What noise do wind turbines make when they are running constantly, and how does this affect noise-sensitive marine mammals like porpoises? These questions were investigated in the RAVE project Operational Noise. Two turbines in Alpha Ventus were equipped with underwater microphones and acceleration sensors for measuring vibration. The operating noise was measured at partial load and full load of all the turbines in the wind farm. During strong winds the ambient noises, which in particular arise as a result of shipping, are as loud as the operating noise of the turbines, which are at around the hearing threshold of porpoises and seals. In terms of airborne sound, under water in a wind farm is as loud as lunchtime in a university canteen. The foreseeable noise accumulation resulting from the ongoing operation of many new offshore wind farms, especially during the erection phase (construction noise), and the shipping noise means that independent underwater measurements will be necessary in future.