LARYNGEAL SYSTEMS
Contents:
CONTROL OF VOCAL INTENSITY, FUNDAMENTAL FREQUENCY AND EFFICIENCY
VOCAL
INTENSITY AND EFFICIENCY
FLUCTUATIONS
AND PERTURBATION IN VOCAL OUTPUT
FUNDAMENTAL FREQUENCY AND PITCH
What's the difference between fundamental frequency and pitch?
Although there is a subtle difference;
Pitch is the
psychological correlate of frequency. It is a perceptual measure; in other
words, pitch must be heard and measured by ears connected to a brain. Pitch is
closely related to frequency i.e. as frequency increases, pitch increases and
vice versa.
Optimal
pitch:
It
refers to the pitch of vocal fold vibration that is optimal or most suitable
for each individual. It is also known as natural pitch/ optimum pitch
level. It is determined by the physical characteristic of the
individual voice mechanism.
This frequency of vibration is the most efficient for a given
pair of vocal folds and is the function of the mass and elasticity of vocal
folds.
Optimal
frequency varies as a function of age and gender. An adult female has a Fo of
approximately 212Hz; an adult male is around 132Hz. This difference in Fo is
due to variation in tissue mass and length of vocal folds. Children of both
genders have Fo of about 300Hz, which changes during puberty.
According
to Fairbanks, the natural pitch is located about 1/4 th of the way up the total
range.
Habitual pitch:
Refers
to the frequency of vibration of vocal folds that is habitually used during
speaking. In an ideal condition habitual pitch is same as optimal pitch. Some
individuals alter their everyday pitch in speech beyond the range expected for
their age, size, and gender. The use of an abnormally higher or lower Fo is
often not a conscious decision; it has an effect on phonatory efficiency and
effort.
Fundamental frequency is the rate at which vocal folds vibrate. F0, by
definition, does not have to be sensorially perceived. Rather it is a
measurement of sound wave's base frequency that can be quantified by an
instrument such as a spectrograph. Thus, fundamental frequency – or F0 - is the
more accurate scientific term.
THE PITCH CHANGING MECHANISM
Fundamental
frequency change comes from stretching and tensing of vocal folds using the
cricothyroid and thyrovocalis muscles. Pitch changes is brought about by
changes in vocal fold length, mass of vocal fold, and thickness.
VOCAL FOLD LENGTH AND PITCH
Pitch is directly proportional to length i.e., increase in
length of the vocal folds result in a decrease in cross sectional area (mass),
which will result in an increase in
pitch. Mass per unit
length can be changed by spreading the muscle, mucosa, and ligament out over
more distance.
Vocal
fold length for;
Men = 15-20mm,
Women = 9-13mm.
The relative difference between men and women vocal fold length,
appear to be the primary determinants of difference in voice pitch. When
individuals phonate at increasingly higher pitch levels, they must lengthen the
vocal folds to decrease their relative mass and increase their
tension. Increase in pitch appear to be related to lengthening of
the vocal folds with a corresponding decrease of tissue mass and an increase of
vocal fold elasticity. Lowering the pitch is directly related to the shortening
of vocal folds.
Rubin
and Hirt have shown that in falsetto the vibrating length of vocal fold is
systematically shortened as frequency is increased.
VOCAL FOLD THICKENESS AND PITCH
FO is directly related to how many vibratory
closings - openings vocal folds make in one second. The rate of vibration is
related to their thickness. Short, thick, lax vocal folds vibrate at a much
slower rate than a long, thin, tense vocal fold. The length of vocal folds
increases almost in a stair step fashion, corresponding to increase the pitch.
Hollien
(1962) found that mean thickness or mass of the vocal folds systematically
decreased as voice pitch increased. The relationship between the length and
frequency in vocal folds can be explained, considering the vocal folds more
like an elastic band with which vibrating frequency is dependent on the mass of
the band and tension exerted on it. Increase in length produces decrease in
mass of the vocal folds thus permitting higher vibrating frequency. The
systematic decrease in vocal folds mass continued until about the start of
falsetto frequencies. There after mass remained constant as higher
Fo were produced in falsetto. This is analogous to stretching the elastic band
until it can be stretched no longer.
VOCAL FOLD TENSION AND PITCH
Tension
can be varied over a considerable range, by stretching them tighter or relaxing
them. According to Van Den Berg tension is responsible for variation
of F0. Van Den berg reported that in chest voice a small amount of tension
produces a fairly large amount of elongation.
For
frequencies produced in so called mid voices, moderately large range tension
produces a small amount of elongation, where as in falsetto voice large
increase in tension produces very little elongation. Thus tension on the vocal
folds is much more dominant in producing F0 change, than in the chest or modal
voice in which there are large changes of vocal fold length.
INTRINSIC LARYNGEAL MUSCLE ACTION
Modifications
in the length and tension of the vocal fold necessary to produce an increase in
pitch are mediated through the interplay of intrinsic laryngeal muscles:
1. cricothyroid
2. thyroarytenoid
3. posterior
cricoarytenoid
Cricothyroid muscle:
Origin:
anterolateral arch of the cricoid cartilage
Insertion:
into thyroid cartilage
Course: as an oblique and vertically
directed
rectus bundle.
Contraction of the rectus bundle causes rotation about the
cricoythyroid joint which decreases the distance between the cricoid and
thyroid cartilages anteriorly. This results in an increase in the distance
between the arytenoids cartilages and the thyroid cartilage at the angles.
Since
the vocal folds extend from the arytenoids to the thyroid cartilage, it follows
that contraction of the rectus bundle of the cricothyroid muscles elongates the
vocal folds and makes them thinner.
Contraction
of the oblique fibers may slide the thyroid cartilage forward on the
cricothyroid joint, and this action if unopposed, will result in elongation of
the vocal folds with the negligible increase in their tension (Green, 1957)
With
no opposing muscular forces acting upon the vocal folds, contraction of the
cricothyroid muscle may simply elongate them and make them thinner. In either
case, little or no increase in tension (& pitch) will result.
