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jdowning
Oud Junkie
Posts: 3485
Registered: 8-2-2006
Location: Ontario, Canada
Member Is Offline
Mood: No Mood
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In the meantime.
The latest MIT paper on the air resonance power efficiency in violins has a supplementary paper freely available here:
http://rspa.royalsocietypublishing.org/content/royprsa/suppl/2015/0...
... and includes a diagram at the beginning comparing the air resonance frequencies of a long, narrow S shaped violin soundhole with a round
soundhole of the same area and a round soundhole of the same peripheral length. Not surprisingly - all else being equal, same air volume, resonance
chamber stiffness etc. - the larger diameter soundhole (same peripheral length) results in a higher air resonance frequency than the smaller diameter
soundhole (same area).
The MIT results show an air resonance frequency for the violin S shaped soundhole falling between the round soundhole max/min values.
Using my #1 resonance chamber, I plan to run a series of trials to compare the measured air resonant frequencies of a simple long, narrow rectangular
soundhole (of variable length) with round soundholes of equivalent area and peripheral length - just to see if similar comparable results might be
obtained. See attached sketch - soundholes drawn to the same scale.
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jdowning
Oud Junkie
Posts: 3485
Registered: 8-2-2006
Location: Ontario, Canada
Member Is Offline
Mood: No Mood
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An update on resonance test chamber #3.
A 2cm (3/4 inch) thick plywood flange or rim has been cut, fitted and glued to the chamber with waterproof construction adhesive (polyurethane). This
will allow the test sound boards to be clamped to the chamber.
The chamber exterior has been spray painted for cosmetic reasons - green colour because that is what I had available in stock!
A water drain valve has been added to the bottom of the chamber for convenience as water will be used to vary the chamber air volume for the
trials.
I found a welded steel support frame - made for another project - that has been sitting outside for a few years so is pretty well rusted but will
serve the purpose with some wooden blocks added to fit the chamber. I might even wire brush and paint the frame to look pretty - although not
essential.
All components and materials have been found in stock so there has been no expenditure apart from a little of my time.
The air volume of the chamber empty works out to be a maximum 36,825 ml (cc) with an internal diameter at the flange of 40.2 cm. The volume was
accurately determined by filling the difficult to calculate dome end with a measured amount of water (8,650 ml) and then by calculating the volume of
the cylindrical portion from measured diameter and height.
This larger diameter chamber is to investigate the air resonance frequency responses of multiple sound hole geometries - particularly the large/small
diameter configurations typically found on ouds and on some types of historical lutes represented in the iconography (but not in surviving examples).
The testing will be more complicated than single sound hole acoustic testing due to the increased number of variables - multiple sound hole diameters
and spacings with different air volumes.
I will likely simplify test parameters by focussing first on single large sound hole/single small sound hole configurations - just to see where that
might lead.
Not a top priority project for me at the present time, however.
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