Stop the transmission of direct sound and make the transmission sound attenuate sufficiently, and the influence of transmission sound can be ignored. Therefore, the sound insulation effect of the sound barrier can be generally expressed by noise reduction, which reflects the two kinds of screening capacity of the sound barrier. A sound barrier is inserted between the sound source and the receiving point, which is infinitely long. Sound waves can only be diffracted from above the barrier, and then form a sound shadow area, just as light is blocked by an object to form a shadow. In this area, people can feel that the noise is significantly reduced, which is the noise reduction effect of the sound barrier.