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About
the Voice
How
does your voice work?
This page covers:
Anatomy
101: Parts of the Voice
Physiology
101: How it Works
Anatomy 201:
Cartilages and Muscles of the
Larynx
Anatomy 202:
Structure of the Vocal
Fold
Anatomy 301:
The Role of the Nervous
System
Physiology 201:
Vocal Fold Vibration and
Pitch
Physiology
202: Vocal Fold Vibration and
Loudness
Acoustics
101: Sound Waves and How they Move
Anatomy 101:
Parts of the Voice
1. Larynx (pronounced LAIR-inx, not
LAHR-nix)
The larynx is the voice box. The vocal folds (also called
vocal cords; refer to our explanation
to clarify this terminology) are part of the larynx. The
vocal folds vibrate to create the sound of the voice.
(for more detail see Anatomy 201:
Anatomy and Muscles of the Larynx)
2. Pharynx (pronounced FAIR-inx)
The pharynx is the throat. It goes up from the larynx and
divides into the laryngopharynx (just above the larynx), oropharynx (going into the mouth) and nasopharynx (going into the nose).
3. Trachea (pronounced TRAY-key-ah)
The trachea is your windpipe. It's the tube that connects
your lungs to your throat. The larynx sits on the top of the
trachea.
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Figure 1:
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Some other nearby organs:
4. Esophagus
The esophagus is your food pipe. It's just behind the larynx
and trachea. Your pharynx carries both air and food/water.
The air goes through the larynx and trachea, and food and
water go into your esophagus.
5. Spinal column
The spinal column is behind the esophagus. You can feel
it by pressing the back of your neck.
6. Diaphragm
The diaphragm is underneath the lungs, inside the rib
cage. It's shaped like a dome. The diaphragm is your main
muscle for controlling respiration (breathing).
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Figure 2:
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Vocal
FOLDS or
Vocal
CORDS???
You've probably heard the term "vocal cords"
used to describe the part of the body that creates
sound for the voice. You've probably also heard the
term "vocal folds" in the same context. So what's
the difference between the two? Well . . .
Vocal folds are the same
as vocal cords.
The two terms refer to the exact same part
of the body performing the exact same functions.
The term "vocal cords" is less technically correct
but more often used among singers and
laypersons.
Why, then, do voice scientists and
otolaryngologists refer to them as vocal
folds? If you refer to Anatomy
202 "Structure of the Vocal Folds" you'll see
that years ago, vocal folds were thought of as
being two cords stretched across the airway, like
strings on a piano (hence the term "cords"). Now we
know that vocal folds are multilayered folds of
tissue that are continuous with other tissues in
the throat. Therefore, vocal "folds" is a more
accurate term, but it's OK with us if you call them
vocal cords. Just don't get confused and call them
vocal "chords!"
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Physiology
101: How it Works
1. Air comes out of the lungs, through the trachea, and
into the larynx.
2. The air makes the vocal folds vibrate.
3. When the vocal folds vibrate, they alternately trap
air and release it.
4. Each release sends a little puff of air into the
pharynx; each puff of air is the beginning of a sound wave
(see Acoustics 101: Sound Waves and
How They Move).
5. The sound wave is enhanced as it travels through the
pharynx; by the time it leaves the mouth, it sounds like a
voice.
Figure 3: Check back soon to see a picture of this
process.
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To see how the vocal
folds vibrate, purse your lips and blow; this is
similar to vocal fold vibration. Or, hold two
pieces of paper so close together that they almost
touch, and blow through them. Surprised? They don't
blow apart -- they vibrate together.
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Anatomy 201:
Anatomy and Muscles of the Larynx
This is as much as you need to know about
vocal anatomy, but it's simplified. If you want more detail,
check out an anatomy textbook.
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* laryngeal [pronounced lah-RIN-jul
or lair-in-JEE-al] = having to do
with the larynx
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Figure 4: In the picture on the right,
part of the thyroid cartilage is cut away to show
the extent of the cricothyroid muscle, insterting into the
inside of the thyroid cartilage.
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Anatomy of the
Larynx:
1. Cricoid (rhymes with "thyroid")
cartilage - As the top ring of the trachea, the
cricoid cartilage is shaped like a signet ring, wider in the
back than the front.
