WO2004009255A1

WO2004009255A1 - An air-driven low frequency sound generator and a method for regulating the piston in such a generator

Info

Publication number: WO2004009255A1
Application number: PCT/SE2003/001234
Authority: WO
Inventors: Mats Olsson
Original assignee: Mats Olsson
Priority date: 2002-07-22
Filing date: 2003-07-21
Publication date: 2004-01-29
Also published as: DE60303608D1; SE0202291D0; SE0202291L; EP1542810B1; EP1542810A1; SE524605C2; ES2257695T3; DK1542810T3; AU2003247312A1; ATE317731T1; DE60303608T2

Abstract

The invention relates to an air-driven, low frequency sound generator comprising an open resonator (2) working as a sound emitter and also comprising a feeder unit, for the generation of standing, gas-borne sound waves, which sound waves produce a varying sound pressure inside said resonator. The feeder unit includes a cylinder (5) with one open end facing said open resonator and communicating therewith, further including a piston (4) performing a reciprocating movement inside said cylinder, and also including a surge tank (7), at least partly surrounding said cylinder and connected to a first pressure gas source (9), for feeding pressure gas into the surge tank and further into said cylinder via at least one connection opening (10) arranged in a wall of said cylinder. The sound generator further comprises means (14, 15) for bringing said piston (4) to a stand-still position, in which said connection opening is blocked by the piston, by exerting an additional pressure on the upper end of said piston. The invention also relates to a corresponding method.

Description

AN AIR-DRIVEN LOW FREQUENCY SOUND GENERATOR AND A METHOD FOR REGULATING THE PISTON IN SUCH A GENERATOR

The invention relates to an air-driven low frequency sound generator, as is described in the preamble of patent claim 1. The invention also relates to a method for regulating the stand-still position of a piston included in a low frequency sound generator, as described in the preamble of patent claim 6.

In particular, the present invention relates to a low frequency sound generator of the type described in WO-A1 -8807894, in which a feeder unit comprising a movable piston is used to generate a standing gas-borne sound wave in a resonator. The piston performs a reciprocating movement inside a cylinder, which is surrounded by a surge tank connected to a pressure gas source. The piston regulates a connection opening between the cylinder and the surge tank. Since this connection opening of the cylinder is located close to an open end of the cylinder, which is facing and in communication with an open end of the resonator tube, the resonator tube is also in communication with the interior of the surge tank.

The dimensions of the resonator are chosen in such a way that a standing sound wave may be generated in the resonator by the pressure gas supplied via the surge tank. Preferably, the resonator is a quarter wave resonator, in most cases a quarter wave resonator tube. When a standing sound wave has been generated in the resonator, this standing sound wave has its maximum sound pressure amplitude where the feeder unit is situated. This sound pressure works on the end surface of the piston, resulting in a reciprocating movement of the piston. The other end of the piston is spring-loaded, and the piston can move in phase with the variations in sound pressure of the standing wave under the condition that the resonance frequency of the oscillating mechanical system is higher than the frequency of the standing sound wave in the resonator.

Sound generators of this type are, for example, used for cleaning big boilers, heat exchangers and other apparatus where it is difficult to get access to the surfaces that need to be cleaned.

In many of their applications, these sound generators are used intermittently. When the sound generator is to be shut off for a certain period of time, this has, according to prior art, been achieved by shutting off the supply of pressure gas to the surge tank of the feeder unit. In such a stand-still position, the piston will normally be located such that the connection opening between the surge tank and the inside of the cylinder is not completely blocked by the piston and thereby gas will be flowing back from the resonator tube into the cylinder and further into the surge tank. This gas flow has the disadvantage that it may cause corrosion on the sliding surfaces for the piston inside the cylinder when the gas flows from the resonator tube via the cylinder and into the surge tank. Since there is only a very small gap between the external sides of the piston and the inside of the cylinder, the build-up of corrosion on the inside of the cylinder could be quite detrimental to the functioning of the feeder unit. If there is too much corrosion, there is even a risk that the movement of the piston is inhibited.

Another disadvantage with this prior art sound generator is that the start-up of the feeder unit does not result in that the piston immediately starts moving with its working displacement amplitude. Instead there is a progressive start of the movement of the piston that will take up to approximately one half of a second before it has reached its working displacement amplitude.

The object of the present invention is primarily to solve the problems discussed above. This is achieved, according to the present invention, by means of an air-driven low frequency sound generator provided with the new features as defined in the characterizing portion of patent claim 1 , and by a method as defined in the characterizing portion of claim 6.

