The sound of musical instruments changes when you make even slight alterations to their design and construction. Small changes to the bridge or nuts can alter the sound of a mandolin. The bridge of the mandolin supports the strings above the sounding board and elevates them so that a musician can play a note or chord by pressing on one or more strings along the neck of the instrument. To achieve a clear sound from each string without any rattle or buzz, the slots for the strings on the bridge must be cut in the correct manner.
Determine the location of the bridge on the sound board. This is at twice the distance between the nut and fret 12, plus 1mm. Place the bridge at a location in the centre of the sound board at the calculated distance.
Calculate the location of each of the eight strings of the mandolin and mark them on the top of the bridge in pencil. Find the locations of the two outer strings on the bridge by pinning a string between the nut and the bridge. Measure 2mm in from the edge of the nut and stretch the string to the bridge. Examine the string as it passes over the fingerboard and ensure that there is at least 2mm between the string and the edge of the fingerboard. Wherever the string passes over the bridge is the location of that outer string. Repeat the process for the other outer string and mark its location. The distance between these two points is the spread of the strings.
Mark the location of the other two sets of two strings on the bridge. Separate each set of two strings from the next set by 7 to 9mm and the two strings in a set by 2 to 4mm measured by the centres of the strings. Place pencil marks on the bridge to note the location of the strings.
Use a half-round file of the correct size to make a U-shaped slot in the top of the bridge for each of the strings. You will use a different file for each string. The slot should be filed to the depth of half of the string's diameter and no wider than the diameter of the string. If necessary to ensure that the slot is not too tight against the string, widen the slot slightly. File the slots with the back of the slot lower than the front. Do not file the front lip of the slot because this serves to stop the string with a clean and crisp sound. The string will follow the slot as it drops towards the tailpiece of the mandolin.
Restring the mandolin and check each string for sound. If there is a rattle or buzz, move the string and file the back groove more, but don't over-file. Replace the string and check the sound again until it doesn't rattle or buzz.