Sound designing classical guitars through metamaterials

An approach is presented to alter the sound of a classical guitar using a metamaterial structure. By adding additional masses using magnets in a row between the bridge and the soundhole, a bandgap of around 300 Hz appears. Alternation of magnet masses and placement determine the location and strength of the bandgap. A similar approach was presented with a frame drum R. Bader, J. L. Fischer, M. Münster, and P. Kontopidis, “Metamaterials in musical acoustics: A modified frame drum,” JASA 145(5), 3086–3094 (2019). There, the lower and upper limits of the bandgap are caused by the relations between the frequency wavelength and the magnet distances. With the guitar, the bandgap is caused by a shifting and attenuation of modes. Interestingly, the added masses cause a decrease in the guitar top plate mode frequencies, which is similar to membrane behavior and contrary to the expectancy of a plate. Such a guitar sound is unique due to the strong bandgap and cannot be achieved with typical alternations of changed fan bracing or top plate thickness. Magnets can easily be applied by guitarists within minutes and, therefore, qualify as a guitar sound design chosen by players.