Cello Bridge Design
Cellos vary hugely in their tonal and responsive personalities, from the darkest sounding English cello to the brightest French instrument, and from a swift, responsive Guadagnini to a slower, more demanding Montagnana. An intelligently cut bridge (along with a well fitted and adjusted sound post) can transform a cello with potentially unbalanced qualities into a useful and rewarding instrument which is well adapted to the work being asked of it, whether orchestral, chamber or solo performance.
The bridge and sound post are at the heart of a cello’s set up, forming the fundamental link between the resonant body of the cello and the strings. The design of a bridge therefore has a profound influence an instrument’s tone and response. The design of the bridge should complement the inherent tonal characteristics of a cello, bringing the projected and ‘under the ear’ sound of the cello as close as possible to the player’s ideal. The bridge contributes to the quality of support and resistance experienced under the bow and is used to achieve the desired balance between power and flexibility of response. Bridges can be cut to influence the timbre of a cello’s sound, making it more or less open, balancing the registers of the cello to create an even tonal response and adding brilliance or depth to the sound.
The two most familiar bridge designs are French and Belgian. The legs of the French bridge account for approximately half its height and within this basic design there is plenty of latitude for the luthier to choose slightly different shapes and thicknesses to control the tonal outcome. The French bridge is often a good choice for bright-sounding cellos. The Belgian bridge has longer legs than the French model and a more compact heart and upper body. The reduced mass of wood above the Belgian heart produces a sound which is brighter and more open than the French bridge – and often louder. The Belgian bridge emphasises the upper register of the cello and can also be used to make the sound of gut G and C strings more crisp and clean. Cellos with an inherently dark sound often benefit from the fitting of a Belgian bridge.
Despite the common misconception that the Belgian bridge is a modern innovation, it has been in use for many years and is a direct descendent of the bridge model commonly used in England in the heyday of English cello making circa 1800. There are many examples of this c.1800 bridge model which is now usually called a ‘Forster’ bridge but was also used in other fine workshops such as Betts, Hart, Fendt and Panormo. It is therefore likely that most English cellos made from 1800 to 1840 would have started life with the equivalent of a Belgian bridge which could have given a brighter and more open sound to English cellos than the French bridge design which became fashionable during the 20th century.
Another frequent misapprehension is that Belgian bridges put more tension on an instrument than French bridges. Belgian bridges can appear taller than their French counterparts, due to their different proportions and longer legs but in fact, the choice of a Belgian as opposed to French bridge will have no effect on the string clearances, string spacing, bowing curve or the string tension on the cello.
A good cello bridge is made from unfigured maple with an extremely fine growth and a very strong cellular structure. We believe that the a cello’s set up should be treated with the greatest respect, only making changes when it is clear that the original bridge and post are not functioning well. When an old bridge needs replacing, we follow as closely as possible the elegant, organic models passed down to us by earlier craftsmen. Our bridges are cut from a reliable and consistent stock of wood and we produce only first class cello bridges so that every piece of work contributes to our working knowledge of tonal control.
The design of a bridge should be a natural complement to the cello. A French bridge is often a good choice for bright cellos where extra depth and interest is desired in the sound. By contrast, cellos with an inherently dark sound often benefit from the fitting of a Belgian bridge which emphasises the upper register of the cello and which can also be used to make gut G and C strings sound crisper and cleaner. Depending on how they are designed and cut, it is quite possible for there to be a tonal overlap between Belgian and French bridges and in fact many cellos could have very satisfactory set ups with either bridge model. The Belgian bridge that we use is a particularly moderate design and is made from a custom bridge blank designed by Robin and not available as a standard product from bridge manufacturers.
When assessing a cello we focus on the existing set up and try to understand the player’s tonal ideal. If the current bridge has clear shortcomings, we will consider our portfolio of bridge designs and decide which model will come closest to producing the player’s ideal sound. We will then adjust the design of this model to suit the individual instrument and player, cutting the bridge to influence darkness and brightness, resistance and flexibility.
As with every other vibrating part of a cello, the bridge will improve with age provided that it is looked after well. When fitting a bridge we always make a special wooden measuring gauge which fits between the end of the fingerboard and the bridge. This gauge is a useful tool for checking that the bridge is standing straight and, if used regularly, will help to maintain the cello’s sound adjustment and prevent the bridge from warping.
