One of our newest projects is an investigation of the physics of the Nigerian slit-log gong, usually referred to as the slit-log drum (although it is actually an idiophone and not a membraphone). These two-toned drums are beautifully hand carved and deceptively simple looking, yet we have only begun to understand the important design parameters of these drums.
We have determined the acoustically important modes, studied the power spectra of three different drums, and built a computer simulation that allows us to investigate how changing the design parameters changes the sound. The computer model is a finite element model that allows us to predict the effects of changing the physical design. The current model considers only isotropic materials, while wood is orthotropic, but it is still very useful for studying the physics of the drum. The next step is to add the anisotropy of the wood grain to the model.
Below is a picture of the actual drum beside an interferogram of it vibrating in the lowest normal mode. Along with these is the computer model and the predicted shape of the lowest normal mode. Clearly there is already good agreement between the model and the experiment. We have used this model to investigate the effects of changing the geometry of the holes and the placement of the slit. Our analysis shows that the holes and slit are optimally placed to ensure overlap between the Helmholtz resonance of the cavity and the resonances of the lowest body vibrations.
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