The position of the vocal chords, in other words the nature of the glottis (i.e. the opening between the vocal chords), modifies the breath in many ways.
When they are apart, in what we may call the rest position, the breath passes through unhindered. When we want a particularly large supply of breath, as in blowing, we keep them still more apart. When we wish to "hold our breath," we close them firmly. When we wish to "clear our throat," we press them together and then let the breath come out in jerks; if this is done violently and (as a rule) unintentionally, a cough is produced; sometimes we do it slightly before the opening vowel of a word spoken emphatically (this is commonly the case in German, and is known as the "glottal stop").
We may also close only the fleshy ends, and leave the cartilaginous ends open; then we speak in a whisper.
7. If we neither leave the vocal chords apart nor bring them together quite closely, but let them touch lightly, then the air as it passes out will make them vibrate; and breath accompanied by this vibration is voice[5] in the narrower application of the word. In ordinary speech this vibration is an essential part of all vowels and of many consonants. They are accordingly called voiced[6] sounds; those produced without vibration of the vocal chords are voiceless.[7]
The vibration can be felt in several ways. Utter a long s and then a long z (the sounds at the beginning of seal and zeal respectively), again long s, again long z, and so on; at the same time put your fingers to your throat, or put your hands to both ears, or lay your hand on the top of your head, and you cannot fail to notice the vibration every time you utter z. Try it also with f v f v f v, etc., and with the sounds written s in sure and z in azure, and the sounds written th in thistle and th in this. Then proceed to p and b, t and d, k and g (as in go). Lastly, utter a long ah with full voice, and then whisper the same sound softly. Ascertain in each case which sound is accompanied by vibration of the vocal chords.
Utter a long f and suddenly separate the lower lip from the upper teeth, and nothing more will be heard; but utter a long v and again suddenly separate the lip from the teeth, and you will hear the "voice," with a sound like the [ə] described in § 38. (It is the sound uttered when we hesitate in our speech, and is usually represented in writing by "er ... er.")
It is important that the vibration should be good. If it is slow, the pitch will be low; if it is quick, the pitch will be high. But whatever the pitch, the vibration must be uniform. To practise this, dwell on various voiced sounds for a long time, emitting the breath slowly and regularly.
Only the voiced sounds can be produced with varying pitch; they are musical, the rest are noises. Notice, in church for instance, how the tune is carried by the voiced sounds; the voiceless ones seem to break the course of the tune.
When the vocal chords are short they vibrate more quickly than when they are long, and quicker vibrations give a higher pitch. This explains why the average pitch of a woman's voice is higher than that of a man. When a boy's voice "breaks," this is due to certain changes affecting his vocal chords; it is important that the voice should not be subjected to any excessive strain when it is in this stage.
Certain affections of the throat interfere with the action of the vocal chords, and they become incapable of vibrating; then we "lose our voice." When we "lower the voice," we make the vibrations slower, and lower the pitch. When we "drop the voice to a whisper," we are intentionally preventing them from vibrating. This much diminishes the carrying power of the voice, and we thereby ensure that our words are heard only by those who are quite close. A peculiar variety is the "stage aside," when the actor tries to convey the impression that his words are not heard by those near him, yet desires them to be heard by the spectators, many of whom are much farther away. This is a very loud whisper; it naturally requires a considerable effort and is very tiring.