All existing acoustic string instruments are essentially a vibro-acoustic system that transmits the vibration energy of strings to the resonator (board, sound box, and resonator) through a bridge, efficiently converting it to a sound radiation energy. In other words, they have physically coupled strings and a resonator and the efficiency of the energy transfer from the vibration energy of the strings to the sound radiation energy generated by the resonator decreases as the impedance of the bridge foundation increases: a small impedance means the strong vibro-acoustic coupling between the strings and resonator.
In this work, an innovative concept of an acoustic string instrument, named decoupled acoustic string instrument, was proposed and verified with a prototype. The most noticeable feature of this idea is that the vibrating strings are separated from the resonator and fixed on a relatively rigid foundation.
The decoupled acoustic guitar prototype was constructed to demonstrate the proposed idea. Two piezoelectric sensors measured the varying forces in two orthogonal directions at the string ends, which were then input to an electro-dynamic actuator mounted on a separated resonator of a commercial acoustic guitar for sound generation.
It was shown the completed prototype had the 2 times longer sound sustainability and 2 times more balanced sound, compared with the coupled system using the same resonator. The nasality and clarity of the generated sound were shown to be controllable by changing the mixing weights for each measured signal.