Patent ID: 7352870

Claim:
An active sound muffler for reducing a sound to be reduced as emitted from a sound source located at one of opposite sides of a sound insulating wall and diffracted by a top end of the sound insulating wall and transmitted to the other side, the muffler comprising: an additional sound source arranged at the top end or the other side of the sound insulating wall and adapted to output a control sound with a predetermined amplitude and a predetermined phase; a sound source gauging device arranged on a straight line connecting the top end of the sound insulating wall and a sound receiving point to be located at the other side and receive the diffracted sound, and adapted to gauge a sound pressure or an acoustic intensity of the sound to be reduced and that of the control sound; and an additional sound source controller configured to control the output of the additional sound source so as to minimize a sound pressure or an acoustic intensity of the sound to be reduced at the sound receiving point, whichever appropriate, based on the outcome of gauging of the sound source gauging device, wherein the additional sound source shows a line sound source characteristic and wherein the sound pressure detected by the sound source gauging device is expressed by the equation P=Q p *Z p +Q s *Z s , where P is the sound pressure, Q p is an intensity of the diffracted sound, Z p is a space propagation characteristic from a position of a diffracted energy to the sound source gauging device, Q s is an intensity of sound emitted from the additional sound source and Z s is a space propagation characteristic from the additional sound source to the sound source gauging device, wherein Zs=pjω·e −jkLs , where p is the air density, j is the purely imaginary number, ω is the angular frequency, and the number of waves is k (k=ω/c, c: sound velocity).