The Directive 2000/14/EC, amended by the Directive 2005/88/EC, formulates requirements for a number of machine groups for use outdoors. Measurement procedures and operating conditions are specified for measurement of the sound power level. The Directive itself describes the measurement procedures in detail but does not formulate the way of calculating the uncertainty K which is to be added to the median sound power level. Due to this fact, various different formulas are being used throughout Europe to determine uncertainty K, leading to noteworthy diverse results and to a distortion of the internal market. Results of data evaluations showed that the difference between the lowest and highest value was at approximately 2 dB. This round robin test on 5 types of machines (or more if possible) shall provide further statistical knowledge in order to establish coherent methods so that all examinations are based on the same technique and lead to comparable results.