Seating/false beats

[Ron Nossaman]

At 01:10 PM 4/21/97 -0700, you wrote:
>Steve,
>
>You wrote:
>
>
>>	My own suspicion is that there may be more expanding and
>>contracting than just the bridge or bridge cap. How about the soundboard?
>>Does it crown up enough in high humidity to produce this effect, or at
>>least contribute to it? Would that help explain why I notice the effect
>>more in the mid-treble than anywhere else?
>>
>
>
>The answers here are yes, and yes.
>
>And I'm not sure why none of us have made an issue of this aspect before.
>
>Thanks.
>
>Horace
>
>




Sure the soundboard is a factor. It accounts for the major portion of noted=
 pitch raising and lowering with humidity swings. I don't see, however, how=
 the rising soundboard, increasing downbearing (wrong direction), could in=
 any way push strings up bridge pins. The bridge swelling in that same high=
 humidity period, however, would. Here's a good place to make the=
 observation that the combined effect of the rising soundboard adding=
 downbearing, and the bridge swelling and forcing the string up the pins=
 puts much more pressure on the bridge surface than either alone would. The=
 higher string tension from the rising soundboard increases the sidebearing=
 force on the bridge pins and makes it more difficult for the bridge to=
 elevate the strings. That's double the wood crush for the money. The two=
 humidity reactions (board/bridge) are inseparable in practice, but the=
 board isn't what lifts the string on the pins. It's the bridge. All the=
 lifting action takes place in the area between the bottom of the bridge=
 pin, and the top of the bridge. The rising board pitch increase masks the=
 swelling bridge (increased stagger) increase or someone would have brought=
 this whole thing up a long time ago.


As to why it happens more in the mid-treble, I have a few more ideas. First,=
 that's the area with the least downbearing (usually). Regardless of what=
 the manufacturer intended, that's what I find in practice. This might be=
 that it's the most likely spot to find the soundboard flat, generating the=
 retched noises that makes me take readings in the first place. This may/not=
 be a factor. Second, the string section between the rear bridge pin and the=
 aliquot, or hitch, is closer to being the same length as the speaking=
 length in the mid-treble as anywhere else in the piano. The downbearing=
 forces are more balanced. The shorter the moment arm, the faster the=
 bearing angle changes with bridge elevation. In the bass and low tenor, the=
 bearing angle changes on the back of the bridge much more than on the front=
 with a humidity cycle. Maybe this (and more vibrating string mass) pulls=
 and jiggles the string back down on the bridge as it dries and shrinks. In=
 octave 5 1/2 - 6, the forces fore and aft are more evenly split, and the=
 string mass is considerably less. I think string mass probably has more to=
 do with it than anything else. Thoughts?

 Ron Nossaman