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

[Vern <jevern@xxxxxxx>]

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.  

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.

See http://www.seykota.com/rm/spool_card/spool_card.htm <http://www.seykota.com/rm/spool_card/spool_card.htm> for a demonstration of the bernoulli effect.

Q.  What does “compression” in a harmonica mean to you?

Vern



> On Apr 5, 2016, at 1:43 PM, Bob Laughton <bob@xxxxxxxxxxx> wrote:
> 
> RE: Thickness of reed plates
> 
> I've always assumed it's just physics - the pressure vector having a
> (slightly) greater distance to apply force to the reed.
> 
> Think of two pea shooters, one twice as long as the other. With equal
> pressure the longer one would expel the pea with a (slightly) higher
> velocity.
> 
> Maybe it is more complicated than that?
> 
> Bob
> ---
> On Mon, April 4, 2016 12:28 am, Robert Hale wrote: