Sound absorption and acoustic surface impedance

The normal sound absorption coefficients of two different test objects in an impedance tube are measured with two different methods: The standing wave ratio method (ISO 10534-1:1996) and the transfer function method (ISO 10534-2:1998). From the latter is the specific acoustic impedance ratio calculated. The test objects are two cellular plastics: AC 30 S and FIRE 30 VF (both are 30 mm thick), the latter has a thin film on the front side. The effect on absorption coefficient when using a cavity behind the samples is investigated. The cavity results in a displacement towards lower frequencies. The difference when using the FIRE 30 VF is that the absorption coefficient has a wide peak at 350 Hz (which is displaced to 300 Hz with cavity behind). Over 600 Hz is the absorption coefficient reduced when using FIRE 30 VF. There are discrepancies between the two methods except when comparing AC 30 S over 1200 Hz where the methods agree. The transfer function method is in general more accurate.