Tutorials - Voice Production

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3 Levels of Intensity Control: Lungs, Glottis & Vocal Tract

One of the key tools vocalists use to be expressive is the ability to control the loudness, or intensity, of their singing. There are three distinct ways in which this is done:
  • changes made above the larynx (adjustments in the vocal tract)
  • changes made in the larynx (activity in the laryngeal muscles)
  • changes made below the larynx (breath control, changing lung pressure)
We will cover each of these mechanisms in turn, but first, a quick review of some of the sound-related terms involved.

Loudness: a perceptual quantity which can only be assessed by an auditory system, including the brain. Perceived 'loudness' varies according to pitch, because the human ear is not uniformly sensitive to all frequencies. For instance, the ear is most sensitive to pitches in the 1000-3000Hz range. Lower or higher pitches, even if sung/produced at the same volume, will sound softer by comparison.

Acoustic power: a measure of the amount of energy produced and radiated into the surrounding air, per second, measured in watts. Note that unless the vocal sound makes it out of the vocal tract and into the environment, it doesn't count. Thus, singers and speakers who are more efficient in getting as much of their sound out of their mouths as possible will produce more acoustic power than other vocalists, all other factors being equal.

Volume: an arbitrary descriptive term for the 'amount of sound', as perceived by an average listener.

Intensity: a measure of the radiated power (covered above) per unit area. Intensity decreases as the distance from the sound source increases, since the area through which the sound is being sent grows ever larger.

Intensity Control in the Vocal Tract
The resonances of the vocal tract, also known as formants, selectively boost the energy of harmonics of the glottal source spectrum. Harmonics which happen to be at frequencies close to these formant frequencies will be made louder; other harmonics, which lie between the formant frequencies, are made softer. The vocal tract can never increase the overall energy being radiated. The biggest boost occurs if a harmonic happens to precisely match a formant frequency; this is known as formant tuning.

Intensity Control in the Larynx
The degree to which the vocal folds are adducted also effects the power radiated by the voice. If the folds are not held close enough together, this results in an overly breathy timbre, which diminishes power. Too much adduction, or a pressed voice, also diminishes power. The ideal amount of adduction is somewhere between the two of these; the center of the spectrum occurs when the folds are touching for exactly half of each cycle of vibration. Current research seems to indicate that the ideal may be slightly on the breathy side of this center, but additional research is needed.

Intensity Control from Lung Pressure
Lung pressure: vocalists can also increase volume by putting more air through their vocal instruments. Glottal source power increases by 6 dB for every doubling of the lung pressure above the minimum pressure necessary to start sustained phonation; this pressure is known as the phonation threshold pressure. It also increases 6 dB with each doubling of pitch (Fo), which equates to a difference of over 12 dB over the average opera singer's range (assuming 2+ octaves). So, high notes sung with high lung pressures will tend to be the loudest sounds a vocalist can produce; this matches well with common sense.

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