Compared to a centralized system for acoustic source localization using WSN, a distributed system has many advantages in terms of scalability, power consumption, and response time. Complexity is an important factor to consider when implementing a distributed system because of the limited resource of WSN devices. The complexity of the source localization algorithm is related to the area required for location estimation. Since reducing the size of the estimation area influences the accuracy of the source localization system, a policy should be carefully designed to reduce the size. In this paper, we use a Voronoi diagram to define an estimation area. The Voronoi diagram-based mechanism may yield an optimized estimation area and accuracy, but creating a Voronoi diagram is not suitable for WSN devices due to the high implementation overhead. This paper proposes an efficient mechanism to create the estimation area, which can be implemented in WSN devices and achieve reasonable accuracy, compared to the ideal Voronoi diagram-based one. The proposed algorithm has been applied to a distributed source localization algorithm, and compared with a Voronoi diagram through extensive simulations.