The vibrations of the strings are transmitted to the resonator by the bridge, which plays an important role in the processing of vibrations. Placing a mute on the bridge (thereby toning down the high frequencies) suffices to hear the difference in power and timbre.
The bridge is an acoustic filter. The signal it transmits to the top plate will undergo transformations, resulting in a loss of the signal that will generally be greater for high frequencies.
The essential qualities of a bridge depend on the following characteristics: density, celerity, elasticity. Performance should be sought in thickness, shape, cut-outs, and frequency.
The bridge also absorbs part of the vibratory energy: optimization of it is therefore necessary.
(2) Hold the bridge by a rubber band looped through the heart or around one foot. Tap the bridge (preferably, with an object made of plastic) to hear its frequency.
Bridge blanks have a fundamental frequency from 260 Hz to 395 Hz and weigh from 3 to 4 g.
The bridges available on the market are standardized; consequently, the heavier bridges have higher density. The best ones have high density, elasticity, and frequency.
The thickness for the feet of a violin bridge is between 4 and 4.2 mm, depending on the characteristics of the wood. Width is from 41 to 42 mm.
The feet must be adjusted to a perfect fit with the curvature of the top plate’s arching. A good bridge must sit on the top plate naturally. The standardized height is 32 mm maximum. The back side of a properly adjusted bridge must be perpendicular to the line of the ribs.
The thickness of the bridge depends on its density and frequency.
A fitted violin bridge weighs from 2 to 2.3 g.
A free bridge should be tuned between 260 Hz and 360 Hz. It is advisable to tune the bridge to approximately 270 Hz, which corresponds to the relative top plate coupling frequency, as well as the A0 cavity mode frequency “with a sound post”. A frequency of 360 Hz gives the violin maximal power, but can reveal undesirable high ranking harmonics. Compression by the violin strings does not modify the frequency of the bridge: after installation, it remains identical.
The frequency of the bridge is inversely proportional to that of mode B1+. When the latter is above 550 Hz and the sound is aggressive, harsh, and raspy, the bridge’s frequency should be brought down as far as 260 Hz, leaving more wood in place, to yield a sweeter sound.
Give the bridge the standard thickness, then adjust the feet on the top plate. If the feet are left
too high, there will be a shortage of wood at the head of the bridge; if the feet are too short, there will be too much wood above the heart. In both cases, the frequency of the bridge and the sonority of the violin will be modified.
After fitting the feet to the top plate, adjust the height of the bridge. Note down its frequency, refine the final thicknesses, and remove wood from the various regions in order to obtain the desired frequency for the free bridge.
Thinning a bridge blank to the standard thickness lowers its frequency by 45 to 65 Hz.
Reducing the height of the bridge head to its standard value lowers its frequency by 45 to 65 Hz.
Irrespective of the properties of the bridge, once its thickness has been adjusted, its frequency rises by as many hertz as it dropped when the height of the bridge was reduced.
Removing wood from the various regions of the bridge yields the following outcome:
¨ (C) - raises the frequency by 5 Hz
¨ (E) - raises the frequency by approximately 10 Hz
¨ (F) - raises the frequency by approximately 10 Hz
¨ (D) - lowers the frequency by 10 Hz
¨ (B) - lowers the frequency by 10 Hz to 15 Hz
¨ (G) - lowers the frequency by 25 Hz to 30 Hz
¨ (A) - Thinning the bridge lowers the frequency by approximately 5 Hz.
To obtain satisfactory results, the frequency of the bridge blank must be at least equal to that
of the desired adjusted bridge.
Tuning the free bridge is necessary. However, according to the regions from which wood is removed in order to tune the bridge, the violin’s sonority and equilibrium are modified. Enlarging the heart and the eyes gives a clearer timbre, enhancing the higher harmonics. Beyond a certain limit, the sound becomes sour and aggressive.
A string lifter is absolutely necessary for improving a violin’s sound by successive bridge adjustments. The device must remain in place throughout the process until the strings are retightened and the bridge is once again under tension.
Slacken the strings: the tension on the neck and plates relaxes, increasing the B1- mode frequency and lowering the B1+ mode frequency by a few hertz. Once the strings have been retightened, the violin gives the impression that it breathes, has lovely sound and a perfectly adjusted bridge. But two days later, once the instrument has stabilized under the tension of the strings, the bridge setting is no longer correct, nor is the violin’s tone (which occasionally is even worse than with the previous bridge).
N.B. The vibrations transmitted to the bridge by the strings do not function like the frequency of an electric current, but more closely resemble a trepidation or an oscillation of the bridge, making the plates vibrate top plate mechanically and the back plate via the sound post in a multitude of nodal and antinodal zones depending on the frequency, as well as certain parts of the ribs.
Not all vibrations pass through the bridge. The strings also make the neck, fingerboard, tailpiece, upper and lower blocks vibrate by setting a part of the ribs in vibration and the top and back plates extremity (mode C2 or CBR).
After assembly of the violin, as each piece of the “puzzle” is set in vibration, the whole forms
a resonator whose outcome depends on several variables: quality of the materials, thicknesses, weight, tension, load and deformation of the materials, moisture content in the wood, A0 mode frequency, B1- and B1+ mode frequencies and the delta between them, and varnish.
Violin strings’ tension: 30 kg to 40 kg (66.14 to 88.18 pounds) depending on the diapason and the type of strings.
Calculation of string pressure on a violin top plate with a 158° angle and a 2.5 ratio.
Lower nut height: 6 mm (0.24 inches) – Bridge height: 32 mm (1.26 inches)
Example: 32 kg ÷ 2.5 = 12.8 kg pressure on the top plate, and via the sound post, on the back plate.
(70.55 lbs ÷ (2.5 = 28.22 lbs)
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