2. Thyroid cartilage - the thyroid cartilage fits
over the cricoid cartilage, and is hinged so that it can
slightly rock forward and downward. The thyroid cartilage
comes to a point in the front; this point is termed the
thyroid notch, but is commonly called the Adam's Apple. The
vocal folds (also called vocal cords; refer to our explanation
to clarify this terminology) attach at the inside of the
thyroid notch.
3. Arytenoid (pronounced ah-RIHT-uh-noid)
cartilages - These sit atop the back of the cricoid
cartilage and hold the back end of the vocal folds. The
arytenoid cartilages can rock, glide, and pivot, thus
controlling the movement of the vocal folds.
4. Vocal Folds (Vocal Cords) - These remarkable
structures provide a valve for the airway and also vibrate
to produce the voice. The vocal folds are multilayered
structures, consisting of a muscle covered by a mucosal
covering.
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IMPORTANT
TERMS
Abduction - The vocal folds abduct (come
apart) in order to let air in and out of the lungs
during breathing.
Adduction - The vocal folds may adduct (come
together) to trap air in the lungs. They may also
adduct to vibrate to produce vocal sound.
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5. Glottis - This is the space between the two
vocal folds. When the vocal folds adduct, the glottis
closes; when the vocal folds abduct, the glottis opens. The
adjectives "glottal" and "glottic" are used to describe many
aspects of vocal fold movement. The glottis opens and closes
during vibration. Refer to the corresponding pictures.
6. Epiglottis - This soft cartilage serves as part
of the protective swallowing mechanism. It folds backward
over the glottis during a swallow so that food and water do
not go into the lungs. It is not involved in normal voice
production.
7. Hyoid (rhymes with "thyroid") Bone - This
horseshoe-shaped bone is positioned slightly above the
thyroid cartilage and is the only bone in the body not
connected to any other bone. The hyoid bone provides the
attachment for many of the muscles of the tongue, jaw, and
neck.
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Figure 5:
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b a c k
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This is a view from above
the larynx. In this view, mucosa covers the muscles
and cartilage.
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The vocal folds are
stretched horizontally across the larynx from front
to back. They open in a V-shape, with the wide part
of the V at the back and the point of the V just
inside the thyroid cartilage (Adam's
Apple).
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Muscles:
Intrinsic Laryngeal Muscles
These are the muscles within the larynx
itself.
All these muscles are paired. That is, they are
symmetrically arranged on the left and right sides of the
larynx.
Posterior cricoarytenoid - These are the only
muscles involved in abduction. They open the glottis by
pulling the back ends of the arytenoid cartilages together.
This pulls the front ends (where the vocal folds attach)
apart, therefore pulling the vocal folds apart.
Lateral cricoarytenoid - These are adductors. They
close the glottis by pulling the back end of the arytenoid
cartilages apart. This pulls the front ends together, making
the the vocal folds come together.
Thyroarytenoid - These are the muscles that form
the body of the vocal folds themselves. They shorten the
vocal folds by pulling the arytenoid (back) end of the vocal
folds toward the thyroid (front) end. This shortens the
vocal folds and bunches them up, which causes them to
vibrate more slowly, thus lowering pitch. The thyroarytenoid
muscles also have a force to strengthen glottic closure.
That is, they help bring the vocal folds together and keep
them together to resist the airstream from the lungs.
Cricothyroid - These are the vocal fold
lengtheners. They pull the thyroid cartilage down and
forward on its hinge, which increases the distance between
the arytenoids and the thyroid notch (the Adam's Apple),
thereby lengthening and tightening the vocal folds; this
causes them to vibrate faster, thus raising pitch.
Interarytenoids - There are 2 sets of these: the
transverse arytenoids and the oblique arytenoids. They bring
the two arytenoid cartilages together to provide medial
compression for the vocal folds. In other words, the vocal
folds squeeze together tighter to resist the air pressure
from the lungs (shown on figure 7).
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Figure 6:
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b a c k
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This is a view from above the
larynx. In this view, muscles and cartilage are
exposed. Arrows indicate the direction each muscle
contracts.
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Extrinsic Laryngeal
Muscles
These are the muscles of the
front of the neck and jaw that surround the larynx.
The muscles of the front of the neck are also
collectively referred to as the "strap" muscles.