Hence anterior sliding
movements of the arytenoids cartilages will be limited by the stout posterior
cricoarytenoid ligament and by contraction of the posterior cricoarytenoid
muscle.
The thyroarytenoid muscle, acting without opposition, will simply
decrease the distance between the arytenoids cartilages and the thyroid
cartilage to produce a shortening and relaxation of the vocal folds.
Pitch
increases therefore are brought about by the antagonistic action of the
cricothyroid and thyroarytenoid muscles (vocal fold tensors)
The
F0 is primarily affected by applying more or less longitudinal tension to the
vocal fold via cricothyroid muscles.
Secondarily
affected by adjustments applying more or less vertical tension to the vocal
folds via the muscles which can elevate or depress the larynx by applying more
or less intrinsic tension in the vocalis muscles themselves.
EXTRINSIC LARYNGEAL MUSCLE ACTION
Few
extrinsic muscles and supplementary muscles play a role in producing tones near
the extreme ends of the pitch range and to facilitate rapid changes in pitch.
Rapid changes in laryngeal position, during phonation of high and low pitched
tones, are brought about by laryngeal elevators and depressors, and by
supplementary musculature which attaches to the hyoid bone.
Pitch
changes brought about by extrinsic muscles is not very well understood.
Electromyographic evidence has shown heightened activity of the sternothyroid
muscle when the larynx is depressed and of the thyrohyoid muscle when the
larynx is elevated.
REVIEW OF LITERATURE
1. Rouberau B,Cheverie-Muller
C, Lacau Saint,Guily, J Acta otolaryngology, 1997 may 117(3) 454 – 64.
(Electromyographic activity of strap and cricothyroid muscles in pitch change)
The EMG activity of the cricothyroid muscle and the three
extrinsic laryngeal muscles (thyrohyoid, TH, sternothyroid ST, and sternohyoid
SH) were recorded through the voice range was examined using rising and falling
glissandos (production of a sustained sound with progressive and continuous
variation of Fo.) Muscle activity was observed at various pitches during the
glissandos. The strap muscle activity during the production of glissandos
appears to be synergistic.
At
the loudest frequency, CT is inactive but the strap muscles (TH, ST, SH) are
active. As frequency increases, strap muscles activity decreases while the CT
controls frequency in the middle range.
At
higher frequencies the strap muscles once again become active. This activity
might depend on the vocal vibratory mechanism involved. The role of the strap
muscles at high pitch is a widely debated point but it sums that in some way they
control the phenomena relevant to rising pitch. The phasic type strap muscle
activity contracts with the tonic type activity of the CT. The CT
closely controls the frequency, while the straps are not directly linked to the
pitch but rather to the evolution of the frequency of voice production
(speaking voice, ringing voice, held notes, glissandos, trillo, vibrato etc.)
SUB GLOTTAL PRESSURE AND FUNDEMENTAL FREQUENCY
Sub glottal pressure and Fo are the two
primary control variables of phonation; where sub glottal pressure is related
to loudness of the sound produced and Fo to the perceived pitch of the voice.
When a greater discrepancy exists between the
air pressure below the vocal folds and above the vocal folds, they will be
blown further apart. This will increase intensity or loudness. So amplitude
and, F0 rise together when sub glottal air pressure is increased. It has a
minimum at some frequency and increases as frequency increases or decreases
from that value. (Kataoka, and Kitajima, Ann. Otol. Rhinol. Laryngol. 2001,110,
556 – 561).
Early
research on relationship between sub glottal pressure and pitch was conducted
by famous physiologist Johannes Muller (1843) and later by Likovius (1846), who
concluded that pitch rose in response to increased air pressure.
Negus
(1929) said that, in phonation, elastic tension of the vocal folds and air
pressure are associated in such a way that a slight increase in air pressure
causes a considerable rise in pitch.
Ohala
(1970), measured rates of increase in fundamental frequency and sub glottal
pressure, keeping the muscular tension of the larynx constant. Experiment
involved having a subject maintain a steady pitch, and then pushing on the
chest or abdomen at unexpected moments. The rate was 2 – 4Hz/cm H20 for the
modal voice and 7 – 10Hz/cm H2O for falsetto.
Wullestein
(1936), used freshly excised human larynx, found that F0 rose from 85 to 115 Hz
when air pressure was doubled.
Although
rises in pitch may be accompanied by increases in sub glottal
pressure, increases in sub glottal pressure need not produce rises in
pitch.
Cause
and effect relationship:
Brodnitz
in (1959) in his study has noted that when a subject is singing an upward
scale, the sub glottic pressure increases coz the greater stiffness of the
stretched vocal folds offer increased resistance to airflow. Hence sub glottal
pressure must increase.
Experiments
conducted on animals revealed: as long as vocal fold tension is held constant,
increases in sub glottal pressure do not result in increases in pitch.
Timcke
et al (1958) and Van Den Berg (1957): reported a simple experiment to
demonstrate the effect of sub glottal pressure on pitch. A sudden
push on the abdominal wall of a subject during the production of a sustained
sound raises the intensity of the voice and produces an increase in pitch.
Kunze
(1962): measured intratracheal pressure (sub glottal) directly as a group of
subjects phonated at various pitch levels at a moderate intensity level.
Purpose was to relate intratracheal pressure to F0 and to
relate glottal resistance to Fo. The data suggest that the larynx offers
increased resistance to air flow as the vocal folds are placed under increased
tension to raise the frequency of their vibration. An increase in sub glottal
pressure is required to overcome the increase in glottal resistance.
To
conclude; pitch changes are mediated primarily through modifications in glottic
tension and mass; however, an increase in sub glottal pressure with laryngeal
tension held constant , will produce a negligible raise in pitch.
(A) Relationship between mean intra tracheal pressure and Fo for
10 adult males.
(B) Relationship between glottal resistance and Fo for 10 adult
males.
