---------------------- multipart/alternative attachment
To preface any further discussion, I think what Ron has found frustrating
in my thinking is his perception that I keep changing my question once he
has answered it. In fact, I probably do. What seems to be one topic is
not. I don't think we agree totally on what Negative Front Bearing is,
what it does, what causes it, or what can or should be done about
it. Ron's comments force me to think about and struggle to refine and
clarify my thoughts and words. I am not, however, playing games. There is
something in all this that I too find rather frustrating. I would like to
think that I could acknowledge errors in my understanding if these can be
shown to me. And, while I make my replys (mostly) to Ron, I still assume
there are readers other than Ron, though, at some point, I may just have to
admit it's me an' him. After that, who can say? Finally, though I tried
to delete parts of the exchange which I thought to be less relevant or
urgent, I still apologize for the length.
At 07:35 AM 4/2/2004 -0600, Ron Nossman wrote:
>DS
>>I understand Ron to be saying that it is not necessary for the string to
>>terminate simultaneously at both the pin and the bridge edge. If that
>>understanding is accurate, I wonder if that opinion is generally shared.
>RN
>The notch edge in front of the bisection of the pin (into the speaking
>length) can cause tonal clarity problems. A notch edge behind the pin
>bisection (into the bridge) won't unless the pin is loose in the cap.
Ron, in your previous post you ended by asking that we keep the issue of
false beats created by loose bridge pins separate from the discussion of
other termination distortion. So, for now, let's not be concerned with
loose bridge pins.
I accept that a vibrating string contacting the notch before the pin will
cause tonal deficiencies that would not be present if the pin were to be
contacted before the notch, however, what I'm talking about in this thread
is not a defined notch whose contact with the string occurs closely after
the bridge pin, but rather a tangent relationship between string and a
rounded or mis-planed bridge surface, the contact points of which could
(when string is not seated) take place 1/4" or more to the rear of the
pin. In such a case, it would seem that the horizontal bridge surface can
only be supporting the position of the string on the pin,(as a later quote
of Ron's states) but making no direct contribution to the string energy
termination. If that support is far enough from the pin, it would seem to
allow some slight amount of vertical sliding movement at the pin, even with
a 10 degree angle. This movement would cause accelerated wear and
flattening which could, in turn, cause tonal distortion.
>>>>>At the same time, compression on the bridge top (exacerbated by
>>>>>tapping down on the strings) lowers the contact point on the bridge.
>>>> DS
>>>>Assuming enough downbearing ( at some point in time) to compress the
>>>>wood fibers, I would place more responsibility on the seasonally
>>>>induced increase in downbearing more than the unsubstantiated certainty
>>>>of aggressive tapping. And, of course, there is the speculation that
>>>>the bridge surface itself rides up the pin in humid conditions, in
>>>>turn, pushing the string further up the pin.
>>> RN
>No. It isn't primarily downbearing that is crushing bridge notch edges.
>It's more cyclic seasonal dimension changes of the bridge cap pushing the
>string up the pin against the friction of the string against the pin, and
>pin angle.
I hadn't thought of this before, and it makes sense, but, just to show the
way my mind functions (or dysfunctions), Ron says "it isn't PRIMARILY
downbearing..." and "it's MORE cyclic..." . So, of course, I wonder just
how much of each it is. In other words, the piano presumedly starts off
with downbearing which, over time, will, by itself cause some wood
compression. Presumably, even without angled bridge pins, a seasonal
increase in the height of the bridge will increase the compressing force
unless the pitch is maintained, ie. lowered. This net force probably then
remains the same, except a greater approach angle of the string would put
more of that force on the extreme edge of the bridge. Then too, if, as is
common, the rear string segment has negative bearing, the up-force would be
relaxing as the bridge top moves upward.
So then, let's assume that the pitch IS allowed to go sharp, generating
more downward force by the string. If the front bridge pin were straight,
not angled, it would only be the direct increase in downbearing force that
would be acting to compress the wood. (Probably a good argument for Wapin
right there), But the bridge does ultimately push the string upwards on
the pin. Even in the case of negative net downbearing, where the seasonal
increase in bridge height and board height should, theoretically lower the
pitch, the force required to overcome the vector friction (pin angle)
would probably be enough to cause some wood compression.
DS
>>>>The question there would be whether the string then follows the board
>>>>back down in the dry season.
>>> RN
>>>This can also be measured.
>>
>>Is this conceptual model commonly shared by other readers?
>
>What, the concept of measuring rather than taking a consensus?
You're right. My question wasn't clear. In fact, neither question was
clear. In a positive front bearing situation, it's not whether the string
follows the board, rather, whether it follows the bridge surface as it
recedes down the pin, or whether the friction between string and pin is
sufficient to overcome the strings natural tendency to return to a straight
line and the vector force of the angled pin. If negative bearing exists,
there is no reason to expect the string to follow the bridge.
>>>No pin will remain bottomed in the hole with the bridge changing overall
>>>height with humidity swings. The point of zero relative movement between
>>>the bridge and the pin is typically somewhere near the bottom of the cap
>>>- depending on the type of capping material used. That means that as the
>>>bridge top is going up the pin, the bottom of the hole is getting
>>>deeper, and moving away from the base of the pin.
>>
>>Ron, I can picture what you're describing but I can't grasp the
>>methodology by which you determined it. Can you explain it?
>
>A model bridge section, with vertically laminated root and solid quarter
>cut cap, taken from about 4%MC to about 12%MC -
>
>Root height went from 0.842" to 0.868", or +0.26" height change.
