Re: [Harp-L] Double thickness reed plates? [getting long]

Vern wrote:

I have a very different interpretation of the physics of reed motion and sound production.

Because it is virtually constant throughout the cycle, static pressure of your breath on the reed does not cause it to vibrate. It is the bernoulli effect of the air moving past the reed. The air rushing through the slot has a lower static pressure that pulls the reed towards the slot. However, when the reed is in the slot, air flow is shut off and so is the force acting on the reed. Think of the reed as an airplane wing that is acted on by the FLOW of air near it.

A reed must have a gap so the air can begin rushing through the slot and create the bernoulli force to start the reed moving towards the slot. When the reed has no gap, the static breath pressure is there but it doesn’t cause the reed to vibrate. For this reason, I posit that your peashooter analogy isn’t valid. It is more like a child pumping in a swing or a clock pendulum.

When the reed is above the plate, the slot is open and there is a puff of air through the slot that pulls the reed down towards the slot.
When the reed is in the slot, there is no air flow and no bernoulli force acting on the reed.  It is “coasting”.
When the reed is down below the plate, the slot is open (perhaps not as much) and there is a puff of air that pulls it up towards the slot.

This meets the requirement for a mechanical oscillator…a variable force out of phase with motion of the mass. When the reed is up, the air pushes it down and vice versa.

It is these puffs of air that constitute the sound.
This is a nice explanation of the fluid dynamics of air flowing through a reed slot and over the reed. It is born out by research done by Dr. Antaki (TurboDog) and others. There are several examples of germane research on the R&D page of his company's web site.

Of interest are his wind tunnel experiments that visualize the Bernoulli effect that Vern describes.

There are also supportive computer simulations in a research presentation that Antaki refers to in these slides.

Scroll down to the "Fluid Dynamics of Harmonica Reed Tip" slides to see exactly what Vern is talking about. (As they say, a picture is worth a thousand words.)

Vern goes onto say:
IF the plate is thicker, the puffs are briefer because the time between them is longer. This could conceivably change the wave shape of the sound generated and subtly affect the tone. However, I don’t hear much difference in the tones of standard and thick plates.
But unfortunately I'm not following that part. So I have yet another interpretation.

I think it is a simple matter of the closing reed swinging wider and then the greater energy stored in it as a cantilevered spring causing a like increase in the magnitude of the opening reed's travel in the opposite direction, etc. This results in a louder sound because more air flow is chopped by the reed but causes no real change in the timbre of the sound compared to a single reed plate.

If you are interested in learning more about the physics and fluid dynamics of the harmonica, I highly recommend that you spend a week or so studying the landmark paper by Bahnson and Antaki. Every time I read it I learn more. Then if you're really into this stuff, that article cites prior research.

There was a recent long and involved thread on Gussow's Dirty South forum in which I cite several scientific papers and present a number of relevant illustrations and videos. Many other forum members chimed in, including some original computer simulations, making that one thread a sort of modern treatise on the subject of reed shape and physical dynamics of air flow inside a harmonica.

All pretty riveting stuff!

At least that's the way I see it. 8^)


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