REPRODUCIBILITY OF SOUND ABSORPTION AND SURFACE IMPEDANCE OF MATERIALS MEASURED IN A REVERBERATION ROOM BY ENSEMBLE AVERAGING TECHNIQUE USING A PRESSURE-VELOCITY SENSOR

摘要

In earlier papers, the developments of measurement method with a pressure-velocity sensor (pusensor) for measuring absorption coefficient and surface normal impedance of materials using ensemble averaging: EA method have been researched. The present study proposes a reproducibility of sound absorption and surface impedance of materials measured in a reverberation room by ensemble averaging technique using a pressure-velocity sensor. A round robin test measurements by experimenting the sound absorption characteristics in three reverberation rooms is conducted. The target frequency range is 200 - 2500 (Hz). The measurement results of two materials are reported here. In an early stage, EA method was targeted mainly at constructing impedance boundary conditions of sound fields that used in numerical methods such as finite element method. Therefore, its frequency range was set as 100-1500 Hz. Takin into account the recent progress of computational resources, absorption characteristics in a rather higher frequency region are required. Herein, to raise the measurable frequency upper limit of EA method with a pu-sensor up to 3000 Hz, geometrical improvements are applied to an acoustic tube. The effectiveness of the improvement was confirmed using a series of EA method measurements conducted in three reverberation rooms. In all measurements, relative humidity was monitored carefully based on earlier findings by some of the authors. Using the improved acoustic tube, a series of EA method measurements of the glass-wool panel and a needle felt sheet were conducted in three reverberation rooms at different places with the improved calibration including relative humidity consideration. Those of sound absorption coefficient values are satisfactory in the entire frequency region from 100-3000 Hz, although agreement of impedance values among three different reverberation rooms are fair for frequencies below 200 Hz. The results presented here confirm the reproducibility of EA method using improved pu-sensor calibration with consideration of relative humidity.