Prior to Han Dynasty expansion into southern China and northern Vietnam, there was a prominent bronze culture that originated in both Yunnan and the Red River Delta of Vietnam. The earliest bronze drums are categorized as Phase 0, and were found only in Yunnan and northern Vietnam. Phase 1 drums were more numerous and were found in a few places outside of the region of origin. Phase 2 occurred when Han Dynasty territory expanded into Yunnan and northern Vietnam, and this was about the same time the bronze drums expanded into Indonesia. At the decline of the Han Dynasty, phase 3 drums appeared but was isolated only in present-day Guangxi province. It seems the expansion of drums into Indonesia was due to extensive trading with the Han Dynasty.




Source: http://www.nguyenthaihocfoundation.o...ji-Imamura.pdf