Physiological and physical basis of speech

Speech production

Speaking is in essence the by-product of a necessary bodily process, the expulsion from the lungs of air charged with carbon dioxide after it has fulfilled its function in respiration. Most of the time one breathes out silently, but it is possible, by adopting various postures and by making various movements within the vocal tract, to interfere with the egressive airstream so as to generate noises of different sorts. This is what speech is made of.

The vocal tract comprises the passage from the trachea (windpipe) to the orifices of the mouth and nose; all the organs used in speaking lie in this passage. Conventionally, these are called the organs of speech, and the use in several languages of the same word for the tongue as a part of the body and for language shows the awareness people have of the role played by this part of the mouth in speaking. But few if any of the major organs of speech are exclusively or even mainly concerned with speaking. The lips, the tongue, and the teeth all have essential functions in the bodily economy, quite apart from talking; to think, for example, of the tongue as an organ of speech in the same way that the stomach is regarded as the organ of digestion is fallacious. Speaking is a function superimposed on these organs, and the material of speech is a waste product, spent air, exploited to produce perhaps the most wonderful by-product ever created.

Relatively few types of speech sounds are produced by other sources of air movement; the clicks in some South African languages are examples, and so is the fringe linguistic sound used in English to express disapproval, conventionally spelled tut. In all spoken languages, however, the great majority of speech sounds have their origin in air expelled through the contraction of the lungs. Air forced through a narrow passage or momentarily blocked and then released creates noise, and characteristic components of speech sounds are types of noise produced by blockage or narrowing of the passage at different places.

If the vocal cords (really more like two curtains) are held taut as the air passes through them, the resultant regular vibrations in the larynx produce what is technically called voice, or voicing. These vibrations can be readily observed by contrasting the sounds of f and v or of s and z as usually pronounced; five and size each begin and end with voiceless and voiced sounds, respectively, which are otherwise formed alike, with the tongue and the lips in the same position. Most consonant sounds and all vowel sounds in English and in the majority of languages are voiced, and voice, in this sense, is the basis of singing and of the rise and fall in speaking that is called intonation, as well as of the tone distinctions in tone languages. The vocal cords may be drawn together more or less tightly, and the vibrations will be correspondingly more or less frequent. A rise in frequency causes a rise in perceived vocal pitch. Speech in which voice is completely excluded is called whispering.

Above the larynx, places of articulation in frequent use are between the back of the tongue and the soft palate, between the blade of the tongue and the ridge just behind the upper front teeth, and between the lips. Stoppage and release (technically, plosion) at these places form the k (often written as c, as in cat), t, and p sounds in English and, when voicing is also present, the g (as in gift), d, and b sounds. Obstruction at these and other places sufficient to cause noise gives rise to what are called fricative sounds; in English these include the normal pronunciations of s, z, f, and v and the th sounds in “thin” and “then.” A vowel is characterized as the product of the shape of the entire tract between the lips and larynx, without local obstruction though usually with voicing from the vocal cords. It is contrasted with a consonant, though the exact division between these two categories of speech sound is not always easy to draw. Different shaping of the tract produces the different vowel sounds of languages.

The soft palate may be raised or lowered. It is lowered in breathing and allows air to pass in and out through the nose. In the utterance of most speech sounds it is raised, so that air passing through the mouth alone forms the sound; if it is lowered, air passes additionally or alternatively through the nose, producing nasal sounds. All but a few languages have nasal consonants (the English sounds m, n, and ng as in sing), and some, such as French, have nasalized vowels as well. A few people regularly allow air to pass through their nasal passages while they speak; such persons are said to “speak through the nose.”

All articulatory movements, including the initial expulsion of air from the lungs, may be made with greater or less vigour, giving rise to louder or softer speech or to greater loudness on one part of what is said.

Every different configuration and movement of the vocal tract creates corresponding differences in the air vibrations that comprise and transmit sound. These vibrations, like those of all noises, extend outward in all directions from the source, gradually decreasing to zero or to below the threshold of audibility. They are called sound waves, and they consist of rapid rises and falls in air pressure. The speed at which pressure rises and falls is the frequency. Speech sounds involve complex waves containing vibrations at a number of different frequencies, the most complex being those produced by the vocal cords in voiced sounds.

The eardrum responds to the different frequencies of speech, provided they retain enough energy, or amplitude (i.e., are still audible). The different speech sounds that make up the utterances of any language are the result of the different impacts on one’s ears made by the different complexes of frequencies in the waves produced by different articulatory processes. As the result of careful and detailed observation of the movements of the vocal organs in speaking, aided by various instruments to supplement the naked eye, a great deal is now known about the processes of articulation. Other instruments have provided much information about the nature of the sound waves produced by articulation. Speech sounds have been described and classified both from an articulatory viewpoint, in terms of how they are produced, and from an acoustic viewpoint, by reference to the resulting sound waves (their frequencies, amplitudes, and so forth). Articulatory descriptions are more readily understood, being couched in terms such as nasal, bilabial lip-rounded, and so on. Acoustic terminology requires a knowledge of the technicalities involved for its comprehension. Both sorts of description and classification are important, and each has its particular value for certain parts of the scientific study of language.