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.
The radiation patterns of 32 guitars are investigated. Therefore, the top and back plates are measured using a 121-microphone array, back-propagating the recorded sound field onto the guitar top and back plates. Both, the eigenvalues and the forced oscillation patterns are measured, the latter by plucking the guitar strings for all possible notes. For each note, the forced-oscillation radiation pattern is calculated for 20 partials up to 4 kHz. These radiation patterns are then forward-propagated into the surrounding space around the guitar. Considerable differences appear between the different guitars within the same frequency region in terms of shape and intensity. Also, for similar frequencies, different patterns may appear, depending on the string and note played.
Acoustic metamaterials have properties not found in nature, like negative stiffness, cloaking behavior or frequency bandgap damping. This is achieved by complex geometries often constructed out of multiple subunits in sub-wavelength size. Although also musical instruments often have complex shapes, like guitar or piano soundboards with regular fan bracing, metamaterials have not explicitly been used here. As an example, a modified frame drum is proposed with increased sound possibilities by adding masses to the drum membrane arranged in a circle. Such structures have been shown to have cloaking behavior. Using microphone-array and laser interferometry measurements it is shown that such a drum has a frequency-dependent cloaking behavior. When struck at the center of the added circle most energy above about 400 Hz stays in the circle and decays strongly. Such a sound cannot be produced with a regular frame drum. When struck outside the circle the drum sounds very much like a regular drum without added masses. By gradually changing the playing position from the circle center towards the circle rim, frequencies above about 400 Hz are gradually added. Therefore such a modified frame drum has much more possible sounds and therefore ways of musical articulation.
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