Anatomy and Physiology Series: Laryngeal Physiology (Part 1, Say Yes to subglottal pressure--No to Bernoulli)

I'm going to split up laryngeal physiology into two posts because we've got two major things to discuss.  The first one, and the topic of this post, is the interaction of air pressure laws on vocal fold vibration.  The other, is the ideal interaction of the musculature to encourage efficiency.  I'm starting with air pressure laws because these are vital to understanding why the musculature must maintain such a delicate balance.  So here we go!


There are two main laws of air pressure we're going to talk about, the Bernoulli effect (which many folks speak of) and subglottal air pressure. 

We're going to go through the Bernoulli effect, or Bernoulli's principle, first, since it historically preceeded sub-glottal air pressure as an explanation for vocal fold movement.

Update (08/23/2015)
SCIENCE CORRECTION:  The Bernoulli Principle actually has nothing to do with vocal fold motion.  I know this is going against pretty much everything any of you have ever heard in your vocal pedagogy coursework, but the truth is that you only need the air pressure difference below the vocal folds (subglottal pressure) and above the vocal folds (supraglottal pressure) and the shape of the vocal folds themselves (the mucous lining in particular) to sustain vibration.  The issue with the idea of the Bernoulli Principle is that if that law really applied to vocal fold motion, the vocal folds would have no way to open back up again.  For all practical purposes for a singer, this doesn't really change much, but it is very important for those ENT's and researchers out there looking for ways to repair vocal fold scar tissue surgically and all that stuff.  The other nice thing about this as a singer (for me at least) is this: For all practical technical purposes, it's all about the subglottic pressure, which is great for us since subglottic pressure can be maintained through a lot of difference respiratory coordinations and configurations.  There might be an ideal out there for singers, but this, for me, explains why breathing is so important and such a variable, and sticky, pedagogical area for so many singers.

Subglottal air pressure is the measured pressure below the vocal folds.  This pressure and our intraoral (or inside the oral cavity) air pressure are pretty much equal to the pressure inside the lungs.  If we close the passage with our vocal folds, however, we instantly create an increase in the pressure below the vocal folds.  This is caused by the lungs continuing to exhale air which then "pushes" against that blockage (the vocal folds.)  While that is happening, the closed vocal folds also cause our intraoral pressure to drop to atmospheric pressure.  This increases the pressure difference between subglottal and intraoral pressure.  Now remember, air particles will always move from areas of high pressure to areas of low pressure, so subglottal pressure being significantly higher than intraoral pressure is a critical component to vocal function.  When this increased pressure exceeds a certain amount (3-5 cm H2O), the folds are blown open from the bottom, and the vibration known as phonation begins.* 

Here's how vocal fold vibration really works:  
The vocal folds are set into vibration through two mechanisms:  1. The glottal geometry (or shape of the glottis--the space between the vocal folds) and 2. The inertia of the vocal tract above the vocal folds.  As the subglottal pressure increases below the vocal folds, the vocal folds begin to separate at the bottom.  This is when the glottal geometry is convergent--when the top of the vocal folds are closer to each other than the bottom of the vocal folds.  In this configuration, the intraglottal pressure is high (that's the air pressure between the vocal folds themselves). This causes the vocal fold tissue to move away from the middle of the glottis.  But, vocal folds have elasticity thanks to that mucosa over the muscle.  This elasticity will generate a force that wants to restore the vocal folds to their original configuration.  This force will eventually overcome the force from the increased intraglottal pressure--this step happens from the bottom (inferior) vocal fold edges to the top (superior) edges and results in a rotational motion of the medial surface (middle of the vocal folds).  The vocal folds then take on a divergent configuration--the bottom of the folds are closer together than the top.  At the same time this rotational motion is happening, the intraglottal pressure decreases rapidly.  This causes the vocal fold tissue to change direction and move toward the middle of the glottis--this also happens from the bottom to the top.  The vocal folds either come together or get very close to one another (they don't actually have to make contact to create a sound wave--that's how you make breathy or lightly-voiced sounds) and then the glottis goes back to the convergent configuration and the cycle starts over again.  

From:  Titze, I. (1988).  The physics of small-amplitude oscillation of the vocal folds.  The Journal of the Acoustical Society of America, 83(4), 1536-1552.