Accordingly, by providing a method and means for bringing said piston to a stand-still position, in which said connection opening is blocked by the piston, by exerting an additional pressure on said first spring-loaded end of said piston, the important advantage is achieved that corrosion is avoided on the inner surfaces of the cylinder, due to the fact that any gas flow back from the resonator tube into the surge tank is effectively prevented.

Another important advantage that is achieved by the present invention is that a much speedier start-up of the movement of the piston is obtained. In accor- dance with the present invention, an additional pressure is exerted on the spring- loaded end of the piston, which result in the connection opening between the surge tank and the cylinder being blocked completely, thereby shutting off the pressure gas supply to the feeder unit. When, later on, said additional pressure is made to cease, said connection opening is instantly opened completely, a power- ful gas pulse rushes into the resonator and the generator will start immediately. Tests have shown that the operating sound pressure amplitude can be reached in as short a time as within 1/10 of a second.

In most cases, the present invention will result in a stand-still position where the second lower end of the piston will be located in the vicinity of the open end of the cylinder, next to the resonator.

Further features and advantages of the present invention will be apparent from the remaining dependent claims.

It should be understood that by the expression "low frequency sound" is intended sound of a frequency below approximately 38 Hz. A suitable operating sound frequency would be between approximately 15-35 Hz.

The present invention will now be described by means of a preferred embodiment, given as an example only, and with reference to the enclosed schematic drawings, in which: Fig. 1 is a schematic illustration of a sound generator according to the present invention, during operation, with the piston located at its upper dead centre,

Fig. 2 is a schematic illustration of a sound generator, according to the present invention, during operation, with the piston located at its lower dead centre, and

Fig. 3 is a schematic illustration of a sound generator, according to the present invention, when it is at an intermittent stand-still.

Reference is initially made to Figs. 1 and 2. The illustrated sound generator comprises a feeder unit 1 mounted on a resonator, which in the illustrated case is a quarter wave resonance tube 2. The feeder unit includes a piston 4 that is arranged to perform a reciprocating movement inside a cylinder 5, between an upper dead centre 3, shown in Fig. 1 , and a lower dead centre 8, shown in Fig. 2. The piston is spring-loaded by means of a spring 6, which is attached to the closed end of the cylinder. The cylinder and the piston are mounted in a surge tank or casing 7. A first pressure gas source 9, providing a constant gas pressure

P_d, preferably by compressed air, is connected to the surge tank 7. The cylinder has an open end 11 facing the resonance tube and communicating therewith. The cylinder is also provided with one or several connection openings or ports 0 in the cylinder wall by means of which the pressure gas supplied to the surge tank can also enter into the cylinder and continue down into the resonance tube. At the upper closed end of the cylinder, above the piston, there is a connection opening 18 used for pressure equalisation and it provides a communication either to atmospheric pressure or to a space at the open end of the resonator tube. When the sound generator is in operation, the gas pressure p_g at the rear end of the resonance tube, i.e. that end of the resonance tube that is communicating with the cylinder and the feeder unit, below the piston, varies according to p_g = pi + p o sin 2π-f-t

Where p-i = constant air pressure at the rear end of the resonance tube p o = amplitude of the sound pressure at the rear end of the resonance tube f = frequency of the standing sound wave t = time

If the constant air pressure above the piston, p₂, is equal to the constant pressure at the rear end of the resonance tube, pi, the differential pressure over the piston is p o sin 2π-f-t.

When the differential pressure has its maximum value (+βo), the piston is pressed to the upper dead centre, which is illustrated in Fig. 1. At that position, the ports 10 in the cylinder 5 are open and an air pulse 12 is fed into the cylinder.

When the differential pressure has its minimum value (-β o), the piston is suctioned to its lower dead centre, as illustrated in Fig. 2, and the ports are closed. According to the present invention, at the opposite end of the cylinder, i.e. that end which is not facing the resonance tube and at which end the spring of the piston is fitted, there is a second pressure gas source 14 connected to the cylinder. This second pressure source 14 supplies a constant gas pressure, p_c2, preferably compressed air, which is regulated by a regulating valve 15. This com- pressed air pressure, p_c2, is much higher than the gas pressure p_g. As long as the regulating valve 15 is at the position shown in Figs. 1 and 2, i.e. the supply from the pressure gas source 14 is shut off, the piston will move between the upper and lower dead centres with the frequency f of the standing wave in the resonance tube.

