Diffuseness Quantification in a Reverberation Chamber and Its Variation with Fine-Resolution Measurements

Shuying Zhang,Joonhee Lee

doi:10.3390/buildings11110519

Abstract

Insufficient diffuseness is the major cause of the poor repeatability and reproducibility of building acoustical measurements in a reverberation chamber. Inaccurate results were reported for the prevailing methods in ISO and ASTM standards. Many previous studies, thus, have proposed new methods to quantify the diffuseness of a reverberation chamber more accurately, but there is no general agreement among researchers on the most reliable method. The number of measurement samples required for these diffuseness metrics is also unclear, even though it significantly impacts the robustness of the methods. This study, therefore, aims to compare the performance of the two widely used diffuseness metrics (spatial variation of sound pressure levels and the relative standard deviation of decay rates) in the standards and the recently introduced metric (degree of time series fluctuation). The measurements were carried out with fine resolution microphone positions and varied configurations of acoustic diffusers. The degree of time series fluctuation showed the best correlation with varying diffuser configurations in the low-frequency range. Confidence intervals and coefficients of variation of the three metrics by random sampling also indicated that DTF is more reliable for evaluating the diffuseness in a sound field as it is less influenced by the number of sampling.

Highlights

Accurate measurement of acoustical materials, such as absorption and transmission loss, is crucial for the acoustical design of architectural spaces
The maximum coefficients of variation (CV) of srel and σSPL are almost three times the CV of degree of time series fluctuation (DTF) when only five microphone positions are utilized, which indicates that the DTF is more robust than srel and σSPL when a small number of measurement locations are utilized
This research work aimed to find an effective method to quantify the diffuseness of the reverberation rooms and determine the optimal number of measurement samples required for accurate spatial sampling

Summary

Introduction

Accurate measurement of acoustical materials, such as absorption and transmission loss, is crucial for the acoustical design of architectural spaces. According to the relevant standards ASTM C423-17 [1]; ISO 354:2003 [2], sound absorption coefficients need to be measured in a reverberation chamber that closely approximates a diffuse sound field. The diffuse sound field requires acoustic energy distributed uniformly throughout the space (homogeneity) and equal sound intensity over all directions (isotropy), which are impossible to obtain in actual reverberation chambers. In a room with unequal absorption and low scattering, especially in rectangular rooms, the measured reverberation time can deviate up to 50% and more from the value by the Sabine equation [3]. Accurate measurements of absorption or transmission loss of the specimen require averaging values measured over multiple locations in the reverberation chamber. The minimum variation across the chamber (homogeneity) is crucial for the accuracy of acoustic quantities

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