Abstract: Air-driven, low-frequency sound generator with a positive feedback system, comprising, as a sound emitter, an open resonator (11) for the generation of standing gas-borne sound waves which produce a varying gas pressure inside the resonator; and feeder (10) with a tube (14) for the supply of pressure gas to the resonator and a back and forth movable springing valve slide (15) whose position remains unaffected by the pressure gas and which regulates the gas flow from the tube while creating a modulated flow of pressure to the resonator. The tube is surrounded by a surge tank (13), connected to the pressure gas source, and the valve slide is arranged as a piston movable inside the tube and is set to regulate a connecting opening (20) between the surge tank and the inside of the tube. This opening is situated at one end surface (19) of the piston and said end surface is being exposed to the inside of the resonator by means of the one end of the tube communicating therewith.

Abstract: Method and apparatus for increasing the turbulence in a gas exposed to low frequency sound. The gas is exposed to sound produced by a sound generator (10) which has a tubular resonator (11) having an open end and is operated at one of the resonance frequencies of the resonator having a maximum frequency of 150 Hz. The gas is flown into the tubular resonator through the open end thereof and is discharged from the resonator substantially in a region, where the sound pressure of the standing wave in the resonator has a node, after having passed through a substantial length of the resonator.

Abstract: Method and apparatus for infrasonically intensifying a glow bed (13), supported by a grate (12). In order to improve the beneficial effect of sound on combustion the bed of fuel, located on the grate, is exposed to a high particle velocity of a sound positively produced by an external low frequency sound generator (11) the frequency of which is determined by the sound generator, to provide a reciprocating movement of combustion air and combustion gas through the glow bed. The dimensions of the grate in a plane transverse to the reciprocating movement of combustion air and combustion gas are less than a quarter of the wave-length of the sound generated by the sound generator.

Abstract: An infrasound generator including a quarterwave resonator tube having a closed end and an open end and being of substantially uniform diameter, the open end of the tube being adapted to communicate with a space containing the heat exchange surfaces of a boiler through an aperture in a wall of the space and having means for supplying pulsating pressurized air to the closed end of the resonator tube. An outwardly opening conical diffuser is connected between the open end of the resonator tube and the space and a sleeve extends back from the wall and surrounds at least a portion of the length of the tube, the sleeve being of greater diameter than the tube with resilient closure means connected between the diffuser and the sleeve.

Abstract: A low-frequency sound generator for generating intense sound, which generator includes an open resonator tube. Pressurized gas is supplied to the resonator tube as pulses through a feeder system which includes a valve slide member which controls the pressure of the supplied gas. The gas pulses develop standing sound waves in the resonator tube which provides a varying gas pressure. Means in the resonator tube is operable in response to changes in the gas pressure to provide positive feedback of the sound pressure in the resonator tube to the feeder system at a resonant frequency of the resonator tube.

Abstract: A low-frequency sound generator for generating intense sound, which generator includes an open resonator tube. Pressurized gas is supplied to the resonator tube as pulses through a feeder system which includes a valve slide member which controls the pressure of the supplied gas. The gas pulses develop standing sound waves in the resonator tube which provides a varying gas pressure. Means in the resonator tube is operable in response to changes in the gas pressure to provide positive feedback of the sound pressure in the resonator tube to the feeder system at a resonant frequency of the generator.

Abstract: A device for cleaning a fabric filter from adhering dust comprises a loud speaker element forming a dividing wall or a part of such a wall between a chamber receiving the filter, and an open resonator which communicates with the chamber.