Determinação da frequência de transição a partir de funções de transferência e análise de efeitos de isotropia de amostra em medições do coeficiente de espalhamento sonoro de incidência aleatória

Abstract

This research presents developments in the estimation of a crossover frequency (Schroeder’s frequency) from room transfer functions. Analytical responses of a geometry optimised room with different dampening situations were analysed. The crossover frequency was estimated from tests of normality of real and imaginary parts by kurtosis coefficient analysis. For this, different metrics for the coefficient were compared and the G2 metric was chosen due to its lower variance for the coefficient mean upon using more than six transfer functions. The crossover frequency was also estimated from uniformity tests of phase. The maximum deviation from a uniform distribution (DUPH) and KolmogorovSmirnov tests were applied. The phase analysis was preferred, with results closer to the Schroeder’s frequency, which is expected for the analysed geometry, and with good repeatability for analysis with sample sizes greater than 1000. Being statistical tests, the importance of testing with different sample sizes for a higher confidence was reinforced. It was noticed that lower values for the constant in Schroeder’s equation may be used for lower dampening values, suggesting a relation between Schroeder’s frequency and degree of diffusion. Also, developments in the investigation of sample isotropy effects on the measurement of scattering coefficients in the reverberation chamber are presented. The hypothesis development is presented through bibliographic review alongside the indication, by means of a numerical method developed in this work (symmetry function), of an increase of isotropy in samples composed by randomly distributed hemispheres in conformity with an increase in coverage density and/or elements downsizing. More experiments are needed for the replication of results obtained in previous research that highlight the isotropy effect. However, it was also noted an inverse area effect, probably due to edge effect. Additionally, an algorithm for the impulse response cropping for the calculation of T4, as required by ISO 17497-1, was developed.