Available shields are of a certain thickness that has an influence on sound diffraction. Judging by the research results, the influence of shield thickness is insignificant if the thickness is less than the length of a sound wave. A thick shield plate or a wide shield has two edges, and this increases noise reduction at the expense of double diffraction. Solution of flat monochromatic wave diffraction has been analyzed. The compact formula of sound insulation at uniform field in front of the shield has been obtained.