Piano World Home Page Forums Our Most Popular Forums Piano Tuner-Technicians Forum Why does a piano go out of tune?

OK, I'll try. But this is my understanding as a non-pro, so anyone is welcome to correct me.

Firstly: to be clear, I'm speaking of compression-crowned soundboards. I'm not sure whether this logic would apply to rib-crowned soundboards as well. I would imagine that it applies to any soundboard which has its grain running perpendicular to the ribs.

So this is my logic:

At higher humidity, the soundboard absorbs some extra moisture. Wood swells almost exclusively perpendicularly to the grain. Hence, the soundboard expands in the longitudinal direction of the ribs. But the ribs remain roughly the same length, because wood hardly expands along the grain. This results in tension on the ribs, and compression on the soundboard, i.e. increased (compression) crowning, which in turn results in a higher pitch.

At lower humidity, the SB loses some moisture and shrinks slightly, resulting in a slight loss of crown, and correspondingly lower pitch.

If the SB panel could withstand this cycling without taking harm, then it wouldn't crack. But the fact that it does crack tells me that at times of high humidity, the wood fibres are actually compressed irreversibly. Then, when humidity falls, the SB panel is actually too small for the ribs. This puts the SB under tension (at least locally), and the ribs under compression, and hence a crack forms in the SB.

Each period of high humidity (high crown) does a bit of irreversible fibre damage, resulting in a slight loss of crown compared to the previous year. Therefore, the pitch this summer is also not quite where it was last summer.

Once the SB is so compression-damaged that no further damage is done by high humidity, I would imagine (this is conjecture!) that the pitch would finally become stable from year to year.

Again, all of this is just a layman's understanding. I'm not a pro, and gladly stand corrected.

When a soundboard developes cracks, it's because it has shrunk enough for the crown to flatten completely, sending it concave instead of convex. Once this happens the grain of the wood is pulled apart (causing the split(s)). The ribs keep the soundboard from total collapse under the emmense pressure of the string tension, but the tuning stability is totally lost. Instead of the soundboard being a solid mass where the grain is compressed, it becomes like a springboard. The only way to cure this is to remove the strings, and insert fillets of spruce having first restored the "crown" by applying sufficient force from underneath the soundboard.

No, that is not the reason. A soundboard develops cracks because the cross-grain motion due to moisture cycling is constrained by the ribs. You could take a perfectly flat board and glue ribs across its grain, and with enough moisture cycling, it will crack. Whether there is a crown or not is immaterial.

With the greatest respect BDB - The whole reason for a soundboard having a crown is so that the grain is subjected to a huge amount of compression - this compression makes it virtually impossible for cracks to appear and also adds to the sound transference properties. Of course a flat board will split .... it's not under compression, and likewise a soundboard that has lost it's crown will split because it's grain will be torn apart by the down force exerted by the tension of the strings. Your statement that "Whether there is a crown or not is immaterial" is utterly wrong I'm sorry to say.

You can read just about any woodworking book written in the past 30 years and you will find what I wrote.

You can also go argue with Del Fandrich about whether soundboards should be compressed or not.

It does not matter to me. I have a good formula for Grem-B-Gon!

Thanks Mark R for your detailed description. I am trying to imagine and conjecture a long term process in my mind with little experience or knowledge about the material behavior, but the interplay between ribs and soundboard under the conditions is a bit unclear to me.

BDB, If a soundboard has no crown then what will stop the string compression on the bridge from pushing the soundboard and ribs downwards and tearing the soundboard apart as Jonkie said, and shearing from the ribs?

You should ask that to manufacturers that use reverse-crown soundboards. Rippen is out of business, but I think there are others these days.

...this compression makes it virtually impossible for cracks to appear...

And yet they do appear. Your explanation is incomplete unless it includes the reasons for cracking. Like which comes first, loss of crown or appreance of cracks, and why?

Perhaps the big names in piano manufacture should revise their designs of soundboard technology so that their instruments become as good as Rippen ! Oh hang on a minute though ..... weren't Rippon piano's so bad that everyone regretted buying one? I also wonder why the design of all string instruments have crowns on both the back and belly when it would be such a good idea to not have a crown and watch the instruments weaken and tear the grain apart. The idea of "reverse crown" is cloud cuckoo land.

Johnkie, I have to disagree with your concept of how cracks are formed. Using Steinway as an example, they dry their boards down to about 4% moisture content. That is just about the driest you can get a piece of wood. Then they glue on straight ribs and allow the board to return to normal moisture content. The board swells and creates the crown (being bound on one side). Even if the piano was in the desert, the sound board would not be dryer than the original 4%.

Cracks form when the board is subjected to high humidity. The board swells to the point to where the internal compression exceeds the elastic limit of the wood forming compression ridges. The wood in these ridges is crushed and when the humidity goes back down, a crack appears. The sound board may or may not still be functioning.

I think its pretty rare to see an oil-canned soundboard in my area. Even cracked soundboard pianos are fairly close to pitch, usually, and I would expect an oil canned soundboard to result in a piano that is 3-400 cents flat.

Thanks for that Bill - I totally agree with your point of view. I realise that minor cracks are due to crushing of grain fibers due to excessive humidity ...I was thinking more about the huge cracks (that you could post mail through) where the soundboard has completely lost any crown, leading to structural failure. In any case the fact remains that when a soundboard shrinks or swells, it will go wildly out of tune.

How much crown does a typical piano have? How much higher is the point where the strings pass over the bridge from the point in the same place when the strings would go in a straight line?

Then I could calculate how much losing all the crown would change the tension, length and diameter of the strings, and then also the pitch.

Crown and downbearing are two different things.

Could you elaborate? I guess what I mean is that how much the point where the strings attach to the bridge could lower.

Also, what would be an average speaking length and diameter to use for example for an A4 string? Speaking length is around 55 cm I think but I don't have anything to measure the diameter with.

A PTG conspiracy so tuners can stay employed.

if I may add to the portfolio , thank you.
the variations of building a board are numerous and the
results therefore are too.
the cracking has differnent reasons, depending on
climat, downbearing/tension, quality of wood/selection
etc.
england is a climat where boards swell easely and the
technicans have to deal with that in a very different
way then a technician in arizona.
I replaced a soundboard in a Knabe 120 years old
and was asked to see if I could copy the soft and velvety nature of this sound.
the ripps were crowned to 1cm !!! and the board was
was 7mm thin all around.
but the cracks were everywhere.
I replaced a board in a Ronish 125 years old and the crown was 4 mm and the board was 9 mm in the treble and
7 in the bass. in another grand the ripps were FLAT.
they have been glued with the prinicple of "one sided
gluing, which should have little downbearing.
the last one did not have cracks, but was 85 years old.
all boards had good material. even colored, grained etc.
seems like lots of tension will eventually take its
prisoner. i glued a board together with mateial from one log ! but had to replace on small section later
when I noticed a number of inperfections . the replacement board was 3 years less cured and cracked later.
just a few thoughts that could shed some light n the reasons of cracking soundboards. ( there are more ! )

but their relation is essential !

there are reasons and they vary from climat to climat.
a pdf file is available for serious tuners that have
their customers best interrest in mind.

some big cracks occur close to the glue jonts which inticade too much pressure in the gluing process.
crushing the softer summer fibers.