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Sound Hole Adjustment for best tone ...
...The best advice from the literature seems to be that the tone of the sound holes ("ff" holes) should be between C and D; probably around C#, but that it should not be exactly C# as this might reinforce that note above all others.
...Now the fequency of these notes is:
C 261.63 HZ (vibrations per second)
C# 277.18 HZ
D 293.66 HZ

... I want to suggest that an ideal frequency for the sound holes (on a full sized violin) should be exactly 198 HZ.
.. This 'air' frequency then becomes a pleasing 'carrier' frequency for all the various music made on the violin.
...This frequency seems to answer all the requirements for good tone: in the correct range, not an exact musical note, and a frequency that our ears preceive as especially pleasing.
... The tone is heard by blowing across the "ff" hole opening from the side of the instrument. Each "ff" hole has its' own tone and must be shaped individually to adjust its tone. A larger open area (hole) equals a higher tone. Obviously the shape can be enlarged easier than made smaller so go about this slowly. Also remember that the shape of the "ff" holes is critical to the pleasing appearance of the instrument so they must be reshaped in such a way as to keep the overall appearance of the sound holes intact.
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... I have a whole series of special sanding sticks with convex and concave shapes to help make these adjustments while maintaining the attractive "ff" hole shape. When complete, you may want to color the inside wood surfaces with an oil paint such as 'burnt umber'. I paint it on with a small oil paint brush (after the instrument has been varnished), getting a smooth solid coating. Do not worry about getting any on the top surface of your already varnished instrument, because any that gets on the the top surface is simply wiped off with a paper towel. This leaves a pleasing rich dark shade on the exposed side wood.
... How to adjust the 'blow' sound frequency of each 'ff' hole?
..Could not be easier:
  If the current frequency is too low, simply remove material evenly from the sides of the upper (smaller) hole shape of the 'ff' hole (the side toward the scroll. Be careful to blend the shapes.
..If he current frequency is too high, simply remove material evenly from the sides of the larger (lower end) hole of the 'ff' hole shape.(toward the tailpiece end of the instrument).
..Adjust the sound hole on the other side of the instrument and you are done! ...enjoy.

David Langsather (c) 2017
Modified 10/2017
Page AA_11
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Ideal Tap Tone...
........198 HZ