I. Introduction
Acoustical properties of an environment like reverberation time, speech transmission index (STI), or loudness level may have a significant impact on the listening experience. They can cause discomfort, reduce the attention and learning ability, and result in a negative user experience. Sound absorbers or diffusers are commonly employed to reduce the reverberation and to uniformly distribute the sound energy in a closed space. Hence, a precise evaluation of the material acoustic properties, such as the absorption coefficient and the impedance, is more and more important for the design of buildings, passive noise reduction treatments, but also cars [1]- [3] or theatres [4].