The variations in vocal sounds which give rise to speech are not produced in the larynx, but in the throat, mouth and nose. When unaccompanied by any vocal sound, speech only gives rise to a whisper; but when a vocal tone is at the same time produced, we have the ordinary loud speaking. Since vocal tones can only be produced by expiration, so we can only speak aloud by means of an expiratory current of air; but an inspiratory current may be made to give rise to a kind of whisper.
Speech is composed of two kinds of sounds, in one of which the sounds must be accompanied by a vocal tone, and are, hence, called "vowels;" in the other no vocal tone is necessary, but changes in shape take place in the resonating chambers, so as to give rise to noises called consonants. As the pronunciation of the consonants is always accompanied by some vowel sound, and as the difference between the vowels is brought about by changes in the shape of the mouth, the distinction between the two sets of sounds is rather artificial than real.
The production of the different vowel sounds depends upon such a change being brought about in the shape of the mouth cavity and aperture, that a resonator, with a different individual note, is formed for each particular word.
The sounds called consonants are caused by some check or impediment being placed in the course of the blast of air issuing from the air passages. They may be classified, according to the part at which the obstruction occurs, as follows: -
1. Labials, when the narrowing takes place at the lips, as in pronouncing b, p,f, v.
2. Dentals, when the tongue causes the obstruction by being pushed against the hard palate or the teeth, as in t, d, s, I.
3. Gutturals, when the posterior part of the tongue moves toward the soft palate or pharynx, as in saying k, g, gh, ch, r.
Consonants may also be divided into different groups, according to the kind of movements which give rise to them.
1. Explosives are produced by the sudden removal of the obstruction, as with p, d, k.
2. Aspirates are continuous sounds caused by the passage of a current of air through a narrow opening, which may be at the lips, as in f, at the teeth as with s, or at the throat as in ch.
3. Resonants are the sounds requiring some resonance of the vocal cords, and the air current is suddenly checked by closure of the lips, as in m, or the dental aperture as in n or ng.
4. Vibratory, of which r is the example, requires a peculiar vibration of the vocal cords, while either the dental or the guttural aperture is partially closed.