Negative bearing

[John Delacour]

At 12:46 AM -0500 12/2/01, Greg Newell wrote:

>     What was the effect of the floating soundboard across the 
>bottom? I've often
>wondered why it's necessary to lock it down everywhere. Has anyone 
>ever explored
>a different material joining the soundboard to the rim? I'm thinking of cone
>type stereo speakers where there is a foam rubber of sorts joining the cone
>shaped paper element to the stiffer steel frame. It seems to me that there are
>some parallels here. Probably the tapering of a soundboard is the closest I've
>heard to this.

Greg, there is very little resemblance between the way a soundboard 
works and the way a loudspeaker works.  The cone of the speaker is 
light and stiff and ideally perfectly free to respond to the impulses 
from the solenoid.  It is in no way a medium for the transmission and 
amplification of a sound but a simply a plunger for compressing and 
rarefying air following the dictates of the solenoid.

The soundboard of the "modern" piano, besides acting partly as a 
plunger, has to respond to over 200 separate complex signals from 
different places arriving at different points through various media. 
It is restricted from responding to these signals as a plunger by a 
huge downward force on it up against which it is pressing with an 
equal force by dint of its crown and its inability to spread, so that 
the whole system is in equilibrium with the board in a state of 
compression, ideally at some point below the point at which the 
weakest of the summer growth gives up the fight and the board shears 
(compression marks).  For all this to be possible it is essential 
that the board be fixed for at least most of its perimeter to a 
massive and unyielding structure, which is the framing and specially 
the inner and outer rim.

Sound travels IN the board and not just out from the top and bottom 
of it.  A material needs to be used that will carry sound rapidly 
through the medium.  The winter growth of the fir or spruce 
soundboard carries the waves fast along the length of the boards and 
owing to the slower and less efficient movement of the waves across 
the grain, the board is crossed by bars which transmit fast and also 
compensate for the increased flexibility of the board across the 
grain.  The waves are reflected in all directions through the board 
from the rigidly held perimeter.  I see Ron Overs uses laminae of 
spruce to achieve uniform propagation of the sound.  Properly 
disposed ribs have done the job pretty well for a long while.

That's a very rough description of some what happens, at least enough 
to show that a soundboard and a cardboard cone behave very 
differently.

As to freeing up parts of the perimeter of the board for certain 
tonal reasons (or questions of impedance as some might have it) there 
are numerous examples of this throughout the history of the piano -- 
in fact Cristofori's soundboard was not attached at all to the rim, 
but that was not what we'd call a modern piano.  In most 
straight-strung grands and early overstrungs you will find the edge 
of the board detached from the rim for a length of a foot or so and 
provided with a lath of maple or beech for firmness.  This frees up 
the board to respond better to the low notes on the bass bridge 
which, or whose apron, is fixed quite near the rim.  In my experience 
the effect of this is wholly successful.  Quite a few uprights have 
the board freed up along the bottom.  None of these expedients is 
designed to reduce the overall firmness of the structure but to apply 
topical variations to the stiffness of the structure.  I'd guess that 
90% of all such work is empirical and that 100 faculties working for 
ten years would probably produce no better science.

An interesting extreme case is the old Bösendorfer Viennese grand.  I 
had one of these for a while.  It was barely 5'6" long and was 
terrifically powerful.  The soundboard on these pianos has a very 
pronounced arch and the front of the board, instead of being glued to 
the damm, is held in a shallow arch by a length of maple perhaps 
about 1 cm thick and 6 cm broad -- I forget...it's a long while since 
I burned it!

Differential thicknessing of boards has been common for many years -- 
I think Dolge mentions his having invented the first thicknesser to 
do this.  With an overhead belt sander it is possible to get whatever 
variation you need without the expense of special machinery.

JD