The choice of bridge design also affects the feeling of an instrument under the bow. A French bridge will offer a player more bowing resistance and the Belgian bridge less. Changing from one bridge design to another does require some adjustment on the part of the player, particularly for cellists who have spent many years playing on one bridge model. However, an appropriate change of bridge design can be a very fulfilling and/or liberating experience for the player.
|Physically heavier design||Lighter-weight design|
|Inherently darker sound||Inherently brighter sound|
|Quieter under ear||Louder under ear; may also project more powerfully|
|More bowing resistance (harder work)||Less bowing resistance
|Slower, less immediate response||
Quicker, more immediate response
|More flexible in shaping and colouration of sound||Less flexible in shaping and colouration of sound|
Scientific research. Bridges have been the subject of surprisingly little scientific study but in the late 1980’s O.E. Rodgers and T.R. Masino* carried out some useful research at the University of Delaware. They created finite element analysis models of violin and cello bridges, using engineering software to predict how the bridges would vibrate as structures. To achieve this involved the laborious process of compartmentalising each bridge into a 3-D jigsaw of tiny segments, thus reducing a large complex structure into a series of small, simple structures. A computer was then used to analyse the behaviour of each tiny segment and to compile all the results to predict how the whole bridge will bend and stretch when forced into vibration by the strings.
Rodgers and Masino discovered that there are a series of basic modes of vibration for a bridge. The most important mode is an ‘in-plane’ vibration, a simple side-to-side ‘rocking’ movement, shown in cartoon 1 (left). Another important mode is an ‘out of plane’ movement like an alternate shoulder thrust (2, right). Other modes of less importance are the high-frequency ‘bounce’ mode (3) and the low-frequency ‘skating’ mode (4) both below. This analysis of bridges has proved to be remarkably accurate, as Robin discovered during a series of experiments at Oberlin College, Ohio which were based on the results of this finite element analysis. Knowledge of these modes allows the luthier to visualise how the bridge moves as it is played and provides a theoretical framework on which to base his/her practical experience when cutting a bridge.
Splayed bridge legs For a good tonal response, string pressure needs to be directed to the centre or, even better, towards the outer edges of the bridge feet. It is fairly common for bridge legs to become splayed or over-widened on a cello with very rounded/pointed arching, and this leads to a deterioration in the cello’s tone. Bridge legs also tend to become splayed if a cello lying on its side is accidentally rolled over onto its bridge. Usually no structural damage occurs, particularly if the floor is carpeted, but the cello will sound dreadful because the bridge legs have spread sideways, putting pressure on the inside edge of the bridge feet and causing a temporary but major acoustical problem. If you can see air under the outsides of the bridge feet, they are definitely splayed and you’ll need some help to get the legs aligned again. In this case the luthier will squeeze the legs back towards each other until the bridge feet are in correct contact with the cello, which should ensure that the cello once more sounds like a desirable instrument, rather than the worst student cello you have ever heard.
Lost bridges. We are sometimes consulted by cellists whose instruments used to sound wonderful but have somehow lost their original quality. Even after visiting several different luthiers and having a series of changes made to their instrument, they are still unhappy. When we talk back through the history of such cellos, we frequently discover that the cello first stopped sounding good when the owner had to change the bridge because it was too low, was warped or broken. In some fortunate cases the owner has kept the old bridge, and it has been simple for us to copy the bridge and restore the original sound. Sadly though, it is common for owners to leave their old bridge with the luthier who has cut the replacement, which means that we have to start from first principles when determining what bridge design would best suit their cello. Always hang onto your old bridge as a reference, particularly if you were happy with the cello’s sound when it was on the instrument.
Bridge straightening In the days when all cellists used gut strings, players would routinely straighten their bridges almost every time a cello was tuned, due to the stretchiness of gut and its responsiveness to changes in temperature and humidity. Players knew their cello wouldn’t sound good unless the bridge was standing upright. However, our stable modern steel strings tend to stay in tune from one day to the next, so it is not necessary to straighten the bridge every time we tune a cello and many of us are no longer in the habit of straightening our bridges. However, the top of the bridge is affected by the movement of the strings over the string grooves, particularly if the pegs slip or if new strings are fitted to the cello. Failing to straighten the bridge can lead to the bridge becoming warped and it also causes deterioration in the tonal response of the cello due to the change in the bridge angle.
If a bridge is leaning back very slightly onto its ‘heels’, or leaning forward onto its ‘toes’, this is equivalent to a sound post adjustment and can change the sound of the cello significantly. The posture of the bridge should always remain upright so that the downward pressure is equal on both edges of the bridge feet – neither on ‘toes’ or ‘heels’, just comfortable. If you can see air/light at the back or front of the bridge feet, then your bridge definitely isn’t straight.
If you put new strings on a cello you will need to straighten the bridge as you tune the strings (first tune the string, straighten the bridge and then re-tune the string.) If it is difficult to straighten your bridge under full string tension, this indicates that you need to lubricate the string grooves on your bridge using either dried-out soap or graphite. It’s also advisable to lubricate string grooves when you change a string.
The effect of wood removal on bridge frequencies O.E. Rodgers and T.R. Masino University of Delaware. Newark. DE 19711. USA
Published in the Catgut Acoustical Society Journal Vol. 1. NO. 6 (Series III November 1990)
© Robin Aitchison & Sarah Mnatzaganian 2014
(an extended version of our bridge design article first published in 2006)