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The last time I attempted to discuss this subject with Ron, my apparent=20
inability to express my thoughts precisely enough threatened to undermine=20
the normal civility with which these exchanges usually take place. I am=20
not trying to be purposely argumentative in the questions I raise, nor=20
disrespectful. I apologize for the length and for any continued fuzzy=20
thinking. Even though I direct most of my comments back to Ron, I am=20
interested to know what others think, as well.
At 03:48 PM 4/1/2004 -0600, Ron wrote:
>DSkolnik:
>> I'm asking "why does the string need to contact the front edge of the=
=20
>> bridge?"
>RNossman:
>It doesn't. I suspect you have tuned pianos in which this was the case. I=
=20
>know I have. As long as the bridge pin is solid in the top of the cap,=20
>there won't be a false beat to lead someone to tap the string - or pin.
I understand Ron to be saying that it is not necessary for the string to=20
terminate simultaneously at both the pin and the bridge edge. If that=20
understanding is accurate, I wonder if that opinion is generally shared.
DLove
>>> The assumption is that the strings ride up on the bridge=20
>>> pins. Assuming positive bearing and proper bridge pin angles with a=20
>>> bridge and pins in pristine condition that is not likely.
>> DSkolnik
>>Most of these discussions make these assumptions, or that any evident=20
>>negative front bearing is due to overly aggressive tapping. In my=20
>>experience, these are not self evident. Negative front bearing can exist=
=20
>>(with positive net) from relatively early in the life of a piano, due to=
=20
>>errors in manufacture, bridge roll, or other conditions inducing severe=20
>>compression.
> RNossman
>Then you apparently misunderstood what I was trying to explain to you, or=
=20
>I didn't explain it clearly.
I am not sure what I misunderstood, however, I may have been unclear. I=20
felt that these assumptions seemed to be prevalent in the current=20
discussions on CAUT, starting with Tuning Stability from Jeff Stickney,
DL
>>>At the same time, compression on the bridge top (exacerbated by tapping=
=20
>>>down on the strings) lowers the contact point on the bridge.
>> DS
>>Assuming enough downbearing ( at some point in time) to compress the wood=
=20
>>fibers, I would place more responsibility on the seasonally induced=20
>>increase in downbearing more than the unsubstantiated certainty of=20
>>aggressive tapping. And, of course, there is the speculation that the=20
>>bridge surface itself rides up the pin in humid conditions, in turn,=20
>>pushing the string further up the pin.
> RN
>This isn't speculation. It can be easily enough measured by anyone willing=
=20
>to take the time and trouble to do so.
Am I being irresponsible if I believe you on this without testing it=20
myself? "Speculation" was a poor choice of words. I just wasn't sure that=
=20
any definitive study of this phenomena had been conducted, and was thus=20
reluctant to make an unsupported assertion. I'd love to know if such=20
documented data exists. Should I assume that the remainder of my statement=
=20
concurred with your views?
DS
>>The question there would be whether the string then follows the board=20
>>back down in the dry season.
> RN
>This can also be measured.
Is this conceptual model commonly shared by other readers?
DL
>>>Unseating on the bridge top tends to occur when the contact point on the=
=20
>>>bridge top is lower than the indentation in the side of the=20
>>>pin. Therefore, you are much better off tapping down the bridge pin=20
>>>than the string.
>> DS
>>But this assumes either that the pin is not already bottomed in the hole=
=20
>>or that you can safely drive the pin into the bridge body, like a nail.
> RN
>No pin will remain bottomed in the hole with the bridge changing overall=20
>height with humidity swings. The point of zero relative movement between=20
>the bridge and the pin is typically somewhere near the bottom of the cap -=
=20
>depending on the type of capping material used. That means that as the=20
>bridge top is going up the pin, the bottom of the hole is getting deeper,=
=20
>and moving away from the base of the pin.
Ron, I can picture what you're describing but I can't grasp the methodology=
=20
by which you determined it. Can you explain it? Do you assume that the=20
pin, once so displaced, would return to its bottomed position in the=20
following dry season, or rather that it remains elevated? If so, wouldn't=
=20
the pin eventually work its way out of the bridge? Assuming you cared=20
about having the string contact the front of the bridge, do you agree that=
=20
tapping the bridge pin rather than the string would achieve that end?
DL
>>> At worst, it can create a further disconnect as the contact point on=20
>>> the bridge top is lowered due to further compression of the bridge top.
