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We often hear of the danger of having the untensioned downbearing on a soundboard greater than the crown due to it causing the soundboard to "oilcan" resulting in negative downbearing. I do not recall this ever being challenged, but the more I think about it, the more I doubt the analogy is accurate. Not to say that having so much downbearing is a good idea, nor that you may end up with negative downbearing on some parts of the soundboard, but for the soundboard to act like an oilcan? No, I don't think so.

When pushing on the bottom of an oilcan with the spout down, the bottom is concave against your thumb at first. As you push harder, the outside surface of the metal is under compression while the inside is under tension. There is more and more resistance as you push further until it suddenly gets easier as the inside of the metal comes under tension while the outside is under compression. This is the point where the metal is now convex against your thumb. The shape has inverted. Yet the metal is still pushing against your thumb, just not as hard, until you allow the metal to regain its shape.

Now consider how the analogy with a soundboard doesn't quite work. When the soundboard supposedly "oilcans", the result is negative downbearing. The soundboard is pulling the strings downward. But with an oilcan, you must continue to push on the can to maintain the oilcan effect!

It could be argued that it is still like an oilcan because you could push on the outer circumference and get the metal to invert. And so excessive downbearing on the perimeter of a soundboard could cause the soundboard to invert resulting in negative downbearing in the middle of the board. But would the soundboard need to invert like an oilcan, and more to the point could it? I don't think so.

On every old piano I have bothered to check downbearing on there has been little, or even negative downbearing around the treble strut compared to higher or lower on the bridge. It's a weak point due to the bridge being notched, or at least not having a bridge cap. Does this mean the soundboard is "oilcanned", that the top of the board is under tension and the bottom is under compression? Of course not, and for those of the buttressed arch soundboard model persuasion, the case will prevent the soundboard from inverting anyway.

But couldn't a soundboard invert, like if an elephant stood on it? Well, maybe, but we are talking about the downbearing of strings on a board about 3/8" thick. I would expect on even the flimsiest soundboard that at least 3/8" of downbearing would be necessary to invert a soundboard, where suddenly it would be easier to push the soundboard down. Has anyone ever measured that much untensioned downbearing? And remember, as the strings are tensioned, the downbearing decreases resulting in less, not more, force on the soundboard - just the opposite of what is required for the "oilcanning" analogy to be accurate.

Comments?

Jeff,

As I understand it, the term is used simply to describe the appearance of the board (almost always under the bridge) under tension and DB. Personally, I cannot recall a situation where the SB STAYED that way after being de-stressed. Possibly others have.

However, the very first time (35 years or so ago) that I saw this was in a SS B that had its SB replaced. As it turned out, upon de-stressing it was found to have roughly what you calculated for DB in order to collapse the board, though it showed zero DB in its strung state. Upon correcting the situation the piano turned out to be quite awesome.

It is interesting to me that you hit the nail on the head.

Pwg

Thanks for the reply, Peter. Can you explain more what the situation with the B was? What correction was made? When strung, did ALL the strings show zero downbearing? I can only imagine this to be possible if it was that way before being strung. You say the board was collapsed, yet regained its shape when unstrung. OK, a board in a strung piano with DB would do the same. So is it that the board is "collapsed" (and what does that mean...) or that after being strung some notes have zero or negative DB?

Gosh, I hope you don't think I am trying to badger you. Looking for better understanding of what is said on the subject by striving for clear definitions.

Jeff,
I think your approach of trying to understand soundboards is from the wrong perspective. Which is from a mechanical point of view. It would be very similar to judging a singers appearance before hearing the sound they produce.
Listen first, and then the board will tell you if its working properly or not.
What I listen for is Messa di Voce. If a soundboard has any problems such as loss of crown, excessive downbearing, or age related fatigue, then messa di voce will have problems. It will be either non existent, weak, or pulsating and diminishing. Crown in itself is not a complete indicator either. The main principle of crowning in musical instruments is to get acoustical intensity by creating internal forces. When soundboards are made with the crown parallel to the grain, the soundboard is put under tension. When a soundboard is crowned perpendicular with the grain, compression is used. When a soundboard is carved, a soundpost is used. This is why piano soundboard repairs offer diminishing returns, because they only repair the mechanical aspect, but do nothing towards regaining messa di voce.

