one of the faces of the strip is partially in direct contact with the volatile substance and the other face is partially in direct contact with the environment the volatile substance is preferably an aromatic substance although substances with another type of properties such as insecticide substances for example can also be used the device adjustment means are placed between the air flow and the external face of the permeable strip and are provided with at least one window of a variable area for controlling the amount of air passing through the device and acting on the membrane and as a result the amount of air freshening product supplied to the environment per unit of time the device is optionally provided with side windows the area of which can also be adjusted preferably simultaneously to the aforementioned windows of a variable area the device is complemented with means for fixing to a fixed structure such as the grating of a ventilation air equipment outlet for example such as the ventilation air outlet of a motorized vehicle or an air conditioning equipment in such case the aforementioned air flow comes from the ventilation air outlet of a vehicle or of an air conditioning equipment an evaporator device of great simplicity and therefore of a very reduced cost is thus obtained such that it can be used.