Audible sound is one physico-stimulant factor that could increase the production of algal biomass. This research was done to study the effect of audible sound (music) for Synechococcus HS-9 cell density in simple photobioreactor system. Synechococcus HS-9 is one genus of Cyanobacteria isolated from Rawa Danau Banten hot spring in Indonesia. In this study, we used three photobioreactors; two photobioreactors were exposed with two songs titled "Blues for Elle"(A) and "Far and Wide"(B) respectively, while one photobioreactor remain unexposed to any music as control (K). The songs were played 3 hours every day for 14 days. The growth rate of Synechococcus HS-9 was measured based on cell density which was counted manually under the microscope. The result showed that the peak of Synechococcus HS-9 cell densities in photobioreactor A, B, and K respectively are 7.34×106 cell/mL, 4.32×106 cell/mL, 3.33×106 cell/mL. Yet, based on the growth rate and doubling time of the algal cell, the results showed that Synechococcus HS-9 in photobioreactor B grow faster compared to photobioreactor A and K. The difference in frequency and intensity of music that was exposed to the photobioreactor system might affect the activity of cyanobacterial cell inside it. Hence, audible sound in form of music could increase the cell density of Synechococcus HS-9.