Gemini Special Projects
Carbon Fiber Sound Posts

James Condino installs a Gemini Carbon Fiber Sound Post at the 2012 Oberlin Bass Workshop. See the complete slideshow...

Appalachian Mountain Dulcimer with Carbon Fiber Soundboard

Continuing a tradition of unique applications of composite laminates to acoustics, Gemini recently developed a carbon fiber soundboard for an Appalachian mountain dulcimer. Traditional all-wooden dulcimers produce a sweet, yet relatively quiet tone. The soundboard magnifies the sound made when the strings vibrate by coupling this sound to the body resonance chamber, thereby producing a quality musical tone; however, a wooden soundboard has a high degree of damping, and thus tends not to project well, particularly given the small size of the dulcimer and manner in which the strings are played.

The solution to improving sound projection is to increase the stiffness of the soundboard, but without increasing the fundamental frequency. For example, if solid carbon fiber is used to create a musical instrument, the soundboard natural frequencies will be higher than their wooden counterparts. Although sound amplification is improved, the resulting tones are harsh and unpleasant. Through a combination of FEA analysis and acoustical testing, engineers at Gemini designed a custom composite laminate with drastically increased stiffness, but without increasing the fundamental of the plate. In fact, in the final design the fundamental was reduced by 2 Hz from 149 Hz to 147 Hz, producing an instrument with both excellent sound projection and a warm, full-bodied tone.


PATENT #7,276,868
Carbon Fiber Musical Instrument Sound Board Patent

Gemini owns a patent for a carbon fiber music instrument sound board (US Patnet #7,276,868). This patent details a method of constructing a soundboard for a hollow-body stringed instrument made of layers of carbon fiber and resin surrounding a core of one or more layers of constant-thickness plywood, veneer, or paper. Additional shaped layers are utilized to produce a desired pattern of stiffness within the soundboard, allowing it to be tuned through customized thickness and material mass distribution.

Music Box Passive Amplifier

Amplification of sound is typically done actively using microphones and speakers; however, passive amplification using helmholtz resonation is a very nice alternative. This is particularly true for applications related to music, where an unaltered tone is ideal. Specifically aimed at music boxes and singing greeting cards, Gemini developed a small, yet deceptively complex carbon fiber stage to amplify the sound of these items, particularly in the lower frequency ranges. Due to its low mass and high stiffness, carbon fiber transmits sound extremely well. Interplay from several parameters was investigated using both acoustic and Nastran finite element analyses. The pictures show some of the results from these tests, where the goal was to provide a well distributed range of natural frequencies and clean mode shapes. Essentially acting as the soundboard for the music box stage, multiple composite layups were tested, each with a unique tonality. The effect is quite dramatic, since music boxes and greeting cards typically lack any bass due to their size. The result is an attractive carbon fiber piece with a rich, well balanced tone.

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