Piano World Home Page Forums Our Most Popular Forums Piano Tuner-Technicians Forum FAQ - Tuner-Technicians Forum Fascinating Steinway Phenomena

Yes, compression can and does damage wood. The most extreme and visually dramatic occurrence of this damage in the piano soundboard are the compression ridges that often show up in soundboard panels crowned in this way. As the name implies internal compression is literally forcing the latewood layers up or down as the earlywood fibers fail in shear as the force from internal compression becomes greater than they can withstand.

Even without exhibiting this kind of dramatic failure a wood member held in cross-grain compression over a period of time will physically change shape. In other words a panel that is held in compression across grain will gradually lose some of its overall width. If you were at all inclined toward experimentation I would suggest the following:

Cut a 250 mm long (i.e., with the grain) by 1025 mm wide (across-grain) panel of nice new spruce. Dry this panel to 4% moisture content. (You’ll have to determine this by weighing samples, wood moisture content meters do not read accurately below 6%.) With the panel at 4% quickly cut it to exactly 1,000 mm and put it into a prepared frame that will hold it flat but which will not put any pressure on it at 1,000 mm but which will not allow it to expand beyond that initial 1,000 mm. Now set the whole thing aside in a normal atmosphere for a year.

When you take your panel out of the frame and dry it back down to 4% moisture content you will find it is no longer 1,000 mm wide but somewhat less that that. Just how much less will depend on the specific characteristics of the wood you used and the extent of the humidity swings to which it has been exposed. If you leave it in your frame long enough you will eventually see it actually coming away from the ends during moderately dry periods. This is the effect of compression-set.

This is the piano version of the illustration Hoadley gives on page 114 of his book, Understanding Wood. This illustration shows how constrained wood samples are damaged through the mechanism of compression-set after being exposed to varying amounts of humidity. The experiment I've described above is roughly equilivent to the middle sample. What has happened to the sample on the left is exactly the same as what happens to a compression-crowned soundboard panel over time. The soundboard panel is physically constrained by being solidly glued to all those perpendicular-to-grain ribs on one side of the panel. Those ribs don’t allow the panel to freely expand and the resulting stress-interface between the compressed (trying to expand) soundboard panel and the ribs forms the crown. Without that compression there is no crown.

A free piece of wood like a soundboard panel (or the sample on the right in Hoadley’s illustration) will not develop any internal compression or tension due to changes in humidity. It will simply expand or contract depending on whether it is absorbing or desorbing moisture. It is only when the wood is constrained and not allowed to move that it will develop either internal compression or tension.

Standard woodworking practice calls for wood to be at a minimum of 7% moisture content at glue up. To go below this can lead to starved glue joints as the wood will very readily draw the glue solvent (usually water) out of the glue. This is well above the 4% moisture content called for in the process of gluing up a compression-crowned soundboard assembly.

Gluing up a soundboard panel at the 4% moisture content required to end up with a compression-crowned soundboard assembly is abnormal to the woodworking industry. Several of the wood technologists I have consulted on the subject expressed mild shock and something bordering on disbelief when the process was explained to them.

Like it or not the soundboard panel in a compression-crowned soundboard assembly is under long-term compression and it remains under compression until compression-set has relieved that compression by physically altering the shape of the wood fibers. It is a gradual but certain process.

Del

The force to bend a rib doesn't seem high to you because you are bending the ribs differently. I assume you are grabbing a rib in two spots with each of your hands and then torquing it or maybe bending over your knee (or whatever). BUT, the soundboard isn't doing that. The soundboard is pushing on the rib longitudinally, very close to its centerline. As I mentioned in my previous post, that's what causes the high forces. If you refuse to believe Del, here is a quote from Ron Overs, a piano guru from Australia.

"CC [compression-crowned] rib sound board panels typically are grossly overloaded in compression, even before they are subjected to the downbearing force of the strings. It is not unusual for CC boards to exhibit significant signs of collapse within twenty years, and often much earlier (some overstressed sound boards can exhibit signs of collapse even before the instrument is sold from new). Overs Pianos is currently (2004) replacing a CC concert grand sound board (circ. 1962) from a 'leading' manufacturer, with reverse crown between the bridge and the belly rail in the second top string section (colloquially known as the killer octave)."

Sorry for the duplicate post. Del

The peak compressive forces that can build up in a soundboard panel that is crowned this way is between 1% and 2% depending on the specific wood samples and the atmosphere the soundboard assembly is exposed to.

It is generally accepted within the woodworking industry that to avoid rapid compression stress failure wood should not be under any more than 1% compression (perpendicular-to-grain) for even relatively short periods of time. Significant long-term compression set will occur at much lower levels of compression than this.

Del

Force is measure in Newtons, not percentage.

