A guitar in vacuum: How does it sound?

The loudness of the radiated sound is one prominent quality feature of guitars. However, few studies have been devoted to the ratio between the radiated energy and that which is lost in the wooden structure. In order to evaluate this ratio experimentally, yielding the acoustical efficiency of the instrument, we have compared the vibrations of a guitar at different ambient pressure levels. The guitar is installed in a large tank where the open bass string is automatically plucked. The pressure is varied between atmospheric level and vacuum, which implies a proportionally vanishing radiated power. The decay time of the most energetic component of each partial is estimated. It turns out that decay times are sometimes shorter in vacuum. This can be explained through a model of the coupling between the string and the bridge, the important parameter being the ratio of the real part of the mechanical impedance of the bridge to its imaginary part. The efficiency is measured every 82 Hz between 80 and 2500 Hz. Values are widely scattered between 15% and 60%. A comparison will be presented with acoustical efficiencies of other structures (plates, piano soundboard) and with values obtained on guitars by other methods and authors.