How tight should a sound post be

[Brad H]

1 minute ago, Jerry Pasewicz said:

It is exceedingly rare to ever install a shorter post.

I am confused.  By "shorter", do you mean a post with less tension?   

[Evan Smith]

On this little box with strings, if the strings were supported at the bridge line so that there was no downward pressure on the belly,, the post would fall down as it was brought up to pitch.

From one parameter the post gets looser as the strings are tightened,,,,,

From the other parameter, the  downward pressure of the bridge keeps the top mashed against the post which is being pushed up by the back, because of the relief that the f-holes provide, the top has more freedom of movement than the back.

After a period of tension the ends of the plate raise and the post becomes loose, the sound gets soft, it needs a longer post,, the top gets shorter,, and the top will eventually get narrower, as the cycle slowly raises the top.

On a thick top the post can find a nice place to work much farther behind the bridge than on a thinner top,, this is one trick to getting some better sound out of junk,,just something to remember.

Here is one of my crazy experiments,,, this fiddle used the same post it had in it the whole time, go figure, there is a lot that is not understood,  I cut this fiddle open on October 25 2015, and glued it back on March 24 2018,  The first and last post will tell the story,,, as usual a bit of derailment here and there to wade through,,,,

 

[GeorgeH]

3 hours ago, jacobsaunders said:

I am not aware of the “scientific study” you cite, neither am I sure what you mean with “lateral flexing”. I was talking of longitudinal compression (of the belly) which surely happens. As anecdotal evidence, you may notice that old violins invariably have bellies that are 1 or 2mm shorter than backs after 200 or so years.

"lateral flexing” = "longitudinal compression" 

That "longitudinal compression" happens is not what is being debated, nor whether or not that force has effects on the table over the long term. What is clear is that the "longitudinal compression" does not reduce the pressure of the top on the sound post when the strings are tensioned. The "longitudinal compression" pushing the top up is not greater than the pressure of the bridge pushing down.

All evidence points to the fact that the compression force caused by string tension increases the table pressure on the sound post. Long term micro effects on the table by "longitudinal compression" may be partially the cause of a sound post to eventually become too short, and fall down when string tension is removed. A violin of 200 years old has surely had several new sound posts over the years.

[PASEWICZ]

1 hour ago, Brad H said:

I am confused.  By "shorter", do you mean a post with less tension?   

Nope, I mean a post with less length.

[Brad Dorsey]

Commonly referred to as shorter.

[PASEWICZ]

1 minute ago, Brad Dorsey said:

Commonly referred to as shorter.

[Brad H]

42 minutes ago, Jerry Pasewicz said:

Nope, I mean a post with less length.

OK, so your initial response to my post was a general statement saying that, "Over time, it is rare for luthiers to fit shorter posts in the same fiddle"?

If so, since you made it in reference to my description of fitting a post, would you care to be more specific as to how this relates to my description?   I posted my description because I wanted feedback, so don't worry being critical.

 

[PASEWICZ]

1 minute ago, Brad H said:

OK, so your initial response to my post was a general statement saying that, "Over time, it is rare for luthiers to fit shorter posts in the same fiddle"?

If so, since you made it in reference to my description of fitting a post, would you care to be more specific as to how this relates to my description?   I posted my description because I wanted feedback, so don't worry being critical.

 

Certainly I will, I am mulching at the moment.  And no worries, I do not have a problem being critical.

[David Burgess]

3 hours ago, Evan Smith said:

After a period of tension the ends of the plate raise and the post becomes loose, the sound gets soft, it needs a longer post,, the top gets shorter,, and the top will eventually get narrower, as the cycle slowly raises the top.

I see this quite differently. Most of the need for progressively longer posts (most marked on new instruments which have just been strung up for the first time), results from distortion of the back, with the portion of the back near the soundpost bulging outward on the treble side. On older instruments, this distortion of the back is clearly evident on cross-section CT scans, if one hasn't learned to notice this just by looking at the instrument.

[PASEWICZ]

6 hours ago, Brad H said:

This is a topic which has been on my mind lately.  I will often get ahead of myself and set a post early on in the setup process.     Although I avoid bumping the violin, I never worry about the post falling while moving the violin around in different positions for subsequent setup tasks...and sometimes wonder whether I set them with too much tension.  I tug them into place with a firm pull but try to be vigilant about never raising the top.   When adjusting a post with no string tension, I want there to be enough tension that taps with the setter produce small post movements.   I also do the c-bout squeeze thing that I think Brad D. mentioned in a past thread - the post should fall over when the violin is turned sideways and a slight squeeze is given to the c-bouts.

Any comments?

