sounding board structure and displacement

[Richard Brekne]

Mark Kinsler wrote:

> I believe that a sounding board has a set of parallel ribs that serve 
> to strengthen it and, in some cases, to preserve its dished shape.
>
> So there are two cases to consider:
>
> If the ribs are perpendicular to our line of sight, then the 
> cross-section of the structure will be a beam whose ends taper down.  
> Presumably the glue is secure and has the same flexibility as the 
> wood.  When the center of this beam is loaded, it will bend like any 
> other.  Since the center of the beam has greater depth than the ends, 
> the beam will tend to remain fairly straight, with perhaps the 
> majority of the bending taking place near the ends.  The ends would 
> tend to curl up.
>
> If our line of sight is parallel to the ribs, then our cross-section 
> resembles a thin arch (the thickness is that of the sounding board) 
> with a few lumps beneath it.  When this is loaded at the center, the 
> ends will tend to spread out.
>
Brilliant !!!... A unifiying theory that manages to give everyone a 
little piece of the truth :)... The soundboard both is and isnt... all 
at the same time mind you... an arch.

God I love this list....

> Thus the 'action' of the sounding board varies depending on your point 
> of view.  Presumably the sounding board doesn't care much.
>
> I believe that the bridges lie near one edge of the sounding board in 
> most instruments.  This would tend to reduce the flattening of the 
> sounding board's crown.
>
> The purpose of the 'downbearing' in bowed string instruments is to 
> hold the bridge in place, hold the strings in place, and to ensure 
> that the bridge thoroughly defines one end of the vibrating string.  
> Perhaps this is also the case in pianos.
>
> M Kinsler
>
Cheers
RicB