Bridge Seating / food for thought

[Frank Emerson]

Hi Folks,

I have been a member of this list, off and on, over the years.  I offer the
following observations.

I expect that you are correct, in that if the front bridge pins has become
loose enough to shift its position, it will remain in the furthest position
that it can move, in the direction of reducing the side bearing,
approaching a straight line along the direction of the tension imposed upon
it.  However, I have observed that pressing the front bridge pin in the
direction of correcting the side bearing does, in many cases, reduce or
eliminate the false beat.  You cannot rule out that this condition is
significant in producing the false beat, in many cases.

After removing the strings for restringing, I have observed that there is
usually a very clear impression on the bridge where the strings have
slightly compressed the bridge cap, to clearing show the position of the
string on the bridge.  In a few cases, I have observed a fan-pattern on the
bridge.  At the back bridge pin, there is a clear single-position
impression of the string, but at the front bridge pin, there is a broader,
less distinct, impression from the string.  This suggests a side-to-side
movement of the string across the surface of the bridge.  Other respondents
have pointed out that there is a vertical and horizontal component to the
movement of the string.  In fact, it is a circular motion, constantly
changing from horizontal to vertical, and then from vertical to horizontal.
Returning to my observation of the fan pattern on the bridge, a simplistic
view of this would suggest that the down bearing would limit the vertical
string movement to terminating the vibrating length at the front edge of
the bridge, but with regard to the horizontal component of its movement,
the termination of its movement is the back bridge pin.   Factoring in the
fact that all of this occurs within a matter of mili-seconds, the
frictional factors, the degree of down bearing, etc., the effective
termination point, with respect to horizontal motion might be anywhere
along the distance from the front bridge pin to the back bridge pin.  The
distance from the front bridge pin to the back bridge pin may be as little
as 15mm, or as much as 25mm.  If the difference between the effective
vibrating length in the vertical component and the horizontal component
could be as little as 1mm, and as much as 25mm, this can easily account for
the false beat.

Frank Emerson
pianoguru at earthlink.net


> [Original Message]
> From: Ric Brekne <ricbrek at broadpark.no>
> To: <pianotech at ptg.org>
> Date: 9/9/2006 10:43:01 PM
> Subject: Bridge Seating / food for thought
>
> Hi folks
>
> I just thought I might throw some numbers around to look closer 
> math-wise at this idea that the wobbly pin can change the speaking 
> length enough to account for false beats.  Take the following wire.
>
> 0.9 mm diameter and 150 mm speaking length at a starting tension of 
> 150.95  lbs This yields a frequency of  1218.93 Hz.
>
> To get a  rougly 4 beat per second false beat this string needs to 
> change in length by  around 0.003 mm.   That would require the pin to 
> deflect  nearly 1 mm !!
>
> It would take a 0.15 mm pin deflection to enable a 1 bps false beat.
>
> Realistically a pin might be able to wobble say 0.02 mm. That would 
> yield only a 0.15 Hz change.  Not hearable really. On shorter strings 
> the change would be slightly (and I mean slightly) greater.  On longer 
> strings, less.
>
> Seems to me that  it is obvious that  the pin can not wobble enough to 
> change the length and tension of the string enough to cause any false 
> beats at all. 
>
> There are other problems with this whole idea of wobbly pins causing 
> false beats.. such as the frequency of the supposed wobble itself... why 
> the significant side bearing hasn't already pushed the pin as far 
> sideways as is possible.... etc etc.. but the above in itself should be 
> enough to discard the idea.
>
> Cheers
> RicB
>
>