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ID Code: F5-03
Purpose: Demonstrate a fluid-flow model of the vocal folds.
Description: Blow down between the thin sheets. Air follows the curve of the sheets, according to the Coanda effect. The reaction force on the sheets pulls them together. When the sheets close, air pressure builds up, opening them and restarting the periodic cycle. We use an air gun for this demonstration so that we can keep the geometry uniform; it is easier during a lecture for the demonstrator to simply blow his or her breath into the region between the plates. This demonstration is often incorrectly explained using the Bernoulli principle. According to the INCORRECT explanation, the air flow is faster in the region between the sheets, thus creating a lower pressure compared with the quiet air on the outside of the sheets. This lower pressure causes the sheets to come together, whence the pressure builds up, forcing them apart, etc.

This is demonstrably incorrect. Note that the remaining sheet that is hanging moves toward the center even in the absence of the other sheet, so it does not form a narrow constriction. What is happening here is that the air moves along the surface of the sheet, according to the Coanda effect, leaving in a direction away from the center line of the two sheets. The reaction force on the sheet causes it to move toward the center line of the two hanging sheets.

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