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Casting and forging is a method of formation to produce gamelan instruments. This study aims to determine the effect of forging deformation on the as-cast gamelan bar to changes in the microstructure, hardness, density, porosity, and acoustical properties of the gamelan instrument. The research material was tin bronze Cu-25wt.%Sn. The casting process uses a sand casting method with a pouring temperature of 1100 °C. The forging deformation process was done by reheating the as-cast gamelan bar at a temperature of 600 °C. The thickness of the as-cast gamelan bar is reduced by 30% after the forging deformation process. The observations on the as-cast gamelan bar show coarse microstructure with large grains, inclusions, and increased porosity. The forged gamelan bar shows finer microstructure, elongated grain, α lamellar, α phase like a plate, and decreased porosity. The hardness of forged gamelan bar increased by an average of 30.3% VHN, density increased by 9,56% and porosity decreased by 95% compared to the as-cast gamelan bar. Acoustic properties show an increase in the natural frequency between the as-cast gamelan bar and the forged gamelan bar are 712.5 Hz and 1058 Hz.