[pianotech] Totally glueless

[Ed Foote]

Ed Foote RPT
http://www.piano-tuners.org/edfoote/well_tempered_piano.html

  >>It's the presumption of "acoustic coupling" that is questionable 
here. Good mechanical coupling is necessary for one part to move the 
other to make the system work. What I tried, unsuccessfully again, to 
point out was the difference in scale between the mechanical action 
that is taking place and the imagined tiny vibrations within the 
various individual parts. The basic action is much coarser and simpler 
than is typically believed. Was the O rebuilt because the bass was 
dead? Were the bass strings replaced and the bass was still dead? If 
the screws were too loose to provide a decent mechanical connection, 
that should have been the case. What prompted the rebuild?Ron N<<

The restringing had nothing to do with the bridges.  This is a school 
piano, and lots of broken strings and iffy pins were sufficient reason. 
  The discovery of an unglued bridge was incidental. The bottom of the 
bridge's apron was curved to match the soundboard's curve.

   I submit that the coupling provided by a couple of screws is inferior 
to a full glued surface for several reasons.  The screws are only 
holding from the middle of the bridge to its rear surface, the 
cantilevering is trying to rotate the bridge and the lack of glue means 
the fulcrum is the distal edge of the surface and the upwards force is 
in the middle.  This loses all attachment in front of the screws while 
creating a very short moment arm between the screw and the distal edge 
of that bridge. The pair of screws puts the entire upwards rotational 
pressure of the bridge on two small, unsupported spots on the 
soundboard, whereas a glued surface would have the full footprint 
resisting the strain.  The tension of the screws would also vary with 
humidity, eventually loosening enough to allow the bridge to rotate 
lower, (this had happened in this case).  Due to the elastic nature of 
wood,  there is no way for the proximal edge of the apron, under 
stress, to stay in contact with the soundboard when held by these two 
screws, greatly reducing the mechanical contact between the bridge and 
board.
     Luthiers have long known that different ways of attaching necks 
have different sounding results. The integrity of the neck joint is 
widely recognized as having an effect on tone.  I have observed this, 
myself, on numerous guitars that have been repaired, and  I am inclined 
to believe that coupling does have an acoustical effect, at least, 
until I am shown otherwise. With only two screws holding the stucture 
together, there will be less coupling than fully glued.  If one is to 
believe that any coupling is as good as another, this won't matter.
   The mechanical coupling is easy to measure, the bridge is still 
there, so it must still be attached....  The acoustic coupling is more 
difficult to measure. If I decide that there must not be any difference 
because I can't hear it, then I am going to find less and less things 
that make a difference because 60 year old ears are no longer hearing 
the same as 30 year old ears.
Regards,