Thanks you Mr. C. Yes you are correct. I am looking at this from a mechanical point of view because that is how the "oilcanning" of soundboards has been described. And perhaps you are also right that this is the wrong way to try to understand soundboards. But I am not trying to understand how soundboards affect the tone. Rather I am questioning the analogy of "oilcanning" when parts of a soundboard exhibit zero or negative downbearing.

I have to admit I had to look up the term Messa di Voce. It made me smile. I remember in HS band competitions, while in the waiting room, the other baritone horn players brushing up the most difficult passages. Instead I would slowly play scales, a long 8 counts each note, from pp to ff back to pp (Messa di Voce). It drove the others nuts and I believe made them less confident. It wasn't my purpose even though it was entertaining. I just didn't want to lose my own confidence by making a bunch of mistakes just prior to the try-outs.

Jeff,

The piano exhibited the following symptom:

1) INCREDIBLE tuning instability

2) Sustain was somewhat short...not a pleasing piano to play

3) Hammers were ROCK HARD, had been juiced to the max, impossible to get needles into

4) DB measured while strung showed no measurable DB...appeared flat

5) Underside showed inverted crown under the bridge, positive crown elsewhere

At some point I had replaced the hammers (I believe this was before I started looking at DB and crown). The result was that there was no volume in the piano. I ended up having to juice them to the max JUST LIKE THE PREVIOUS SET.

After I found the DB/crown situation I pronounced the piano as DOA needing a rebuild, on the ASSUMPTION that the soundboard had collapsed and was useless. This was a school and they opted to wholesale the piano to a rebuilder colleague of mine (I made sure he knew it was a basket case) and just get another piano.

One must bear in mind here that I had only been in the business about 10 years at this point and my diagnosis and rebuilding abilities were not what they are now.

Well, i got a call from him sometime later after he had fixed it. He wanted to thank me profusely as the only problem with it was that it had MASSIVE DB and was "oil-canning" the board, resulting in the symptoms I described. He said all he needed to do was fix the DB (by shimming the aliquots), and the piano was fantastic! He made a lot of $$$ on that piano.

I have subsequently learned that this symptom picture (as described above in total) is an indicator of too much DB, uneven DB, or a combination of both. I have proved it several times and suspected it several more times.

Ask away!

Pwg

"Ask away!" Thanks, I guess I will.

The 2nd hand info about the SS-B just defies logic in my mind and so I must remain skeptical. Sure, I agree lack of DB can result in instability and a thin tone, regardless of the reason. But for there to be no DB (anywhere?) until the piano is unstrung, and then there is massive DB??? Perhaps something was done with the nose bolts...

But you say you have proved similar symptoms to be caused by too much DB, etc. OK, can you give one or two examples?

The DB was squashing the board at the bridge, giving the ILLUSION of no DB. It had GOBS of DB...too much, such that it was distorting the board and it would convulse wildly with the slightest change in humidity. Also, there was so much pressure the the board could not vibrate properly which was the reason for short sustain and lack of volume with normal hammers (thus the need to juice them up to get anything at all like a B).

At any rate, this and other cases have caused me to conclude that it is impossible to get a TRUE picture of the DB situation in a strung piano. It needs to have the pressure off, look at the deflections, look at the crown...then decide.

Pwg

You asked for examples after this one. Initially I did not come to my conclusions based on just that one experience. I was baffled for a while.

Some time later, actually it was when the space shuttle exploded (I remember because I was stringing the piano when I heard the news), had opportunity to restring a fairly new SS M. I had been tuning this piano since brand new, but was perplexed by its seeming extreme tuning instability (even with humidity control). I had no explanation other than still kind of new, and it's just that piano. Due to an accident with a cleaning person that turned the bass strings black, the owner said restring the entire piano (money was no object). So it came into the shop.

Well, upon de-stringing I found what I considered to be excessive DB throughout the treble bridge. Not yet correlating the previous piano I simply reset the bearing to what I saw fit (basically equal to the amount of available crown) by shimming the aliquots, and restrung it.

To my surprise, this piano soon became VERY stable in its tuning, and seemed to blossom in its tone. The only thing I could attribute it to was correcting the bearing. That was when I started putting 2 & 2 together. Could they be related? I believe so.

