The Bernoulli principle and its misapplication in vocal pedagogy
How physics is often misunderstood in the voice studio
The Bernoulli principle, put simply, is that as the speed of a fluid or gas increases, pressure decreases. It is relatively well-known due its application in the physics of flying. An easily understood example of the effect in action: when the air moving past the upper surface of an airplane wing travels at a greater velocity than the air moving past the lower surface, Bernoulli’s principle dictates that the resulting differential between the lower pressure above the wing and higher pressure under the wing will create lift.
How is this applicable to the singing voice and in which circumstances does Bernoulli’s principle apply? It is common for voice teachers to refer to the Bernoulli principle to argue that a greater velocity of airflow will necessarily suck the vocal folds together because of the consequent lower pressure created at the glottis. The common example is for the teacher to blow air from the mouth onto two pieces of paper held loosely together, which will, of course, come together along the length. Case closed, right? The trouble is that, for the example to make sense in the context of phonation, there would have to be slower moving or unmoving air present on each external edge of the vocal folds; that is, a higher pressure in relation to the low pressure of the faster air flow through the glottis. This is self-evidently anatomically impossible. Therefore, the example of the voice teacher with two pieces of paper is specious. When they blow, they are indeed demonstrating the Bernoulli principle at work, just not in a way that could ever apply analogously to the vocal folds. As in the case of the airplane wing, the papers are sucked together because there is a pressure differential between the lower pressure of the rapidly blown air and the higher pressure of the still air inside the room.
This misunderstanding possibly derives from the misapprehensions of very early voice science or, perhaps less innocently, from a desire to legitimize and justify certain technical ideas and aesthetic ideals.
Trusting that the demonstration accurately represents the physics of the vocal tract, a singer may employ a phonatory mode with a loose initial vocal fold adduction and abundant airflow through the glottis, assuming that the velocity of the airflow in itself will close the folds fully to phonate efficiently. While it is definitely possible to vibrate the vocal folds with breath in this more opened configuration, this (with a touch of irony) is the exact phonatory mode in which the Bernoulli principle does not strongly apply. Consequently, the abundant airflow will do extremely little to suck the vocal folds together and the singer will be left with rather inefficient adduction of the folds.
Bernoulli’s principle applies strongly to phonation in cases where glottal closure restricts the airflow. This may or may not be the subjective experience of the singer, but this is the objective reality. Some explanation is required:
The singer breathes, taking air into the lungs
The singer seals the glottis, preventing the air from escaping
The singer compresses the air inside the torso with the action of the diaphragm and associated muscles. Compressed air is even less in motion than air inside a room. According to Bernoulli’s law, we therefore have a high level of pressure
As the singer phonates, the first vibration begins when the high pressure air blows open the bottom half of the vocal folds. This puff of air is now moving rapidly. Therefore, according to Bernoulli’s principle, the pressure of the air has lowered.
This low pressure left behind sucks the bottom of the vocal folds back together.
The moving air now separates top of the vocal folds, the remaining low pressure in turn immediately sucking them back together again.
The glottal seal, having been reestablished, along with the continuing high pressure of the compressed air inside the torso allows the process to continue through the entire vocal phrase.
Therefore, the Bernoulli principle’s application to phonation applies strongly in the case of technical practices based on firm adduction of the vocal folds and a high level of breath compression. A looser fold adduction and flow-based use of breath can well be a valid approach depending on genre and personal preference, but in these cases the singer is not harnessing the Bernoulli principle efficiently in aiding phonation.
Finally, as a matter of opinion, I find the mode of phonation that better leverages the Bernoulli principle preferable for high level acoustic singing. It bears a far greater similarity to widely accepted healthy speech practices, it protects the vocal folds from potential damage inflicted as a result of poor approximation, and it safely enables a vast increase in the intensity of phonation.
*For more detailed information concerning not only the Bernoulli principle in singing, but anatomic and acoustic explanations in general, I recommend visiting voicefoundation.org
This is really interesting on this subject and I probably need to read it a few more times to really understand the physics. I am one of those teachers who uses the blowing through two pieces of paper as an example but my emphasis is on the fact that the initial moment of sound production is a gentle drawing together of the folds rather than blowing them apart. I focus a great deal on a clear coordinated onset and use gestures/movement and emotion/imagination triggers to play with timing and encourage that slight holding back of the breath and find different onset qualities. I also demonstrate with a balloon squeal where the air moves more slowly with the elastic recoil of the balloon and buzz of the vibration. Plus relating it to a lip bubble/trill, oboe reed etc. I thought I understood the physics of it but am starting to doubt myself as read mixed messages and don’t want to give out wrong information. The vocal tract is much more complicated than these other things of course and I accept that Bernoulli isn’t the full story but do find it a useful image with children to help them to understand not to over blow. Very interested to know your thoughts.