Monotonic infrasound with stable peaked frequency of 0.77 Hz was recorded at Volcan Villarrica in January 2010. Similar monotonic infrasound had been previously reported at Villarrica (e.g., Ripepe et al. [2010]). Using joint infrasound and visual observations from a suspended camera we demonstrate that the likely source of infrasound is Helmholtz resonance produced from a cavity with volume 10 5 m 3 that separates the active convecting lava lake from an overhanging spatter roof. Spatter roof dimension (65 m diameter) and vent diameter (10 m) in the roof are constrained from video observations. Assuming a cylindrical cavity we infer a cavity height of 31 m that is corroborated by video records of spatter drips. The drips take as long as 2.2 s to fall from the roof into the lake, corresponding to a height of more than 24 m, which is in good agreement with the observed resonance frequency.