Voiced sounds.
The vocal folds originate at the angle of the thyroid cartilage, course horizontally backwards and are inserted into each of the arytenoid cartilages. By adduction (i.e. drawing these cartilages towards each other), the slit between the folds, called the ‘glottis’, is narrowed, and an airstream can set the folds into vibration. A vibration cycle can be described as follows. When the glottis is slightly open an airstream from the lungs can pass through it. This airstream throws the vocal folds apart and at the same time generates a negative pressure along the edges of the folds. The sucking effect of this negative pressure along with the elasticity and other mechanical properties of the folds closes the glottis again. Then the air pressure difference across the glottis throws the folds apart, thus starting the next vibratory cycle. The frequency of a vibration is determined by the transglottal air pressure difference and the mechanical properties of the folds. A high pressure difference or tense and thin vocal folds, or both, give a high vibration frequency; converse states give a low frequency. The mechanical properties of the folds are regulated by a series of muscles that vary the length and stiffness of the folds by manipulating the positions of the laryngeal cartilages. Thus these muscles are used to regulate the vibration frequency. As the vibration frequency determines the pitch perceived, these muscles are often referred to as the ‘pitch regulating muscles’. An increase of the subglottal pressure raises the amplitude of the sound produced and also increases the vibration frequency, raising the pitch. Thus, in order to perform a crescendo at a constant pitch a singer has to raise the subglottal pressure and simultaneously compensate for the pitch increase by adjusting the pitch-regulating muscles.
By vibrating, the vocal folds repeatedly interrupt the airstream from the respiratory system. Thus they act as a valve oscillating between open and closed positions: the result is a chopped airstream corresponding to a complex sound, the fundamental frequency of which is equal to the vibration frequency of the folds. The glottis is schematically shown as a function of time in
The horizontal portion of the curve corresponds to the closed phase of the glottal vibration cycle, and the triangular portion is the open phase. As the air flow generally increases more slowly than it decreases, the triangular part of the curve is asymmetrical in the figure. In trained voices the glottal closure is often observed to be more efficient than in untrained voices. Also, the vibration pattern appears to vary considerably less with pitch and vocal intensity in trained voices than in untrained ones (see Sundberg, Andersson and Hultqvist, 1999).
The sound generated by the chopped transglottal airstream is built up by a great number of harmonic partials whose amplitudes generally decrease monotonically with frequency, roughly by 12 dB per octave at neutral loudness. It is noteworthy that this holds as an approximation for all voiced sounds. Partials of measurable amplitude in the source spectrum are generally found up to 4–6 kHz. This means that a tone with a fundamental frequency of 100 Hz may contain between 40 and 60 partials of appreciable amplitude. However, the amplitudes of the source spectrum partials vary with pitch and vocal intensity (see Sundberg, Andersson and Hultqvist, 1999).
Voiceless sounds.
The sound source in this case is noise generated by a turbulent airstream. The narrow slit required for the noise generation can be formed at various places along the vocal tract, the lowest position being at the glottis itself, which can be kept wide enough to prevent the folds from vibrating and narrow enough to make the airstream turbulent. This is the oscillator used in the ‘h’ sound. Another place used in some languages is the velar region, which can be constricted by the tongue hump. The resulting sound is used as the voice source in the German ‘ach’ sound. In most remaining unvoiced sounds the tongue tip constricts the vocal tract in the palatal, alveolar or dental regions as in the initial phonemes of ‘sheep’, ‘cheap’ and ‘sip’. In the ‘f’ sound the upper incisors and the lower lip provide the slit.