I have written in the past about Sir C.V. Raman’s work on understanding the Indian drums–the tabla and the mridangam–and their peculiarity at being percussive instruments that can achieve harmonic overtones. The secret is the black patch that is seen on every tabla or mridangam that makes the centre of the circular membrane denser than the periphery. In fact, CNR Rao points out that Raman did this work while he was an accounts officer in the British government, and that this work got him elected to the Royal Society.
This is all good. However, as the percussionist U. Sivaraman and Dr. T. Ramasami, Director of the Department of Science and Technology of the Govt. of India explained in the thirteenth ISRO-Satish Dhawan lecture at JNCASR on October 16th 2012, the materials that are traditionally used to make these drums do not satisfy phytosanitary constraints–island nations will not allow you to carry untreated leather without decontaminating it first. Fumigation effectively destroys the stuff that holds the black patch together — usually something like cooked rice.
The solution, then, is to use materials that reproduce the properties of traditionally used materials while meeting safety norms. Simple enough? Not even close: the traditional materials have been fine-tuned over millennia. And nobody knows exactly why the peculiar combinations of materials used in the making of these drums work.
The drum-heads have three layers, two of which are annuli. The layer that is entire is made of goat-skin. Goat skin from Bengal is preferred to goat-skin from Rajasthan. This layer also has the black patch on it. The black patch is iron oxide, silica and manganese held together by some adhesive. The silica and manganese are provided by river-bed sand. River-bed sand from Tanjore gives the drum a different sound quality compared to sand from Varanasi. The three layers are held together by ropes made from buffalo hide that tie the membrane down at exactly sixteen equally spaced points on circumference. The shells that these membranes are mounted on are made of particular types of wood–jackfruit tree wood is the best kind.
All of these things are necessary. Change any of them and the result is bad sound. Unless you know what each component does. U. Sivaraman and T. Ramasami have spent a decade looking for the answers.
The annular layers are there to cut out the highest frequencies, while still retaining up to nine overtones. This particular detail is actually due to CV Raman and a 1935 Indian Academy of Sciences paper.
Untanned goat-skin is used in the mridangam because it has the highest strength (per unit thickness). The annular layers are usually cow-skin.
Also, monitor-lizard skin is used in the kanjira, U. Sivaraman pointed out. But monitor-lizards are endangered, and we now use something else.
The shells are made of particular types of wood because these types of wood have the best sound radiation properties.
With modern materials, and Fourier analysis, Dr. Ramasami and company have managed to make novel versions of the Indian drums that reproduce the sound quality that connoisseurs enjoy, while also keeping the tabla travel-safe. This even includes reducing temperature-sensitivity letting musicians play in air-conditioned halls. They haven’t been able to make the drums humidity-resistant, though, which is an important shortcoming.