FREQUENCY CONTROLLED MOTOR DRIVEN LOW FREQUENCY SOUND GENERATOR

Abstract: The invention relates to a frequency controlled motor driven low frequency sound generator. The low frequency sound generator consists of an open resonator (2) arranged as a sound emitter for generating standing gas-borne sound waves which produce a varying gas pressure inside the resonator, and a feeder unit (1) for a modulated supply of air pulses to the resonator. The feeder unit (1) comprises a motor driven piston (8) with an approximately sinusoidal volume velocity. The piston is mounted on a piston rod (7) which is connected to a flywheel (6). The flywheel, in its turn, is mounted on a shaft (5) which is driven by a motor (3). The motor speed and thereby the frequency of the piston is regulated by a special electronic control unit.

Description: TITLE OF THE INVENTION: FREQUENCY CONTROLLED MOTOR DRIVEN LOW FREQUENCY SOUND GENERATOR This invention relates to a frequency controlled, motor driven, low frequency sound generator. A low frequency sound generator with a positive feedback system is described in SE B, 446 157 (corresponding to EP, B1 , 0 006 833). It consists of an open resonator arranged as a sound emitter for generating standing gas-borne sound waves, which produce a varying gas pressure in the resonator, and; a feeder unit with a pipe for supplying pressurized gas into the resonator and, a back and forth springing movabl valve slide, the position of which remains unaffected by the pressurized gas. The valve slide regulates the gas flow from the pipe while supplying a modulated flow of pressurized gas into the resonator. The valve slide is constructed as a sleeve, which is axially displaceable inside or outside the pipe and controls an opening arranged in the pipe wall for the supply of pressurized gas from the source. In SE, A, 8701461-9 (publication No 457 240, corresponding to PCT/SE88/00172) a low frequency sound generator is described, which is a further development of the above mentioned generator. The slide valve, in this case, is designed as a piston movable inside a pipe/cylinder, said piston being arranged in order to regulate a connecting opening between an air surge tank and the inside of the cylinder at one o the end surfaces of the piston. The air surge tank surrounds the cylinder and the feeder unit and is also connected to the pressurized gas source. One end of the cylinder is open towards the interior of the resonator and the connecting opening ma communicate with the interior of the resonator depending on the position of the pisto The basic principle for the operation of both these low frequency sound generators is when the sound pressure in the resonator is higher than the surrounding atmospheri pressure, the valve slide will move in such a direction as to free the opening, and the air having a higher pressure than the sound pressure will be forced into the resonator When the sound pressure in the resonator is lower than the surrounding atmospheri pressure, the valve slide will be forced to move in the opposite direction with the res that the opening is closed completely. The above described low frequency sound generators are both air driven. In a feed unit forming a part of the sound generator, working according to the above describe principle, it is essential to supply a large volume of air through the opening during a very short period of time and with a minimum loss of pressure during the passage o the air into the resonator. The supplied pressurized gas has so far been generated a blower, which is both space demanding and expensive as well as having the disadvantage of the supplied air being relatively hot. The purpose of the present invention is to generate sound pressure pulses in the resonator without the use of blower. The low frequency sound generators according to above mentioned docu¬ ments have a positive feedback system which means that the movement of the val slide and the subsequently generated pressure gas pulses are automatically adjust to one of the natural frequencies of the air column inside the resonator. This way, adjustments can be made according to variations in the frequency depending on e. changes in the temperature. The apparatus, according to the present invention, is equipped with a control system which is normally used in such a way that a maxim sound pressure is obtained in the resonance unit in the same way as when using a positive feedback system as described above, but it can also be adjusted in such a way that a lower sound pressure may be obtained. The invention, with examples of embodiments, will now be explained in detail with reference to enclosed drawings, in which: Fig 1 is a side view of the entire low frequency sound generator including resonator Fig 2 shows the driving part of the feeder unit in enlargement Fig 3 shows the air pulse generating part of the feeder unit in enlargement Fig 4 shows a block diagram of the control system. Fig 1 shows a low frequency sound generator with a feeder unit 1 and a resonator 2, only fragmentarily shown in the figure. The resonator 2 preferably consists of a quarter wave resonance tube open at one end and closed at the other end, or a half wave resonance tube which is closed at both ends. In connection with the closed en of the resonator there is a feeder unit 1 installed. The main parts of the feeder unit consist of a driving part with a motor 3, whose drive shaft 11 via a clutch 4 is connected to a shaft 5. On the shaft 5 there is a flywheel 6 attached, which in its turn equipped with a number of holes for optional mounting of a piston rod 7. The piston rod 7 is attached to a piston 8, which is movable inside a cylinder 9 surrounded by a cylinder block 10. The airpulse generating part of the construction consequently consists of the piston 8 and the cylinder 9. It is the reciprocating movement of the piston 8 and the resulting, approximately sinusoidal, volume velocity of the piston th generates air pulses at the closed end of the resonator 2. Fig 2 shows the driving part of the feeder unit with a motor 3, which is carried by a support fastened on to the cylinder block 10. The drive shaft of the motor 11 is connected via the clutch 4 to the shaft 5. The clutch 4 is e.g. of rubber or other flexibl material in order to absorb any angle, axial and/or radial play that may occur betwee the drive shaft 11 of the motor 3 and the shaft 5. It also carries torque variations, whi are caused partly by the inertia of the piston and partly by the sinusoidal load consisting of pressure variations in the resonance tube which have not already been eliminated by the flywheel. The shaft 5 is carried by a bearing housing 12 which in its turn is fastened, with a right angle bracket 13, to that end of the cylinder block 10 which is turned away from the resonance tube 2. The bearing housing 12 can e.g. be mounted on the bracket 13 with a bolted joint or it could be welded on to it. On the en of the shaft 5 which is turned away from the motor 3 there is a flywheel 6 detachably connected. This flywheel is, at different distances from its centre hole, equipped with holes for optional, detachable connection of the piston rod 7. The piston rod is mounted with bearings on a screw 14, with which it is attached to the flywheel 6, and by means of the screw 14 being drawn through one of the holes in the flywheel 6 made for this purpose, the screw being fixed with the help of a locking nut 15. Fig 3 shows the cylinder 9 and the piston 8. The other end of the above mentioned piston rod 7 runs through the piston 8 and is attached to its top, whose end surface may be bellowing outwards. The piston 8 is movable back and forth with low friction inside the cylinder 9 due to the fact that there is a small radial play between the pist and the cylinder. Furthermore the piston may preferably be equipped with holes in order to, among other things, lessen its weight and thereby also the mentioned friction. The holes also contribute to an improved cooling of the piston. The cylinder located in the cylinder block 10, said cylinder block being mounted in connection wi the closed end of the resonator 2. Through the reciprocating movement of the piston 8 and the approximately sinusoi volume velocity of the piston, sinusoidal air pulses are generated and will propagat into the resonance tube 2. Through reflection of these air pulses there is a standing sound wave building up in the resonance tube, said sound wave having its maximu sound pressure where the feeder unit is located. This sound pressure works upon t end surface 16 of the piston and generates a force working on the piston equal to t sound pressure multiplied by the area of said end surface. In order to obtain as hig a sound intensity as possible, it is desirable that the reciprocating movement of the piston, which is determined by the flywheel 6 and the shaft 5, takes place with a frequency that, as far as possible, is the same frequency as a certain chosen frequency corresponding to one of the natural frequencies of the air column inside t resonance tube 2. Fig 4 shows the control system for controlling the flywheel and thereby the movem of the piston. The control system is based upon the utilization of the phase displacement between the sound pressure measured at the end of the resonance organ 2 which is turned towards the cylinder block 10, and the speed of the piston. Said sound pressure is measured preferably with at least one gas pressure transducer 17 and the phase of the piston speed is preferably measured with at lea one level indicator 18 mounted in connection with the flywheel. The phase for the piston speed corresponds to the phase of the position of the piston with a 90° displacement. The measured values are compared by means of a signal comparat 19, which then will send a control signal influencing a speed regulation device 20 connected to the motor 3. The transducer as well as the signal comparator and spe regulation device are preferably electronic. Maximum interaction between the movement of the piston and the resonator is obtained when the frequency of the piston is chosen so that the mentioned phase deplacement is equal to nil. During possible fluctuations in the standing wave frequency, due to e.g. temperature variations, the frequency of the piston is automatically adjusted. Due to this arrangement, the piston may also be forcibly controlled by means of chosing to give the piston a somewhat different frequency than the one corresponding to the frequency of the standing sound wave. This can be done either completely manually or automatically controlled by a predetermined factor, e.g. the temperature, or controlled through other electronic equipment such as a computer. It is also possible to use a design where the speed regulation device is controlled directly by the gas pressure transducer without any level indicator being used. Evidently, also other kinds of designs may be used within the frame of the idea of th invention. E.g. it is possible to exchange the shaft 5, the flywheel 6 and the piston ro 7 with a more conventional arrangement including a crankshaft and connecting rod.

