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Abstract: The sounds from the spherical interior cavity as well as that from outside the cavity (which is what one listens in the normal playing) of an Indian classical rhythmic instrument, called Ghatam, are simultaneously recorded, analyzed and are compared. Although both sounds are emanating from the same vibrating system, they are from different parts of the system, and hence some similarity is expected. The Ghatam, called also the clay-pot, is essentially an acousto-mechanical system with the structure of the instrument providing the mechanical part and the closed spherical cavity and the external environment extending up to the listener, providing the acoustic system. The mechanical system is a spherical clay pot and has a narrow opening with a short neck. The instrument is placed with the mouth facing upwards on the lap of the artist sitting cross legged on the floor. Rythmic syllables are produced by impacting the spherical doubly curved surface of the instrument with the player’s hands and fingers. There are five (some use six or more) basic finger and hand formations that provide the impact excitation at different points of the Ghatam. These 5 strokes are denoted by corresponding sound syllables, tha, dhi, thom, num, and num(2). The Ghatam instrument is constructed for a specific fundamental frequency by choosing the proper design parameters of the clay-pot such as the volume of cavity, the thickness of the vessel.At the end, a “garland of basic strokes”, composed on the spot by the artist, called the “thani” was played for about 5 minutes. The sounds from outside and the interior cavity sounds were measured and Fourier transformed in order to obtain their spectral characteristics that would show their spectral content. A comparison of the recorded internal and external sounds shows that, in general, different resonance frequencies of the vibrating system acquire prominence with different stroke excitations and the subjective experience of these corresponding sounds are perceptibly different. The interior sound is more resonant and sharp, and gives out a high pitched metallic sound.
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