1.2. Vocal Mechanism

Before analysing the linguistic function of phonetic units we need to know how the vocal mechanism acts in producing oral speech.

Human speech is the result of a highly complicated series of events. Let us consider the speech chain, which may be diagrammed in simplified form in Table 1:

Table 1

Speaker's brain	Speaker's vocal tract	Transmission of sounds through air	Listener's ear	Listener's brain
Stage 1	Stage 2	Stage 3	Stage 4	Stage 5
linguistic	physiological	acoustic/physical	auditory	linguistic

The process of speech production is realized in the following scheme:

1. the message is formed and encoded in the brain of the speaker (linguistic/ psychological level);

2. it is transmitted to the organs of speech and some sounds are articulated (physiological stage);

3. the movement of the organs of speech produces sound waves (physical/ acoustic stage);

4. the sound waves are perceived, identified and decoded by the listener. So the last stages are the reception of the sound waves by the listener's hearing physiological apparatus, the transmission of the spoken message through the nervous system to the brain and the linguistic interpretation of the information conveyed.

In accordance with their linguistic function the organs of speech may be grouped as follows (Picture 1).

Picture 1

The respiratory or power mechanism furnishes the flow of air which is the first requisite for the production of speech sounds. This mechanism is formed by the lungs, the wind-pipe and the bronchi. The air-stream expelled from the lungs provides the most usual source of energy which is regulated by the power mechanism.

From the lungs through the wind-pipe the air-stream passes to the upper stages of the vocal tract. First of all it passes to the larynx (Picture 4) containing the vocal cords which began to vibrate.

The opening between the vocal cords is known as the glottis.

When the glottis is tightly closed, as in Picture 2, and the air is sent up below it the so-called glottal stop is produced. It often occurs in English when it reinforces or even replaces [p], [t], or [k] or even when it precedes the energetic articulation of vowel sounds. The most important speech function of the vocal cords is their role in the production of voice. The glottis can have a narrow opening which can make the vocal folds vibrate producing the “voiced sounds”. Finally, it can be wide open (Picture 3), as in normal breathing, and, thus, the vibration of the vocal folds is reduced, producing the “voiceless sounds”.

The height of the speaking voice depends on the frequency of the vibrations. The more frequently the vocal cords vibrate the higher the pitch is. We can alter the amplitude of the vibration which causes changes of the loudness of the sound heard by the listener.

Picture 2. Closed glottis. Picture 3. Open glottis. Picture 4. The larynx.

From the larynx the air-stream passes to supraglottal cavities, that is to the pharynx, the mouth/oral and the nasal cavities. The velum is the part responsible for the selection of the cavities. Through the oro-nasal process we can differentiate between the nasal consonants ([m], [n], []) and other sounds.

Finally, the articulation process is the most obvious one: it takes place in the mouth and it is the process through which we can differentiate most speech sounds. In the mouth we can distinguish between the oral cavity, which acts as a resonator, and the articulators, which can be active or passive: upper and lower lips, upper and lower teeth, tongue (tip, blade, front, back) and roof of the mouth (alveolar ridge, palate and velum). So, speech sounds are distinguished from one another in terms of the place where and the manner how they are articulated.