REVIEW OF LITERATURE
1. Robert E. McGlone,
Thomas shipp, the journal of acoustic society of America, July 1970, vol 48,
issue 1A, pg 118, “Changes in Sub glottal Air Pressure Associated with Changes
of Fundamental Frequency”
Sub glottal air pressure was recorded with the
aid of an intratracheal catheter as three young adult males
produced an upward and downward glissando. From a graphic
display of both the pressure and the phonation, changes in
pressure (Ps) and fundamental frequency (Fo) were determined. The mean Fo/Ps ratio for both phonatory activities was 7.67 semitones per
centimeter of water. This is considerably larger than reported
by previous investigators. Moreover, when only the range of
fundamentals most often used for speech was considered an even
larger ratio was found. Even though the importance of sub
glottal pressure to the generation of a laryngeal tone cannot
be denied, the present data suggest that changes in the
fundamental of that tone are probably more directly related to
laryngeal adjustments such as changes in mass and length of the
vocal folds. ©1970Acoustical Society of America
2.
Philip Lieberman, Ronald Knudson, “Determination of the Rate of Change of
Fundamental Frequency with Respect to Sub glottal Air Pressure during Sustained
Phonation”
The rate of change of fundamental frequency with
respect to Trans glottal air pressure was determined for a
single male speaker by sinusoidally varying buccal air pressure
while the speaker sustained short episodes of phonation at
various fundamental frequencies. The speaker phonated at two
levels of "effort," i.e., "soft" and "loud"
phonation with and without auditory feedback. The sensitivity
of the larynx to variations in Trans glottal air pressure
varied from 3 to 18 Hz/cm H2O. Fundamental frequency
was most sensitive to variations in Trans glottal air pressure
at high frequencies and in the "soft" mode of
phonation. The minimum trans glottal air pressure for sustained
phonation was 2–3 cm H2O. These results are consistent with
some earlier studies and a recent theoretical model of laryngeal
activity. These results further indicate that variations in sub glottal air
pressure and adjustments in laryngeal muscular tension both play a
role in regulating fundamental frequency during normal speech. ©1969 Acoustical
Society of America
Research Findings on Pitch – Raising mechanism:
As
the vocal folds are tensed and elongated for the production of higher pitched
tones the following changes occur:
Vocal
folds lengthen and change from round, thick lips to narrow bands.
At
natural pitch level, vocal folds are relaxed and almost flaccid and high
pitches they appear stiff and rigid.
The
glottis appears as more of a variable slit and only the medial edges of the
folds seem to vibrate.
At
extremely high pitches, vocal quality becomes breathy due to failure of vocal
fold to approximately completely in the area of the vocal processes.
Hollien in (1960a) found a relationship
between general vocal fold length and natural frequency of phonation. Persons
with larger larynx and long vocal folds tend to phonate at a lower pitch level
than persons with smaller larynx and shorter vocal folds.
Hollien
and Curtis (1960) employed x – ray laminography study of the larynx during
changes in vocal pitch. Results indicated that the folds became less massive
and thinner as frequency was raised, with larger changes occurring in the low –
frequency portion of the subjects ranges.
CONCLUSION
Pitch
changes are mediated primarily through modifications in glottic tension and
mass; however an increase in sub glottal pressure, with laryngeal tension held
constant, will produce a negligible raise in pitch.
SUMMARY
There
are three physiologic parameters that control the Fo of vibration in the vocal
folds:
Length
Mass
Tension
Role of length in Fo change is minimal. Mass is more dominating
at low fundamental frequencies, especially those produced in modal (and pulse)
register. Tension is dominant at frequencies produced in the falsetto register.
PITCH LOWERING MECHANISM
Increase
in tension and concomitant decrease in the mass of the vocal folds is primarily
responsible for an increase in pitch. Similarly a decrease in tension and/or an
increase in mass per unit length of the vocal folds lowers the pitch.
The
glottal margins can be relaxed by two mechanisms:
The
inherent elastic property of the tissue
Tissue
elasticity cannot satisfactorily explain how pitch can be lowered beyond the
habitual pitch level.
Further
decrease in tension is produced by active forces which shorten the vocal folds,
thus relaxing and thickening them.
Pitch lowering is brought about by the musculature of the vocal
folds. Unopposed by other muscles, the thyro arytenoid muscle draws the
arytenoid and thyroid cartilages toward one another, to shorten and relax the
vocal ligament. Medial compression at low pitches is probably facilitated by
the lateral cricoarytenoid muscle.
Extrinsic muscles and pitch
The
larynx rises and falls in position during phonation of high and low pitched
tones, which are brought about by laryngeal elevators and depressors, and by
supplementary musculature, which attaches to the hyoid bone.
Electromyographic
evidence has shown heightened activity of the sternothyroid muscle when the
larynx is depressed and of the thyrohyoid muscle when the larynx is elevated.
Contraction
of the inferior pharyngeal constrictor will not greatly influence laryngeal
position.
CONCLUSION
The laryngeal structures are complex and that muscle may
complement each others activities one moment and counteract them the next. Any
changes brought about in the larynx are the results of the algebraic (vector)
sum of the various forces in action.
INTESITY CHANGING MECHANISM
Intensity changes are an important part of our everyday verbal
behavior, and the extremes in intensity of vocal tones span a considerable
range even during conversational speech.
As
pitch is the perceptual correlate of frequency, loudness is the perceptual
correlate of intensity. Intensity (or its correlate, sound pressure level) is
the physical measure of power (or pressure) ratios, but loudness is how
perceive power or pressure differences.
CONTROL OF INTENSITY AND EFFICIENCY
There
are two gestures important of significance for increasing vocal intensity:
increased sub glottal pressure and medial compression.
Sub glottal pressure:
The intensity of voice, perceived loudness of
the voice, is directly related to changes in sub glottal and trans glottal air
pressure.
Hixon
and Abbs (1980) have written sound pressure level, the primary factor
contributing to our perception of the loudness of voice, is governed mainly by
the pressure supplied to the larynx by the respiratory pump.