At any rate, it doesn't make any difference. As far as I know, I've had only two pianos fail after restringing, and the reason that they failed is because they were subjected to excessive heat. That was in the Oakland firestorm, when it got up to a couple thousand degrees. Nothing was left of them except the strings, so I hear. I doubt anyone else's pianos would do any better under those circumstances.

Del was referrring to how much the wood was compressed in distance. For example, if a piece of wood measuring 100 cm was compressed 1%, its compressed dimension would be 99 cm.

Should have done this earlier. Just want to thank you, Del, for responding to my question earlier in this thread and for continuing to visit PianoWorld. We piano people appreciate your willingness to share your knowledge.

Jeanne W

I second that emotion Jeanne W.

Thank you to all you contributors - Educating us all. Taking the time to write thoughtful insights from your years of experience. None of this shill-baiting nonsense that drives my crazy. Threads like this remind me why I love Piano Forum. And why I will start subscribing to keep it going. Thank you all.

I've sent this one into Frank B. to have this thread join the FAQ page. Hopefully he'll listen.

Hmmmmmm.....pretty harsh. I've been restoring, and rebuilding pianos since 1977. I've never replaced a soundboard. When I come across old Steinways, irrespective of how the SB looks or sounds, I ALWAYS tell the customers that restoring Steinways is a specialized art, and that the SB may need, and probably should, be replaced. I refer them to a shop that does that. I don't offer that service directly, although I am willing to sub that out if necessary to accomodate a complete rebuilding.

Many customers don't want, no matter what, a soundboard replaced. There is the family heirloom factor to consider. It comes up all the time in the field. Some clients simply want THEIR piano rebuilt...If, after properly advising a client as to their options, they want me to rebuild their piano, and assuming I can do a good job for them, we will take in the contract.

You run a factory, where you have a different onus. However, I have two people besides myself in my shop. We do beautiful restoration work on old pianos here. No corners cut. And, I ALWAYS advise customers to consider buying a new piano instead of rebuilding their older one...new pianos are immediately available (whereas we are 1-2 yrs. backlogged) and the results are predictable in purchasing a new (or newer) piano.

That said, if I were to refuse work on pianos that might be improved by replacing SB's, I would be virtually guaranteeing that many old, quality, pieces of family history would be basically thrown out.

I think we've found a great balance in how we bid on work. I am brutally honest with my customers about their old Brazilian Rosewood Chickering grands that need restoration. In all cases so far, those customers have wanted their pianos restored, rather than tossed. Those customers, having the option described to them, chose NOT to have the SB's replaced.

Would their pianos be better with new SB's installed?? Yes, in most cases. But I have generally found that neither the will, nor the budget, to get that done exists with the pianos I restore.

Somewhere within this issue there IS a fine art to making a piano with an original SB a beautiful restoration.

BTW, we replace virtually ALL action parts, and purchase high quality strings and hammers for our restorations...we don't cut corners...

...but neither am I a "crook" for being willing to assist my customer in bringing their fine, old, family heirloom piano back to life...

You run a factory and obviously keep high standards. If I were spec-ing on Steinways or MH and offering them for sale, replacing a SB would be no contest...it would happen virtually every time.

We live in different worlds. There are hack shops out here, to be sure...it drives me nuts too...but my clients are my best source of rebuilding referrals...they know I bring to the table a very discriminating ear, a willingness to assist them at their level, and a standard that frankly costs more than the competition last time I checked. None of our restorations have ever left a client feeling "stiffed". Never happened, never will. They get the whole picture before we start the work, and make their informed choices.

I think we come from two different worlds. I respect yours, and if you were here in Michigan I would probably refer certain rebuilding customers to you...especially those who have pianos that deserve new SB's....er, that is, if you could quit implying that anybody who does what I do is a crook....

RD

The sound coming from this 1917 O Steinway is from a 92 year-old sound board with 92 year old treble strings and new GC bass strings.

This piano only required proper voicing and action regulation. The sound board has no cracks. I was told that cracks only mattered if they caused a buzzing sound and that it can be corrected with shims.

Action and voicing were the main concerns for the expert Steinway tech that brought it to life.

Recorded with a DM-10 and a cheap condenser mic. The humidity over the past few months this summer knocks some of the unisons out of whack after about a week, but it's still acceptable. You will notice the bass is a bit stretched because they are recent replacements and I only bring them back into alignment once every ten days or so when they get too far off.

Glen

Snippet of Moonlight Sonata Mvt 1
http://www.box.net/shared/zv6h8ej07a

C-min Improv
http://www.box.net/shared/elyyhcuzsb

F-min Improv
http://www.box.net/shared/v70msif9xr

Schumann Warum
http://www.box.net/shared/g70giix57h