Re: [Harp-L] Tolerances on Harmonicas and the Manufacturing process.

[Winslow Yerxa <winslowyerxa@xxxxxxxxx>]

Vern - 

~9 percent is not an insignificant number.

Specific locations of leakage may be critical. Leakage (or lack thereof) in air flow around the edge of the reed being played may be more significant than leakage elsewhere.

It may be that an embossed slot has aerodynamic properties that are different from those of a slot manufactured to the same tolerance. A manufactured slot will present something close to a right angle with a sharp edge followed by a vertical drop, while an embossed slot will have been deformed by pressure so that it slopes down and inward to its edge, with an underside that slopes back outward to the original slot edge.

it would be interesting if someone could do a controlled test to compare the behaviors and overtone profile of otherwise idnentical reeds and reedplates, with one set having embossed slot edges.

Winslow

Winslow Yerxa

Author, Harmonica For Dummies ISBN 978-0-470-33729-5

--- On Tue, 5/26/09, Vern Smith <jevern@xxxxxxx> wrote:

From: Vern Smith <jevern@xxxxxxx>
Subject: Re: [Harp-L] Tolerances on Harmonicas and the Manufacturing process.
To: "Harvey Berman" <cscharp@xxxxxxxxxxxxx>, harp-l@xxxxxxxxxx
Date: Tuesday, May 26, 2009, 12:31 PM

I agree to all that Harvey has said about tolerances and manufacturing processes.  I also agree that small reed tolerances are better than large ones.

However, I raise the following questions about the importance of variations of the clearance between the reed and slot:

 If the nominal clearance is .001". Reeds vary in length from .75" to .30" for an average of about .50".  The width of a reed is about .08" Thus the clearance area around the average reed is (.5 + .5 + .08)" * .001"  = .00108 sq in.

However, consider the gap of the opening reed in the same chamber.  A typical gap for a .5" long reed is .008"  The area through the gap (two triangles of .5 * .008" plus .08" * .008" at the tip) is .00464 sq in.  ...almost five times the clearance area.  If by means of tolerances or embossing you close or open the clearance by .0005" or 50%, you have changed the clearance area by  .00054 in sq.  However, you have changed the total leakage area by only  .00054 / (.00108 + .00464)  =  9.4%

Not only that but the opening reed (especially the long ones) open even farther when you blow or draw.

Thus the gap of the opening reed contributes the vast majority of the leakage area in an unvalved diatonic.  I think that this remains true even if you quibble with my exact numbers.

Q. What then is the big deal with clearance tolerances and embossing?  If they are significant, it must not be as simple as total leakage.  It must be how completely the flow is shut off when the reed is passing through the slot.

Q. Could it be that we subjectively perceive differences in performance that are not there?

Q. Has anyone measured differences in clearance and then compared performance in a test using "blind" players?

Q. What have I overlooked?

Vern