Negative bearing

[Delwin D Fandrich]

----- Original Message -----
From: "Don" <drpt@sk.sympatico.ca>
To: <pianotech@ptg.org>
Sent: December 02, 2001 11:46 AM
Subject: Re: Negative bearing


> Hi all,
>
> The vibration properties of primary interest for constructing a soundboard
> are, the speed of sound and the damping capacity or
> internal friction.
>
> The speed of sound in wood varies directly with the square root of the
> modulus of elasticity and inversely with the square
> root of the density. The speed of sound in wood also decreases with an
> increase in temperature or moisture content.

Actually, the speed of sound through wood is of no consequence in the
function of the soundboard panel except that it is a function of the
materials stiffness--as you say the material's MoE.

Sound, the sound that we hear, does not travel through the soundboard
material. The soundboard vibrates in response to the vibrating energy
transferred to it through it by the bridge. Ideally this would cause the
soundboard panel to vibrate as a perfect diaphragm, and ideal which is,
obviously, impossible. There is mass involved and some finite stiffness.
Stiffness could be added by making the wood panel thicker or by making the
ribs heavier, both of which would add an unacceptable amount of mass. So, we
use a wood which has an excellent stiffness-to-weight ratio, we add some
crown and string downforce (while it's debatable whether the crown is really
necessary it seems the string downforce definitely is) to load the assembly,
making it even stiffer, and hope for the best.

>
> When wood is strained, some energy is dissipated as heat. This is done
> through internal friction. Internal friction increases
> as the moisture content increases.

This may be a bigger factor than is sometimes thought. Several materials
which might otherwise be good candidates for soundboard panel have such
radically lower internal friction characteristics that the resultant sound
no longer sounds like a 'normal' piano.

Del