This study aims to generate a quiet zone in a three dimensional enclosure by controlling its boundary condition. It is well known that the boundary condition affects the enclosure sound field. This implies that we can modify the sound field by changing a wall’s acoustical property, e.g. admittance. Attaching absorptive materials on a wall can fulfill this alteration. In this context, a theoretical analysis is performed to understand the influence of the absorptive material arrangement on the interior sound field. Then, to find its optimal arrangement, a simulation program that combines BEM and genetic algorithm is developed. The program has following features. The arrangement of absorptive materials is expressed as a vector form. The vector is defined as an AMA (absorptive material arrangement) vector. In addition, for the practical application, the program determines the element of the AMA vector from the predefined set of available absorptive materials. We believe that the newly defined vector certainly provides a way to understand the role of a sound absorptive material’s arrangement on making a quiet zone in an enclosure.