JIMRPT@aol.com wrote: > > ... To respond to Danny's question re: polyurethane........I have finished a > few boards with polyurethane and was, and am, under the impression that the > sustain was slightly adversely affected. I have no data to prove this, just > my impression. I no longer use polyurethane for this purpose. > Just some thoughts > Jim Bryant (FL) Jim, I rather doubt that it was the polyurethane that affected the sustain characteristic of the soundboard. Sustain is a function of the mechanical (wave) impedance of the soundboard system. This is a highly complex function, but basically it is determined by the mass and the elasticity (or "springiness") of the system. As soundboards age under the load of the string downforce, the springiness component of the equation changes—especially with compression crowned soundboards. The wood cell structure within the soundboard panel gradually crushes as it is repeatedly subjected to the swelling of the wood during the humid climate cycles. While not usually catastrophic in the short term—when it is catastrophic, we call the result a “compression ridge”—over the years the wood cells loose their resiliency. As this happens the soundboard looses some of its springiness. The soundboard now presents a lower acoustic load to the energy coming to it from the strings which means that energy is transferred from the string to the soundboard at a faster rate; hence, sustain time decreases. The visible appearance of the soundboard is not an indication of the ability of the soundboard panel to continue functioning as a piano soundboard. Until something better comes along, our criteria is primarily sustain time and our experience in judging that time. As I’ve said before, any soundboard over 40 years old is suspect. And any soundboard showing evidence of compression ridges is suspect. Not automatically rejected, but certainly suspect.
This PTG archive page provided courtesy of Moy Piano Service, LLC