Graphing Phonation: Waves & Spectra
Glottal Flow
Waveform
Study the two cycles of a typical glottal
flow waveform above. The
variables shown are as follows:
| Tp |
the part of the glottal cycle
in which airflow rate is increasing (thus,
the slope of the curve is going upward during
this time). Think of 'p' for positive. |
| Tn |
the part of the glottal cycle
in which airflow rate is decreasing (downward
slope in the curve). Here, 'n' represents negative. |
| To |
the length of time during each
cycle in which air is flowing (i.e., the folds
are open) |
| T |
the total duration of each vibrational
cycle |
| uo |
the average rate of airflow |
| uac |
maximum rate of flow |
In addition to the variables shown in the graph,
two other useful variables can be derived, which
define how the waveform is shaped:
- Qo, the open quotient, which
is equal to To / T. This can be expressed
simply as the percentage of time in each cycle
during which the folds are open.
- Qs, the skewing
quotient, which
is equal to Tp /
Tn. This
is the portion of
time in each cycle
during which the
folds are moving outward,
divided by the time
in which they are
moving inward. More
simply, the skewing
quotient is a number
that explains how
far from symmetric
(how skewed) the
waveform "bump" is.
The shape of the glottal waveform helps to determine
the loudness of the sound generated, and also its timbre.
A 'jagged' waveform (with a large skewing quotient
or small open quotient) which represents sudden changes
in airflow will produce more high frequencies, and
result in a 'brassy' timbre. A smoother curve, representative
of gradual changes in airflow (open quotient and skewing
quotient both high) will tend to produce a more 'fluty'
sound.
A vocalist can control the open quotient (To)
by moving the vocal processes (tips of arytenoid
cartilages at one end of each vocal fold) closer
together or further apart during phonation.
Secondary Sound
Sources in the Larynx
In addition to the desired vocal sound produced by the larynx, other sounds
are also often produced. Turbulence, a 'non-smooth' flow in the glottal
airstream, can create a 'hissing' sound, which is called aspiration if
it is combined with sound from vocal fold vibration, and whisper if
it is not. Aspiration is a contributing factor to 'breathy' voice.
Other sources of extra sounds in the larynx include
glottal 'clicks' or 'stops', which are produced by
an especially effortful and sudden opening or closing
of the vocal folds. Also, any extra mucus or other
fluids which are on or near the vocal folds can produce
a rough or 'gurgly' noise as they move around during
phonation.
Audible Frequencies
in Sound Sources
Generally, human detection of sound is limited to waves having frequencies
from 20 Hz to 20,000 Hz. The precise limits of hearing vary on an individual
basis, and tend to degrade with age and other causes of hearing loss. The sensitivity
of human hearing is not constant across this frequency range; frequencies in
the thousands of hertz (1,000 to 10,000 Hz) are usually more easily heard than
those at the extremes of the audible range. Audiophiles enhance these less
discernable frequencies with various graphic equalizers, woofers, and tweeters.
Frequency Spectra
All of the sounds we hear in everyday life are made up of many component frequencies,
intermixed in varying proportion to one another. A sound spectrum is
an array of these components of an sound, separated and arranged in order
of frequency. Each component frequency is a pure sine-wave tone; the graph
of the shape of such a pure tone is called a sinusoid. In general,
an infinite collection of sinusoids is needed to construct any complex sound.
Spectral Slope
The spectral slope of a given spectrum describes how rapidly the amplitudes
of successive partials (component frequencies) decrease as they get higher
in frequency. Three examples of spectral slopes are shown below:
Spectral slope influences the timbre of the sound,
just as waveform shape does, as described above.
A spectral slope of around 6 dB/octave, the least
severe slope in the graph, results in stronger high
frequencies, which yield a more 'brassy' or strident
sound. The middle slope depicted, 12 dB/octave, is
that of a normal vocal quality. The most extreme
slope shown, 18 dB/octave, would result in a more
'fluty' sound; it has stronger low frequencies, as
compared to the higher ones, which rapidly drop off
in strength.
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~ National Center for Voice and Speech, www.ncvs.org