When the valve 15 is switched into the open position shown in Fig. 3, the high compressed air pressure, p_C2, generated by the second pressure gas source 14, will act upon the upper end of the piston and serve to press down the piston. This gas pressure p_c2 should be so high, as compared to the gas pressure p_g, that the piston is pressed down to a position such that its upper dead centre will be located below the ports 10. Thereby said ports will be blocked by the piston and the^gas pressure p_cι from the first pressure gas source 9 will be shut off. In this position, the sound generator will come to a stand-still since the movement of the piston will stop when the compressed air from the first pressure gas source 9 cannot enter into the resonance tube 2. In the advantageous embodiment illustrated in Fig. 3, the piston will be pressed down to a seat 16 arranged at the lower end of the cylinder 5. When it is desired that the sound generator should enter into operation again, the position of the valve 15 is changed back from the position illustrated in Fig. 3, to the position illustrated in Fig. 1. The spring will then move the piston to a position above the ports, and the piston will be free to move again between its upper and lower dead centres. The pressure gas from the first pressure gas source 9 will then be allowed again to enter into the cylinder and further down into the resonance tube. in order to control the operation of the regulating valve 15, some type of control means are preferably provided. These control means may be manual means or some kind of automatic control means, known per se. The inventive concept, according to the present invention, should not be limited to the embodiment illustrated here, by way of example only, but may be modified and varied within the scope of the invention as defined in the appended patent claims.

Claims

PATENT CLAIMS

1. An air-driven, low frequency sound generator comprising an open resonator working as a sound emitter and also comprising a feeder unit, for the generation of standing, gas-borne sound waves, which sound waves produce a varying sound pressure inside said resonator, said feeder unit including a cylinder with one open end facing said open resonator and communicating therewith, further including a piston performing a reciprocating movement inside said cylinder and having a first sprwg-ioaded end and a second opposite end facing said resonator, and also in- eluding a surge tank, at least partly surrounding said cylinder and connected to a first pressure gas source, for feeding pressure gas into the surge tank and further into said cylinder via at least one connection opening arranged in a wall of said cylinder, and said piston functioning as a valve for regulating said connection opening, characterized in that it comprises means for bringing said piston to a stand-still position, in which said connection opening is blocked by the piston, by exerting an additional pressure on said first spring-loaded end of said piston.

2. A low frequency sound generator according to claim 1 , characterized in that it comprises a second pressure gas source connected to said cylinder for feeding pressure gas to a space inside said cylinder and behind said piston in order to exert said pressure on the spring-loaded end of the piston.

3. A low frequency sound generator according to claim 2, characterized in that it comprises a regulating valve for regulating the connection of said second pressure gas source.

4. A low frequency sound generator according to claim 3, characterized in that it comprises control means for opening said regulating valve when the sound generator is to be brought to an intermittent stand-still and for closing said valve when the sound generator is intended to enter into operation again.

5. A low frequency sound generator according to any one of claims 2-4, characterized in that a seat is arranged at the open end of said cylinder and that said second end of said piston is brought to abutment against said seat in its stand-still position.

6. A method for regulating the stand-still position of a piston included in a low fre- quency sound generator comprising an open resonator working as a sound emitter and also comprising a feeder unit, for the generation of standing, gas-borne sound waves, which sound waves produce a varying sound pressure inside said resonator, said feeder unit including a cylinder with one open end facing said ces Dator and communicating therewith, further including said piston performing a reciprocating movement inside said cylinder, said piston having a first spring- loaded end and a second opposite end facing said resonator, and also including a surge tank, at least partly surrounding said cylinder and connected to a first pressure gas source, for feeding pressure gas into the surge tank and further into said cylinder via at least one connection opening arranged in a wall of said cylinder, and said piston functioning as a valve for regulating said connection opening, characterized in that said piston is brought to a stand-still position in which said connection opening is being blocked by the piston, by means of exerting an additional pressure on said first spring-loaded end of said piston.

7. A method according to claim 6, characterized in that said additional pressure is exerted in the form of a gas pressure, which gas pressure is fed into said cylinder from a second pressure gas source connected to said cylinder.

8. A method according to claim 7, characterized in that the connection of said pressure gas source is regulated by a regulating valve.

9. A method according to claim 8, characterized in that said regulating valve is opened when the low frequency sound generator is to be brought to an intermittent stand-still and in that it is closed when the sound generator is intended to en- ter into operation again.

10. A method according to any one of claims 6-9, characterized in that said second end of said piston is brought to abutment against a seat located at the open end of said cylinder by means of exerting said additional pressure.