We won't provide detail on all
the extrinsic laryngeal muscles, but there are many
to give the head and jaw their wide variety of
movements.
Though each of the strap muscles
is responsible for a single specific movement, when
there's tension in the neck, the muscles tend to
contract as a unit (all at once). This tension can
make it harder for the intrinsic laryngeal muscles
to do their job. Extrinsic laryngeal muscle tension
is a factor in many voice disorders (see
Voice
Disorders).
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Figure 7:
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t o p
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b o t t o m
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This is a view from behind
the larynx. In this view, muscles and cartilage are
exposed.
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More
Terms:
Phonation - Phonation is the word we use
for making noise with the larynx, or producing
vocal sound; phon- is a root word meaning
sound.
Dysphonia - Dysphonia is poor voice
quality; dys- means bad, and phon-
means sound. We often use the term dysphonia even
if the sound is acceptable but the person has
discomfort while phonating.
Adduction - Adduction
occurs when the vocal folds come together to close
the glottis (think of adding the vocal folds
together to remember this term).
Abduction - Abduction is
bringing the vocal folds apart to open the glottis
for breathing.
Mucosa - Mucosa is the kind of tissue
that lines the entire inside of the mouth, throat,
etc. It is soft and wet, and should always be
covered by a layer of secretions (saliva). The
mucosal covering of the vocal folds is very
special: it is made of several layers of collagen
fibers. Each layer is arranged differently in order
to give different kinds of strength yet flexibility
for vibration. In most phonation, it is the mucosa
that vibrates, not the entire vocal fold. See
Figure 9.
Vocal Tract - This term refers to
everything from the glottis to the lips. The vocal
tract is the passage for the sound wave. Most people call it
their throat!
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Figure 8a: A picture of
abduction as we see it through
an endoscope.
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Figure 8b: A picture of
adduction as we see it through
an endoscope
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Here's
something you probably never thought
of…
Your larynx is called your voice box, but
phonation (production of vocal sound) isn't the
primary biological purpose of the larynx.
The most important thing your
larynx does is . . .
protect your lungs from food and water.
The vocal folds form a valve that closes tightly to
protect the airway. When you swallow, the glottis
closes tightly --that is, the vocal folds adduct
(come together) tightly. Also, the epiglottis folds
over the glottis, and the larynx rises up while the
esophagus opens to let the food/water enter. You've
probably gotten water "down the wrong pipe"
sometime in your life. That's because the water
went down too fast for the glottis and epiglottis
to protect the airway. When the water hit the vocal
folds, they went into a cough reflex to expel the
water and keep it out of the lungs. (Food or water
in your lungs is a very bad thing.)
The second most important
thing your larynx does is . . .
trap air inside your lungs. This
provides what we call "thoracic fixation," also
known as the Valsalva maneuver. That is, trapping
air inside the lungs provides air pressure, against
which you push for excretion and childbirth, or
stabilization for lifting.
We share these laryngeal attributes with air-breathing
animals. The fact that our larynx can also make
beautiful sounds is one of our very special human
attributes.
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Anatomy 202:
Structure of the Vocal Fold
Vocal folds used to be called vocal cords (and are still
often referred to that way) because it was thought that they
vibrated much like strings on a violin. This has been shown
to be untrue.
The figure below shows the cross section of a vocal fold.
As you can see, it is a multilayered structure. The
innermost layer is the thyroarytenoid (a.k.a. vocalis)
muscle, which runs the entire length of the vocal fold, from
the thyroid cartilage to the arytenoid cartilage. The
thyroarytenoid muscle is the most dense portion of the vocal
fold.
Surrounding the thyroarytenoid muscle is a sheath of
mucosal tissue, that varies in stiffness from the stiffest
portion surrounding the muscle, to the outermost layer,
which is quite floppy.
The mucosal covering is divided into three sections,
deep, intermediate, and superficial. All three layers are
composed of collagen fibers, but the fibers are arranged
differently in each layer. The fibers of the superficial
layer are sparsely arranged, and are like thin, loose
threads. The intermediate fibers are arranged along a
different direction, and are more like a bundle of soft
rubber bands. The fibers of the deep layer lie in yet a
different direction, and they are arranged in dense, stiff
bundles, like clumps of cotton thread. The overall looseness
or stiffness of the mucosa depends on the state of
contraction of the laryngeal muscles, but in general, the
mucosa is similar in consistency to soft-set Jell-O.
