Physiology

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: 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.
   
   

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.

The nerves coming from the brain to the brain stem, and then out to the head and neck, are called the Cranial Nerves. There are 12 of them.

The nerves coming from the brain to the spine, and then out to the lower parts of the body, are called the Spinal Nerves. There are 31 of them.

The 10th Cranial Nerve is called the Vagus Nerve. Vagus means wanderer, and it does go to many parts of the body, including the larynx. Google it – it’s very cool!

Two branches of the Vagus Nerve are Recurrent Laryngeal Nerve and the Superior Laryngeal Nerve. See Figure 10. The Recurrent Laryngeal Nerve is called recurrent because on the left side, it 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). It then comes back up and attaches to the larynx. It innervates all the muscles of the larynx except the cricothyroid (the vocal fold lengtheners). Injury to this nerve can result in Vocal Fold Paralysis (see more in the section on Specific Voice Disorders).

The Superior Laryngeal nerve innervates the cricothyroid muscle only. Both nerves carry information about sensation back from the larynx to the brain.

Figure 10

Vocal Fold Vibration & 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 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 children and 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.

Vocal folds 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.

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.

Left: Glottic Cycle for a Soft Voice
Right: Glottic Cycle for a Loud Voice

Figure 13: This diagram shows cross sections of the vocal folds.

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 vocal folds resist the air pressure from the lung longer, because of contraction of the thyroarytenoids, lateral cricoarytenoids, and interarytenoids (muscles in the larynx that bring the vocal folds together). Those muscles work in coordination to squeeze the vocal folds together more strongly. 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.

Explanations of why all of this happens are beyond the scope of this website. For more information on these subjects, please refer to a more advanced voice textbook.

The MOST Important factor affecting loudness!

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.  How the sound wave is affected is explained more in the section on Acoustics.  It's pretty complex, but very cool.  Stay tuned for more content on this important area.