Urban Acoustics is a conceptual smartphone application, which enables users to interact with their urban environment in a new, creative way. The application deals with the topics acoustics and music and features two different layers: An active and a passive one. The active layer enables users to record and upload rhythms and melodies created on urban surfaces, etc. to an internet community. In the passive layer, one can experience these sounds in an interactive way. By moving through the city, you can listen to rhythms and sounds created by other users at the places you are passing. These form a musical composition created by the community, which is in a state of permanent development and variation.