Sample rotational symmetry in the random-incidence scattering coefficient measurement

Sant’Ana, L. H. Postgraduate Program of Civil Engineering, Universidade Tecnológica Federal do Paraná, Curitiba, PR, Brasil, luis.2014@alunos.utfpr.edu.br

Gomes, M. H. A. Academic Department of Mechanical Engineering, Universidade Tecnológica Federal do Paraná, Curitiba, PR, Brasil

Larcher, J. H. K. Postgradute Program of Mechanical Engineering, Pontifícia Universidade Católica do Paraná, Curtiba, PR, Brasil

Catai, R. E. Academic Department of Civil Engineering, Universidade Tecnológica Federal do Paraná, Curitiba, PR, Brasil

Abstract

It is well known that samples with rotational symmetry present an underestimated scattering coefficient when measured through the reverberant chamber method. The ISO 17497-1 even states that symmetric samples should have its center displaced from turntable's center for the measurement. However, it hasn't been discussed if sample isotropy may underestimate the measured coefficient as well. Analysing previous measurement results, this seems to be the case. To investigate the matter, the case of surfaces composed by distributed hemispheres was chosen for study. A method for calculating a sample's symmetry function was developed and is presented. It shows an increase of isotropy as more elements are introduced or the smaller the elements are (in other words, the more "noisy" the surface is). Measurements were made in order to further study the phenomenon. It seems, indeed, that sample isotropy causes an underestimation of scattering coefficients, although more measurements are recommended. Also, it was noticed that even in the case of the studied samples, which have not a strict edge, there are edge effects at play. It can be explained by the greater influence elements further away from the center have on the sound field during the turntable rotation, accentuated by the higher circumference-area ratio the smaller the sample. Accordingly, the sample edge strip, when present, also results in lower measured scattering coefficients. Since there is no strict sample edge, this could be an artifact created by the strip. These effects are discussed alongside other phenomena that affects sample representativeness - a key concept in understanding how the measurement method interprets the sample.

Keywords:

acoustics

sample representativeness

edge effect

isotropy

autocorrelation.

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