Homepage News & Articles New Technology Pianos. A Dealer's Personal View

New Technology Pianos. A Dealer's Personal View
Article published on 30 December, 2006

Phoenix Agraffe Systems,


Conventional bridge system

It is long established practice for the strings of pianos to pass from tuning ping via a fixed point normally on the frame, which defines the beginning of the sounding length, and thence over the bridge with which the string is held in contact and through which the vibration energy in the string is transferred to a sounding board.  In a grand piano the string is set in vibration by striking it from beneath by a felt covered hammer.  The blow tends to lift the string and thus imparts a separating force between the string and the bridge cap.  It is therefore necessary to provide a downwards force on the string, called down bearing, to ensure it remains in contact with the bridge cap or vibration energy transfer would be interrupted if the string parts from the bridge cap.  The surface area of the strings is inadequate to excite significant sound waves in the air surrounding the instrument. Thus no substantial sound would be heard unless the soundboard itself receives the vibration energy input.

 Two principal means are conventionally used to keep the string in firm contact with the bridge cap.

The string changes angle in the vertical plane as it passes over the bridge cap. Thus it creates a downward pressure onto the bridge cap.  The change of angle is typically about 1 to 5 degrees and may be varied across the registers of the piano so that optimum contact force is developed for best sound in each register.  Each string is located in the horizontal plane on the bridge by two bridge pins so positioned that the string is wrapped around each pin in the horizontal plane. The first pin defines the end of the sounding length. The two pins are so positioned that the string line is displaced sideways as it traverses the bridge.  The string changes angle by about 15 degrees in the horizontal plane, which is the amount of wrapping round the bridge pin.  This change of angle combined with the tension in the string creates a lateral force against each bridge pin.  The bridge pins are inserted in the bridge cap at about 15 to 20 degrees to the vertical, an angle that causes the string to tend to slide down the pin and thus press against the bridge cap. This develops an additional pressure contact between string and bridge cap without, in this case, causing a down bearing load on the soundboard.

 

Disadvantages of the traditional bridge system

This traditional system has consequent disadvantages that affect the performance and durability of the piano.

 

The down bearing force resulting from the change of angle of the string in the vertical plane as it passes over the bridge from about 200 strings in a typical piano amounts to approximately a half tonne.  That force is applied constantly throughout the life of the piano to the soundboard.  The soundboard must therefore be so designed to withstand the load for the life of the piano.  This entails creating a soundboard stronger and stiffer than would be ideal for optimum acoustic performance.. If the designer creates a thin light soundboard he can enhance the piano acoustic performance at the expense of a shorter instrument life.  As the crowning of the soundboard sinks with time due to this load, the contact force between strings and soundboard decreases and the efficiency of transfer of vibration energy of from strings to sound board becomes compromised.  The piano in consequence loses its acoustic performance.  In extreme cases the string may part from the bridge cap when the hammer strikes and then buzzing sounds develop

 

The downwards loads from the strings to the sound board may also compromise the freedom of the soundboard to respond to the vibration energy from the strings and this may result in degraded sound volume and quality, the duration and the harmonic content of the sound developed is spoiled.  It is for example well established that down bearing load on the bass bridge of a piano will suppress the performance of the middle treble range of the registers because the bass bridge is located near the soundboard zone that needs to respond to the middle treble register frequencies.

 

Because the strings are struck from underneath it is found that with forceful playing they tend to slide up the inclination of the bridge pins and be held away from firm contact with the bridge cap by friction between the pin and the string.  Piano technicians will need to tap them down again after vigorous playing or the sound performance will suffer.

 

The bridge pins are inserted in the bridge by drilling inclined pilot holes of controlled depth.  It is critically important that each pin is firmly located and the hole into which it is fitted is not bell mouthed which would allow the pin to flex at its upper end.   An insecure pin may result in a badly defined length of the sounding portion of the string.  In that case each string may produce a varying frequency known as a false note.

 

The two sets of bridge pins are mounted about 20mm apart.  Because of the wrap angle round the pins the string line is displaced sideways where the string traverses the bridge.  In consequence of this asymmetry it is found that a piano string that is initially excited to vibrate in the vertical plane will begin to develop a component of vibration in the horizontal plane.  The plane of vibration rotates with time.  The quality and volume of sound when the string is vibrating horizontally is degraded and the overall sound quality of the instrument may be compromised by a weak or variable note quality.

 

Where the string traverses the bridge cap it lies in contact with the bridge cap surface.  When striking with the hammer excites the string, most of the vibration energy is in transverse movement, but a proportion also appears as longitudinal vibration.  Longitudinal rubbing between the string and the bridge cap results in loss of vibration energy in the string and reduction in the efficiency of use of the available vibration energy produced in the string.