 

Hello Brad,

    My comment about never putting in a shorter soundpost:  I was taking from your description that you were doing new set-up work which makes my comment more relevant as instruments stretch a great deal early in their lives.   Even without the new set-up, your description of being able to drop the post with a squeeze is a little loose, especially as we are headed into summer where the RH will cause the top and back to expand.  I have found that a soundpost needs to have a minimum tension in order to get predictable results from adjustment, and to hold a reasonable adjustment through small changes in RH.  Unfortunately, I do not know of a shorthand description of what that tension is, and I have lamented so in conversations about learning from the internet.  If I could say one thing that would help, it would be to closely control the RH in the workshop, and when you go to adjust an instrument where you set up and adjusted it before and recently, make note of the RH where the instrument is currently living.  Through time the changes in adjustment that correspond with the changes in RH will become clear.....higher RH relating to “looser” soundposts.  Once again, it is essential to have a good initial tension, but eventually cause and effect of adjustment will be clearer.   

[Don Noon]

1 hour ago, David Burgess said:

I see this quite differently. Most of the need for progressively longer posts (most marked on new instruments which have just been strung up for the first time), results from distortion of the back...

Although I mostly agree with this, I see it in a third way:  whatever is moving around on a new instrument, the soundpost will not change length, and therefore movement of the top and back (in the soundpost location) MUST be exactly the same.  If the soundpost fit becomes looser, then it means that the stresses in the back have relaxed more than in the top, or in other words, the maple has a higher creep than the spruce, and the distortion is more permanent in the maple than the spruce.

That matches what I understand about the wood.  Spruce is far more creep-resistant than maple, especially if you are dealing with highly figured maple.  And yes, I have noticed the bulge in the backs of old violins.

[Brad H]

Thanks, Jerry, for the explanation and info on RH.   

 

 

[Danube Fiddler]

What´s about this distortion effect, whicht lifts easily and often lifts the lower f-flap until 5 mm in old instruments ? The axis of this leverage effect should most probably be in the top area directly below the right bridge foot or between bridge foot and post. Not directly above post, because the post-surface area is to big and therefore not suitable as axis.  If axis should be so, as I assume, the post area of top will also be slightly lifted.

[David Burgess]

30 minutes ago, Danube Fiddler said:

What´s about this distortion effect, whicht lifts easily and often lifts the lower f-flap until 5 mm in old instruments ? The axis of this leverage effect should most probably be in the top area directly below the right bridge foot or between bridge foot and post. Not directly above post, because the post-surface area is to big and therefore not suitable as axis.  If axis should be so, as I assume, the post area of top will also be slightly lifted.

That happens too, and can also be clearly seen on older instruments which haven't been re-arched recently.

[scordatura]

Overall what do you all feel is more important in the effect of sound post fitting--tension (length) or position? Yes they are interdependent...My experience is tension/length has overall more effect on sound.

[Michael Jennings]

4 hours ago, David Burgess said:

That happens too, and can also be clearly seen on older instruments which haven't been re-arched recently.

David, could you expound a bit on "re-arching", or direct to information[link] to how this is done?

I have a fiddle of unconfirmed origin but appears to be early 19th century. Very extensive, well done repairs, including replaced wood around both ff's and crack repair, SP patches belly and back. The belly arch is "distorted" so that the high point is under the treble foot of the bridge. I suspect that the distortion is partly due to a "long" SP that has been in there for decades.

Play well and sounds sweet and probably not worth a lot more attention...... but I am curious .

[David Burgess]

18 minutes ago, Michael Jennings said:

David, could you expound a bit on "re-arching", or direct to information[link] to how this is done?

 

This is typically done by affixing the top or back to a flat piece of plywood. Then using various sorts of shims, clamps, and jackscrews to  push and pull the shape closer to where you want it to end up. Then making a plaster mold while the plate is still jacked into shape. Making remaining corrections by adding or removing material from the plaster cast. Then using heat and some moisture repeatedly to get the plate to conform to the mold, and stay in that shape.

[Michael Jennings]

^^^^^Thanks.....

[Gavin R]

I am new to setting up violins hence I have checked out this forum. I am a physicist so my view of the mechanics would be that the tension on the strings will compress the body slightly along the length of the belly and back. This must happen due to the longitudinal curvature of belly and back. I would think the flexibility of the glue and compressibility of the wood would be a factor in the bodies resistance to this longitudinal compression as the belly/back near the ribs will compress as the middle of the belly bows outwards. The exact thickness, design, wood type/ age of instrument, humidity, stiffness of the bass bar (and flexibility of glue?) must be critical in reducing any change of shape. All these factors would affect the increase in distance between belly and back when strings are tightened. However, the strings also push down on the bridge which would tend to counteract this expansion. This balance of forces is surely why violins last hundreds of years compared to say guitars where the string tension eventually destroys the top sound board.  The construction variations and the wood / glue used would affect the balance of the two competing forces on the belly. Some belly’s may bow outwards under compression more than others and perhaps some not at all (unlikely I would think). This would depend on the uncompressed curvature of the belly which must vary slightly between violins. It does, I have two old violins in-front of me and they are different. The force down on the belly from the bridge would depend on string selection, bridge height and length from end pin to nut. In terms of the mechanics it’s the angle the strings deviate by as they pass over the bridge that affects the downward force.  With all these variables it doesn’t surprise me at all that some sound posts fall when you tighten strings and some get tighter when you tighten strings. I guess the best method has to be checking the ‘tightness’ once tuned as this is where it’s obviously going to count. Other than this there seems to be little point in employing the same routine for all violins which will result in a massive variation in final sound post compressive force.  The only other logical method is of course trial-and-error to see what sounds best. Which brings us back to the original question, how tight should a sound post be ! 