It
appears that the trained voices of actors or singers increase intensity by
increasing both sub glottal pressure and airflow rate with only minimal
increase of glottal tension. (Bouhys, Proctor and Mead 1966)
Kunze (1956) confirmed the positive relationship between sub
glottal pressure and intensity of voice and stated that “the sound intensity
level of the voice will increase by about 8 to 12 dB when sub glottal pressure
is doubled.”
Relationship
between sub glottal pressure and intensity is shown graphically below:
(A) Relationship between mean rate
of air flow and vocal intensity.
(B) Relationship between
intratracheal pressure and vocal intensity.
The duration of closed phase of the vibratory cycle increase
with vocal intensity and sub glottal pressure also increases with increase in
intensity. The relationship between sub glottal pressure and the actual sound
output of the vocal folds depends on the speaker. However, for every doubling
of sub glottal pressure there is an increase of between 8 to 12 decibels in
vocal intensity.
Medial compression:
In a
cycle of vocal fold vibration, there is an opening stage, closing stage and
closed stage. In modal phonation at conversational intensities, the vocal folds
spend about 50% of their time in opening phase, 37% of the time in closing
phase and 13%of the cycle completely closed.
When
the vocal folds are tightly adducted for increased vocal intensity, they tend
to return to closed position more quickly and to stay closed for a longer time.
The opening phase reduces approximately 33%, while the closed phase increases
to more than 30%, depending on the intensity increase.(Fletcher, 1950).
Point to remember:
To
increase vocal intensity, the vocal folds are tightly compressed, it takes more
force to blow them open, they close more rapidly, and they tend to stay closed
because they are tightly compressed. As a result more energy is required to
hold the folds in compression; the release of the folds from this condition is
stronger. Each time the folds open, they do so with vigor, producing an
explosive compression of the air medium. The harder that eruption of the vocal folds
is, the greater is the amplitude of the cycle of vibration. As the amplitude of
the signal increases, so does the
intensity.
MECHANISM OF LOUDNESS VARIATION
Sound
is the variation of air pressure at a rate to be audible. The sound pressure
level of sound produced by the vocal folds is directly dependent on the air
pressure during the vocal folds into vibration. The greater the air pressure,
greater the sound pressure level of the sound.
There are two basic mechanisms that can be
used to produce greater force beneath the vocal folds.
1. First,
the thorax can increase the flow of air and can create a greater force beneath
the vocal folds.
2. The
vocal folds remain closed for a greater proportion of their vibratory cycle and
produce a greater pressure beneath them even when the force from the lungs
remains constant.
Both mechanisms are used during phonation, although one might
expect that greater thoracic pressure are used to achieve large changes in
sound intensity and variation of sound intensity, such might be found during
the production of a sentence.
GLOTTAL RESISTANCE AND VOCAL INTESITY
Greater closed time of the vocal folds produce a greater
resistance to vibration, which appears to be the major physiologic parameter
controlling vocal intensity for phonation produced in the modal register.
Resistance
is defined as the ratio of the air pressure level divided by the airflow.
Ishiki reported for frequencies produced in chest voice increase
in air flow. In this airflow is related to increase in vocal intensity. There
is considerable muscle activity in falsetto that would produce high level of
glottal resistance even at low intensity levels. Additional increase
in glottal resistance may be difficult to achieve in this register. Airflow
rate may be more important to produce variation in vocal intensity. The range
of vocal intensity is much small in falsetto than in chest or modal register.
In modal one can produce ranges of vocal intensity between 40 and 50 dB (from
60 to 110 dB SPL). In falsetto the range of vocal intensity is about 15 to 20
dBSPL (from about 60 to 75 dBSPL). When phonation are produced at equivalent
fundamental frequencies in both registers, those produced in modal register
have a range from 25 to 32 dB, whereas those produced in falsetto have a range
from 9 to 20 dB. Where less than the increase in sound intensity.
Glottal
resistance increased with increase in sound intensity. Glottal resistance is
the major controlling parameter of vocal intensity for low pitched phonation.
To increase glottal resistance there must be increase contraction by muscles of
the larynx, especially those that will force the vocal fold to the midline.
Increased
muscle contraction that close the vocal folds, will increase the vocal fold
resistance, requiring greater pressure beneath the vocal folds in order to
sustain vibration. When the vocal folds open, the air pressure released into
the vocal tract will be greater producing greater sound pressure levels.
Phonations
produced in falsetto voice do not show systematic changes in the glottal
resistance with changes in vocal fold intensity.
Musculature responsible for changes in vocal intensity
Four laryngeal muscles, along with forces of exhalation are
responsible for changes in vocal intensity. Forceful adduction of the vocal folds
is accomplished by simultaneous contraction of the lateral cricothyroid and the
arytenoids muscles, while increase in glottal tension is mediated by
thyroarytenoid muscles or the cricothyroid muscle or both. Increase in pitch
that often accompanies increase in intensity of phonation can be accounted for
by the greater tension of vocal fold.
Literature review
1. Pavoo Alku, juha VInturri, Erkki Vilkman, Speech communication
archive, vol. 38, November 2002, issue 3-4, pg no
321-334.(measuring the effect of Fo raising as a strategy for increasing vocal
intensity in soft, normal and loud phonation.)
The study revealed that, in producing loud voice, speakers use
Fo to increase the number of glottal closures per time unit, which increases
rapid fluctuations in the speech pressure wave form, which, in turn, raises
vocal intensity. The increase of SPL due to this active use of Fo was
approximately 4 dB in loud speech produced by both male and female
speakers.
2. Sundberg J, Titze I, Scherer R. J
voice. 1993 mar;7(1):15-29
“Phonatory control in male singing: a study of the effects of sub
glottal pressure, fundamental frequency, and mode of phonation on the voice
source”.
This article describes experiments carried out in order to gain a
deeper understanding of the mechanisms underlying variation of vocal loudness
in singers. Ten singers phonated at different pitches and different loudness.
