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Why sound post fit matters


actonern
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Having full contact of the post ends with the top and back plates is established wisdom about getting the best from an instrument.

 

I'm curious why that should be.

 

For example, when playing a natural harmonic on a string, the width of one's finger touching the string doesn't matter... why should it be so critical to good sound for a post to have full contact across its diameter, do you think?

 

E

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Ok, I'll have a go at this.

 

Usually it's just an established fact of set up - not too tight - not to loose, and well fit ends.

A soundpost that is standing up straight with no string or bridge pressure pushing down on the top. But barely, it's just snug. Not really tight, not really loose. The ends are usually fitted as well as possible to the interior plate contours, and the closer they are to matching the top and bottom surfaces, the better off they are for tone - generally.

If you don't believe it, go ahead and try different ideas and see what happens. It's simple to find out either way.

 

That is, most usually...

There are occasionally times where tweeking a well fit post, or taking it slightly off kilter, can be found to cause a real tonal improvement.

but that doesn't change the basic requirements for the vast majority of cases, really. 

 

Who knows? you may find out differently.

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While this subject will no doubt be argued extensively, I feel that the most likely explanation is that flush and orthogonal equals "perfect" in the minds of most woodworkers (it would probably not occur to most of us to do anything else without excellent reason), and I certainly see no harm to be done by careful fitting of anything.

 

Edit--my point is that fine fitting was probably done in the best fiddles from the first insertion of a sound post on without the matter being further considered, and that the acoustic and structural arguments are all post hoc.   Good workers do good work.  It never occurred to Da Salo and the others to hammer in a rough ended random length of doweling at an angle just because they could, and we've quite rightly copied them ever since.

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One non-acoustic reason for having full post contact with the top is to minimize or avoid damage to the inside of the top.  More post area in contact with the top means less post pressure on the top (because pressure is force per unit area), and less post pressure on the top means the post is less likely to dig into, gouge out and otherwise damage the top.

 

This reason does not apply to the back, because maple is harder, and thus more resistant to post damage, than the top.

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Carl is right. If the post doesn't fit all around, then the resulting gap may cause a disharmonic in the system. The system won't work as intended. The different problems(*) that may come about because of this includes intonation problems, buzz at certain frequences, whistling strings (especially the E string). That's why it's important to insert the post correctly, since it becomes increasingly difficult to fit the post correct once damage is inflicted to the top.

 

(*) These problems may also be caused by other factors. But the soundpost is a common trouble spot.

 

 

 

P.S. Linkman's image shows both damage to the top and a poor fit. D.S.

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I don't know why the fit is important for sound, I just know that it is.

One thought is that when one edge is raised, versus total contact, the net or average position of the contact has changed quite significantly. One edge tilts up, and it's as if the soundpost had been moved a couple of millimeters. Another thought is that the wood is elastic enough that the amount of contact area will influence how much the contact area cyclically deforms with vibration.

 

Curious1, do you have any of your swivel-end soundposts made up yet? I'd love to try one!

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No, not yet David. I do think they would be useful and it is hard to imagine why/how they would be less effective than a spruce sound post if designed properly.

Personally, I think fit is the least important element in sound post setting. Position, tension and fit would be my order. A poorly fit post WILL fit eventually. See linkman's post. The top will fit to the post! Also the load will distort the top to fit the post. Seen that plenty of times! These are reasons enough to fit the post well.

Tension will also fit over time! Too tight or loose will eventually settle in. The arching will distort off course but it will all work out.

Position is the only thing that is a constant here.

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Interesting, Curious1.  I had this idea that a certain amount of tension was important to maintain.  The effect of this tension over time, as you say, was that the instrument would deform and equilibriate, at which point the tension wasn't tonally ideal,and a longer post had to be fit.  Cycle repeats until the post area becomes the lowest point of the arch, and the instrument then heads for the sand bags, and the process repeats again.

 

Is that not right?

