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.
Tutorials
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~ National Center for Voice and Speech, www.ncvs.org
