Improved method for determining the absorption coefficient of high reflective surfaces

Invited paper

Wout Schwanen


Wednesday 3 june, 2015, 09:40 - 10:00

0.2 Berlin (90)

The demands for the exterior noise test tracks defined in the ISO 10844:2014 are more demanding than the demands in the ISO 10844:1994 version. Among others, this certainly holds for the acoustical absorption of the road surface. This may not exceed 8% in each 1/3rd octave band between 315 and 1600 Hz, while in the former one the limit was 10% on an average value. The mandatory test for acoustic absorption checking is the spot method prescribed in ISO 13472-2 that applies a two microphone impedance tube (standardized in ISO 10534-2). Straightforward application of this method causes complications since for each microphone position destructive interference will take place at certain frequencies. This introduces a significant error in the determination of the transfer function between the two positions and thus in the determination of the absorption coefficient. We developed a procedure where signals are measured continuously at three microphone locations. Between each of the three pairs of positions the coherence function is measured. Occurrence of destructive interference at one position will result in loss of coherence for each pair that shares that position. Then we select each individual frequency that pair with the highest coherence, and use that for the calculation of the transfer function, leading to a much better estimate of the absorption coefficient. This has shown to be a major improvement that reduces errors over the total frequency range and allows the determination of absorption coefficients below 2%.

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