[Harp-L] reed setup
Sun Jun 19 18:26:25 EDT 2016
> On Jun 19, 2016, at 1:26 PM, Robert Laughlin <harmonicaman1968 at xxxxx> wrote:
> ……..Mostly, I'm referring to the proper gap adjustment, and what I would call the "shaping" of the reed for best performance……….
I have made several optical instruments for accurately measuring gap without touching the reed. I have run sets of reeds on an air table with a manometer to to measure pressure. I regard a reed as a machine whose operation can be understood by means of physics and experimentation. I believe that the fundamentals of gapping can be understood and used by anyone who wishes to adjust reeds. I do not believe that reeds have a mystique and gapping is an art that is acquired by gurus only after long years of experience.
My aim here is to give any beginner a starting point that will allow the reed to vibrate freely over a substantial range of pressures.
WHY IS A GAP NEEDED? The Bernoulli effect exerts a force on reeds. The higher velocity of air flowing through the slot has a lower static pressure and draws the reed toward the slot. If the gap is too small, there is insufficient flow to create this pressure difference and the reed never develops any speed. Static breath pressure pushes it into the slot and it just stays there.
If the gap is too large, the reed won’t start with gentle pressure. It may not start at all because it never reaches the slot. It is the change in Bernoulli pressure as the reed enters the slot and shuts off the flow that excites the vibration.
SHAPE: I rely on the experience of Sissy Jones. Although she didn’t use these exact words, she described the following shape: At the rivet end, the reed is parallel to and at the same altitude as the plate. At the tip, the height above the plate is the specified gap. Between these points, it has a smooth curve. This makes sense because it minimizes air flow at the stiff end where it can have minimal effect and concentrates it at the free end where it can have maximum effect. She achieved this shape in a fraction of a second by pressing on the reed near the rivet with her left fingertip and lifting the free end with a blade or fingernail.
GAP: I have measured the reed gaps of about a dozen reed plates that played well and have found the median* gap for reeds of various lengths. I have prepared a chart of pictures that show the gaps in proportion to .080” reed widths. By comparing the gap seen on the reed plate to the picture of the gap on the chart, the gap can be determined with sufficient accuracy.
* Half of the gaps are larger than and half are less than the median value. This minimizes the effect (a property of the mean) of extremely large or small gaps within the set.
If you are gapping a reed among others that play well, the gap will be smaller that of its longer, lower pitched neighbor and larger than that of its shorter, higher-pitched neighbor.
I cannot attach the chart to a Harp-l post. Send me an email off-list and I’ll send you a copy.
The optimum gap covers a fairly broad range. If you use the above specified shape and gap, the reed will almost certainly vibrate freely. If you are trying to optimize bending, then you will wish to use a bit less gap. How much less is where the art comes in. If you are setting the harp up for a hard-blowing player, the you will wish to use somewhat larger gaps.
The above won’t make you a gapping guru, but you will be able to adjust every reed for free vibration and and to know which direction to go to make desired changes.
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