Experimental Analysis of the Dispersion in the Measurement of the Absorption Coefficient with the Impedance Tube

Porous materials are widely used as means for reverberation and noise control. One of the ways to characterize its acoustical performance is with the aid of the impedance tube. Measurements can be taken in accordance with the standard ISO 10534-2. It is relatively well known that even for homogeneous samples, there can be a relatively large dispersion on the measurement of the absorption coefficient. Such dispersion can be caused by compression of the sample, different impedance tubes used in the measurement, deviation when mounting different samples of the same material and spatial variation along a given material. This paper analyses this experimental dispersion for those scenarios, with the aid of several experiments. Furthermore, different types of materials (foams, felt, glass fiber) are measured. A statistical analysis is made on every set of measurements to test for normal distribution (for each frequency), to calculate mean value, standard deviation and the confidence interval.