Re: [Harp-L] Reed Chamfering

[David Payne <dave@xxxxxxxxxxxxxxxxxxxxxx>]

Reeds are springs. When you change something about the spring it bends in a different way. I don't think chamfering the base of the slot can have anything to do with aerodynamics.  Up high, I don't know. There's a lot of things I don't know, except that there's virtually no spring action going on at the top (free end) of the reed.

 
David Payne
www.elkriverharmonicas.com
www.hetrickharmonica.com


________________________________
 From: Vern <jevern@xxxxxxx>
To: Rick Epping <rickepping@xxxxxxxxx> 
Cc: "harp-l@xxxxxxxxxx" <harp-l@xxxxxxxxxx> 
Sent: Tuesday, January 1, 2013 3:22 PM
Subject: Re: [Harp-L] Reed Chamfering
 
Hi Rick,

Thank you for your response.  It confirms my expectations in all respects but one.

As you say, there are different explanations for the excitation of the reed by the flowing air. Although I have my own, I don't presume to have the last word on the subject.  Certainly I do not know the details of the local conditions around the reed.  Neither do I claim to be the originator of the following theory.  From what I have read and  experienced, this is what makes sense to me:

I take the following as givens:
- Bernoulli's Principle states that the static pressure is less in moving air.  This is what makes the airfoils of airplane wings, propellors, and turbines work.
- Because the pressures in a harmonica are low, the effects of compressibility are negligible. 
- To excite vibration of a mechanical system, (the reed), a variable force out of phase with displacement must be applied. Think of pushing a child in a swing. You push in the opposite direction (out of phase) of the swing's displacement from chains-vertical zero.
- The constant static breath pressure will cause a constant reed displacement but will not excite vibration. 

Consider the variable forces on a reed in three positions:
1. Above the plate, there is air flow under the reed and through the slot.  The static pressure is reduced on the bottom side of the reed, forcing it toward the slot.
2. In the slot there is hardly any flow and thus hardly any vertical force on the reed. The reed is moving at maximum velocity and the inertia carries it on to the bottom of its swing.
3. Below the plate, flow in the slot reduces the static pressure on the top of the reed forcing the it upward. 

This meets the criteria of a variable, out-of-phase force. It is very close to the analogy of the swing.
The chamfer will not much effect the reed above the plate but will facilitate flow below the plate.  This is the mechanism that makes chamfering effective.
This is a weak effect. It needs a resonant embouchure to work.  I have made a chamber with a connected syringe to act as the embouchure. It can be placed over a reed on an air table.  Various settings of the syringe radically affect reed response. When sounding reeds 

On Jan 1, 2013, at 3:47 AM, Rick Epping wrote:

> ...........  Do you think that the reduced thickness of the reed's edges achieved through chamfering may also act as a means of pressure relief as the reed approaches the upper edge of the reed slot?...................

No. I think that it facilitates the flow through the slot when the reed is below the slot.  The opening to the sides and end is larger for a longer period of time. The openings at the top of the plate are unaffected by chamfering.  It would follow that there would be no effect at pitches so high that the reed does not exit the slot on the bottom of the plate. Chamfering of the bottom edges would seem to affect the reed at the upper edge of the slot.

I claim only that the above explanation is an unproven hypothesis that seems consistent with what I know of physics and the reed behavior that I have observed.  There may be things that I haven't taken into account. Certainly there are experts that know far more than me about aerodynamics, vibrations, and acoustics.

It was my intention to make the above brief and succinct.  I apologize for failing to do so.

Vern