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Bungee Cord Sound Post Retainer
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Irv
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October 6, 2018 - 1:40 pm
Member Since: December 23, 2017
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A while back I posted a thought that a bungee cord with a plastic end ball (sold by Harbor Freight and others to secure garden hoses and other loose objects) might be used to secure a sound post on a violin while changing strings.  I thought that all of the resulting structural distortion would occur in the front and back plates, and bocaholly thought that the ribs would take up the deflection.  I had the necessary measuring instruments, so a simple experiment was made to determine what would happen.

The test violin was a 1/2 sized German made violin without a sound post.  A piece of painter’s tape was placed across the top of the violin just under the circles on the upper portion of the f holes in the center bout area of the violin.  The middle of the top plate was marked on the tape.  The following measurements were made.

      97.03 mm across top

       54.46 mm front to back

A simple loop knot using both strands in the bungee cord was made to secure the ball to the cord so that the unstretched length was the width of the c bout.  The cord was stretched around the violin at the c bout and the violin body distortion was measured.

     97.25 mm across top

      53.17 mm front to back

The stretched bungee cord decreased the distance between the top and bottom plates of the violin, so it should be capable of securing a sound post while changing strings.  The deflection of the front plates was compensated by a corresponding expansion of the ribs.

While I was at it, I expanded the loop and secured it in the depression under the points on the lower bout (reasoning that this was a little closer to the sound post).  No noticeable deflection was obtained.  This is obviously a much more rigid area on the violin.  It also is a region of solid material on the front plate, since the f holes do not extend this far down.

A picture of the bungee cord retainer follows.  Various sizes of violin can be accommodated by the use of a longer length of bungee and knot as appropriate.E97FE824-50A5-45C2-8722-FBE0396AD93A.jpegImage Enlarger

Research is what I’m doing when I don’t know what I’m doing.  —Werner von Braun

I consider any plane that I design a success if it rises high enough to crash.  —RA Heinlein

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Gordon Shumway
London, England
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October 6, 2018 - 2:05 pm
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I'm pretty mystified by soundposts. I assume from what I've read that they are loose and just held in place by friction from the compression. Why aren't they glued in place?

I always wanted to be a juvenile delinquent but my parents wouldn't let me.

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bocaholly
Boca Raton, Florida
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October 6, 2018 - 2:34 pm
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Very cool, @Irv. Glad it worked out.

A+ on knot proficiency. You're now ready to sail away with these folks...
aQAAAPZAADAcW1tb2QAAAAAAAAGEAAAoC4AAAAA0OXuAAAAAAAAAAAAAAAAAAAAAAA=

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Irv
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October 6, 2018 - 2:45 pm
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Hi Andrew Fryer and others.  I have seen a few string basses with a retaining cup positioned around the sound post on the plates for ease of positioning, but nothing like that for a violin.  A small shift in position and length of the post can make a huge change in the sound, something that I have not yet tried.  Tradition is also big in violin making.

Research is what I’m doing when I don’t know what I’m doing.  —Werner von Braun

I consider any plane that I design a success if it rises high enough to crash.  —RA Heinlein

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Fiddlerman
Fort Lauderdale
October 8, 2018 - 12:21 pm
Member Since: September 26, 2010
Forum Posts: 14138

It is normally not a problem to change all the strings at the same time without having the soundpost falling. However, it's recommended to change one string at a time to retain enough tension on the top of the instrument.

I can take off all the stings and squeeze the sides of the instrument and the soundpost will fall almost every time if I want it to.

"The richest person is not the one who has the most,
but the one who needs the least."

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