 

 

The Phoenix bridge agraffe system addresses all these issues and results in a more efficient transfer of vibration energy from string to soundboard.

 

Down bearing

A key feature of the phoenix agraffe is manner in which an adjustable amount of contact pressure between the string and the bridge cap is arranged. Without applying any down bearing load on the bridge or soundboard.

 

The string passes first over the knife-edge, which is retained in position in the horizontal plane, by location in a slot in the agraffe base.  The knife-edge is however free to move in a slide in the vertical plane. Contact pressure is developed between the knife-edge and the bridge cap by a change of angle in the string line in the vertical plane as it passes over the knife-edge.  The string is deflected downwards by a roller located under a clip in the bridge agraffe..  downwards deflection of the string by the roller causes the change of angle of the string as it passes over the knife edge.  The amount of string deflection and thus the load on the knife edge  can be adjusted by changing the roller diameter.   Depression of the string by the roller causes an upward force on the roller and therefore the agraffe body, which is equal to the sum of the downward force on the two knife-edges.  it is therefore necessary to provide an equal and opposite force to retain  the agraffe in contact with the bridge cap.  This is done by a screw  which attaches the base of the agraffe to the bridge cap .  the screw is secured in the bridge cap using an insert in the wood to ensure the load of some 75 kgms can be held.  Because the knife edges are independent of the agraffe body,  the upwards pull of this screw on the bridge cap exactly equals the downwards force applied to the two knife edges.  There is therefore no resulting down bearing load on the bridge cap or the sound board if the plane of the strings does not change at the bridge cap.  This is ensured by providing adjustable height hitch pins.

 

In practice a small amount of down bearing may sometimes be advantageous  and may be used to match the note quality on adjacent notes.  The adjustable hitch pins serve this purpose..  It is also found that by changing the roller diameter and thus the pressure between the knife edge and the bridge cap, the note quality can also be modified advantageously. 

 

Side draught

In the Phoenix Bridge agraffe system there is no side draught in the horizontal plane of the strings. In consequence the strings have reduced tendency to develop a rotation of the plane of vibration.  The clarity and purity of the note is thus improved.  It is also observed that the symmetry of termination of the sounding length at surfaces in the same plane is advantageous in this respect.

 

Bridge Pins

The Phoenix system does not incorporate bridge pins.  The tendency for development of falseness  due to insecure pins is eliminated.

 

Compliance

 Efficiency of conversion of finger energy to sound energy is a critically important matter in determining the acceptability of the artist/instrument interface of any piano.  The efficiency of transfer of vibration energy is affected, inter alia, by the compliance of the contacting surfaces, the pressure of contact between the surfaces , and the area of the surfaces.  The Phoenix system allows optimisation of the factors affecting energy transfer. The compliance of a wire lying on the comparatively soft wood  surface of a bridge cap is not favourable to efficient energy transfer .  The compliance of the relatively larger area of the base of the Phoenix knife edge is significantly less (i.e.better)

Experience  has demonstrated that as the compliance of the contacting surfaces is reduced there is a tendency for enhancement of higher harmonics in the piano sound.  The Phoenix system incorporates a harmonic moderator pad under each knife edge which is conveniently applied by a complete covering over the top surface of the bridge cap, to permit adjustment of the compliance for sound optimisation.  This pad may be  made of fibre , rubber,  metal, wood, plastic, felt or any combination of these materials. The softer the material the less the enhancement of higher harmonics in the instrument sound and the lower the efficiency of transfer of vibration energy to the soundboard.

 

Application of the Phoenix System to other instruments

 

The phoenix agraffe system can equally be applied to stringed instrument with bridge system. for example a violin or double bass or a harpsichord.  Such instruments are particularly adversely affected by down bearing loads from the bridge onto the resonance box.

 

Alternative configurations of the Phoenix system

  

In the prefered arrangement the  location of the knife edges relative to the roller in the agraffe is achieved by slot recesses shaped to receive the knife edges  in the base of the agraffe.  Another option is for the knife edge to be located in a sheet template cut and shaped to receive and position  each knife edge correctly and independently on the bridge top. This configuration allows the agraffe body to be made significantly smaller and shorter.

 

Alternatively the knife edges can be positioned in the horizontal plane by a thin leaf spring attached to or integrated with the base of the agraffe that permits free movement of the knife edge in the vertical plane without  developing forces that would necessitate production of significant downbearing on the sound board..

Back to articles index
Site Map
Hurstwood Farm Piano Studios
The Hurst Crouch, Borough Green, Seven Oaks, Kent TN15 8TA, United Kingdom     T: 01732 885050    F: 01732 883030     E: info@hurstwoodfarmpianos.com

Web Design & Development by Doublard Design Ltd