APPARATUS FOR GENERATING IN PARTICULAR LOW-FREQUENCY SOUND

Abstract: An apparatus for generating in particular low-frequency sound, comprising a drive unit (10) and an open resonator (11) connected thereto. Said driving unit (10) comprises a housing with a movable member (14) opening and closing an aperture (13) provided in said housing and opening in said resonator (11). The member (14) is operated by means of compressed air whose actuation of said member (14) is controlled by means of a valve (21) activated by electrical pulses from a current source, the frequency of said pulses being manually or automatically controllable. For the removal of soot from furnaces, several apparatuses of this type may be coupled in parallel to a common current source, such that they operate in phase, and a considerable sound amplification is achieved.

Description: The present invention relates to an apparatus for generating in particular low-frequency sound, comprising a drive unit which is adapted to generate sound by the supply of compressed air, and an open resonator which is connected to said drive unit and preferably is in the form of a straight pipe. It is known to remove soot from furnaces and to clean for example filters by means of low-frequency sound generated by means of low-frequency generators provided with resonators. For efficient soot removal and cleaning, considerable sound pressures are required, and this in turn means that large and cumbersome units must be used. Frequently, several such units must be used together. The low-frequency generators normally are of the pneumatic type, the frequency generation being integrated with the oscillator (the diaphragm) and the frequency control being integrated with the horn. The sound transmitter is self-adjusting and operates on the system's own resonance frequency, and the frequency cannot be changed. Existing pneumatic sound transmitters with controlled frequency are highly sophisticated. The air supply valve conforms slavishly to changes of optional signals. The control is, so to speak, an "analog" control. Coupling in parallel of such known devices may result in phase shifts reducing the efficiency. It may also happen that entire systems or at least parts thereof are subjected to heavy resonant vibration. It is the object of the present invention to eliminate the above mentioned disadvantages of prior art technique and to provide an extremely simple apparatus for generating low-frequency sound, said apparatus being provided with means for controlling the frequency of the sound generated. In this manner, it is possible without difficulty to couple in parallel a desired number of apparatuses of relatively small size. This object is achieved in that the drive unit comprises a housing having a chamber with an aperture opening in the resonator, a member oscillating within said chamber and having a first actuating surface which is located within a first space of said chamber and by means of which said member is movable, by supply of compressed air to said first space, in a direction away from said aperture, such that said aperture is opened, and a second actuating surface which is located in a second space and by means of which said body is movable, by supply of compressed air to said second space, in a direction toward said aperture, such that said aperture is closed, that said second actuating surface has a larger area than said first surface, and that said second space communicates with the atmosphere via a passage which is opened and closed by means of an electrically operated valve. The invention will be described in more detail below, reference being had to the accompanying drawings illustrating embodiments. Fig. 1 is a lateral view, partly in section, of an apparatus designed in accordance with the invention and adapted to generate low-frequency sound, and Fig. 2 illustrates an modified embodiment of such an apparatus. Fig. 3 illustrates how a number of apparatuses according to the invention can be coupled in parallel for cleaning the combustion space of a furnace. The apparatus according to the invention as illustrated in Fig. 1 comprises a drive unit generally designated 10 and having means for generating low-frequency sound. Connected to said drive unit 10 is a resonator 11 which may have any shape suitable in the context, for example flaring exponentially, although it preferably is in the form of a straight pipe. The drive unit 10 has a chamber 12 which is connected to the resonator 11 via a duct or aperture 13. The chamber 12 houses an approximately cylindrical member 14 provided at one end with a sealing means 15 which, in Fig. [, as its slightly raised peripheral edge engaging a portion around the aperture 13. Nearest the sealing means 15, the member 14 has a smaller cross-sectional area than the chamber 12, such that a first space 16 is formed. At some distance from the aperture 13, the member 14 widens to the cross-sectional area on the chamber by means of an inclined annular portion 35 forming an actuating surface. The