Abstract: A device for underwater generation of low frequency sound comprising a coally shaped explosion chamber open at both ends, a firing chamber having inlet and outlet passages at opposite ends thereof, ignition means in the firing chamber, a restricted orifice between the outlet passage of the firing chamber and the small open end of the explosion chamber, a mixing chamber opening into said inlet passage, and two supply lines for introducing gases to form a combustible mixture in the mixing chamber, the combined resistance the supply lines and the mixing chamber offer to the flow of gas in the upstream direction being slightly greater than the flow resistance of said restricted orifice, the arrangement being such that when the gases in the firing chamber are ignited with the device submerged in water the flame spreads down stream through the explosion chamber and also upstream into the supply lines thereby extinguishing the flame.