Their voice source characteristics were analyzed. It was found that the main
physiological variable underlying loudness variation is sub glottal pressure
(Ps). Increases in the voice amplitude are achieved by (a) increasing the pulse
amplitude of the flow waveform; (b) moving the moment of vocal fold contact
earlier in time, closer to the center of the pulse; and (c) skewing the pulses.
The last mentioned alternative seems dependent on both Ps and the ratio between
the fundamental frequency and the first formant. On the average, the singers
doubled Ps when they increased fundamental frequency by one octave, and a
doubling of the excess Ps over threshold caused the sound pressure level (SPL)
to increase by 8-9 dB for neutral phonation, less if mode of phonation was
changed to pressed. A shift of mode of phonation from flow over neutral to
pressed was associated with a reduction of the peak glottal permittance i.e.,
the ratio between peak trans glottal airflow to Ps. Flow phonation had the most
favorable relationship between Ps and SPL
3. A M Sulter, F W J Albers. Clinical otolaryngology, Aug 1996, vol. 21,
issue 4, page 324- 327. “The effect of frequency and intensity level on glottal
closure in normal subjects”.
The degree of glottal closure during phonation has an influence on
voice quality and it is related to the robustness of the voice source. Investigation
of 47 healthy men and 92 healthy women with no vocal complaints was done using
videolryngostroboscopy. Results indicate that men have better glottal closure
than women (p<0.001). An increase in glottal closure is related to improved
glottal closure (p<0.001), and in women a negative relationship was
established between pitch and glottal closure (p<0.001). Normal glottal
closure in men is a complete closure of at least 90% should be attained. If
these percentages cannot be established during loud phonation, it suggests the
presence of less robust larynx.
To evaluate and quantify the function of the voice source, in
clinical practice the larynx should not be observed at only one intensity
level, but at a variety of intensity and frequency
levels.
RELATIONSHIP BETWEEN PITCH AND INTENSITY
Although increase in vocal intensity are mediated by increased
compression of vocal folds and heightened activity of respiratory mechanism,
intensity ranges are frequency dependent. Increasing the sub glottal air
pressure, keeping the other things constant, can increase vocal intensity. If
sub glottal pressure is increased without muscular adjustment of the vocal
folds, the fundamental frequency as well as the intensity will increase. During
phonation of a steady tone, a gentle punch on the stomach, the tone not only
gets louder but increases in pitch.
Increase
vocal intensity is due to greater resistance against increased airflow. The
vocal folds are blown wider apart releasing a larger puff of air which sets up
a sound pressure wave of greater amplitude. The vocal folds not only move
farther apart from each vibratory cycle of increased intensity, but they stay
adducted for a larger part of each cycle.
At higher sub glottal pressure, the vocal folds
remain closed for a greater proportion of the vibratory cycle and close more
rapidly, frequency and intensity tends to increase
TRANSGOTTAL PRESSURE DIFFERENTIAL
An important factor related to voice production is the pressure
differential across the glottis. A sub-glottal pressure from to 2-3cm of water
will sustain phonation and assume that the supra glottal part of the vocal
tract offered little or no resistance air-flow.
Thus,
when supra glottal pressure is about the same as atmospheric pressure and
sub-glottal pressure is above atmospheric, the trans glottal pressure
differential will be approximately equal to sub glottal pressure.
A
constriction in the supra glottal part of the vocal tract should cause
intraoral and pharyngeal pressures to be elevated, the effective sub glottal
pressure will be diminished, and this will be reflected in a drop of
the trans glottal pressure differential.
Trans
glottal pressure differential is equal to sub glottal pressure (Psg) minus
supra glottal pressure (Po) or
TPD = Psg – Po.
Hence,
if supra glottal pressure should approximate sub glottal pressure, the pressure
differential at the laryngeal level approaches zero and the vocal fold
vibration will be arrested. Since the total pressure drop along the vocal tract
is equal to sub glottal pressure. We see that articulatory constrictions during
conversational speech are continually influencing the pressures available to
the glottis.
INFLUENCE OF ARTICULATION ON TRANSGLOTTAL PRESSURE DIFFERENTIALS
The trans glottal pressure differential is influenced by the
oral airway opening. Airway opening is largest for vowel production, and supra
glottal pressure is nearly atmospheric under these conditions. A closed airway
for voiced plosives results in approximately equal supra and sub glottal
pressure.
Perkell
(1969) and Kent and Moll (1969) have shown that voiced stops are produced with
larger supra glottal volumes than their voiceless cognates. Ex: supra glottal
volumes is largest for the production of /b/ than it is for /p/.
Kent
and Moll found that pharyngeal expansion was accompanied by depression of hyoid
bone, an active process that would relax the walls of the pharynx.
The
total resistance (Z) to airflow in the speech mechanism is equal to the sum of
the resistance offered by the component parts such as the larynx, tongue, the
lips etc... Or
Z
total = Z glottic + Z supra glottic
Significance of Trans glottal Pressure Differential
As the respiratory system plays a role in influencing the
laryngeal behavior to modify pitch and intensity, and how the larynx and
respiratory system work in concert during pitch and intensity changes.
Similarly, the processes of articulation impose constraints upon laryngeal
behavior. During respiratory compensation, when intraoral pressures are
elevated due to articulatory constrictions, the trans glottal pressure
differential will drop, and result in decrease in pitch, intensity, or both.
Thus there is an interlinking of the subtle and exquisite interplay between the
respiratory, laryngeal, and articulatory systems.
CHECKING ACTION AND AIR FLOW RESISTANCE
Pressures
in excess can be generated by the inflated thorax unless sustained contraction
of the inspiratory musculature checks the thoracic rebound.
Reciprocity
between the need for checking action and the resistance to airflow, offered by
the speech mechanism:
The
extreme case is complete blockage of air flow by the lips or tongue, or by the
vocal folds, following a deep inhalation. The resistance or impedance is
infinite; no air flow can take place, and so there is no need for checking
action by the inspiratory musculature.
