Speech

Man has developed a very sophisticated method of communication. By combining various sounds in patterns we can convey, more or less, any number of thoughts to other people. This is speech. The process of forming words is an extremely complicated procedure. The brain decides what it wants to say and chooses the appropriate words. Messages are then sent to the respiratory muscles, larynx, mouth and tongue so that they work in unison to produce the correct sequence of sounds. Because we usually speak at a rate of 150 words per minute this has to be a very rapid process.

Learning to talk

Speech is learned at an early age. Most children can say single words by one year of age and speak meaningfully by the age of two. Learning starts when babies hear words spoken by adults. They then associate the words they hear with objects. During this time babies also make random noises themselves. Sometimes these noises correspond with words that they have heard and they get a positive response from the adults and this encourages them to repeat the sound. As they get older children gain more control over the organs associated with speech and so can reproduce sounds more readily.

When we listen to ourselves on tape recorders we think that our voices sound different. This is because when we speak our voice travels to our ears via the bones in our skull and so has a different quality to when it travels through the air. The sense of hearing is an important factor in controlling speech, as can be illustrated by the difficulties that deaf people experience with speech. This is not only because they have difficulty in learning the association between an object and a word, but also because they receive very little or no feedback from their speech. The interaction between speech and hearing has a great influence on the ability to vary the pitch of the voice, the timbre and the loudness. It is by these aspects that emotion is shown. Speech lacking in varied intonation is usually considered to be mono-tonuous and emotionless.

Speech organs

All the organs of respiration take some part in the formation of speech. The respiratory muscles are used to inhale before starting to speak. They then direct a stream of air across the vocal cords, enabling a sound to be produced.

The larynx, or voice box, contains the vocal cords which break up the stream of air into pulses, which, on hitting the eardrum are heard as a sound. In order to produce a sound the vocal cords act in the following manner. They are first pulled together by the laryngeal muscles, so that air pressure builds up behind them. The air pressure then causes the cords to be forced apart allowing a pulse of air to escape. As this happens, the pressure falls and the cords are pulled together again. Once more, the air pressure builds up and the cycle is repeated. To make the voice louder the pressure behind the cords has to be increased so that the pulse of breath, when released, has more force. Pressure is increased by the respiratory muscles in the chest. The frequency at which the vocal cords open and close determines the pitch of the voice. The muscles acting on the vocal cords can pull them tighter – which makes their edges thinner. This tightening and thinning of the cords causes them to open and shut more rapidly. The opening and shutting is a vibration – as is sound -and the faster the vibration the higher the pitch of the voice. Relaxing the cords lowers the pitch. In men the frequency of the voice is, on average, 150 vibrations per second and in women the frequency is double this.

The vocal cords vary greatly in dimension because they depend on the size of the entire larynx, which in turn depends on age, sex, size and build. Young boys and girls have about the same voice pitch. During puberty, the male larynx grows considerably under the influence of male hormones so that eventually it is about one-third larger than the female larynx. In males this becomes visible as the ‘Adam’s apple’. Because this enlargement results in a lengthening of the vocal cords, the voice pitch will get lower by about one octave. At the beginning of this process, boys often have a difficulty in controlling their voice: they often switch between the high and the low pitch.

This is called the ‘breaking of the voice’. The larynx with the vocal cords are – contrary to what most people believe – not the most important organs involved in speech. On whispering the cords fall totally apart, allowing the air to pass freely, without a build up of pressure behind them. People who have had a laryngectomy (surgical removal of the larynx, usually to treat cancer) can learn to speak by means of their oesophagus. Another conjecture that dos not hold quite true is that speech is only possible on exhalation. Although this does apply to western countries, there are certain African tribes who have learned to speak while inhaling. When the voice leaves the larynx it is no more than an unmodified, monotonous sound. It is changed by the upper respiratory passages when the pharynx, nose and sinuses act as resonance chambers. They change the sound and give people their individual characteristic vocal qualities.

The role they play is obvious and can be illustrated by the change in the sound of the voice when the nose is blocked as in a cold. The produced and modified sound is then broken up into different sounds that can be recognized as words or parts of words. The organs that do this are the tongue, the soft palate and the lips. The soft palate is at the back of the hard palate, forming a partition between the nose and the mouth. It is mobile and can be raised to block the passage of air between the nose and mouth. The tongue is almost entirely composed of muscles; its amazing flexibility enables it to move into a great many different shapes. The position of the tongue in the mouth can be altered by a set of muscles that are attached to other structures in the mouth, one of which is the hyoid, or lingual, bone.

The lips contain a muscle which completely encircles the mouth. Contraction of this muscle causes the lips to purse and form an O shape. There are also muscles attached to the angles of the mouth which can pull the mouth wide into a grin.