Do Pianos Mature?

[John Musselwhite]

Hi Folks.

We have been talking about whether a piano can "mature" and Richard brought 
up the point that scientific evidence of why or how this might happen 
seemed to be lacking. While perusing the "Wood Handbook - Wood as an 
Engineering Material" I ran across this on "creep and relaxation", which 
may or may not have something fundamental to do with the phenomenon.

________________________
"Creep and Relaxation

When initially loaded, a wood member deforms elastically.
If the load is maintained, additional time-dependent deformation
occurs. This is called creep. Creep occurs at even very
low stresses, and it will continue over a period of years. For
sufficiently high stresses, failure eventually occurs. This
failure phenomenon, called duration of load (or creep
rupture), is discussed in the next section.
At typical design levels and use environments, after several
years the additional deformation caused by creep may
approximately equal the initial, instantaneous elastic
deformation. For illustration, a creep curve based on creep as
a function of initial deflection (relative creep) at several stress
levels is shown in Figure 421; creep is greater under higher
stresses than under lower ones."
(Wood Handbook - Wood as an Engineering Material p. 4-24)
________________________

If I'm interpreting this correctly, after a rim is bent or a lever is 
loaded additional time-dependent deformation in that rim (rim creep?) 
continues to occur long past the time the rim becomes part of the case, 
going on for at least several years until it settles in to being a rim. 
While this period isn't specified I imagine it is highly dependent on the 
species from which the "member" is made, the manner in which the parts are 
joined and the kinds of stresses involved in the parts.

It goes on to say that any structural members under stress, which I would 
take to be the inner and outer rims, beams, bridges and soundboards and 
even the wooden action parts, would be subject to creep of some kind. 
According to the book, if the stresses are high enough over time "creep 
rupture" or failure will eventually occur.

Stress cycles apparently decrease the time it takes for creep rupture to 
happen, and while total failure of most action parts is rare, the 
continuing cycle between a light load at rest and a heavy load as a note is 
being played would seem to make action parts vulnerable to this kind of 
stress failure as time goes on.

The book also says that changes in climatic conditions increase the rate of 
creep and shorten the duration during which a member can support a given 
load. It doesn't specify the "climatic changes" required, but we know that 
pianos are shipped all over the world and in shipping can be subject to 
gross climate changes along the way.

What effect will all this have on an instrument that was just lumber a 
relatively short time before? If the lumber came from the tropics does the 
climatic changes to the Northern Hemisphere after it is worked have a 
greater creep effect than lumber that comes from here? Furthermore, do 
different wood species "creep" differently when moved from one area of the 
world to another that's totally dissimilar?

This raises other questions such as the effect this has on mass-produced 
pianos where the structural members are made from wood of the meranti group 
like lauan. While the book says it has structural properties favorable to 
Northern Red Oak and it's usually well-dried (a whole other related topic), 
could long-term climate-induced creep failure be part of the explanation of 
why people appear to think that Asian pianos don't last as long as a 
domestic instrument?

Also, are domestic pianos made from domestic woods such as maple and spruce 
less affected here by the same kind creep failure while perhaps benefiting 
to some extent from the effects of creep itself as they mature? While maple 
rims and spruce beams appear to be structural overkill when lauan parts 
appear to perform well enough over the short term, do they seem to outlast 
the "select hardwood" because they're under less stress and tolerate 
long-term creep failure much better?

To anthropomorphize this slightly, I would take all this to mean that 
according to the standard engineering reference source, at least for "a 
period of years" depending on the design, structural materials, execution 
and servicing, the longer a piano is a piano the more it wants to *be* a 
piano. This maturing process is complex, time dependent and could even be 
said to be somewhat "creepy" in nature. It is, to some extent, measurable 
and predictable, taking maturity in a piano from the philosophical realm of 
art and opinion and placing it within the world of science.

For more details on this including charts, drawings and photos, please see 
the "Wood Handbook - Wood as an Engineering Material" previously mentioned 
here and available on the Internet. I encourage comments as to whether I've 
drawn the correct conclusions from it as to how this might affect pianos.

Regards...

                 John



John Musselwhite, RPT    -     Calgary, Alberta Canada
http://www.musselwhite.com  http://canadianpianopage.com/calgary
Pianotech IRC chats Tuesday and Thursday nights and Sunday Mornings
http://www.bigfoot.com/~kmvander/ircpiano.html