Pwg

Hmm... interesting.
Never thought that downbearing issues could affect tuning stability.

I had a piano once that was completely dead. Took the strings off, and the soundboard was completely flat. Popped the board out, and immediately had a full crown. My guess is that it was under full compression the whole time, but it could never crown up because of it being glued in prematurely.
-chris

The way I look at it, Keith, is by imagining two people pulling on opposite ends of a rope. If I grab the middle of the rope and pull sideways an inch, it takes very little effort to draw the two people a little closer. But the next inch takes more effort, and if I want to deflect the middle of the rope a foot, it takes even more effort.

And so when there is little or no DB it takes less movement of the soundboard to change the tension of the string than if there is reasonable DB. In a case as Peter just described, where excessive DB in one part of the soundboard causes less DB in another part, a change in humidity would change the string tensions very differently from one part of the scale to another. Same thing if you are pitch raising after such a humidity change.

Speaking of which, when I pitch raise (aurally), I generally give more of a pitch raise around the treble break because I can count on that area dropping in pitch more than the lower tenor and higher treble as I go up the scale. That is even with "distributing" the pitch raise by raising one string on all the unwound F's, then F#'s, etc. I rough in a temperament octave first, of course.

Thanks, makes much sense, especially correcting the DB in the treble with aliquot shims. Wouldn't work in the tenor, of course - the strings are too long. Which tells me that your friend who worked on the SS-B did the same, although at first read it sounded like the DB problem was across the entire scale and the shims were installed across the entire scale to correct the problem, too. That made me doubtful of the narrative...

So I am gathering now that the "oilcanning" as described by you and others is a temporary inversion of crown at the bridge, mostly in the treble, caused by excessive DB in that area. This also causes a temporary loss of crown and DB in the tenor affecting the tone and stability. But the problem could exist without any inversion of crown. Careful measurement in either case would surely show DB in the treble.

I can't seem to post a message. Don't know why.

Pwg

Okay, well that one went through. I've attempted several earlier that triggered a "database" error. I'm not fond re-writing substantial posts. I will try again later.

Pwg

I recently removed a board from a small SS
Prior to removal with strings on and up to pitch there was very slight (almost not measurable) bearing in the treble, everywhere else zero.
There was zero crown on the board - used straight edge to detect.
The board was not inverted, it was as flat as my straight edge.
Now after sitting for a couple months (out of the piano) the board is very inverted - or oil caned.
Scratches head wondering why it didn’t revert and give some positive crown?? Why negative???
My assumption is the board suffered compression set, failed, lost any support capability it may have had and the ribs now dominate the shape of the assembly.
Why flat sawn ribs would move in the opposite direction I could only guess but it does suggest that this sort of design has an undesired effect of working against crown.

I’ll add one more bit of data for what it’s worth: the board was removed during hot dry California Sierra Foothills weather and now it’s cold rainy and damp.

I think its a poor assumption. First of all, you have no idea whatsoever if it ever had extreme compression. or much compression at all. And relating to soundboards, Compression set, may just be a theory, otherwise Craig Hair is using damaged material. I remember looking at the numbers of the compression values that it would take to create compression set in spruce, and I recall thinking that it was pretty near impossible. But in tension across the grain Spruce is very weak. Regarding the ribs inverting, i'd be thinking the Bellyman were distracted that day, and installed ribs that were not properly kiln dried.

Compression crowned boards are known to lose crown faster than any other method of crowning.
They all (Every method of crowning) do eventually.
Many times a new piano right from the factory has flat dead areas caused by excessive compression as a result of the crowning process.
I haven’t done the math for a long time but the elastic limit (FPSL) for Sitka vertically cut, across the grain is very low. The compression crowning process can easily reach this limit before the piano is strung.
My assumption is not so poor.
Consider the 1 percent compression limit rule: compressing wood beyond its FPSL will permanately deform it perpendicular to grain.
And the ribs are designed to restrain the board in order to induce crown. There is no bearing load capacity designed into the system.
Once the elastic limit is exceeded, the board has very little resistance to being oilcanned in either direction.

Everything Peter is reporting I have experienced as well.