## 1 Introduction
### Overview of Measuring Methods  
Most of the investigations were performed under the acoustic plane wave incidence conditions, by means of the Impedance Tube Method (ITM), also called the Standing Wave or Kundt's Tube Method. This measurement approach typically determines the \(nTL\) and sound absorption (\(\alpha\)(-)) (or reflection \(r\)(-)) coefficient. The impedance tube apparatus is commonly used for the characterization of locally reacting material samples. The sound absorption determination by ITM can be done by measuring maximum and minimum pressure amplitude in the tube by means of the moving microphone probe (the method using the standing wave ratio, as described in [28]), or by determination of the pressure transfer function between wave components at two or more microphone positions at a given distance from the specimen along the tube [29]. The disadvantage is that the ITM does not allow for oblique incidence measurement. Another standardized laboratory method which is used very often is to determine the statistical sound absorption coefficient \(\alpha_{S}\)(-) in a diffuse field in a reverberant room, according to ISO 354 [30]. The test results obtained by this method are often considered the more realistic for characterizing specimen absorption properties in situ, as compared to the ITM approach. However, this method requires a reverberant room and a flat specimen with a total surface of 10-12 m\({}^{2}\). From this expensive measurement procedure \"just\" the absorption coefficient is obtained (and not a complex valued reflection coefficient, as is the case in the ITM) and the measurement uncertainty is relatively high. The absorption coefficient is determined based on [PERSON]'s formula and is dependent on the reverberation time measured in the reverberation room with and without the specimen, the room volume and the surface area of the specimen. There are several microphone or p-u probe free-field methods. The basic principles are explained in, e.g., [PERSON] and [PERSON] [31]. Methods differ regarding the required sample size, wave decomposition methods, measurement setup composition and procedure (number of microphones--from two to array, distance of microphones from specimen, angle of incidence, distance of source, etc.). Free-field methods consider the incident sound wave as a plane wave (implying that a well-defined acoustic field is required, demanding the absence of spurious reflections and edge-diffracted waves [32]). A good accuracy can be achieved by means of free-field methods in the frequency range above 290 Hz (for oblique incidence above 400 Hz [33]). [PERSON] and [PERSON] [34] found that the measurement does not yield accurate results for low values of \(kR\) (for \(R\) = 3 m and \(k\) < 5.5 m\({}^{-1}\)), where \(k\) is wave number and \(R\) is the distance of the sound source from the specimen surface. They also found that phase mismatch between microphones plays a crucial role, especially in the measurement of low frequencies or highly reflective materials. Assuming the non-standardized, impulse-response based microphone methods, the work of [PERSON]'s [35] can be mentioned, which describes a method to determine the sound absorption from frequency 80 Hz onwards considering Fresnel zones. A different technique for sound absorption determination, by means of a p-u probe where the measured sound pressure and the normal component of the particle velocity determines the absorption, was introduced for the first time by [PERSON] and [PERSON] [36]. Several approaches exist to determine the sound transmission loss. The simplest, which are the same as in the sound absorption case, make use of ITM. For example, the three-microphones method to measure the high \(nTL\) (up to 100 dB) acoustic samples was developed by [37] (the usual limit is around 50 dB in the frequency range below 500 Hz [38]).