In view of the practical importance of the problem of the reduction of noise at a point due to an unavoidable source of sound situated at a second point, certain aspects forming a logical sequence have been selected for a theoretical treatment from as general a point of view as possible. The first relates to barriers, the second to the enclosure of the source, and the third to the enclosure of the point of observation. In all cases connected with a simple source of sound and a point of observation the Helmholtz reciprocal principle holds, and two applications of this are given, in connexion with the sound transmitted round a barrier, and in connexion with the problem of an enclosed source.