During strombolian eruptions, large bubbles break at the surface of the lava column and produce sound. Acoustic pressure recorded during several explosions on the Eastern vent of Stromboli volcano, shows a pattern consistent between explosions. The well-marked oscillation contains only very low frequencies (around 7 Hz), and is followed by a signal containing now both higher frequencies and lower frequency (less than 4 Hz). The generation of sound is explained by the vibration (7 Hz) and bursting (higher frequencies) of large bubbles, followed by the drainage of magma down in the conduit (less than 4 Hz). The interpretation suggests that: (1) the bubble vibrates before bursting and (2) the vibration prior to bursting yields more energy than the bursting itself. The radius of bubbles responsible for explosions is determined to be close to 1 meter, in agreement with observations of a few conduit sizes.