An arrangement to reduce noise of a wind turbine rotor blade is provided, including a wind turbine rotor blade and a noise reduction device. The noise reduction device includes a serrated extension for at least reducing noise generated from the wind turbine rotor blade, the noise reduction device attached to the trailing edge section. The serrated extension has a number of first and second teeth. The designs of the first and second teeth are differently compared to each other such that suction side noise and pressure side noise are both reduced by the teeth of the serrated extension. Advantageously, the first teeth and the second teeth are arranged in an alternating, periodic pattern and differ from each other by an angle of inclination with regard to a trailing edge streamline and or by its planform shape. Furthermore, a method to reduce noise of a wind turbine rotor blade is provided.