Sediments are grains carried by the flow of water to a place where the speed of water flow is low. There are several ways in sediment testing in rivers, one of which is by using a microphone. The hydrophone is a device made from iron pipes and is used for measuring sediment transport by applying waves originating from the transport of these materials. The type of sediment that can be processed by hydrophone is a basic sediment (bedload) that always moves on the river bed and has a large size. This is because the location of the hydrophone is on the riverbed. The data produced by the sediment transport is in the form of sound and then processed into a wave (pulse). The study was conducted in Gemawang River on the Code River to determine the grain size of the sediment in the river using hydrophone. This test aims to determine the grain size of sediments based on the wave size produced. This test is carried out by flowing sediment granules which have the same size so that the resulting waves have almost the same size. The test is carried out by flowing sediment granules with an average size of 1.18 mm, 2.38 mm, 4.75 mm, 9.5 mm and 12.5 mm with the flow rate used which is 0,0004 m³ / sec and 0, 0002 m³ / second. From the test results, the grain size of the sediment that can be read by hydrophones is > 2 mm.