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The sound absorber is a combination of an acoustical transformer with an actively simulated acoustical impedance. The acoustical transformer transforms the high active impedance into a low impedance at the mouth of the transformer. The absorber is tuned by changing the active impedance.

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1. Active sound absorber, comprisinga) an acoustical transformer,b) a device for simulation of an acoustical impedance, which comprisesc) an electroacoustical transducer with a membrane, wherein said membrane is acoustically coupled to said acoustical transformer,d) pressure sensing means, being arr

1. Active sound absorber, comprisinga) an acoustical transformer,b) a device for simulation of an acoustical impedance, which comprisesc) an electroacoustical transducer with a membrane, wherein said membrane is acoustically coupled to said acoustical transformer,d) pressure sensing means, being arranged within said acoustical transformer substantially close to said transducer's membrane, for measuring the air pressure and producing signals indicative of said air pressure,e) driving means, for moving said transducer's membrane with a momentary membrane speed which depends on said measured air pressure according to a predetermined impedance function, wherein said driving means receive said signals produced by said pressure sensing means,wherein said acoustical transformer and said device for simulation of an acoustical impedance are dimensioned such that said sound absorber is an acoustical resonator with predetermined resonance frequencies. 2. Device of claim 1, in which said acoustical transformer is a Helmholz resonator, and in which said pressure sensing means is arranged within said Helmholz resonator's cavity. 3. Device of claim 1, in which said acoustical transformer is a pipe resonator. 4. Device of claim 1, in which said acoustical transformer is an acoustical horn. 5. Device of claim 1, in which said acoustical transformer is an acoustical transmission line. 6. Device of claim 1, in which said acoustical transformer is a combination of at least two devices selected from the group consisting of the acoustical horn and the pipe resonator and the Helmholtz-resonator and the acoustical transmission line. 7. Device of claim 1, wherein said impedance function is adaptable. 8. Device of claim 1, wherein said impedance function contains terms which change periodically. 9. Device of claim 1, wherein said driving means comprisea) a controller, which receives said signals produced by said pressure sensing means,b) a power amplifier, which receives signals from said controller and which drives said electroacousticaltransducer. 10. Device of claim 9, in which said acoustical transformer is a device selected from the group consisting of the acoustical horn and the pipe resonator and the Helmholtz-resonator and the acoustical transmission line. 11. Device of claim 9, in which said acoustical transformer is a combination of at least two devices selected from the group consisting of the acoustical horn and the pipe resonator and the Helmholtz-resonator and the acoustical transmission line. 12. Device of claim 1, wherein said driving means is a closed-loop control system, further comprisinga) movement measuring means for measuring the movement of said transducer's membrane and producing signals indicative of said movement,b) calculating means, which receive the signals produced by said pressure sensing means, for calculating setpoint values for movement, wherein the setpoint values for movement are calculated from said signals produced by said pressure measuring means in accordance to a predetermined impedance function,c) a controller, which receives said signals produced by said movement measuring means and said setpoint values produced by said calculating means,d) a power amplifier, which receives signals from said controller and which drives said electroacousticaltransducer,wherein said controller is dimensioned to drive via said power amplifier said transducer's membrane such that the actual values of movement of said membrane are substantially equal to said setpoint values of movement,and wherein said resonance frequencies are tunable by changing said impedance function of said device for simulation of an acoustical impedance. 13. Device of claim 12, in which said acoustical transformer is a device selected from the group consisting of the acoustical horn and the pipe resonator and the Helmholtz-resonator and the acoustical transmission line. 14. Device of claim 13, wherein said impedance function is electronically adaptable. 15. Device of claim 13, wherein said impedance function contains terms which change periodically. 16. Device of claim 12, in which said acoustical transformer is a combination of at least two devices selected from the group consisting of the acoustical horn and the pipe resonator and the Helmholtz-resonator and the acoustical transmission line. 17. Device of claim 16, wherein said impedance function is adaptable. 18. Device of claim 16, wherein said impedance function contains terms which change periodically. 19. Method for tuning the resonance frequency of a resonant sound absorber, comprising the step of acoustically coupling an acoustical transformer to a device for simulation of an acoustical impedance, which further comprises the steps ofa) arranging an electro-acoustical transducer with a membrane, such that said membrane is acoustically coupled to said acoustical transformer,b) arranging pressure sensing means within said acoustical transformer , for measuring the air pressure and producing signals indicative of said air pressure,c) arranging driving means, for moving said transducer's membrane with a momentary membrane speed which depends on said measured air pressure according to a predetermined impedance function, wherein said driving means receive said signals produced by said-pressure sensing means,d) dimensioning said acoustical transformer and said driving means such that the device works as acoustical resonator at predetermined resonance frequencies,e) tuning said resonance frequencies by adapting said driving means. 20. Method according to claim 19, further comprising the steps ofa) arranging movement measuring means for measuring the movement of said transducer's membrane and producing signals indicative of said movement,b) arranging calculating means, which receive the signals produced by said pressure sensing means, for calculating setpoint values for movement, wherein the setpoint values for movement are calculated from said signals produced by said pressure measuring means in accordance to a predetermined impedance function,c) arranging a power amplifier, which drives said electroacousticaltransducer,d) arranging a controller, which receives said signals produced by said movement measuring means and said setpoint values produced by said calculating means, which drives said power amplifier such that the actual values of movement of said transducer's membrane are substantially equal to said setpoint values of movement,e) dimensioning said acoustical transformer and said driving means such that the device works as acoustical resonator at predetermined resonance frequencies,f) tuning said resonance frequencies by adapting said impedance function. 21. Method for transforming the acoustical impedance of a device for simulation of an acoustical impedance to other values,comprising the step of acoustically coupling an acoustical transformer to a device for simulation of an acoustical impedance, which further comprises the steps ofa) arranging an electro-acoustical transducer with a membrane, such that said membrane is acoustically coupled to said acoustical transformer,b) arranging pressure sensing means within said acoustical transformer, for measuring the air pressure and producing signals indicative of said air pressure,c) arranging driving means, for moving said transducer's membrane with a momentary membrane speed which depends on said measured air pressure according to a predetermined impedance function, wherein said driving means receive said signals produced by said pressure sensing means,d) dimensioning said acoustical transformer and said driving means such that the device works as acoustical resonator at predetermined resonance frequencies.