cylindrical member terminates at a distance from the rear end of the chamber 12 to form a second space 17, and it appears that the side of the member 14 facing the space 17 has a much larger area than the transitional portion 35. The member 14 is provided in its rear side with a bore housing a spring 19 which strives to urge the member 14 toward the aperture 13. Extending between said first space 16 and said second space 17 is a duct 18 having a relatively small cross-sectional area and opening in the bore of the member 14. In the rear side of the drive unit 10, which is constituted by a removable lid, two passages 20, 22 are provided, the first of which extends from the space 17 and opens in a cavity in a valve 21 mounted on said drive unit 10. This cavity of the valve 21 communicates with the atmosphere through the second passage 22. The valve 21 is a solenoid valve having a coil 23 which is connected to a current source by means of wires 24 and within which an armature 26 is movable back and forth in conventional manner in response to the activation of the coil 23. The armature 26 is connected with a valve member 25 which, in its advanced position, closes the entrance to the passage 22 communicating with the atmosphere. It will be seen that, by displacing the valve member 25, the communication between the space 17 and the atmosphere is opened and closed. It should be emphasised in this connection that the valve 21 is but schematically shown and may be replaced by other valves; the important thing is that the communication between the space 17 and the atmosphere can be opened and closed electrically at the desired frequency. A transverse bore provided in the drive unit 10 and opening in the space 16 houses a connection 27 for connecting a compressed air line 29 to said drive unit 10. Sound is generated by the apparatus according to Fig. 1 by supplying compressed air to the drive unit 10 via the line 29 and the connection 27. The compressed air first enters the space 16 and then goes to the space 17 via the duct 18 and the bore. Since the compressed air acts on a much larger area of the rear side of the member 14 than on the front side of said member, and since this force furthermore is supported by the force of the spring 19, the member 14 will be held with the seal 15 i engagement around the aperture 13, as shown in Fig. 1. If an electric voltage is applied to the coil 23 which thus moves the armature 26 away from the drive unit 10, the valve member 25 is lifted from its seat surface, thereby exposing the opening of the passage 22 in the valve cavity so that the space 17 will communicate with the atmosphere via the passages 20, 22. As a result, the pressure in the space 17 is reduced, and by the action of the compressed air on the actuating surface 35 the member 14 is moved to the right in Fig. 1 whereby the aperture 13 is opened toward the chamber 12. When the valve member 25 again closes the passage 22, the pressure in the space 17 is again built up so that the member 14 is moved to the left in Fig. 1 and the aperture 13 is closed. This movement of the member 14 in the drive unit 10 generates the low-frequency sound which is amplified by the resonator 11. It will be appreciated that the reciprocating movement of the member 14 is controlled by means of the valve member 25 whose movement is dependent upon the movement of the armature 26 and, thus, by the activation of the coil 23. The movement of the member 14 and thus the frequency of the sound generated may be readily controlled electrically by connecting the lines 24 of the coil 23 to a suitable current source which is capable of supplying pulses of a frequency readily adjustable, preferably a square wave voltage. Fig. 2 illustrates a modified embodiment of the drive unit, a diaphragm 14 forming the reciprocable member. As will be seen, an annular ridge 28 is provided around the aperture 13 to the resonator 11, and this ridge is engaged by one side of the diaphragm 14 in the state corresponding to the one shown in Fig. 1. In this embodiment, the space 16 is formed by the annular space between the ridge 28 and the periphery of the diaphragm 14, and the space 16 communicates with the space 14 on the rear side of the diaphragm 14 via the duct 18 which here is formed in the diaphragm proper. The diaphragm 14 is mounted such that it engages the ridge without actuation by compressed air, and when compressed air is supplied via the passage 29 and the connection 27, the air will flow, as in the embodiment previously described, into the space 16 and from there via the duct 18 into the space 17 which is vented in the same manner as in the earlier embodiment, the diaphragm 14 being urged away from the ridge 28 by the compressed air in the space 16. As has been indicated above, the resonator preferably is a straight pipe 11 because this will give a resonance curve having a relatively wide peak. If, on the other hand, the horn flares exponentially or in accordance with a Bessel curve, the degree of efficiency will be affected, and the