|
[About Us] [Guestbook] [Contact] [Info] |
A R T I C L E S
The Placement and Fit of Archtop Instrument Bridges
by Bruce Morrison
The owners of Archtop fretted instruments are well aware of the importance of having the bridge correctly located for optimum intonation. Yet, two of the most important aspects of Archtop instrument bridging, the fit of the bridge feet to the soundboard and having the bridge stand perpendicular to the soundboard, are frequently overlooked. When either of these particulars of bridging is incorrect, an instrument's ability to produce its best sound is impaired.
Placing the bridge of an Archtop instrument for the optimum intonation is a two-step operation. The first step measures the bridge's mathematically correct position, and then adds an approximate amount of compensation. Using that position as a reference point, the second step determines the bridge's correct compensation position. Use the metric scale to make these measurements. Because metric uses smaller units of measurement, it's more accurate. Also, it's easier to read because there are no fractions to correlate.
To determine the bridge's mathematically correct position for the first string, measure the distance from the front face of the nut to the middle of the twelfth fret. Double it. The doubled figure is the bridge's mathematically correct location. Now we add a little extra to approximate the compensation. This is done to get us as close to the bridge's correct compensated position as possible so that we'll have to move the bridge as few times. For a short scale instrument such as the mandolin, add 2mm. Longer scale instruments such as the guitar require the addition of 4mm. For the bridge's location on the lowest string, add 2 to 3mm to the figure for the first string on a short scale instrument, and 3 to 5mm for a long scale instrument. Once you've recorded those figures, tune your instrument down to about three-quarter tension before repositioning the bridge. A bridge is much easier to move with the strings at reduced tension.
To set the bridge in its correct compensated position, chime the first string at the twelfth fret. Then sound the string fretted at the twelfth fret. Compare the two notes. If the fretted note is flat, move the bridge away from the nut. Repeat this process until the two notes match. Do the same with the lowest string. That puts the bridge in the correct compensated position for the highest and lowest strings. You may find that it sits at a slight angle, not perpendicular to the strings. That's OK. Once it's in place, bring the instrument back up to concert pitch, being careful to keep the bridge perpendicular to the soundboard. Double check the intonation at concert pitch. You may need to readjust the bridge's position. Again, make any adjustments at reduced tension. As the fine tuning of the inner strings may require alteration of the bridge saddle, in most cases it's best to have a qualified repairman make those changes.
When the bridge feet do not fully contact the soundboard, string energy is dissipated into the air rather than being passed on to the soundboard. The result is an instrument that doesn't sound its best. When the bridge feet are properly fit, you should not be able to slip a .002" feeler gauge between them and the soundboard.
The bridge must stand perpendicular to the soundboard. Even though the bridge feet may be in full contact with the soundboard, having the bridge saddle lean forward or backward prevents the strings from exerting their maximum downward pressure on the soundboard. If that pressure is not achieved, the instrument will not produce its best sound. This needs to be checked every so often as tuning up tends to pull the bridge saddle forward. You'll notice that this will probably happen as you return the instrument to pitch after moving the bridge. Depending on the arch of the soundboard, there are cases in which the bridge feet may be well forward of the saddle even though the bridge is standing perpendicular.
To review, there are three key factors of Archtop instrument bridging which help an instrument sound its best. First is having the bridge correctly placed for intonation. Second is having the bridge feet properly fit to the soundboard. Third is having the bridge stand perpendicular to the soundboard.
Back to Top of Page Return to Articles by Bruce Morrison index page]
[About Us] [Guestbook] [Contact] [Info]