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Compensated Bridges
Compensated Bridges
As promised, a few words about compensated bridges. The why and the wherefore!
The whole idea of bridge compensation, as noted last month, is to try and get closer to perfect intonation. Because the strings have thickness, which tends to inhibit their ability to vibrate, the actual length needed, for a string to vibrate at the exact frequency required to produce a perfect note, is a little longer than the theoretical scale length. Also, you want the strings to give you different notes, so to achieve this they are all different thicknesses. Consequently all need slightly differing lengths over which to reach perfect pitch. Hence….. compensation.
You would think that there would be a standard formula for bridge compensation, taking into account different scale lengths and string gauges, but I’ve seen nothing simple that approaches this to date. It is as simple to start a bridge’s compensation off in general terms, and fine tune to suit individual string choice and scale length, which remember can vary a fair bit!!
So what are my general ground rules? Well, I use the following as a start, with a normal set of 4 plain and 4 wound strings….
- E: no compensation
- A: 2mm back (.079 inch)
- D: 1mm back (.039 inch)
- G: 2mm back (.079 inch)
- (The D is further forward than the A because the core, which is most crucial part, is thinner.)
Apparently Embergher’s compensation on the bridge was very similar to this. I also find that the bridge itself, often needs to be 1-2mm further back than is theoretically correct. Generally this movement of the bridge as a whole will bring you as close as you need to perfect intonation, without further tweaking.
If further work is needed, fine tuning the compensation can be finished on the instrument, with a small file. I normally check the amount of compensation needed, by playing the harmonic on fret 12, once the string itself is in tune at the 'correct' length. If the harmonic is sharp, you need to push the compensated part of the bridge back a little and try again.
It can be seen from this why the bridges on old Neapolitans are sometimes seen to be angled, with the bass side further back. It is an approximation of compensation, for instruments with brass saddles, for instance, on the very low shelf bridges.
The thing to remember is that only the most attuned ear will be able to hear small variations in pitch, so get as close as you can. What with temperature and humidity changes, absolutely perfect pitch is virtually impossible to attain, and definitely impossible to maintain.
Best regards, Dave
Tip by Dave Hynds, July 2007