>Cap height went from 0.253" to 0.119", or +0.004" height change.
>Pin height above the cap went from 0.026" to 0.119", or -0.008 height change.
>
>If the pin height lessened by 0.008" as the cap height increased by
>0.004", the point of zero relative movement between the bridge and the pin
>is somewhere a couple of thousandths into the bridge root below the cap.
>As the root height increases, the hole the pin was installed in increases
>in depth, to the same percentage of overall height change the root
>experienced. Since the pin doesn't change dimension with humidity changes,
>the bottom of the hole it's in has to recede from the bottom of the pin.
Ron, this could be interesting, but you have some errors in your
numbers. Root height changed +0.026", not 0.26". One of the cap height
numbers is wrong as is at least one of the pin height number.
Also, the other value that would establish an accurate picture would be the
height differential of the board.
>>Assuming you cared about having the string contact the front of the
>>bridge, do you agree that tapping the bridge pin rather than the string
>>would achieve that end?
>
>I do care about the string contact with the front of the bridge, but I do
>not agree that tapping the pin will achieve that end. That's the whole
>point of all this. If the string isn't contacting the notch edge, it's for
>a reason that tapping neither string, nor pin will cure.
While I basically agree with you, I'm finding it difficult to reconcile
with your response to Wim's "More Wood, less pin..." questions:
>Wimblees:
>>There has been a lot of discussion about tapping the pin to create better
>>tone, less distortion, etc. But what are we doing? Is the better
>>termination caused because by tapping we are driving the pin deeper into
>>the wood at the bottom of the hole, thus creating a more stable pin,
>Ron N:
>Partly, but I think mostly dragging the string down with the pin to the
>notch edge.
and
>WimB
>>So what is the real reason for tapping? More wood, or less pin?
>RonN
>Or seating the string by proxy?
>>I think we may agree, but I'm not sure. If you conceive of Front Bearing
>>as that part of the total downbearing which pulls the bridge mass
>>forward, then I can understand the configuration you describe above.
>
>Front bearing is the angle between the string segment on the bridge top
>and the speaking length segment.
My contention is that, since the string segment on the bridge displays
considerable curvature, it is misleading to think of angles or to assume
that the imaginary straight line between front and rear bridge pin is
meaningful in defining the angle actually formed by the two string segments
as they converge at the front pin.
>>As it relates to termination however, Front Bearing, as I understand it,
>>is precisely about the relationship of the vibrating string segment to
>>the front edge of the bridge and those few millimeters behind the
>>edge. If that edge is below the line formed by the apex of the bridge
>>and the front string terminus, then a seated string will describe a
>>negative front bearing angle. Any noise elimination achieved by such
>>seating will be temporary, as you point out.
>
>As I point out. I've also pointed out that this local negative front
>bearing from tapping a string down isn't to be considered to be overall
>front bearing. It results from doing something that isn't indicated from
>the cause of the symptoms - seating strings.
This is where I either lose you or disagree. When you say "this local
negative front bearing from tapping a string down...". I'm not sure how
to interpret this. Yes, if you don't seat the strings to the bridge, you
won't read it as negative, but you have the potential problems discussed
above, relating to excess string movement at the pin. If you do seat it,
you eliminate, temporarily the sliding movement on the pin, but introduce
an unstable condition which will eventually return the string to its
neutral, elevated position. But, at the beginning and end of this comment,
you seem to again assert that the condition is caused by string seating. I
see no reason to support such an assertion.
>>I'll admit however, I'm confused. You say the string does NOT climb the
>>pin, yet, in the above example you point out that the tapped string will,
>>in fact "straighten back out". Wouldn't this qualify as climbing the
>>pin, since I don't believe you are saying that either the bridge surface
>>or the pin is responsible for the string's re-elevation.
>
>No, that doesn't qualify as climbing the pin. The string segment, courtesy
>of the tension it's under, merely returns to as straight a line between
>it's end supports (in all planes) as it can. If the notch edge just
>happens to be below that horizontal plane, the string won't be contacting
>it. Of course the bridge surface is responsible for this. It's one of the
>horizontal support points for the string. It's just that this support
>point doesn't necessarily correspond with the notch edge.
Then I understand your use of "climbing the pin" to refer to a string
moving upward on a pin in spite of positive bearing angle.
>>The question remains for me, whether there is any audible or measurable
>>difference between the vibrational pattern of a string which terminates
>>simultaneously at pin and notch, and one which lacks a defined,
>>horizontal edge.
>
>Depends on where the termination is in relation to the pin, as described
>above, and whether or not the pin is tight in the bridge.
Again disregarding the issue of possible loose pins, I clarify... I wonder
whether any electronic device can measure a difference in the vibrational
envelope created by a string which terminates simultaneously at the pin and
notch, and one where the horizontal termination (or support) lies some
distance behind the pin.
Of course, my other ongoing concern regarding negative front, or for that
matter, areas of negative net bearing, is whether it should, if accurately
determined, constitute a reasonable basis upon which to reject the
acquisition of an instrument. But as Ron says, one at a time.
David Skolnik
---------------------- multipart/alternative attachment
An HTML attachment was scrubbed...
URL: https://www.moypiano.com/ptg/caut.php/attachments/9f/ff/62/32/attachment.htm
---------------------- multipart/alternative attachment--
This PTG archive page provided courtesy of Moy Piano Service, LLC