Investigation and correction for the calibration error of using two-microphone impedance tube method on low absorption materials

Abstract

The two-microphone impedance tube method has been widely used for determining the sound absorption coefficients of materials. In this method, a calibration procedure is performed to correct the amplitude and phase mismatch of the two microphones, through repeated measurement of a specimen with the two channels interchanged. Both ASTM E1050-12 and ISO 10534-2 suggest the use of a highly absorptive material as the calibration specimen and the calibration, once complete, is valid for all successive measurements. When following the Standards, however, test materials of low absorption capability exhibited a pattern of peaks and dips in the absorption coefficient curve at certain frequencies, which are obviously not a part of the material’s own behavior. This paper will discuss the cause of these peaks and dips and also suggest a simple solution to the issue. Using the new method, one is able to obtain smooth absorption coefficient curves for low absorption test materials just as expected.