Taking up the slogan of the teams of architects “for happy architecture” for the design of the new E line of the metro in the Lyon area (France), this paper defends the idea of a happy atmosphere as a design tool. The research presented here focuses on the sound dimensions of the architectural project. Principles of a sound charter are discussed and ambiance creation is thought through the design of tensions between the material, sensitive and symbolic conditions of the experience of a back plan and the conditions of appearance of sound events. Ambiance design is a means of liberating the perception and representation of metro’s users and a method to design sensitive worlds for the public, participating in this way in the renewing the ordinary experience of the metro.