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Bridge agraffes (I)
an alternative to traditional bridge pins
by Calin Tantareanu

1 October 2006

 

1. Introduction

 Y ou might ask, why does anybody need a new type of string termination? The quick answer would be, to prolong the piano's sustain and to enhance the contents of high partials in the tone. An improved sustain would be most desirable in the highest treble notes of a piano, whose sustain is often too short to be musically pleasing.

A good string termination is one which allows the vibration of the string to be transmitted via the bridge to the soundboard and then be transformed into sound waves, with as little loss as possible. Losses occur because the string termination is not stiff enough, thereby absorbing and wasting part of the energy it is supposed to transfer. This results in a shorter sustain and a weak sound. While this happens for any note of a piano's scale, it is most noticeable in the treble, where the strings have little mass and little energy to drive the soundboard.

 T he traditional method for constructing the string bearing point on the bridge is a notch in the wood with two metal pins, which form a sort of horizontal zig-zag clamp, holding the string against the wooden bridge.

◄ Typical metal bridge pins on a wooden bridge (bass bridge of an August Förster concert grand). [click on the thumbnails to see larger pictures]

This system has a weak point which cannot be avoided: it acts like a damper for very high frequencies. The reason is the following: the thin string bears directly on the wooden surface, which is inherently softer then the metal bridge pin, and has a certain flexibility. Because of this, the very high frequencies produced in the top notes of a piano (the topmost note, C88, has a fundamental frequency exceeding 4000 Hz), the relatively flexible string termination absorbs a good part of the string's energy, which means that much less remains there to reach the soundboard and be converted into sound. Other weaknesses of bridge pin terminations are:

• The pins getting loose over time, which can happen, given the high stresses the bridge wood is subjected to, even with the best workmanship possible. This makes the pin move and absorb even more energy from the string. The stress can be great enough to start cracks forming in the bridge cap.

• The strings cutting grooves into the wood, which gives an imprecise termination and again leads to wasted energy.

▼ The grooves cut in the wooden bridge cap by the strings (Steinway model O grand):

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2. Theory behind bridge agraffes

 A  radically different way of building a stiffer string termination is to use a bridge agraffe. While these are not a new invention (they have been used since the latter part of the 19th century), they are little known and until very recently they were not used by modern piano manufacturers.

A bridge agraffe is a metal device which couples the strings to the wooden bridge. It has been made in various shapes, but all have several features in common:

• The strings bear on metal edges, which are much stiffer and transmit high frequency vibration better when compared to the bare wood.

• The device has a large footprint on the wooden bridge and eliminates energy and tone loss because of wood deformation. Such deformation occurs easily when a thin metal string bears directly on wood (which is the case when using traditional bridge pins).

Bridge agraffes can be divided into 2 main types:

2bp - Agraffes with 2 bearing points for the strings. The strings goes under and over the metal bearing points. Depending on the construction of the device, it can induce a rotating force on the bridge, because the string is angled vertically.▼	3bp - Agraffes with 3 bearing points for the strings. The string goes over, under and again over the metal bearing points (see picture below). This cancels out the rotating force on the bridge. The device can also be constructed with the string angle reversed (under-over-under, not pictured).▼

While agraffes with only one bearing point (1bp) are possible, if they are combined with string downbearing, I haven't found any clear examples so far.

 

Continue to part II

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© Calin - Do not reproduce or distribute without my written consent.
Last update: 18.11.2006