During
phonation of a neutral vowel, at a conversational pitch and intensity following
a deep inhalation, larynx offers minimal resistance to the air flow, the vocal
tract is an extremely low impedance system, and if air flow is to be regulated,
checking action is essential.
The
reciprocity between checking action and glottal resistance was investigated by
Holstead (1972). She found that checking action decreased as laryngeal
resistance increased.
FLUCTUATIONS AND PERTUBATIONS IN VOCAL OUTPUT
Small
fluctuation in frequency, amplitude and wave shape are always present in voice
signal, reflecting the internal voices of human body. The larynx is especially
susceptible to small fluctuation in normal, vascular, respiratory, lymphatic
and other transport system.
A
perturbation is usually thought to be a minor disturbance/ small changes from
an expected behavior. Fluctuation, suggest more severe derivation from a
pattern.
In
voice perturbation is talked about in two
parameters:
Pitch
perturbation (jitter):
Cycle to cycle variation in period that occurs
when an individual sustain phonation at a constant intensity, some degree of
jitter occurs in the laryngeal signal as it is quasiperiodic.
(T1-
T2) + (T2- 3) + (T3- T4)
J = --------------------------------------
3
T1,T2, T3, T4 are periods of 4 consecutive
cycles in glottal waveform.
Amplitude
perturbation/ intensity perturbation ( shimmer):
Cycle to cycle variation in amplitude
that occurs when an individual attempts to sustain phonation at a constant
frequency.
(A1- A2) + (A2 – A3)
S = ------------------------------ dB
2
Variability:
ability of someone or something to vary by design or by accident. It
may cause the final result to be far from expected results. But final results
can also be better than expected.
Fluctuation, perturbation, and variability have no physical
definition. No numbers or units of measurement are attached to them. But secure
a purpose in describing physical process.
Periodicoty :
events are termed periodic if they could not be distinguished from one another
by shifting time forward or backward by a specific interval ‘T’ i.e, period.
Source of fluctuation and perturbation
Different sources of fluctuation in voice signal are found
at several stages of speech production chain:
1. neurologic
2. biomechanics
3. aerodynamic
4. acoustic
Neurologic sources:
Sustained
muscle contractions are made of large number of individual twitches of groups
of muscle fibers called motor units. Single twitch associated with a given
motor unit lasts for a fraction of a second.
The
firing of motor unit depends on arrival of neural signals at myoneural
junctions, junction between muscle fibers and nerve ending. Neural signals vary
in their filling intervals, hence interval between twitches also vary.
Increased
amounts of jitter and shimmer at the extremes of intensity and fundamental
frequency range may help the listener detect when the voice is approaching its
physiologic limits.
Orkiloff
and Kahane (1991), Titze (1991) worked out a mathematical model of jitter,
manipulating various aspects of neuromuscular function of the vocal folds and
found that the level of the jitter in the human voice is around 2%. Research
show that normal jitter ranges from 0.2% to 1%. Jitter value above this
indicates pathology. Jitter has also been used as an index of vocal maturation
in children and as an index of vocal aging.
Some
neurological disease e.g.: Parkinson’s disease exhibit abnormally large amount
of tremors. It is known whether some CNS disorder exaggerate the normal
physiologic tremor or more peripheral mechanisms is responded for the
uncontrolled oscillations.
Biomechanical sources
This
can be observed in the absence of all neural connections to the larynx. As
vocal fold are made up of liable tissue with an exterior layer of mucous, they
do not repeat their pattern of vibration from cycle to cycle. Various
irregularities in tissue geometry and mechanical properties cause slightly
different forces and motions every time glottis opens and closes. A major
contribution to this condition is asymmetry across glottal midline.
In
normal phonation vocal folds are trained to move synchronously, small
asymmetries are not disturbing. If asymmetry becomes too severe and vocal fold
get out of step and don’t meet the same way every cycle leads voice changes.
Sometimes a sub harmonic appears which indicates periodicity is achieved only
every 2 and 3 cycles.
Asymmetry
is not only in the geometric point of view but also the difference in the
tension of effective mass. Two vocal fold may look similar, yet be dissimilar
from mechanical point of view.
Another
bio mechanical source – pulsative nature of blood flow. Periodic contraction
and expansion of volume of tissue, changes, shapes of vocal folds and their
characteristic stiffness. Lung pressure cannot be kept steady over long stretches
of phonation, which may introduce low frequency variation in average and
frequency of glottal pulse. Articulators have an influence on vocal fold
vibration by mechanical coupling.
Aerodynamic sources
Results
from an instabilities in glottal air flow (Kaises, 1983, Lingenerants 1991).
Air from glottis behaves like a jet emerging from a nozzle of a pressurized
garden hose. Depending on velocity of jet and configuration of glottis, the
direction of jet can flip flop to sides. Turbulence can be generated if glottal
opening and particle velocity are excessive; perceptually turbulence creates a
breathy voice.
Acoustic sources
Acoustic
processes from vocal tract are feedback into glottis therefore different vocal
tract shape create different driving pressure in vocal fold. Change in rapid
vocal tract shape results in irregular component in vocal fold vibration. Thus
one could expect vocal jitter and shimmer to be greater during changing
articulation than during a steady vowel (Baken and Orlikoff, 1986).
CULTURAL FLUCTUATION
Vibrato:
Vocal
vibrato is a stabilized physiologic tremor in laryngeal muscles. Natural vocal
vibrato can be cultivated from a 4 – 6Hz physiologic tremor in cricothyroid and
thyro arytenoids muscles.
Origin
of vocal vibrato stems from the fact that fluctuations are evident not only in
larynx but also sometimes in tongue, jaw and belly. Several recent
investigations give evidence that cricothyroid and thyroarytenoid muscles are
primary producers of vocal vibrato.
