In the second half of the 1990s, the Laboratoire d’Acoustique Musicale at Pierre et Marie Curie University in Paris started a series of investigations on Urban Sound. Initiated at the request of the French Ministry of Environment, these investigations took into account from their very onset the ecological approach to auditory perception, following the footsteps of J.J. Gibson. Care was taken that sound reproduction was ecologically valid, and a whole methodology, based on E. Rosch’s psychological principles of categorization, was developed with the help of psycholinguists in order to evaluate this ecological validity. We called it semio-acoustics. The presentation reviews the theoretical background, the methodology and the main results from a series of 5 doctoral theses on how human subjects talk about and describe soundscapes. It includes examples of free categories, semantic categories, prototypical categories, and graphic categories. More recently, we develop the methodology one step further with the help of semiotics, in order to take into account the imaginaries of the human subjects: we obtained full narratives of ideal urban ambiances. We are presently applying the methodology to Urban Soundscapes.