>> DS
>>Here it is again. What do you imagine (I don't know the answer) the=20
>>differential between the force needed to seat the string and that=20
>>required to further indent an already compressed piece of rock maple? I=
=20
>>find the Compression by Tapping argument suspect.
> RN
>Look up the side grain compression limit of rock maple (1470psi). Then=20
>figure the footprint in square inches of the area under a string being=20
>tapped. Make it effectively about 0.25"long and 0.010"wide. That's 0.0025=
=20
>sq"*1470, or somewhat under four pounds that can be applied to the string=
=20
>before the crush limit of the maple is exceeded. Long term deformation=20
>will happen at a lower figure. The friction level between a new, undamaged=
=20
>bridge pin and a string at a 10=B0 side bearing angle and 160 lbs of=
tension=20
>is over 14.5 lbs, or roughly 3.5 times the force needed to crush the=20
>bridge top under the string. An old grooved pin will have a wider string=20
>contact area and an angled depression for the string to have to be forced=
=20
>up as the pin is driven, so you can expect a considerably higher friction=
=20
>figure. By then, the string depression in the cap will be wider too, and=20
>the psi load will hopefully not be much higher than with a new pin. This=20
>is from the presumably gentler than tapping strings process of driving the=
=20
>pins. What do you suppose the impact psi levels are with someone tapping=20
>strings with a brass drift and a hammer? Maybe a hammer shank, since if=20
>the shank isn't damaged, the bridge won't be either. The compression limit=
=20
>for the end grain of that shank is listed at 17,830 psi, or over five=20
>times the side grain limit of the cap.
Stop here for a moment. I appreciate the data and have no reason to=20
question these figures, unless you believe that they prove that an elevated=
=20
string cannot be rendered to the bridge without further compressing the=20
surface.
RN
>The point is that the edge of the bridge cap has already been crushed by=20
>the expanding bridge before anyone feels compelled to tap either strings=20
>or pins. This means that the edge of the bridge (under the string) is=20
>rounded down to lower than the center, lower than a point even a few=20
>millimeters back from the edge, and lower than a line between that point=20
>and the capo. A bridge with little (but still) positive front string=20
>bearing will have strings terminating on the bridge pin, and a spot some=20
>small distance back from the notch edge, but passing slightly above the=20
>notch edge in a straight line to the capo. This string has NOT climbed the=
=20
>pin, and tapping either it or the pin down will NOT make it stay down. It=
=20
>will straighten back out and again, not touch the notch edge, and if the=20
>pin is loose, will produce a false beat.
I think we may agree, but I'm not sure. If you conceive of Front Bearing as=
=20
that part of the total downbearing which pulls the bridge mass forward,=20
then I can understand the configuration you describe above. As it relates=
=20
to termination however, Front Bearing, as I understand it, is precisely=20
about the relationship of the vibrating string segment to the front edge of=
=20
the bridge and those few millimeters behind the edge. If that edge is=20
below the line formed by the apex of the bridge and the front string=20
terminus, then a seated string will describe a negative front bearing=20
angle. Any noise elimination achieved by such seating will be temporary, as=
=20
you point out.
I'll admit however, I'm confused. You say the string does NOT climb the=20
pin, yet, in the above example you point out that the tapped string will,=20
in fact "straighten back out". Wouldn't this qualify as climbing the pin,=
=20
since I don't believe you are saying that either the bridge surface or the=
=20
pin is responsible for the string's re-elevation.
If you are not concerned about the pin and notch terminating the string=20
simultaneously, as you seemed to indicate at the beginning, then it would=20
seem that negative front bearing is not a problem for you.
The question remains for me, whether there is any audible or measurable=20
difference between the vibrational pattern of a string which terminates=20
simultaneously at pin and notch, and one which lacks a defined, horizontal=
=20
edge.
RN
>>> Furthermore, false beats are usually a product of loose bridge pins=20
>>> and a flagpolling of the pin which creates an oscillation.
>>
>>The false beats created by loose bridge pins is different from the=20
>>distortion caused by faulty termination.
>
>Yes they are, and these discussions need to start, and stay, with one or=20
>the other until some of the questions are answered.
Again, Ron, I'm confused. In the opening comment of your reply (top) you=
=20
said:
>As long as the bridge pin is solid in the top of the cap, there won't be a=
=20
>false beat to lead someone to tap the string - or pin.
Yet, at the end, you seem to acknowledge that there is can be some=20
distortion related to what I called faulty termination, by which I was=20
referring to "unseated" string.
What I can tell you is that before it gets too humid, I'm going to try=20
tapping some bridge pins, just to see. I'm open.
David Skolnik
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