Metamaterials in musical acoustics: A modified frame drum

Mechanical musical instruments have a restricted timbre variability compared to electronic instruments. Overcoming this is the aim of extended playing techniques as well as building more sophisticated musical instruments in recent years. Metamaterials might be a way to extend timbre of mechanical instruments way beyond their present sound capabilities. To investigate such possibilities, a frame drum is manipulated to achieve different sounds. On the drum membrane of 40 cm diameter, a ring of masses is attached in three diameters, 8, 10, and 12 cm with 10 masses each, leading to a cloaking behaviour of vibrations from within the ring into the area outside the ring and vice versa, as shown by microphone-array and high-speed laser interferometry measurements. The resulting sounds have a band gap between about 300 and 400 Hz to about 700-800 Hz, depending on the ring diameter. The 8 cm diameter ring shows the strongest amplitude attenuation in the band gap. Still, when striking the membrane outside the ring, it sounds like a regular drum. This leads to a tremendously increased variability of musical articulations, especially when striking in the ring, as a band gap sound cannot be produced by a regular drum.