The mucosa is covered by a thin layer of epithelial
covering, similar to the loose skin on the back of your
hand. This epithelium contains the mucosa but allows it to
take on an endless variety of shapes.
The interesting thing about the multilayered structure is
that the vocal fold can be stretched or contracted and made
to vibrate at many different lengths. It can also adapt to
different degrees of impact, and can withstand the forces
and strain of extremely rapid vibration by rapidly changing
its configuration.
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Figure 5: |
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This drawing shows a cross
section of a vocal fold. Deeper layers of mucosa
are more stiff than shallower ones. At the core is
a muscle with fibers stretching from front to
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Length of the vocal
folds:
The maximum length of the vocal folds is
about 16 mm for an adult male and about 10 mm for
an adult female.
Source: Titze, Ingo R. 1994.
Principles of Voice Production. Englewood Cliffs,
NJ: Prentice Hall, Inc., p. 178.
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Anatomy 301: The
Role of the Nervous System
Nerves come from the brain to the brain stem (a lower,
more primitive center of the brain) or to the spinal cord,
and then go out to muscles and tissues of the body. Signals
from the nerves activate the muscles and control their
movement. Nerves also carry information about sensations in
the muscles and tissues back to the brain. This two way
process is called "innervation," and nerves are said to
"innervate" the organs. There are two nerves that innervate
the larynx: the recurrent laryngeal nerve and the superior
laryngeal nerve. The recurrent laryngeal nerve is the more
important of the two nerves, and the one most likely to be
damaged. Both the recurrent laryngeal nerve and the superior
laryngeal nerve are part of one of the 12 cranial nerves.
They branch off from the tenth cranial nerve, called the
Vagus nerve. The recurrent laryngeal nerve comes out of the
brain stem and descends all the way down to wrap around the
aorta (the main artery leading out of the heart) on the left
side. It then comes back up and attaches to the larynx.
Figure 8 below diagrams the nerves that connect the larynx
to the brain.
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Figure 10:
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Physiology 201:
Vocal Fold Vibration and Pitch:
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Figure 11: This chart shows how fast
your vocal folds are vibrating (in Hertz) when
you're singing certain pitches. Notice that every
time you go up an octave, you double the frequency
of vibration.
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The term for vibrations, or cycles, per
second, is Hertz. It's abbreviated Hz, but we still
say "Hertz."
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The faster the vocal folds vibrate, the higher the pitch.
Extremely slow vocal fold vibration is about 60 vibrations
per second and produces a low pitch. Extremely fast vocal
fold vibration approaches 2000 vibrations per second and
produces a very high pitch. Only the highest sopranos can
attain those extremely high pitches. In general, men's vocal
folds can vibrate from 90 - 500 Hz, and they average about
115 Hz in conversation. Women's vocal folds can vibrate from
150 -1000 Hz, and they average about 200 Hz in
conversation.
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For all you singers . .
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Different voice types have different average
speaking pitches. Here is a table of average
speaking pitches and frequencies by voice type.
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Soprano
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B3 (246.9 Hz)
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Mezzo-Soprano
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G3 (196.0 Hz)
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Contralto
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F3 (174.6 Hz)
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Tenor
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E3 (164.8 Hz)
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Baritone
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B2 (123.5 Hz)
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Bass
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G2 (98.0 Hz)
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Source: Titze, Ingo R. 1994.
Principles of Voice Production. Englewood Cliffs,
NJ: Prentice Hall, Inc., p. 188.
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Vocal folds (also called vocal cords; refer to our
explanation to clarify this
terminology) vibrate faster as they're pulled longer,
thinner, and more taut. This is done by contracting the
cricothyroid muscle, which pulls the thyroid cartilage down
and forward on its hinge, away from the arytenoid
cartilages, thus lengthening the vocal folds. When they're
lengthened they also get thinner and more taut (like a
rubber band - try it).
Vocal folds vibrate more slowly when they're shorter,
thicker, and floppier. This is done by contracting the
thyroarytenoid muscle, which pulls the arytenoid end of the
vocal folds closer to the thyroid end, thus bunching them
up. The thyroarytenoid muscle is contracted, so it's firmer,
but the mucosa overlying the vocal fold becomes floppier, so
it vibrates slower.
