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Piano Bridges

Bridges transmit the energy of the vibrating strings to the soundboard, where it is reinforced and amplified for more volume and projection.

This section takes you through this process and covers ways to make the bridges more efficient.

The bridges of the piano basically function as a link, or transmitter, between oscillator (strings) and amplifier (soundboard), conveying the vibrations of the string to the sound-board. These vibrations then set the entire sound-board in motion, which, due to its rela- tively massive area, amplifies the sound.

The function of the piano bridge is similar to that of a microphone. The only real difference is that the microphone converts acoustical energy into electrical energy during the transmission process. But the function basically remains the same, that is to bridge the gap between sound source and amplifier.

The bridge also acts to dissipate the energy of a given string along the entire board, putting the whole area into oscillation rather than just a small segment. If the strings were coupled directly to the soundboard with no bridges, there would be too much energy for such a small area. The result would be a very loud tone, too loud for musical purposes, and the duration would be much too short.

The piano designer, or builder can set the optimum amount of soundboard into motion, by positioning of the bridges along the most responsive sections of the soundboard, which in turn directly effects the amplitude, decay rate, and tone quality.

The bridge is glued to the board, taking the concave shape of the board and positioned in such a way that it will give proper speaking length to each string. The speaking length is the actual vibrating area of the string between the two termination points on either side – the bridges on back side of the piano, and the agraffes on the keyboard side.

To insure the tightness and steadiness of the string's contact to the bridge, the string's line of direction is slightly diverted between two staggered bridge pins, angled slightly, to assist in holding the string down. This insures downbearing even if there were little or no contact between string and bridge due to a weak soundboard and loss of crown. Proper downbearing is critical in the production of good tone for several reasons.

First of all, insufficient downbearing would not terminate the vibrating section of the string properly. Rather than reflect the vibrations back towards the speaking length and reinforcing the vibration motion, the string would continue vibrating very slightly over and past the bridge, resulting in energy loss. The tone would sound thin, and fuzzy. Oftentimes this condition can be remedied by tapping down the string on either side of the bridge to create better contact.

Too much downbearing on the other hand, would inhibit the string's and board's ability to vibrate freely, resulting in very loud and harsh, but short-sustained tones. The optimum downbearing for the entire piano should be approximately 800 lbs. or about 4 lbs. for each string. These are just average figures however, and tend to change relative to soundboard crown and string tension, i.e.: high tension should have higher bearing and vice versa. The bearing should also be in proportion to the crowning of the soundboard, both increasing or decreasing proportionately.

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