E

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Run your finger lightly over the surface of the plate just behind the bridge. If you feel a distinct ridge, I would have the post inspected ASAP. This, even after a "competent" shop cuts and/or fits a the post. In bigger shops you may really never know who fit the post unless you personally see it done. A well cut/fit post should feel relatively smooth and solid from the top surface.

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I have seen sound posts that have been in a violin for 30-40, and more, years and still work perfectly. After the initial creep of arching and wood I don't see any reason a violin should continue to move. If you put a "good" post in after that initial settling in it should be good for a long long time. (Assuming the violin is properly made).

If you keep putting in longer and longer posts , for whatever reason, you are going to distort the arching and eventually need the sand bags.

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I agree with curious1.  With the exception of newly made fiddles, or one that moved into a drastically different environment than it was in previously, I tend to replace bridges more often than sound posts...  and if the player takes proper care of those and there isn't some accident or fault in the material, bridges can easily last a decade or more.

 

I guess I don't think of fitting a post in priority order as Curious does... I work the post into position while fitting the ends and adjusting the tension...  and if one particular aspect is off, I toss the post into the drawer of sin and start over.

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Really!! One would want to have Very sensitive fingers to feel the difference between a not well cut and a well cut post from the top surface IMO. I'd be interested to know if anyone else has an opinion on this.

 

Yes, I too would be interested in knowing if a relatively sharp ridge in the top plate above the post is acceptable!!

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I agree with curious1.  With the exception of newly made fiddles, or one that moved into a drastically different environment than it was in previously, I tend to replace bridges more often than sound posts...  and if the player takes proper care of those and there isn't some accident or fault in the material, bridges can easily last a decade or more.

 

I guess I don't think of fitting a post in priority order as Curious does... I work the post into position while fitting the ends and adjusting the tension...  and if one particular aspect is off, I toss the post into the drawer of sin and start over.

Aaron Rosand had the same Wurlitzer bridge on the Kochanski for at least 40 years. That may be an exception but I don't see why a bridge shouldn't last forever too except that players don't take care of them.

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A properly set post has to accomplish all three criteria. But if I had to determine which is most responsible for the best sound I'd put them in that order.

Yes, you can feel a badly set post from the outside and see it.

If I was teaching someone how to cut a post I would reverse the order.

This was along the lines of my question in the bass bar thread. If the plateau bar does nothing to improve the sound, what is the point? Bass bars can be made to be structurally sound in many different shapes. I agree a post should fit perfectly, but if the sound is bad, position and tension should be considered and a new well fitting post made to optimize the sound.
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A properly set post has to accomplish all three criteria. But if I had to determine which is most responsible for the best sound I'd put them in that order.

Yes, you can feel a badly set post from the outside and see it.

If I was teaching someone how to cut a post I would reverse the order.

 

Besides a poorly cut soundpost, when posts become too short (especially as new instruments age), some shops may still attempt to use it rather than cut a new post. This will then require the post to be set obliquely. Not only will this result in a poor soundpost set, it will also result in the palpable ridge I referred to. Why is this relevent to the OP? You need to know a poorly cut post to know a well cut post. And a well cut post may not remain a well cut post, especially on new instruments.

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In a recent discussion on bows -Physics of sound production - does the bow stick actually make a difference?, in post 11, David says

 

'Personally, I think that many instrument "sound adjustment" techniques are effective, because they influence bow "slip-stick" properties more than basic resonating properties of the instrument.  That's partly based on an observation that an optimized adjustment can vary, based on which bow is used. No, I'm not prepared to take enough time away from income producing work to attempt to prove it to skeptics.'

 

This rings true to me, even though I might not set up an instrument for a specific player or bow.

 

With an ill fitting post, there never seems to be that bite and response from a fiddle. 

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Assuming a correctly fitting post I am still interested in how the pressure from the treble bridge foot is distributed via the belly over the surface area of the top of the post( especially in celli). For instance I feel that the further back from the bridge the post is positioned the more weight of the bridge foot could be distributed on the front edge of the top of the post ( i.e the edge nearer the bridge). ....... if not at all times at certain times during performance

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