String Seating
Some basic facts as to its effectivness


Consider the simple triangle below (fig1).  Let line AB represent the line a non-deflected string would make, and point C represents the highest point of deflection the string will experience from a given bridge surface. Lines AC and CB are hereafter refered to as the delfection lines and are not to be confused with line a string will make when stretched across the bridge.



fig 1
Fact : As long as the entire bridge suface remains on or above these deflection lines,  AC and CB, (fig 2), then it  has the potential to be in contact with any string stretched across these three points.




fig 2

In figure 2,  the bridge face is assumed parallel to AB.  If a string is now stretched across ACB then it will be initially deflected to height C at both edges of the bridge surface.  With the passing of  years the surface of the bridge will become indented under the string for a variety of reasons, lowering all or part of its surface. This indentation gets deeper gradually towards the edges of the bridge surface. All other things being equal this process  will continue  until eventually the string line and the  deflection line are identical (or very nearly so).

Regardless of the causes of the indentation, it is clear that while all of the bridge suface EC is above line AC the bridge surface between points E and C will maintain its potential to stay in contact with the string. The same can be said for CF and CB repsectivly of course.  The angle downwards of an indentation itself is not a determinant under normal circumstances.*  Also note that the height of point C is not static. Seasonal changes can vary its height and hence the gradients of  the deflection lines. A permanant lowering of point C can occur because of indentation at the middle of the bridge.  Regardless of the height of point C however, the statements  made just above hold true. 

With this in mind, figures 3 and 4 depict two conditions where string seating would be effective. Whether it is possible for these two conditions to occur or not is delt with briefly later on in this document. However, accepting for the moment, that these two conditions are possible and assuming normal wear circumstances and proper seating proceedures,  string seating will be effective and enduring  because the entire surface of the bridge in both cases lie above the deflection lines



fig 3



fig 4

If / when an area on the bridge surface close to either point E or F actually becomes lower then the deflection lines then that area no longer can stay in contact with the string unless some other force is holding the string down. Figure 5 shows the front edge of the bridge in this condition. It is first when this condition exists that string seating becomes totaly ineffective. The clamping ability of the bridge pins will not be sufficient to hold the string down for more then a short period of time at best.**  It goes without saying that string seating will also be futile if the surface of the bridge is not able to deflect the string at all.



fig 5


The issue of the effectivity of string seating procedures largely rests on what degree anyone holding an opinion on the matter accepts whether or not a string may find itself in need of seating in the above conditions.  If one refuses to acknowledge that the conditions depicted in figures 3 and 4 can exist, then one is bound to conclude that any string seating proceedure is in the end futile and will exhasperate the condition that caused the need for string seating in the first place. If one on the otherhand acknowledges that these two conditions can exist, an explanation as to what causes the strings to move up the bridge pins may be desired but is not strictly necessary. To show whether either of these conditions is in fact the case one only need confirm that the string does in fact need seating, and that the bridge surface is in fact in its entirety over the deflection lines.*

This text does not attempt to show any kind of a mathematical <<proof>> one way or the other as to whether or not any given condition can or can not exist.  It is doubtful a model sufficient to describe adequatly all the various forces involved on the bridge surface, strings, and bridge pins under various climatic conditions, including the dynamics of the system could be contrived with out a great deal of effort and resource.  It is worth noting however that in addition to what has already been mentioned it is possible to observe and measure a decrease in distance between the strings and the top of the bridge pins during periods of increasing humidity. This points clearly to a string being pushed upwards by the surface of a bridge expanding due to taking on humidity.  Normal bridge compression under the string  will also occur as the face of the bridge strains against the downward pressure of the string downbearing and the partial clamp the bridge pins provide. Under normal circumstances this will result in a curved bridge surface that does not preclude any string seating effectiveness.*  The combination of these processes, the dynamics of any particular piano can result in any of the above conditions. This fact only underlines the complexities of the interactions taking place.


Richard Brekne, RPT

* --  The angle downwards at which an indentation of the bridge can take can be artificially made severe enough to cause the equivalance of a bridge pin inserted beyond the notch edge.  This can only be caused by over aggressive and erroneous string seating proceedures, and not by cyclic seasonal factors or by carefull and correct string seating.  A string stretched over a bridge will try and form a straight line from its last unclamped point of contact with the bridge to the termination point.  But the string will also attempt to conform to  the deflection lines.  Eventually left to its own, the tendancy towards the later will  equalize any and all conditions where the  former sees any part of the string  not  in contact with the bridge. This is true as long as the entire surface of the bridge remains above the deflection lines.  Some manufacturers purposely insert bridge pins just beyond the notch precisely because of this tendancy.

Seating a string with an indentation (or notch) behind a bridge pin and below the string line can  force the string into a zig zag shape which will be unstable unless appropriate regard to the condition is taken.   Judicious seating of strings in this situation however can cure the condition.  The techician can seat the string in such a way as to cause an indentation that becomes even graduated curve over the suface of the bridge that does not have a higher gradient then the string line from the end of the bridge to the termination points. In extreme circumstances this can necessitate a lowering of point C.  This underlines the need to excercise care and correct proceedure when seating strings in the first place.  A peice of  medium hard wood just less then the width of the bridge surface is used to tap gently on the strings behind the bridge pins and on the bridge surface itself.  Any downward pressure on the string outside of the bridge surface is bound to cause damage and risk this zig zag condition.

Seasonal changes alone (even assuming an absolute clamp at the notch  by the pin) will cause the string to indent the bridge with a curvature that decreases proportionaly with the distance from the clamp i.e. bridge pins. The resulting indention will  leave the string in contact with the bridge at the notch, curving gently over a now rounded bridge surface. As long as this curve in its entirely lies above the deflection  lines the any needed string  seating will be effective and enduring.

** -- During seasonal changes the height of the bridge changes. After many years it is possible for the condition depicted in fig 5 to be evident durning dry periods only. String seating will be of marginal value.

copyrights reserved
May 2005