Source: Land NRW (2017) Datenlizenz Deutschland -Namensnennung - Version 2.0 (http://www.govdata.de/dl-de/by-2-0)

The noise map indicates the noise load according to the European directice 2002/49/EC. The noise load considers the traffic noise from major roads (here in particular the highways and major local streets). The noise contribution of lower roads and the aircraft noise from the air of Merzbrück is not taken into account.

More information (in German)  http://www.umgebungslaerm.nrw.de/laermkartierung/index.php.