I have all the equipment so I am going to experiment. I have heard that I need a ‘swear box’ if a newbie to fitting sound posts’!

And by the way, I think the sound post may have multiple functions. It creates a node in just the right place to alter the harmonics you hear. It will change the resonant frequencies of the body. It may damp unwanted resonances. It may persuade belly and back to vibrate at the same frequency? It certainly must also transmit vibrations directly to the back of the violin. This vibration will have a phase difference depending on the density of the post (and compression). This phase difference will influence how the belly and back oscillate with respect to each other and the vibrating air within the violin. The phase difference is caused by the time it takes sound to travel through the post and arrive at the back. This phase difference is different for different frequencies and could cause reinforcement or cancellation of certain frequencies. This is why different tightness/ thickness/ material of sound post causes the changes in sound that many have described. 
Its quite funny that I had an old broken violin many years ago and I thought the post was just a structural strut!

[Dr. Mark]

On 10/27/2024 at 8:00 AM, Gavin R said:

I am a physicist

That's not enough for admission to Clown College, although you may now go to a good restaurant in a bad suit and not get thrown out if you carry a photocopy of your GPA.  You still need: 

1. To illustrate your mastery of forming pre-conceived notions - to be evidenced through prior work, testimony, or practical examples which will be selected by the College.

2. Comparatively little experience with, or knowledge of, the subject matter of questions you choose to answer.  Note answers of sufficient detail will make you eligible for a bonus point towards your admission.

3. Unfamiliarity with prior research, including computer modelling and various failed alternative explanations

4. Proof of your ability to draw conclusions prior to carrying out experiments or to forego experimentation altogether.  Alternatively you may illustrate for the board a minimum level of experimental ineptness.

5. Evidence through written examination by the board that you thoroughly understand the following: asking any pertinent question is a sign of weakness and incompetence

If you can provide all of the above you will be eligible for an interview with the College and, if admitted, forfeit your right to be taken seriously.  I see that an essay follows your opening statement and perhaps here we'll learn more...

[Pierrick584]

On 4/28/2018 at 7:36 PM, PASEWICZ said:

It would be nice if we had some sort of screw mechanism or gauge this....

if I remember right, animanova makes carbon fibre soundpost that can have its tension adjusted while in position. so it kinda exist already

[Michael Darnton]

7 hours ago, Dr. Mark said:

That's not enough for admission to Clown College, although you may now go to a good restaurant in a bad suit and not get thrown out if you carry a photocopy of your GPA.  You still need: 

1. To illustrate your mastery of forming pre-conceived notions - to be evidenced through prior work, testimony, or practical examples which will be selected by the College.

2. Comparatively little experience with, and knowledge of, the subject matter of questions you choose to answer.  Note answers of sufficient detail will make you eligible for a bonus point towards your admission.

3. Unfamiliarity with prior research, including computer modelling and various failed alternative explanations

4. Proof of your ability to draw conclusions prior to carrying out experiments or to forego experimentation altogether.  Alternatively you may illustrate for the board a minimum level of experimental ineptness.

5. Evidence through written examination by the board that you thoroughly understand the following: asking any pertinent question is a sign of weakness and incompetence

If you can provide all of the above you will be eligible for an interview with the College and, if admitted, forfeit your right to be taken seriously.  I see that an essay follows your opening statement and perhaps here we'll learn more...

 @Gavin R  Actually, I don't think that you did too horribly in your speculations. 

Regarding the original question: the proper post pressure is the one that makes that instrument work for that musician playing whatever that music of the moment is. It's pretty straightforward.

[Dr. Mark]

1 hour ago, Michael Darnton said:

Actually, I don't think that you [Gavin R] did too horribly in your speculations

God and the devil are in the details.

[Michael Darnton]

12 minutes ago, Dr. Mark said:

God and the devil are in the details.

Or one's actual experience, perhaps.

[VicM]

On 10/27/2024 at 2:00 PM, Gavin R said:

And by the way, I think the sound post may have multiple functions. It creates a node in just the right place to alter the harmonics you hear. It will change the resonant frequencies of the body. It may damp unwanted resonances. It may persuade belly and back to vibrate at the same frequency? It certainly must also transmit vibrations directly to the back of the violin. This vibration will have a phase difference depending on the density of the post (and compression). This phase difference will influence how the belly and back oscillate with respect to each other and the vibrating air within the violin. The phase difference is caused by the time it takes sound to travel through the post and arrive at the back. This phase difference is different for different frequencies and could cause reinforcement or cancellation of certain frequencies. This is why different tightness/ thickness/ material of sound post causes the changes in sound that many have described. 

The post does three things in order of importance :

1. Makes the violin louder

2. The other thing

3. Equalizes the volume through the  range.