Reproducibility of sound absorption and surface impedance of materials measured in a reverberation room using ensemble averaging technique with a pressure-velocity sensor and improved calibration

The authors have been developing a method using pressure-velocity sensors (pu-sensors) to measure the absorption coefficient and surface normal impedance of materials using ensemble averaging: EApu method. The improvement of geometrical configurations for pu-sensor calibration using an acoustic tube is presented first. Because no clear guideline for pu-sensor positioning in acoustic tube has been reported to date, such improvement is important. Moreover, it enables EApu method measurement to be conducted properly at frequencies region up to 3000 Hz. The tube has two openable hard end-walls that help us to make inside humidity conditions similar to those of the outside room and to realizing a one-dimensional sound field at calibration. With such improvements and with consideration of humidity, a round robin test using EApu method was conducted in three reverberation rooms to prove the reproducibility of measurements of absorption characteristics of glass-wool and needle felt. Results revealed excellent agreement of absorption coefficient values for reverberation rooms at frequencies of 100-3000 Hz. Results also demonstrated that the maximum difference of absorption coefficient values is less (almost equal or less than half) of those found in the authors' earlier study of the 125-1250 Hz frequency region.