In the framework of the agreement between Institute of Marine Environment and Resources with Hydrosphere Atmospheric. Research Center (Nagoya University), Japan) about "Drifting buoy observation along Vietnamese coast in summer 2010", the observation of water temperature and salinity (T-S) according to depth was done by CTD equipment in Danang-Quynhon area. The dada shown that the speed of sound in seawater was maximum with value of 1543.8 m/s in the Danang transect
the water layer with the suddenly change of sound speed was at 5 to 10m depth in near shore (from station D1 to D4), and at 10 to 15 m depth in offshore (in D5 and D6 station). In the Quynhon transect, the maximum value of sound speed in seawater was 1545.2 m/s and the minimum value was 1515,.2 m/s. The maximum value of sound exists at surface layer to 30 m depth in near shore and to 40 m depth in offshore
at that water layer, the speed of sound is quite identical, and under this water layer is suddenly change layer with the fast change of the sound speed.