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.

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.

Figure 1:

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).

Figure 2:


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? 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!"


 

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.

 

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.


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.

 

* laryngeal [pronounced lah-RIN-jul or lair-in-JEE-al] = having to do with the larynx

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.

 

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.

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.

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.

 

Figure 5:

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This is a view from above the larynx. In this view, mucosa covers the muscles and cartilage.

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).

Muscles:

Intrinsic Laryngeal Muscles
These are the muscles within the larynx itself.

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).

All these muscles are paired. That is, they are symmetrically arranged on the left and right sides of the larynx.

Figure 6:

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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.

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).

 

Figure 7:

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This is a view from behind the larynx. In this view, muscles and cartilage are exposed.

 

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!

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.


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.

Figure 9:
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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 back.

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.

 

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.

 


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.

 

Figure 10:


 

Physiology 201: Vocal Fold Vibration and Pitch::

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.

 

The term for vibrations, or cycles, per second, is Hertz. It's abbreviated Hz, but we still say "Hertz.""

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.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.

For all you singers . . .

Different voice types have different average speaking pitches. Here is a table of average speaking pitches and frequencies by voice type.

Soprano

B3 (246.9 Hz)

Mezzo-Soprano

G3 (196.0 Hz)

Contralto

F3 (174.6 Hz)

Tenor

E3 (164.8 Hz)

Baritone

B2 (123.5 Hz)

Bass

G2 (98.0 Hz)


Source: Titze, Ingo R. 1994. Principles of Voice Production. Englewood Cliffs, NJ: Prentice Hall, Inc., p. 188.

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.

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).

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. 

Glottic Cycle for a Soft Voice

Glottic Cycle for a Loud Voice

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.

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.

 

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.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.).

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.

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.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.

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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…

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.

Confused? Pictures help!!

A picture of vibration as it proceeds through time is called a waveform. A waveform shows properties of vibration.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).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 whole cycle is now completed

 

Figure 17a: Sine waves of different frequencies - the sine wave on the right has a frequency that is 3 times greater 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 phasee
 

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 online! 

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.

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.

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.

 

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.

 
Remember the waveform - the picture of a sound wave??

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.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.

Wavelength - One more concept to think about: 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.

 

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Deirdre D. Michael - micha008@umn.edu
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