[Harp-L] RE: Reed gapping questions

["Aongus MacCana" <amaccana@xxxxxxxxxx>]

Thanks Vern for this further piece of technical enlightenment. I am copying
and pasting it immediately to the Word document I had already created from
your initial post.

If I can work myself up to it I shall retrieve my homemade carburetor
balancer from the garage, wipe the cobwebs off it and have a go. If and when
I do this you shall be the first to hear the results.

I am sometimes (slightly!) concerned about my tendency for soaking up all
this theoretical knowledge and then not actually doing anything with it or
about it. 

Beannachtai

Aongus

 

From: Vern [mailto:jevern@xxxxxxx] 
Sent: 15 September 2013 20:20
To: Aongus MacCana
Cc: Harp-L List
Subject: Re: Reed gapping questions

 

I posit that a reed has optimum response when it spends about half of each
cycle in the slot.  The following explanation still holds if the optimum
time-in-slot is more or less than 50%.

 

In addition to its vibrational displacement, a reed has an average
displacement in response to static breath pressure.  IF you are a hard
blower, the reed needs to start farther away from the plate so that the
average deflection places it at the position for optimum time in the
slot...and vice versa.

 

Because there must be some flow through the slot at startup to begin
oscillation, the gap cannot be zero. IF the gap is so large that the reed
never reaches the slot, oscillation will not begin.  

 

In two-reed draw-note bending, the lower-pitched opening blow reed  sees
reverse static pressure, pushing it away from the slot.   Thus the optimum
gap will be lower than it would be in the normal closing mode. 

 

In an attempt to dispel the mystique of gapping involving the secret
knowledge of experts and trial-and-error methods, I have made several tools
for measuring gaps and have made many measurements of gaps on real reeds.  I
believe that if you can accurately measure, record, and set gaps, you can
achieve similar reed performance in different harps without special
expertise or trial and error. That said, the effect of gap can be obscured
by the effect on resonance of the player's embouchure.

 

Many feel that this explanation fails to consider other (unexplained)
variables.  I welcome alternative explanations for discussion.

 

Placing a tube from your manometer in your mouth while you play would give
you a fairly accurate measure of breath pressure.  This reasonably assumes
negligible pressure drop across the mouthpiece. Bear in mind that the tube
will affect your embouchure.  Another way would be to connect the manometer
to a tiny hole in the back of the comb. I made most of my pressure
measurements with the manometer connected to a pressure table with bare reed
plates. I would be interested in the pressures that you measure. 

 

Vern

 

 

On Sep 15, 2013, at 8:00 AM, Aongus MacCana <amaccana@xxxxxxxxxx> wrote:





I liked your informative post on this topic. From what I have gleaned from
this list and elsewhere, it seems that large gaps are for hard players and
small gaps for more gentle players. From what I understand this is only a
matter of response - in whether the reed decides to vibrate or not and that
it does not have much or anything to do with tone. What I find interesting
is that this is the first time I have seen anyone suggest an objective
physical measurement of soft and hard blowing:  "0.5 ins. Water gauge is
gentle playing and 10 -12 ins. Water gauge is going fairly hard".

I made a simple U Tube manometer from transparent fuel tubing years ago for
the purpose of balancing twin carburettors on VW and Porsche engines. I am
beginning to wonder whether I might be able to stick the end of the tube
into my mouth ahead of the tin sandwich and establish where I stand in the
"hard blowing stakes"

Beannachtai

Aongus Mac Cana