Bivalves have a crucial have great social, economic, and ecological importance in coastal regions. The mussel Perna perna has a wide distribution throughout the Brazilian coast and is intensively used in farming activities. This study aims to characterize the sound emitted by mussels under laboratory conditions and estimate the variation in their acoustic activity rate according to the population density. Two groups of treatments were designed (total and partially submerged), each one with three different population densities: 10, 40, and 80 individuals. The recordings occurred in an acoustically insulated tank. A total of 4,344 acoustic activities were recorded. The sounds were associated to the valve movements and classified as impulsive activities. The acoustic activity was mainly concentrated in the 4–6 kHz band. The maximum sound pressure level (max SPL) ranged from 43 to 105 dB re 1μPa. The longest acoustic activity was detected in the class of 0.01–0.02s. According to the Kruskal-Wallis and Nemenyi tests, the acoustic activity rate was density-dependent, increasing with the mussels density. The results allowed to estimate the population density of Perna perna based on its acoustic activity which might be an important step toward the population monitoring over a spatial and temporal scale.