In
all these investigations increased activity of intrinsic laryngeal muscles
preceded the fundamental frequency use by a considerable lead time, suggesting
that the influence may be casual rather than reactionary. Vibrato extent decreases
when rapid pitch changes occur. The perception of vocal vibrato is complicated
by the fact that intensity and timbre fluctuate when fundamental frequency
varies. Hence the amount of vibrato used in any production is largely dictated
by style and personal rotate.
Trill:
The
basic difference between a vocal trill and vibrato is that the average pitch is
raised in trill but not in vibrato. In trill there is an attempt to alternate
between a base note and higher note were as in vibrato the attempt is to remain
on same note. Trill is more demanding on pitch changing mechanism than vibrato
because a larger fundamental frequency change must be executed in a shorter
amount of time.
Trillo:
Is rapid repetition of same note which
includes repeated voice onset and offset. It is executed with laryngeal
adductor and abductor muscles, lateral and posterior cricoarytenoid, rather
than with CT and TA muscles. Since adductor- abductor muscles are extremely
fast in response, trills can be executed at unusually high frequencies.
Makes, Slipp, Doheraty (1987) reported trills in 9 – 10Hz range.
VOCAL
REGISTERS
A register refers to differences in mode of vibration of the
vocal folds. Musically, a register refers to a particular part of
the range of pitches of a voice or instrument.
The
entire range of Fo in the human voice is enormous, from under 60Hz in the basso
voice to over 1568Hz in the soprano voice (Zemlin 1998).
In singing, registers have been assigned particular names, thus
chest register indicates a mid range of pitches, and falsetto refers to much
higher range of pitches. Head register is described as mixture between chest
and falsetto. (Titze, 1994).
In
terms of voice production, the range of Fo is divided into three registers;
pulse, modal, and falsetto.
1. Modal register/ modal phonation:
Refers to the pattern of phonation used in the
normal conversational speech.
There are two modes of phonation;
Vertical
mode of phonation: the vocal folds open from inferior to superior (bottom to
top) and also close from inferior to superior.
As in the figure, the vocal folds are approximated at the
beginning of a cycle of vibration, followed by air pressure from beneath
forcing the vocal folds apart in the inferior aspect.
Anterior
– posterior dimension :
Anterior – posterior mode is less stereotypical when compared to
vertical mode.
Zemlin (1998) reported that the vocal folds tend to open from
posterior to anterior, but that closure at the end of a cycle is made by the
contact of the medial edge of the vocal folds, with the posterior closing last.
The minimum driving pressure in modal phonation is approximately
3 – 5 cm of water subglottal pressure. If pressure is lower than this the vocal
folds will not be blown apart.
Clinical relevance/ importance: a client who cannot generate 3 – 5 cm of water and sustain it
for 5 seconds will not be able to use the vocal folds for speech.
Glottal fry/ pulse register
Pulse
register, refers to a range of very low Fo, which perceptually creates a creaky
popping sort of sound.
Pulse register is also called vocal fry, glottal fry, and
strohbass. Where ‘pulse’, ‘fry’, ‘strawbass’ all refer to is the crackly,
“popcorn” quality of the voice.
Glottal
fry is the product of a complex glottal configuration. The vocal folds vibrate
at a rate ranging as low as 30Hz, up to 80 or 90 Hz, with an average of
approximately 60Hz.
The vocal folds are closed for a much longer period during each
cycle of vibration, i.e., around 90% of the cycle and opening and closing
movement’s together account for only about 10% of each cycle.
In
glottal mode of vibration, requires low sub glottal pressure to sustain it (2
cm water), and tension of the vocalis is significantly reduced, so that the
vibrating margin is flaccid and thick. The lateral portion of the vocal folds
is tensed, so that there is strong medial compression with short, thick vocal
folds and low sub glottal pressure.
If either vocalis tension or sub glottal pressure is
increased, the popcorn like perception of glottal mode of vibration is lost.
Acoustically the waveform of an utterance produced in
pulse register look like what Titze (1994) called a series of wave packets.
After each vocal fold closing, there is a burnt of acoustic energy. Thus energy
dies out, leaving a temporary interval during which there is no acoustic
energy, this interval has been dubbed by Titze as temporal gap.
Below about 70Hz, the human ear seems to be able to detect these
bursts of acoustic energy followed by gaps of silence, within each glottal
cycle. Above 70Hz or so, a continuous sound is perceived, rather than the
individual acoustic pulses and temporal gapes.
Falsetto:
In falsetto, the vocal folds lengthen and become extremely thin
and stiff. During vibration they vibrate along the tensed, bowed margins. Brief
contacts are made between the vocal folds as compared with modal phonation and
the degree of movement is reduced. There is dampening of posterior portion of
the vocal fold, so that the length of the vibrating surface is decreased to a
narrow opening. In contrast, pitch level is elevated in modal phonation, which
involves lengthening the vocal folds. The perception of falsetto is one of an
extremely “thin”, high- pitched vocal production.
Because
of both the high speed of vibration and the less complex manner of vibration of
the vocal folds during falsetto, the quality of the tone is almost flute like.
(Zemlin 1998). When the Fo is very high, the harmonics are widely spaced,
giving the sound a thinner quality, compared to the richer quality of a low
pitched sound. Another factor that contributes to the distinctive quality of
falsetto voice is the slightly breathy component that results from the vocal
folds never quite closing during vibration.
Whispering:
Whispering is not really a phonatory mode, as
there is no voicing. It is a non vocal sound production. The difference between
vocalization and whispering is in the configuration of the non vibrating vocal
folds during exhalation and the resulting acoustic product.
During
normal phonation, the arytenoids cartilages are approximated so that their
medial surfaces are in direct contact. The vocal folds lie parallel to one
another. In whispering, the arytenoids are slightly abducted and “toed in”,
creating a small triangular chink in the region of the cartilaginous glottis.
When the breath stream is released, the turbulence occurs in the chink, and
frictional sounds are generated.
Air
flow through the glottis plays a very important role in the production of a
whisper, amounting to about 200 cm3/sec for a forced whisper. (Monoson and
Zemlin, 1984).
