The effect of changes in atmospheric conditions on the measured sound absorption coefficients of materials for scale model tests

The absorption of scale model materials is generally measured in a scaled reverberation chamber, with frequency scaled according to the scale factor. Moreover, the acoustic absorption of the chamber needs to be reduced, especially the absorption of air. It depends not only on the volume of the chamber, but also on the air parameters: pressure, temperature and relative humidity. Most frequently, in order to reduce the air absorption, the air inside the model is dried or replaced with dry nitrogen, or a digital air absorption compensation is used. Air-drying methods are time consuming and require specialised equipment; digital compensation may in turn result in errors, hence the question arises whether scaling the air absorption is really necessary to achieve good accuracy of the sound absorption coefficient measurements. In order to answer this question, the authors firstly ran a theoretical analysis of the issue based on ISO 9613-1 formulas describing air acoustic absorption. Based on this analysis, the influence of particular air parameters on the total air absorption in the model reverberation chamber, Aair (m2), was examined, together with the possibilities of scaling down the Aair values. This indicated, that the Aair may be scaled by manipulating temperature or relative air humidity. For the needs of the study, the relative air humidity was chosen, as it was used by the predecessors and may be easily manipulated. In the next step, the experimental studies of sound absorption coefficient measurements were conducted in a 1:8 scale reverberation chamber for representative specimens at different relative air humidity values (3-45%). The gathered data was then statistically analysed. The results showed that the influence of relative air humidity on the accuracy of sound absorption coefficient measurements in model tests is negligible and therefore such measurements can be performed in the ambient air humidity of the test room. As a consequence, a conclusion was drawn that the Aair value does not need to be scaled. Moreover, an attempt was made to identify the cause of the inaccuracy of the measurements - the analysis indicated that the values of the intensity attenuation coefficient m calculated according to ISO 9613-1, at model frequencies are not sufficiently accurate.