Signal analysis in the sound absorption measurement procedure: the importance of time subtraction and reference surface corrections

The sound absorption coefficient measurement (following ISO 13472-1 normative) of highly absorbing materials, which are tipically employed in room acoustic correction, presents some inherent difficulties. These materials present highly sound absorption coefficient indeed, but usually low sound insulation index and are installed over highly reflective surfaces. This leads to some measurement problems, partially solved by means of the reference surface method. In this work some measurement examples on the same materials but with different contour conditions are analyzed and the improvements on the results, due to reference surface normalization and time domain subtraction of free field response, are discussed.