Spectral characteristics of registers:
Spectrally, the different registers have different shapes. Pulse
registers with its low Fo and many harmonics has a shallow spectral slope. The
spectral slope is steeper for the high pitched voice of falsetto with its high
Fo and few harmonics.
Review of literature:
1. Mc Allister A, Sederholm E, Sundberg J; Logoped
Phoniatr Vocol; 2000; 25(2): 63 –
71. “Perceptual and acoustic analysis of vocal
registers in 10- year- old children.
Register transitions were identified by a group of five experts on
child voices in recordings of 15 10-year-old children who sustained the vowel
[a:] at different pitches throughout their vocal range. Two had mutational
voices, seven had deviant voices and six were controls with normal voices. The
control group had one register transition at a mean fundamental frequency of
511 Hz, or about 25% higher than in adult voices. This difference between adult
and child voices may be because of the difference in trachea length, as
proposed by Titze (J. Voice 1988; 3: 183-194). Children with either functional
or physiological voice deviations exhibited a transition at a mean frequency of
417 Hz. A second transition was found in four voices at a mean frequency of 902
Hz. No correlation was found between the occurrence of register transitions and
discontinuities in the upper and lower voice range profile contours.
2. Tokuda IT, Horacek J, Svec JG,
Herzel H., J Acoustic soc Am, 2007 jul; 122(1):519-31
“Comparison of biomechanical modeling of register transitions and
voice instabilities with excised larynx experiments.”
.
Voice instabilities were studied using excised human larynx
experiments and biomechanical modeling. With a controlled elongation of the
vocal folds, the experiments showed registers with chest-like and falsetto-like
vibrations. Observed instabilities included abrupt jumps between the two
registers exhibiting hysteresis, aphonic episodes, sub harmonics, and chaos
near the register transitions. In order to model these phenomena, a three-mass
model was constructed by adding a third mass on top of the simplified two-mass
model. Simulation studies showed that the three-mass model can vibrate in both
chest-like and falsetto-like patterns. Variation of tension parameters which
mimic activities of laryngeal muscles could induce transitions between both
registers. For reduced prephonatory areas and damping constants, extended
coexistence of chest and falsetto registers was found, in agreement with
experimental data. Sub harmonics and deterministic chaos were observed close to
transitions between the registers. It is concluded that the abrupt changes
between chest and falsetto registers can be understood as shifts in dominance
of modes of the vocal folds.
Use of different registers in singing and speaking
Even
though modal and falsetto registers are characterized by different vibratory
patterns, there is actually a considerable amount of overlap between upper
limits of the modal range and the lower limits of falsetto range.
Most trained singers can produce a high note that is perceived
as being within the modal register and can produce a note with exactly the same
FO that is heard as the lower portion of the falsetto register.
Literature review:
Sundberg J, Hogset C., Logoped phoniatr Vocol,
2001; 26(1):26-36.
“Voice source differences between falsetto and modal registers in
counter tenors, tenors and baritones”.
Vocal registers are generally assumed to be associated with the
voice source, i.e. the pulsating Trans glottal airflow. The waveform of this
airflow was analyzed by inverse filtering in professional singers, four counter
tenors, five tenors, and four baritones singing the syllable [pae:] in soft,
middle, and loud voice in modal and falsetto/counter tenor register. Sub
glottal pressure, estimated from the intra-oral pressure during the occlusion
for the consonant [p], closed quotient, relative glottal leakage, and the
relative level of the fundamental were analyzed. The counter tenors used
comparatively low sub glottal pressures and mostly showed a closed phase in
their flow glottogram waveform. For a given value of the closed quotient, the
fundamental tended to be stronger in falsetto than in modal register. The
observed voice source differences between the registers seem related to a
greater vocal fold thickness in modal than in falsetto register
VOICE QUALITY
The tension of the vocal folds and their mass
per unit length will influence the mode and rate of vibration. As the vocal
folds act in pairs, it is important that the tension and mass be the same for
both. The longitudinal tension, mass per unit length, medial compression, sub
glottal pressure, and physical symmetry, all have an important bearing on the
voice quality. The voice is a good index of general state of health of an
individual.
PARAMETERS OF VOICE PRODUCTION
1. Maximum
frequency (pitch) range
An adult speaker can usually produce tones
that extend over a frequency range of two octaves above the lowest sustainable
tone. By the use of a keyboard or pitch pipe, the lowest tone a person can sustain
can be determined, and the highest tone should be at least two octaves above
the lowest tone.
2. Mean rate of
vocal fold vibration (habitual pitch)
The mean rate of vocal fold vibration represents the habitual
pitch.
3. Air cost
(maximum phonation time)
A healthy larynx, vibrating appropriately can utilize from about
100 – 200 cc of air per second. An adult speaker should be able to sustain
comfortable phonation, for about 15 – 25 seconds. Maximum phonation time (MPT)
is commonly employed as a test of vocal efficiency.
4.
Minimum – maximum intensity at various pitches
The
test requires a sound pressure level (SPL) meter to determine the minimum and
maximum sound pressure levels that can be produced by a speaker at a various
points along the frequency range. At midrange, a minimum – maximum SPL of 50dB
is within normal range.(Coleman, et al., 1977).
5. Noise
The
term noise refers to the quality of a voice as a consequence of a periodicity
or a random distribution of acoustical energy in the voice spectrum.
Vocal roughness refers to a voice quality that is detected,
defined, and described based on the basis of a listener’s auditory impression
(Toner et al., 1990).
References :
Zemlin W.
R (1988). Speech and Hearing Sciences: Anatomy
and Physiology.
Ingro. R.
Titze. Vocal fold Physiology; Frontiers in basic science.
J. A. Seikel, David
D, Paula Seikel. Essentials of anatomy and physiology for
communication disorders.
Michael S.
Benninger, Barbara H.Jacobson,Alex F. Johnson. Vocal Arts Medicine. The
care and prevention of professional voice disorders.
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