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Figure 12: The vocal folds on the left
are singing rather high (733 Hertz) while the vocal
folds on the right were singing much lower (about
200 Hertz).
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It sounds pretty simple, but it's
usually more complex than that.
The cricothyroid muscle and thyroarytenoid muscle coordinate
with each other to create different pitches. They can also
coordinate differently to produce the same pitch with a
different sound quality. The amount of airflow from the
lungs also impacts the pitch. In addition, the other muscles
in the larynx can affect pitch and loudness adjustments in
very complex ways.
Physiology 202:
Vocal Fold Vibration and Loudness:
IMPORTANT:
If you haven't read Anatomy
201 yet, read that first. You have to
understand the glottis before you read about loudness.
Don't worry. It's not that hard.
Loudness is pretty complex -- lots of factors affect
loudness.
The loudness of the sound coming directly from the vocal
folds (also called vocal cords; refer to our explanation
to clarify this terminology) has to do with one thing: the
strength of the explosion of air into the glottis (the space
between the 2 vocal folds) each time the glottis opens
during a cycle of vibration. The loudness of the sound
coming out of the MOUTH is a different matter. We'll get to
that later.
Figure 13:
This diagram shows cross sections of the vocal
folds.
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Glottic Cycle for a Soft
Voice
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Glottic Cycle for a Loud
Voice
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Remember that the vocal folds alternately trap
and release air; each trap/release is one cycle of
vibration. This cycle is often referred to as the
glottic cycle, and it is divided into phases:
opening phase, open phase, closing phase, closed
phase (see the diagrams on the left and right;
follow along from top to bottom).
During the closed phase, the air pressure builds
up below the vocal folds. When the glottis opens,
the air explodes through the vocal folds, and
that's the beginning of the sound wave. The
strength of that explosion determines the loudness
of the sound coming directly from the larynx.
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Keep in mind that,
depending on the pitch of the sound, each cycle of
vibration can be occurring within one sixtieth of a
second or at any speed up to nearly one
two-thousandth of a second! Regardless of how fast
the vocal folds are vibrating, each cycle is still
divided into phases, and those phases can have
different proportions.
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What causes stronger explosions of air going into the
glottis?
The longer the closed
phase, the more the air pressure builds up -- thus the
stronger the explosion. With soft phonation the
closed phase is proportionately short, and air pressure
doesn't get as much chance to build up. The explosion is
weaker. With loud phonation, the closed phase is
proportionately longer, and the air pressure builds up
more. Therefore, the explosion is stronger.
How does the closed phase get longer?
The muscles in the larynx that bring the vocal
folds together contract more strongly, squeezing the
vocal folds together harder, so they can resist the air
pressure longer. Those muscles are the thyroarytenoids,
lateral cricoarytenoids, and interarytenoids. The muscles
in the neck may also help provide stabilization, or may
actually help produce the squeezing effect.
What happens to the opening phase when more air
pressure builds up?
When the air pressure builds up for a longer
time, not only does the air explode more strongly through
the larynx, but the vocal folds are blown more strongly
apart, and that opening phase is more sudden. The open
phase is actually shorter, because the forces that suck
the vocal folds back together are stronger. The closing
phase, therefore, is also more sudden, and the vocal
folds snap back with high impact. The sudden closing
phase helps produce a brighter, "ringier" sound but it is
also harder on the vocal folds (see our pages on
Voice Disorders or Singers,
etc.).
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Explanations of why all of this happens are
beyond the scope of this website. For more
information on these subjects, please refer to an
acoustics textbook.
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The other thing that affects loudness is how the sound
wave is enhanced by the vocal tract. Think of blowing into
the mouthpiece of a trumpet, and then blowing into the
mouthpiece when it's connected to the rest of the trumpet.
This difference in sound is similar to the difference of
when the sound leaves the glottis and when it leaves the
mouth.
Acoustics
101: Sound Waves and How They Move
Acoustics is the study of sound. In this section
we're going to explore how sound waves travel from the
vibrating vocal folds, through the vocal tract (throat), and
out into the air.
Sound happens when air molecules get moved, and the
movement continues in a wave (much more on that later). When
the sound is continuous (like the human voice), it's because
there's some vibration that moves, or disturbs, the air
molecules. In the case of the human voice, it's the vocal
folds that are vibrating. You actually know a lot about
vibration, even if you've never thought about it.
