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TedN

soundpost crack

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I agree with Doug about shock or too much tension but think post position can also be a factor if it is outside normal parameters. Too close to the bridge foot no, but set outside the foot on either side or more than a post's width back, yes. Also the fit of the post is definitely involved and if the top of the post is at even a slight angle in relation to the top a crack will eventually occur.

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I would add that the shock or trauma can easily be caused by a bad-fitting case that allows the bridge to hit the inside of the case.  Or forgetting to take off the practice mute before closing the case.

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The previous posters have summed it up pretty well. I might add that if a top is too thin or weak in that area, it will be more likely to develop a crack.

Sometimes you can tell the post is way too tight without touching it with the setter, just by feeling its spot on the outer surface of the top. The tighter the post, the bigger the bump that will appear above it. 

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A soundpost crack comes from the fiddle getting some violent blow, such as being trod or sat upon run over or dropped. The soundpost makes it a place more lightly to get a crack than elsewhere. I would call it a „Sollbruchstelle“ in German, which my dictionary inadaquately translates as „predetermined breaking point“

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I had one violin in my shop for a soundpost patch where you could see the crack in an emerging state. 

As a matter of fact the top of that fiddle was pretty thin.

However, if you really squeeze an ultra tight soundpost into a violin the stress will stress out the top and it will get an infarct sooner or later.

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6 hours ago, jacobsaunders said:

A soundpost crack comes from the fiddle getting some violent blow, such as being trod or sat upon run over or dropped...

I once found out, to my dismay, that a soundpost crack can also spontaneously in appear in a violin sitting in a case not set up if the humidity drops enough.

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I’ve seen someone cause a soundpost crack in a cello top by moving an already overtight soundpost toward the ff. I could see the bulge in the top straining as the post moved and then I saw the white line of the disturbed varnish suddenly appear. Upon closer inspection, the crack was all the way through and needed a patch.

I admit that I was suspicious that the spruce was a little green, but it was clear that the movement of the soundpost into a tighter position caused the damage, as it was a new instrument that was being set up for the first time. 

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13 minutes ago, Jim Bress said:

If soundpost cracks are caused by trauma or poor installation (previous post example), do prophylactic patches on new instruments have any benefit?

-Jim

This has certainly been tried as have thin maple veneers glued on the surface which have the added value of resisting the denting of the top while fitting the post. I personally just leave a smallish thicker area around the post position when making new instruments.

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