During a survey related to acoustic and visual perception of users of urban areas, 614 people have been interviewed in Pisa (Italy). The work aims to identify and quantify the effects of parameters influencing the perception of tranquillity in order to understand the soundscape and to propose a method based on the perception of tranquillity for the detection of quiet areas within urban ones. A linear model that predicts the tranquillity perceived in different environments, based on their visual and acoustic characteristics, is proposed. Users were interviewed by operators inside the areas, using a direct approach of standardized questionnaires and oral questions. Simultaneous noise measurements and soundwalks have been performed, together with visual registrations. The linear model obtained predicts the perceived tranquillity based on the statistical level L A10 (A-weighted noise level exceeded for 10% of the measurement time) the sound sources and visual elements. The perceived tranquillity results negatively correlated to L A10 and to the presence of sound sources or negative visual elements. The presence of beneficial sound sources is positively correlated to the perceived tranquillity. However, the effect of the noise level is regulated by environmental characteristics. Perceived tranquillity is proposed as an indicator to identify quiet areas in the urban environment, according to European Directive 49/2002/EC. The obtained model identifies the areas that would get a higher tranquillity value than a fixed threshold value and therefore would be perceived as quiet. The model can be used as a cost-benefit analysis support tool to identify the best solution between the reduction of noise levels and the regeneration of urban areas, referring to the tranquillity perceived by the users.