Evaluation of Uncertainty Contributions of Measurement Surface and Number of Microphone Positions in Determination of Sound Power Levels

Sound power level of a noise source is determined by means of sound pressure level or sound intensity level measurements performed in accordance to relevant ISO standards. The determination of sound power level according to ISO 3744, 3745 and 3746 standards is used for free field or for approximated free field conditions. Kinds of measurement surfaces, enveloping the noise source, number of microphones and their positions over the measurement surface are stated in the applied ISO standard. The effects of measurement surface and number of microphone positions on the determination of sound power level were investigated theoretically. As a measurement surface; hemisphere, parallelepiped rectangular box and cylindrical surfaces were selected. Key and additional microphone positions were taken into account in the calculations as well. Sound pressure levels of a commercially available reference sound source were measured in hemi-anechoic room using FFT with 4 Hz steps and also at 1/1, 1/3, 1/12 and 1/24 octave bands for all surfaces defined in ISO 3744 and 3746 standards. Sound power level, directivity index and uncertainty contributions resulting from measurement surfaces and number of microphone positions were calculated by using data obtained in measurements. In this paper the theoretical and experimental results are presented.