Noise pollution affects human health in urban areas. A noise map facilitates monitoring of environmental noise pollution. It can promote citizens' awareness of noise pollution, and aid in the planning of urban area. However, state-of-the-art techniques for rendering noise maps by special equipment are expensive in urban areas. In this paper, we present the design, implementation and preliminary testing results of an environment noise monitoring system called DDNM (Day Day Noise Monitor). DDNM is based on smart phones and the public participation, can be leveraged to create an open and inexpensive platform for rendering up-to-date noise maps. We implement DDNM on HTC g18 8G smart phones and Huawei C8812E 4G smart phones. The outdoor experiments demonstrate that DDNM is a feasible platform to assess noise pollution and provide an accurate noise map for the public.