Permeability

Drums are membranophones: instruments equipped with a stretched membrane that is made to vibrate with either indefinite or definite pitch. In the Hornbostel-Sachs (Berliner Gesellschaft für Anthropologie, Ethnologie und Urgeschichte. Verhandlungen; Deutsche Gesellschaft für Völkerkunde 1868) classification, membranophones are organized according to how the sound is produced. We find that “modifying sounds through a vibrating membrane” is considered an unusual mode of sound production relative to hitting, pulling, or rubbing the drumskin. In this project, my desire is to develop an unusual form of sound production on the drumskin by making it vibrate with pitches whose length I can sustain.  

The permeability of a membrane refers to the membrane's ability to be penetrated or passed through, especially by a liquid or gas—or by vibration. The permeability of a drum’s membrane—that is, which sounds are let through the membrane— depends on several factors: The tension of the membrane, the diameter, the thickness, and, in my case, the position and weight of the vibrating speaker and other objects placed on the membrane. I observe, interact, and adjust continuously according to the permeability of the membrane. The possibilities are endless, partly because the membrane’s vibration modes are somewhat unpredictable and hard to control.

As physicist Thomas D. Rossing explains in his book Science of Percussion Instruments (Rossing 1982), the membrane, given its response to vibration, can be thought of as a two-dimensional string. Membranes, being two-dimensional, can vibrate in many modes that are not only harmonics of the fundamental, where the nodal points on a string are substituted by nodal circles and diameters on the membrane. 

In my project, the situation is complicated by my addition of objects and a transducer to the drumhead, which further change the conditions for vibration. This is the main reason I didn't map my drumhead with nodal lines at the beginning of the project and chose instead to examine the diaphragm's vibration phenomenon through playing and listening. I am searching for resonating frequencies on my instruments—that is, frequencies of a certain pitch that pass through the permeable membrane and allow the drum to resonate. To find these frequencies, I send sine waves through the vibrating speaker without touching the skin. I follow the same method as the American composer David Tudor in the first version of his work Rainforest (1968), in which he places transducers inside sculptures. To find the resonating frequencies in the sculpture, he played a sine wave glissando through the transducer, determining by ear which frequencies resonated best (van Eck 2017).

After finding the resonating frequency with a sine wave glissando, I start touching the skin with my hands, skin against skin, moving in circles, which causes a slight alteration in pitch and resonance. I can then apply objects to the membrane. If the object is too heavy, it might strangle the sound, because the object dampens the skin, thus its capacity to vibrate. However, a lighter object can still vibrate due to the force of the vibration, and the frequency is let through in addition to the actual sound of the vibration of the triangle’s natural frequency.  This often generates harmonics and/or distortions. There appears to be an inverse relationship between the weight of objects and the vibrational force generated by the membrane in its ability to produce sound.