Claims: Patent claims 1. Low frequency sound generator, including a resonator (2) for the generation of standing gas-borne sound waves, and a feeder unit (1) for the regulated supply of pulses to the resonator, characterized in that the feeder unit includes a mot driven piston (8) with an approximately sinusoidal volume velocity and where the frequency of the piston (8) is controlled by a control unit. 2. Low frequency sound generator according to claim 1 characterized in th air pulses in the form of approximately sinusoidal sound waves are being generate in the resonator (2) by the reciprocating movement of the piston (8) inside a cylinde (9) surrounded by a cylinder block (10). 3. Low frequency sound generator according to any of the claims 1 or 2 characterized in that the piston (8) is being attached to a piston rod (7) and movable back and forth inside the cylinder (9) with low friction and that there is a small play between the piston (8) and the cylinder (9). 4. Low frequency sound generator according to claim 3 characterized in th the piston rod (7) is detachably mounted on a flywheel (6) which in its turn is mount on a shaft (5) which is driven by a motor (3). 5. Low frequency sound generator according to claim 4 characterized in th the flywheel (6) is equipped with one or more holes for optional fastening of the pis rod (7) with e.g. a screw (14). 6. Low frequency sound generator according to claim 3 characterized in th the piston rod (7) is also a connecting rod which is mounted on a shaft in the shap a crankshaft which in its turn is attached to the drive shaft (11 ) of a motor (3). 7. Low frequency sound generator according to any of the claims 4, 5 or 6 characterized in that the shaft (5) is being carried by a bearing housing (12 and being attached to the drive shaft (11 ) of the motor (3) via a clutch (4). 8. Low frequency sound generator according to any of the preceding claims characterized in that the control unit regulates the frequency of the motor-driven piston (8) with regard to the natural frequency of the resonator (2). 9. Low frequency sound generator according to claim 8 characterized in th the control unit comprises of at least one pressure transducer (17) and a speed regulation device (20) connected to the motor (3). 10. Low frequency sound generator according to claim 9 characterized in t the control unit also comprises of at least one level indicator (18) and a signal comparator (19). 11. Low frequency sound generator according to claim 10 characterized i that the pressure transducer (17) measures the sound pressure at the end of the resonator (2) which is turned towards the cylinder block (10) and the level indicator (18) measures the position of the piston (8) and that the phase deplacement betwe these entities is being registered. 12. Low frequency sound generator according to claim 11 characterized in that the level indicator (18) comprises of a detector mounted in connection with the flywheel (6). 13. Low frequency sound generator according to claim 11 or 12 characteriz in that the phase deplacement is being registered and treated in the signal comparator (19) which then gives a control signal influencing the speed regulating device (20). 14. Low frequency sound generator according to any of the claims 10-13 characterized in that both the pressure transducer (17), the level indicator (1 the signal comparator (19) and the speed regulation device (20) are preferably electronic.

Inventors: SANDSTROEM ROLAND, SANDSTROEM, ROLAND
Applicants: INFRASONIK AB
Application year: 1989
Main classification: B06B1
IPC codes: B06B1, G10K11, G10K15