Soundboards

[Delwin D Fandrich]

robert sadowski wrote:
> 
> Dear List,
> 
>      A technical question. A fellow technician recently had a board
> replaced in a 9' Knabe.  The original board had a gap around the perimeter
> of the board in places up to 1/4". Should a board be glued right up against
> the case and would this have a "coupling" effect on the sound, or isn't
> this important?   I've heard one reason the far-east pianos have a
> characteristic sound is the unavailability of certain hardwoods in case
> parts, etc.  This suggests to me that the case resonates to a certain
> degree contributing to the over-all sound. Hope this generates some
> thoughts.
> 
> Regards,
> Bob Sadowski RPT
> Erie,PA.


Bob,

Assuming that the soundboard was glued firmly to the rim, the gap
between the edge of the soundboard and the inner rim is of no
consequence. Other than for appearance, of course. And for appearance
sake, most soundboard installers, I think, do try to achieve as close a
fit as possible between the edge of the soundboard and the outer rim. We
certainly do. And then we fill any slight remaining gap with an epoxy
filler colored to match the appearance of the soundboard. But,
especially with a piano such as an older Knabe in which the inner rim is
probably somewhere between 45 and 50 mm wide and is made of a wood such
as maple or oak (the 7’ 8” Knabe were putting a board in right now is
oak) a gap of this size will not be a factor in either the pianos
performance or the soundboards longevity . That’s a lot of glue surface. 

On the other side of this coin, several years ago a charge was made that
by not leaving a slight gap between the edge of the soundboard and the
inner rim, the builders were causing the soundboard to somehow
self-destruct. The manufacturer had built and incorporated into their
process a new tool that allowed a better fit between the edge of the
soundboard and the outer rim. This contention, also, was not credible. 

It is important that the soundboard be rigidly attached (both physically
and acoustically) to the inner rim—at least along most of the
soundboard’s perimeter. This is accomplished by gluing and clamping the
board to the top surface of the inner rim. You don’t need a lot of
surface to accomplish this task. Along the treble end of the belly rail
this goal is achieved in only 10 to 15 mm of gluing surface—the typical
width of the treble belly rail extension. A cap is typically glued along
the top of the soundboard along this length and screws are fitted that
extend down into the belly rail to ensure that this joint stays glued,
but it’s the glue that really does the job.

Now, to the second part of your question. First, let me make the
distinction between resonating and vibrating:  
  Res·o·nance (rèz¹e-nens) noun.
    2. Physics. The increase in amplitude of oscillation of an electric
or
    mechanical system exposed to a periodic force whose frequency is
equal 
    or very close to the natural undamped frequency of the system. 
  Vi·bra·tion [vì-brâ¹shen] noun.  
    1. a. The act of vibrating.  b. The condition of being vibrated.  
    2. Physics. a. A rapid linear motion of a particle or of an elastic 
    solid about an equilibrium position.  b. A periodic process. 
All resonating objects are vibrating. Not all vibrating objects are
resonating.

In my opinion, the piano rim must be as massive and as rigid as is
practical. In two otherwise identical pianos, it was easily possible to
both hear and measure a distinct difference in tone, and especially
sustain, between one built with a maple rim and one built with a poplar
rim. This has nothing to do with the “resonance” of the rim. It does
have to do with the mass and stiffness of the rim and its susceptibility
to induced vibration. Any energy that goes into the rim is lost to the
soundboard and, hence, to the air as sound energy. It takes energy to
induce vibration in a piano rim, and piano rims don’t make good
soundboards. How much a rim will vibrate depends on its mass and
stiffness.  

The more massive and rigid a rim can be made, the less energy it will
absorb from the soundboard. Dense and stiff hardwoods such as ash,
beech, birch, maple, oak, etc. make more massive and rigid rims than do
poplar, the soft meranti’s and other similar “select hardwoods.” We
regularly add stiffening beams and additional rim bracing to piano rims
that I deem to be excessively flexible. The goal (and, typically, the
result) is an obvious increase in sustain. Particularly through the
mid-tenor and lower treble portions of the scale.

ddf