The inertance of the vocal tract (#2 listed above) plays a part in this cycle in that as the vocal folds move outward from the middle of the glottis, the airflow through the glottis increases.  This makes sense, cause there's a larger glottis being created so more air particles can rush through.  This rush of air particles accelerates the air above the glottis that was previously relatively still. This increases the air pressure above the glottis (supraglottal air pressure) cause there's a higher number of air particles in that supraglottal space than there was before.  This increase of air pressure creates a force that moves the vocal folds outward.  This is where the elasticity of the vocal folds from above comes back into play, the vocal folds move back toward midline and the glottal flow decreases.  But, all those air particles that rushed through previously continue to move up the vocal tract because the inertia set those particles into motion and they want to stay that way until acted upon by another force.  This causes the pressure both above and within the glottis to decrease, allowing the vocal folds to come completely (or almost completely) together at midline and allowing the air pressure to build up below the vocal folds again.  Thus, the cycle continues as long as there's enough subglottal air pressure and continued air flow through the glottis at each cycle. *

So let's model this with a group of air molecules called Larry.  Larry gets sucked into the lungs during inhalation, he hangs out there a little bit, exchanges his oxygen for some CO2 and the forces of exhalation send him out.  But before he gets to go far, he encounters a blockage through his path.  This causes Larry to get smushed together with some of his other friends right under the vocal folds (like a little traffic jam).  Meanwhile, the air above the folds is decreasing in pressure, making it look like a really nice place to be.  The pressure Larry is under eventually overtakes the medial compression of the vocal folds, and the bottom of the folds begin to open.  Larry rushes through the vocal fold opening (unaware of the elasticity of the vocal folds cause, hey, he's just an air particle!) and then Larry runs into the air particles above the glottis and gives them a "push" to get them moving too.  As Larry and his air particle friends above the vocal folds travel up the vocal tract, the air pressure above and between the vocal folds decreases, the vocal folds come back together, another traffic jam starts up below them and the cycle repeats.  But Larry doesn't particularly care, cause he's already through the traffic stop.  A great visual model of this can be found at The National Center for Voice and Speech's website.  Scroll down for a more scientific explanation (certainly better than my "Larry" one) and some visuals that show this process.

The other nice aspect of taking out the Bernoulli concept is that it makes the concept of different vocal fold configurations for different voicing relatively simple to understand.  The vocal folds do not have to completely contact each other to create a sound pressure wave.  This is seen in regular voice users with videostroboscopy where you can see the vocal folds approximating during breathy or soft voicing and having a lot of contact during loud voicing.  It also explains something about the respiratory systems coordination for classical singing.  At high respiratory pressures, like when you've taken a really big breath, you are usually using high subglottic pressures to voice.  This increases the vocal fold contact time, resulting in greater swings of air pressures and producing louder (higher amplitude) sound waves.  Thus, vocal folds come together and stay together longer when voicing loudly. *

Another interesting aspect of this vocal fold vibration/air pressure relationship is that what is going on above the level of the vocal folds is just as important as what's going on below.  Therefore, if you have excess constriction above the vocal folds, you've changed these air pressure/air flow relationships that alter vibration and vocal fold contact times during vibration.  So, if you've ever had that "squeezing" feeling when singing, you likely had excess constriction above the vocal folds.  How to get that excess tension out is a whole different topic pedagogically, but I think it's enough to say that it requires a "resetting" of the air pressure/air flow relationships during that aria or difficult phrase, or maybe anytime you sing.  This will likely require some amount of "vocal play" between a new respiratory coordination (likely a bigger, lower breath), some amount of inspiratory checking (at least at the beginning of the phrase), and some concept that allows the supralaryngeal space (area above the vocal folds) to release the excess tension (like maybe the concept of an "open throat" or "singing on a yawn" or "inhaling a rose and singing with that space" or some variant of this).  

*Referrences:  

Hixon, T. J., Weismer, G., and Hoit, J. (2013).  Preclinical speech science: Anatomy, physiology, acoustics, and perception (2nd ed.).  San Diego, CA: Plural Publishing, Inc.

Raphael, L. J., Borden, G. J., & Harris, K. S. (2007).  Speech Science Primer:  Physiology, Acoustics, and Perception of Speech.  Lippincott Williams & Williams:  Philadelphia.

Titze, I. (1988).  The physics of small-amplitude oscillation of the vocal folds.  The Journal of the Acoustical Society of America, 83(4), 1536-1552.