Sources of measurement uncertainty in determination of the directional diffusion coefficient value

Values of the directional sound diffusion coefficient, according to recommendations of ISO 17497-2 standard, are measured in the free field of an anechoic chamber. The procedure proposed in the standard for determination of directivity patterns characterizing reflection of sound wave from an examined structure is biased by a measurement uncertainty following from the effect of individual elements of the measurement chain and operations performed on the recorded signal. Further sources of uncertainty can be related to the sample itself due to possible imperfections of workmanship.The paper presents an analysis of possible sources of the measurement uncertainty encountered when the directional sound diffusion coefficient is determined and an attempt to evaluate the effect of individual sources on the expanded uncertainty of the measured parameter. Using the Fraunhofer approach to the wave equation and the Monte Carlo method, the effect of the measurement angular resolution, microphone and sample position, repeatability of generated and recorded signals, the signal-to-noise ratio, and precision of sample workmanship on uncertainty of the directional sound diffusion coefficient was determined. For the value of 0.025 adopted as the admissible maximum of the expanded uncertainty of the parameter, maximum allowable values for uncertainties of individual input parameters were identified.