at the same time in the female.
The pitch compass of the human voice generally ranges from E to e' (8o– to 320–) in Bass voices and c to a" (256– to 853 – in Soprano voices. (The sign – is used to denote the number of cycles a second. This unit is sometimes called a "hertz" after the great German physicist.) These limits are greatly exceeded by many singers. Rethi and Froschels report the abnormal case of a singer with a compass of five octaves, i.e., F (42 ) to f"' – ).
Timbre.—As has already been stated above, the voice is com posed of a fundamental and a series of over-tones in harmonic relation to it. The quality or timbre is determined by the relative strength of these overtones. The phase relationship between them appears to be of little importance for quality. Difference be tween vowels is essentially a difference of quality. Thus when speaking the vowel [a ] certain partials in the region g" (767–) and d"" (1,147 –) receive preferential treatment from the supra glottal cavities while the prominent partials in [i] are in the region and f" ).
The decay in intelligibility of vowels on the suppression of cer tain partials, has been investigated by Stumpf and Fletcher. A quo tation from the latter relating to this is illuminating. Incidentally it shows also that the fundamental may be only subjective. "The vowel 'all' sung on a pitch d' (145 –) is affected only slightly in either pitch or quality when the fundamental and first two over tones are eliminated. Even with the fundamental and first six overtones eliminated, the pitch still very definitely corresponds to the pitch of a pure tone with the frequency of the fundamental, namely, 145–. . The harmonic analysis of this filtered tone shows no frequencies below ,000 . Eliminating all the overtones above the sixth changes the quality by about the same amount as elimi nating the fundamental and first and second overtones. The data also indicate that if the fundamental and all of the upper and lower harmonics except the third, fourth and fifth, are eliminated, the re maining compound tone has the same pitch as the fundamental, although the quality of the sound is very different from that of the sound `ah.' " A rich baritone or contralto appears to be af fected neither in pitch nor in quality by eliminating the fundamen tal and first two or three overtones. The filtering out of higher partials, even of those above the 15th, however, noticeably affects the musical quality of the voice. High harmonics do not appear to be so essential to good quality in sopranos, as in bass, baritone and contralto.