Re: [Harp-L] Gapping/Embossing, am I doing it right?...long.

["Vern Smith" <jevern@xxxxxxx>]

There are customizers who make a specialty of gapping and voicing reeds.  I 
don't doubt that they can do a fine job and none of the remarks that follow 
whould be interpreted as criticism.  That said, I agree with you that there 
is no reliable, objective, articulate information available on gapping.  It 
belongs in the category of a guild secret.  Those who can do it either can 
not or will not give us an articulate description of their proceedure. 
However, the subject interests me and (I don't claim to be expert) I have 
spent a fair amount of time investigating the subject...mostly in relation 
to chromatics.  Here are the approaches with which I am familiar:

1.  The gap should be about as high as the end of the reed is thick.  This 
is a very crude rule-of-thumb and should not be trusted.

2. If only one reed is involved and its neighbors are performing well, then 
its gap should be between theirs.  This is a lot better than #1.

3.  Blow/draw it and see how it works.  If the reed is breathy and hard to 
start, decrease the gap.  If it starts with gentle pressure but chokes at 
high volume then increase it.  Even the experts use some variation of this. 
I am not very familiar with gapping for bending and overblowing but I think 
that gaps should be at the small end of the acceptable range for these 
functions. Many claim that reeds and players are different and no one 
arch-gap geometry is right for them all.  They claim that only long 
experience can give you the art.  I find this distasteful because I think 
that there should be an objective method that can be taught to a beginner 
that will enable him to do good gapping.

4. Put the reedplate on an air table and measure the pressures at which the 
reed begins to speak and chokes.  I find that a properly gapped reed will 
begin to speak at about 1/2" of water pressure and will not choke at 
pressures up to 10" of water. The design of an air table and the means of 
measurement of the pressures is beyond the scope of this post.

5. I made an instrument that used a little solid-state laser (like the ones 
used in pointers) to actually measure the height of the gap without touching 
the reed.  (Feeler gauges don't work because the reed moves before there is 
anything to feel.) For chromatics, I compiled a data base of measurements 
from about a dozen reedplates and plotted them on a length vs gap graph.  I 
observed that there was a fairly large gap range for every length of reed. 
> From this , I concluded that gap is not very critical.  I gapped a CBH2016 
using the middle of the gap range for each reed length and it worked fine. 
I have not yet proved it, but I think that by using my laser measuring tool, 
I could essentially duplicate the performance on a similar harp of any harp 
that I could measure.
      This method has some disadvantages...it cannot be done with 
windsavers in place and it takes about 30 seconds for each measurement.  I 
can share the design of the laser measuring instrument, but not in this 
post.

Arching:  Sissy jones and some others put their finger on the reed and 
prevent it from bending at the base while lifting the tip.  The resulting 
shape is a smooth arc tangent to the reedplate at the root and with the tip 
at the desired gap.  This has the advantage of minimizing airflow near the 
base of the reed where it cannot contribute much to inducing vibration or 
creating sound.  Others may disagree, but I think that as long as the arch 
is close to the above description, the exact shape is not very critical.

Embossing.  I'm a bit skeptical about embossing.  The clearance between the 
reed and the slot is only about .001" to .002"  The amount of air that will 
flow through this clearance is very small compared to the total flowing 
through the slot of a vibrating reed or through the gap of an opening reed 
in the same (diatonic) chamber.  If you really narrow that clearance much, 
the reed (which is not necessarily perfectly centered or aligned) may strike 
the edge.

I hope that this helps you feel a bit less confused about gaps.  It is a 
topic worth much more discussion and about which the acknowledged experts 
could be much more generous with their information.

Vern
Visit my harmonica website www.Hands-Free-Chromatic.7p.com

----- Original Message ----- 
From: "Greg Hommert" <mythacles@xxxxxxx>
To: <harp-l@xxxxxxxxxx>
Sent: Friday, May 05, 2006 11:55 PM
Subject: [Harp-L] Gapping/Embossing, am I doing it right?


> To my knowledge, there are few St. Louis Harp players that expend any 
> specific amount of energy playing "gapped harmonicas" (big picture), or 
> otherwise a harp that is meant to be used for overblowing/drawing (as 
> opposed to a stock harp which is more or less generally not "meant" for 
> that purpose).  As  to my knowledge Sandy Weltman doesn't really do much 
> work on his own harps; so I'm lost as to a specific guide in 
> tuning/gapping/embossing, etc.  I've searched the archives and gathered an 
> abundance of information concerning each matter, however I am still 
> missing some form of a) supervision and b) visual reference.
>
> Would there happen to be some photos floating around of a "properly" set 
> up reed plate that I might be able to at least compare to in a general 
> sort of manner; at least to clear up interpretation issues such as the 
> exact meaning of the word "arch," etc..?
>
> Much Appreciated,
> Greg
>
>
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