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Breathing.
The demands on the breathing apparatus differ significantly between speech and singing. There are two main reasons for this. Firstly, phrases in neutral speech are generally short, typically using no more than 15–20% of lung capacity. In singing, on the other hand, phrases tend to be considerably longer, using twice as much and occasionally nearly 100% of lung capacity. As the recoil forces of the respiratory apparatus vary with lung volume, a singer needs to supply different degrees of respiratory muscle force depending on lung volume. Secondly, the mean over-pressure of air in the lungs, which controls the loudness of phonation, is basically constant in neutral speech, although it is raised for emphasized syllables. In singing, as higher pitches require higher pressures, this air pressure needs to be varied with pitch. As lung pressure affects pitch, failures to reach target pressures result in singing off the pitch. Singers generally use the diaphragm muscle for inhalation, which is reflected in an expansion of the abdominal wall. However, the strategy used for achieving the necessary control of the respiratory apparatus differs between singers. Some contract the abdominal wall, thus raising the level of the diaphragm in the trunk before phonation, while others keep the abdominal wall expanded and thus the diaphragm low in the trunk at the initiation of a phrase. Some even contract both abdominal wall muscles and diaphragm during singing. It is frequently assumed that these different strategies affect the function of the vocal folds and hence the voice timbre (see Thomasson and Sundberg, 1997).
Vibrato.
One of the typical peculiarities of opera and concert singing is vibrato. In Western operatic singing its acoustical correlate is an undulation of the frequencies and amplitudes of the partials. The undulation is almost sinusoidal and has a rate of about 5–7 Hz in good voices. The rate is generally constant within a singer, although it tends to slow down with advanced age. The magnitude of the frequency excursions is of the order of ±70 cents, but greater variation occurs for expressive purposes and at advanced age. Vibrato tends to increase in regularity as voice training proceeds successfully. The frequency and amplitude undulations are synchronous but not necessarily in phase, depending on the frequency distance between the strongest spectrum partial and the nearest formant. If the strongest partial is slightly below the strongest formant, an increase in frequency will cause the amplitude to increase, so that frequency and overall amplitude will vary in phase. The opposite occurs if the partial is slightly higher than the frequency of the strongest formant.
The physiological origin of vibrato is not well understood. EMG (electromyographic) measurements in laryngeal muscles have revealed rhythmical contractions, synchronous with the vibrato undulations, of the pitch-raising cricothyroid muscle. This suggests that the laryngeal muscles produce the vibrato. The neural origin of these rhythmical contractions is unknown. Possibly as a consequence of this, the transglottal air flow varies with the frequency variations, and the resulting vibrato notes tend to consume more air than vibrato-free notes (see Large and Iwata, 1971). In popular singing subglottal pressure seems to be the vibrato-generating mechanism. In some singers the variations in the muscle activity affect the larynx height and even other parts of the voice organ. Pitch seems to be perceived with comparable accuracy regardless of the presence of vibrato for a single note. The perceived pitch agrees within a few cents with the pitch of a vibrato-free note with a fundamental frequency equal to the average frequency of the vibrato note.
Register.
The term ‘register’ is used for groups of adjacent notes that sound similarly and are felt to be produced in a similar way. However, there are a great number of conflicting terms and definitions in common use. In untrained voices in particular a change from one register to another may be accompanied not only by a marked shift in tone quality but also by a ‘register break’, a sudden jump in pitch. In both male and female adults register shifts typically occur in the range of approximately 300–450 Hz. The register above this shift is mostly referred to as ‘falsetto’ in male voices and ‘middle register’ in female voices, while the register below the shift is known as ‘chest register’ or ‘modal register’. A further shift occurs below 100 Hz; this register is called ‘vocal fry’. Registers are associated with certain vocal fold configurations. Thus, in chest/modal register the folds are thick while in falsetto they are thinner. Acoustically, the lowest spectrum partial, other things being equal, has been found to be more dominating in falsetto than in chest/modal register. Also, the ‘heavy’ register in male and female voices has been reported typically to contain stronger high partials than the ‘light’ register. The physiological origin of register is confined to the voice source. According to some experts, a difference between the falsetto and the normal voice in males is that the vocal folds never reach full contact with each other during the vibration cycle in falsetto. Transitions between registers have been found to be accompanied by changes in the EMG signals from laryngeal muscles, and by changes in transglottal air flow. There is reasonable agreement on the importance of the laryngeal muscles to registers, though it has been suggested that a purely acoustical interaction between the glottal oscillator and the resonator is a contributory factor.
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