Patent ID: 6847722

Claim:
A method for attenuating sound in a duct, the sound to be attenuated being detected in the method by means of a detector ( 2 ) and the attenuation being performed by means of two successive actuator elements ( 3 , 4 ), wherein sound is attenuated by means of two successive monopole elements ( 3 , 4 ) in such a way that both elements ( 3 , 4 ) function as a dipole approximation and also produce a monopole radiation needed, a dipole control signal being fed to both elements ( 3 , 4 ) at a phase shift which is 180° between the two elements and a monopole control signal being fed to the elements ( 3 , 4 ) cophasally, wherein the control signal of the first actuator element ( 3 ) is q 1 =½( a/jkd−b/ 2) q i , and the control signal of the second actuator element ( 4 ) is q 2 =−½( a/jkd+b/ 2) q i , where j is an imaginary unit; k is a wave number=ω/c 0 ; ω is an angular frequency; c 0 is sound velocity in a medium; d is a distance between the actuator elements ( 3 , 4 ); q i is the sound pressure to be attenuated, located at the center of the actuator elements ( 3 , 4 ), and converted to a volume velocity quantity; a is a constant or a dipole part control function; and b is a constant or a monopole part control function.