tapping strings

[Richard Brekne]

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David Skolnik wrote:

> Ron N's clarification is worth repeating here
>
>
>> String downbearing is the product of tension and bearing angles. The
>> board
>> is, under all circumstances, exactly resisting the string
>> downbearing force
>> - exactly. The measured crown of the board, compared to the
>> downbearing
>> angles, is an indication of how successfully the board is resisting
>> the
>> string downbearing force.
>
>
> I think it may have been Bob Hohf who used the term 'residual
> downbearing", to, as I understood it, refer to the measurable
> deflection after the string has compressed the board as much as it was
> going to.  On a healthy (ie. crowned board), that 'residual'
> downbearing could theoretically be anything from zero + .0001 (?) and
> up. But if it were zero, you have none, and if you show a negative
> deflection, you enter a different world of forces.  From the point of
> view of measuring this deflection when there is not much to be seen,
> how accurately can it be measured?

Thats exactly my point... Downbearing in pianos per say can not be
directly measured.. It can only be calculated. Re-read Rons very concise
and correct (to my mind) summation. You origionally asked

".., are you saying that it is not possible to take measurements with a
gauge (and string under the 'panel')and make some conclusion about the
downbearing present in the piano?"

And I agree in my answer with what Ron has written.... that yes... taken
together, a measument of positive crown and a measurement of postive
down bearing indicates exactly that there is indeed positive downbearing
and crown...(grin) and as such says the board has at least some spring
in it and that the string plane is not too high relative to the
panel.... but thats pretty much all I think you can discern from those
facts...valuable tho that information in itself may be in some
cirmcumstances.

But you do not know how much down bearing there is because in our strung
and up to tension example because you simply cant measure it and the
soundboard deflection of the strings prevents you from knowing what the
real string deflection angles are.


> Again, in a strung piano, is a test string, run next to, or even in
> the place of, an existing string, going to be able to account for the
> effects of cap indentation?  The problem is a bit quantum...in order
> to truly duplicate the piano string with the test string you must
> recreate the offset created by the bridge pins, and yet, as soon as
> you do, you are potentially distorting the vertical direction  of the
> test string travel.

Grin... well... I wonder if it gets a bit beyond whats practical and
needed in this regard to put so accurate a number on downbearing per se
as to take into consideration string offsets and the angles of bridge
pins....or what ?

>
> So, are all the methods of measuring termination angles equally
> accurate?
>

Probably not... but there are some good ones out there.

> David Skolnik

--
Richard Brekne
RPT, N.P.T.F.
Bergen, Norway
mailto:rbrekne@broadpark.no
http://home.broadpark.no/~rbrekne/ricmain.html


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