How does passing air through a narrow glottis cause vibrations?

Question

I'm studying phonetics as part of a Linguistics degree, and in my textbook, the author discusses how we make our vocal folds narrow, almost touching, such that air passing through vibrates. This is how we produce voiced sounds like /ɑ/ or /b/ or /iː/. The question is: why does simply contracting the vocal folds produce this vibration? The textbook only says:

When the edges of the vocal folds are touching each other, or nearly touching, air passing through the glottis will usually cause vibration.

(Peter Roach, English Phonetics and Phonology: A Practical Course 4th ed, 2009, Chapter 4)

But what physical process means that air passing through will cause vibration?

Answer 1

The process that results in the oscillation of the vocal folds is actually somewhat complicated and may be tricky to grasp if you are "not good at physics", but let's give it a shot!

A somewhat vague explanation is that, due to the way the shape of the vocal tract causes air to flow through the glottis and past the vocal folds, the fluid pressure (where a fluid is a liquid or a gas) just above and just below the glottis is constantly shifting and either causing the vocal folds to come together or pushing them apart. This cycle of the vocal folds hitting together and being pushed apart happens over and over again, many times a second, creating a periodic oscillation or "vibration".

Some more details:

Early models of vocal fold oscillation relied on the Bernoulli Principle, which says that, as the velocity of a fluid increases, its pressure decreases. It was thought that the flow of air through the glottis caused the air pressure between the vocal folds to decrease, causing them to "collapse" together. The full closure of the glottis was then supposed to cause the air pressure below it to build up until it "blew" the folds open like saloon doors, allowing air to flow through it again until the whole cycle started over.

Later on researchers realized that this model was too simplistic and that these forces alone couldn't explain the continual oscillation of the folds, given the vibration-dampening properties of tissue.

The quote below explains how the model was refined to explain what really happens:

When the glottis is closing, the airflow begins to decrease, but the air that is above the glottis does not "know" this, so it continues to move with its same speed (because of inertia). This creates a region just above the vocal folds where the air pressure decreases, because air is not coming from the bottom through the glottis as fast as it is leaving above. When the vocal folds are opening, fluid pressure against the walls is greater than when the vocal folds are close together. Thus, it is the asymmetry of driving force (air) that sustains oscillation.

The quote is from this website, which gives a more in-depth discussion on the topic and also provides some animated graphics to illustrate the various concepts involved. For example, it explains why the vocal folds don't just move together and apart as two uniform blocks but rather oscillate in a wave motion, as shown in this video.

Answer 2

The process is described briefly under Vocal folds in Wikipedia. The air pressure forces the vocal fold out of the way, but then the fold springs back under its own elasticity, interrupting the air again. When that happens repeatedly, it forms an oscillation, which is audible.