Measurement of Sound Absorption Coefficient at Low Frequencies in Reverberation Chamber

Acoustic features of materials are commonly evaluated in the frequency range $100\,\mathrm{Hz}-5\,\mathrm{Hz}$. This frequency range is not sufficient in these days, especially at low frequencies.There are two measuring methods how to measure sound absorption coefficient at low frequencies in a reverberation chamber. Both of these methods are based on measurement of sound pressure level decay. The first measuring method directly uses of modal properties of a room and the decay curve is determined for each natural frequency separately. The second measuring method is based on measurement in accordance with ISO 354 standard. The frequency response of the reverberation room is adjusted by specifically placed a sound absorbing material, which damping isolated natural frequencies of the reverberation chamber.The article will discuss the possibility of combining both of the above-mentioned measuring methods into one. This measuring method will not require additional acoustic treatment and will be based on measurement in accordance with ISO 354 standard. Measuring points will be selected with respect to modal properties of the room and damping of natural frequencies at low frequencies will be realized by the excitation signal. The signal is chosen that there is a minimal excitation of these natural frequencies.