Properties of Vibration
The most basic example of vibration is called simple
harmonic motion. Simple harmonic motion is also known as
sinusoidal motion. You may have heard of sine waves -
they're created by sinusoidal motion.
Figure 14: A sine wave - like the beginning of
"The Outer Limits" for you boomers.
Think of a pendulum. It swings in one direction,
slows and reverses its direction, swings back to its
midline, and then swings in the opposite direction, the same
distance as it moved in the original direction. Then it
reverses direction, returns to midline, and begins the
process again.
Figure 15: Movement of a Pendulum.
There are only three properties that can be manipulated
with a swinging pendulum, and these factors help illustrate
the properties of vibration.
Vibration terms
Frequency is how many times per second the
pendulum crosses its midline. Frequency measures the speed
of vibration. Frequency is measured in Hertz (the
abbreviation is Hz, but we always say "hertz"). In sound,
frequency of vibration gives us the pitch of the sound. The
faster the vibration, the higher the pitch that we perceive.
So, we say that the perceptual correlate of frequency is
pitch. Click
here to see the equivalences of pitch and frequency.
(Yes, vocal folds can vibrate faster than 1000 Hz, or 1000
vibrations per second!)
Amplitude is how far the pendulum swings away from
its midline, in either direction. Amplitude measures the
strength of vibration. Amplitude is measured in decibels
(the abbreviation is dB, you can say decibels or dB, but not
dB's - the abbreviation is already plural).
In sound, amplitude of vibration gives us the loudness of
the sound. The greater the amplitude, the louder the sound
we perceive. So we say that the perceptual correlate of
amplitude is loudness.
BUT, there's not a direct correlation between amplitude
and loudness the way there is for frequency and pitch.
Acoustic scientists distinguish between the amplitude of
vibration, the intensity (or sound pressure level -- we
won't go into detail on this property in this section), of
the sound wave, and the loudness of the sound perceived.
Those are all measured differently, and don't correspond to
each other in a one-to-one fashion. In the case of the
voice, the amplitude of the vibration of vocal folds may
determine the intensity of the sound wave, but many other
factors influence our perception of the voice's loudness.
Read on, and we'll discuss this topic more, but some of the
discussion is beyond the scope of this website.
Phase refers to the moment in time the
vibration starts its first excursion away from midline.
Usually this is in comparison to when other vibrations begin
their first excursion away from midline. Phase doesn't
actually measure any property of vibration. Rather, it's a
property of vibration that exists at any moment in time.
Phase is measured in degrees away from midline, just like
degrees of a circle. If one vibration starts it's excursion
in one direction at exactly the same time another starts its
excursion in the opposite direction, they are said to be 180
degrees out of phase.
In sound, phase doesn't have a perceptual correlate in
the way frequency and amplitude do. If we could hear two
simultaneous sine waves with the same frequency and
amplitude but were out of phase with one another, we might
only hear a slight buzziness to the sound. In the complex
sounds we hear, we don't perceive the phase differences as a
separate entity. Phase differences are very important in
voice though, because different portions of the vocal folds
may vibrate out of phase with each other, resulting in very
complex vibration. More about that later…
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So far we have been talking about listening to
sine waves. In real life, though, we don't actually
hear sine waves or simple harmonic motion. Rather,
sounds that we hear are usually the result of
complex vibration in which there are many
simultaneous frequencies, amplitudes, and phase
differences.
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Confused? Pictures
help!
A picture of vibration as it proceeds through time is
called a waveform. A waveform shows properties of
vibration.
Let's start with the most basic waveform, which is a
picture of a sine wave (a.k.a. sinusoid, or simple harmonic
motion).
Figure 16: Waveforms of sine waves: The horizontal
axis represents time, and the vertical axis represents
displacement from midline (which we'll compare to the
excursion of the pendulum).
A - pendulum swings away from midline in a positive
direction
B - pendulum reaches it's furthest point, reverses itself
and swings back toward midline
C - pendulum crosses midline
D - pendulum swings away from midline, now in a negative
direction
E - pendulum reverses itself and swings back toward
midline
F - one complete cycle is now completed
Figure 17a: Sine waves of different frequencies
- the sine wave on the right has a frequency that is 3 times
higher than the one on the left.
