INTERNATIONAL CONGRESS ON ACOUSTICS MADRID , 2-7 SEPTEMBER 2007 ASSESSMENT OF THE UNCERTAINTY IN ROOM ACOUSTICAL MEASUREMENTS

Every measurement of a physical quantity is affected by uncertainties. For reasons of completeness and in order to make measurements comparable, it is mandatory to state the uncertainty of a measurement. ISO/BIPM’s “Guide to Uncertainty in Measurement” (GUM) standardises the procedure to establish the uncertainty in all areas of measurement. In a number of preceding works it has been found that despite standardisation (ISO3382) the measurement of room acoustical parameters show differences when the results of different measurement teams or repeated measurements are compared. With the goal of establishing a range of uncertainty that is smaller than the just noticeable difference (jnd) and in order to develop a strategy to reduce the range of uncertainty in a measurement, a linear cause-andeffect-model is presented for assessment of the combined measurement uncertainty for room acoustical parameters. The main contributing uncertainties are determined on the basis of special room acoustical measurements that match the model used. It is discussed how the presented procedure can be used to determine the GUM-conform range of uncertainty for measurements in room acoustics. Finally, a practical strategy is derived how the uncertainty of room acoustical measurements can be reduced. INTRODUCTION In architectural acoustics the measurement of room impulse responses (RIR) and the derivation of room acoustical parameters play a key role. This is not only due to the fact that many aspects of the perception of sound in enclosures are related to physical properties of sound fields but also due to the common practice of evaluating the success of constructional efforts in an objective manner. However, even the most carefully designed experiments with state-of-the-art instruments yield results that are still afflicted with various sources of error. In order to quantify the quality of measurements and with the goal of making the results of different measurement teams comparable, it is mandatory to state the range of values, that can reasonably be attributed to the measurand along with its best estimate of the “true value”. In order to propagate this concept the International Standardisation Organisation (ISO) released its Guide to the expression of uncertainty (GUM) [1]. Although GUM-methods are readily applied in many measurements related standards the discussed concepts have not yet found their way into practical room acoustical measurement strategies. BASICS OF GUM Following the principles formulated in the GUM the paradigm of classical Gaussian error calculus is shifted towards the mathematical modelling of a measurement procedure and the estimation of the input quantities with the use of probability density functions (pdf). For practical applications the standard GUM procedure is structured into 7 steps. Some of these segments can be demonstrated using Figure 1. 1. Collecting information on the measurement and its input quantities xi. Information about measurement procedures are often summarised in the relevant standards, e.g. ISO 3382-1:2006 [2] and ISO 18233:2006 [3]. 2. Modelling of the measurement in terms of a model function f. The concept of modelling is based on the idea that a signal is propagated through a