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The Voice Sources
Most sounds made as part of speech and singing result from the vibration of the vocal folds in the larynx. The larynx is thus often considered to be the sound source or voice source.
To initiate speech and singing there has to be a flow of air through the larynx. This flow of air is modulated by the vocal folds that resist the airstream and vibrate. The vibrations cause tiny changes of air pressure in the airstream called compressions and rarefactions. The vibrations are controlled by aerodynamic forces and elastic forces of the vocal folds, together known as aerodynamic-myoelastic forces. The frequency and intensity of these vibrations help determine the pitch and loudness of the resultant sounds. If the vibrations are regular they are considered to be periodic.
We create some sounds without using vibration of the vocal folds as a sound source. Some of these sounds are known as voiceless sounds. Many English consonants fall into this category. If you make the sounds ’s’ and ‘z’ whilst monitoring the larynx externally, you will notice a vibration when making the ‘z’ sound but not when making the ’s’. The ’s’ sound is considered a voiceless sound, whilst the ‘z’ is considered to be a ‘voiced’ sound. The ‘z’ is considered voiced as the vocal folds are engaged to make this sound. The sound source of the ’s’ originates solely from the turbulence around the point of obstruction, the tongue and alveolar ridge. This turbulence causes non-periodic variations of air pressure in the airstream that the ear perceives as higher-frequency noise. Consonants of this type involve no vocal fold involvement at all, they are known as nil-phonation voiceless consonants. The consonant ‘h’, however, does use the vocal folds to create turbulant sound source at the level of the glottis. Some languages include voiceless vowel sounds and utilise this type of sound source known as breath.
Voiced and voiceless sounds use what is known as an egressive pulmonic airstream, that is the air flow originates from the lungs. We are able to make other kinds of sounds using different kinds airstreams. Although we do not use any of these sounds as part of everyday English, some languages of the world do utilise these sounds. We occasionally use some of these sounds for extralinguistic purposes.
Airstreams can be egressive (out of the body) or ingressive (into the body). Most speech around the world uses egressive pulmonic airstreams for the most part. One example of how a pulmonic ingressive airstream may be used as part of everyday speech is during rapid counting: as the speaker comes to the end of the breath, he may countinue to count as he breathes in.
Airstreams can also be glottalic. This is where the glottis is closed and the rapid movement of the larynx creates an air flow in or out of the mouth. Similarly, airstreams can be initiated in the mouth itself through the movement of the tongue using what is known as a velaric airstream. Ingressive velaric airstreams are used in languages that use clicks. We also use a velaric airstream when we use ‘tut, tut, tut’ in disapproval or use a ‘click’ to attract a horse. In buccal airstreams the cheeks are used to generate an airstream, consider a ‘kiss’ or a ‘raspberry’ noise.
Speakers who have there larynx removed can be trained to utilise airstreams generated from the oesophagus by swallowing air and bletching it out. Another kind of airstream utilises the a special valve inserted between the oesophagus and trachea so that the pulmonic egressive airstream can be still be utilised. Both oesophageal or tracheo-oesophageal airstreams use the pharyngo-oesophageal junction as a psuedo-glottis sound source. Artificial larynxes are mechanical vibrators that are placed on the neck and cause the skin of the neck to vibrate like a drum skin and create a sound source.
A final sound source to consider are the false vocal folds. Ventricular phonation is when the false vocal folds are forced together and begin to vibrate as a sound source. This may happen as the result of a severe vocal fold dysfunction and evidence suggests that the false vocal folds may even increase in size to facilitate this kind of phonation. In singing any engagement of the false vocal folds as a sound source is considered unhealthy and should be avoid at all costs.
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