resonance curve will be far more narrow. The resonance frequency is determined by the length of the horn. With low frequencies, the horns must be of considerable lengths and therefore may be curved, for example helically. For optimation of the horns at low frequencies, the horns may also comprise several different sections, such as straight pipes with conical sections. Since the apparatus according to the invention can be controlled with the greatest ease, a plurality of apparatuses may be readily coupled in parallel without causing phase difficulties. If a plurality of apparatuses are coupled in synchronism (in phase), a considerable sound amplification may be achieved. Fig. 3 illustrates an arrangement in which a combustion chamber 30 in a boiler is cleaned by means of several apparatuses according to the invention coupled in parallel and provided with a resonator 11 in the form of a long straight pipe projecting into the space 30 with its end facing away from the drive unit 10. The drive units 10 of the apparatuses are connected to a compressed air line 29 and to a control wire 24 which is connected to a current source 31. The current source 31 produces a square wave voltage of readily controlled frequency, as indicated at 32. It is also possible to provide on a resonator 11 a sensor 34 which senses the resonance frequency of the resonator and is connected, via a wire 33, to the current source 31 for automatic control of the frequency of the square wave voltage. With the arrangement according to Fig. 3, all drive units 10 are supplied with a control voltage of the same frequency and the same phase. For optimation purposes, this control voltage is manually adjustable by means of a knob on the current source 31. It is, of course, also conceivable to supply each driving unit with current from a current source of its own and separately to adjust the oscillation frequency of each drive unit.

Claims: 1. Apparatus for generating in particular low-frequency sound, comprising a drive unit (10) which is adapted to generate sound by the supply of compressed air, and an open resonator (11) which is connected to said drive unit (10) and preferably is in the form of a straight pipe, characterised in that the drive unit (10) comprises a housing having a chamber (12) with an aperture opening (13) in the resonator (11), a member (14) oscillating within said chamber (12) and having a first actuating surface (35) which is located within a first space (16) of said chamber and by means of which said member (14) is movable, by supply of compressed air to said first space (16), in a direction away from said aperture (13), such that said aperture (13) is opened, and a second actuating surface (36) which is located in the second space (17) and by means of which said body (14) is movable, by supply of compressed air to said second space (17), in a direction toward said aperture (36), such that said apertures is closed, that said second actuating surface (36) has a larger area than said first surface (35), and that said second space (17) communicates with the atmosphere via a passage (20, 22), which is opened and closed by means of an electrically operated valve (21). 2. Apparatus as claimed in claim 1, characterised in that the said electrically operated valve (21) is a solenoid valve having an armature (26) which is reciprocable in a coil and connected to a valve member (25) opening and closing said passage (20, 22), and that a current source (31) is connected to said coil (23) for generating pulses of a manually or automatically controllable frequency. 3. Apparatus as claimed in claim 1 or 2, characterised in that there is provided in said second space (17) a spring (19) for actuation of said member (14) toward said aperture (13). 4. An apparatus as claimed in claim 1, 2 or 3, characterised in that a sensor (34) is mounted on said resonator (11) for sensing the resonance frequency thereof and is connected to said current source (31) for frequency control. 5. An apparatus as claimed in any one of the preceding claims,characterised in that said member (14) is an approximately cylindrical member axially movable within said chamber (12) and provided with a sealing means (15) at its end facing said aperture (13). 6. An apparatus as claimed in any one of claims 1-4, characterised in that said member (14) is a diaphragm clamped in said chamber (12) and having its one side facing said aperture (13), said diaphragm being movable into sealing engagement with a ridge (28) provided around said aperture. 7. An arrangement for removing soot in furnaces or the like, characterised in that it comprises a plurality of sound generating apparatuses of the type stated in any one of the preceding claims, said apparatuses being coupled in parallel and connected to a common current source (31), and said apparatus further being operable in synchronism, i. e. with the same frequency and phase.

Inventors: WERTHAJM, MAREK
Applicants: KOCKUM SONICS AKTIEBOLAG
Application year: 1986
Main classification: B06B1
IPC codes: B06B1