Figure 17b: Sine waves of different amplitudes
- the sine wave on the left has an amplitude that is 5 times
greater than the one on the right.
Figure 17c: Two sine waves, 90 degrees out of
phase
A sine wave is a vibration at one single frequency and
one single amplitude. That doesn't actually exist in nature,
and certainly not in the human voice. Real-world vibration
is complex vibration, made up of many sine waves occurring
simultaneously. But to understand complex vibration, it's
helpful to understand how the vibration becomes a sound
wave, and how the sound travels through the air. Let's talk
about that next.
Movement of a Sound Wave
Sound Wave - A sound wave is a disturbance of air
molecules, propagated through air.
For our discussion, assume the disturbance of air
molecules will eventually disturb the tympanic membrane (ear
drum) causing a vibration to go into the brain, where it is
interpreted as sound.
The journey from the eardrum into the brain is very cool.
Go look it up on the web!
Think of a slinky. If you hold one end still, and move
the other in and out in a regular rhythm, each band of the
slinky stays pretty much in place, just moving back and
forth as much as you originally moved the end of the slinky
back and forth. Each band of the slinky will run into the
next band, and then move backward away from it. This will be
repeated along the whole length of the slinky. But you'll
see the bands as they bunch up, and the location of the
bunched up section will continue to move to the other end of
the slinky. It will look like a wave, traveling along the
length of the slinky, over and over, as long as you continue
to move the one end. (Now you want a slinky, don't you? Go
buy one and play with it!)
The wave along the slinky is like a sound wave as it
travels through air. Each air molecule stays in one place,
bouncing back and forth, slamming into the molecule ahead of
it, and then slamming into the molecule in back of it.
Molecules moving in a forward direction will get bunched up,
or compressed, and then more spaced apart, or rarefacted, as
they bounce backward away from each other. The compressions
(bunched up areas) keep moving along, eventually dying out
due to the friction of the air, or ending up on someone's
ear drum.
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In case this sounded too easy…… The
molecules moving forward create compressions, and
the molecules bouncing backward create
rarefactions, BUT, when the backward moving
molecules bump into forward moving molecules just
behind them, that might create another compression,
depending on how soon the next wave starts and how
far the molecules are moving (which depends on the
amplitude of the original disturbance). And, if the
air molecules run into a surface that deflects
them, they can start a whole series of compressions
and rarefactions moving away from that surface. And
in case that wasn't complex enough, air molecules,
unlike bands in a slinky, can move in any
direction. This means that, as all the air
molecules surrounding the disturbance are moved,
the sound wave travels outward in ALL directions;
those compressions and rarefactions can run into
anything, anywhere, and be deflected back from
who-knows-where.
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That all makes sense when you consider the sound you make
when you yell across a large, open space. But you probably
didn't think about the sound wave as it leaves your
vibrating vocal folds and travels through your vocal tract.
That sound wave travels in a very complex pattern of
compressions and rarefactions, and when the pressure hits
structures like the roof of your mouth and your teeth, it's
deflected back, and can exert pressure back onto your vocal
folds, affecting the way they vibrate. That ends up being
pretty important.
AND - all of this happens really fast. The
disturbance of air molecules travels at the speed of sound:
350 meters per second, or 35,000 centimeters per second.
Since the average vocal tract is about 17 centimeters from
vocal folds to the lips, the disturbance whips through your
throat REALLY fast.
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Oh, by the way, if a tree falls in the
forest, there's a sound wave that travels through
the air, whether there's an ear drum to hear it or
not.
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Remember the waveform -
the picture of a sound wave?
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Think of the waveform as depicting compressions
and rarefactions. The positive portion of the
waveform is the compression (where there's positive
pressure) and the negative portion is the
rarefaction (where there's negative pressure). The
waveform shows how the compressions and
rarefactions move through time, but remember that
in any one place, the air molecules are alternately
compressed and rarefacted.
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Wavelength - One more concept to think about
before we get to the really cool stuff: wavelength is how
far a compression travels before the next compression
starts. The compressions travel at the speed of sound, so
the wavelength depends upon the frequency of vibration, that
is, how quickly the next compression starts.
More on acoustics coming soon!
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