## 1 Introduction
### Overview of Measuring Methods
Generally, all ITM methods can be divided into two groups: the first group is based on the wave decomposition (WD) (three or more microphones, one or two acoustic loads) and the second group is based on the transfer matrix (TM) method (one or two acoustic loads). The disadvantage of ITM methods, similarly as in the case of sound absorption determination, is that the size of the test specimen is non-realistically small (which causes problems in relation to the way the test specimen are fitted in the tube)and the \(TL\) is determined under normal-incidence acoustic waves only (\(nTL\)). There are several standardized methods to test \"real-size\" specimens. For example, the most often used laboratory methods are in accordance with ISO 10136-2 [39]. The method is intended to determine the airborne sound insulation \(R\) of separating structures mounted in the transmission suite. In this approach, it is required that sound transmission via flanking paths is suppressed sufficiently well and the sound field in the source and receiving room are assumed to be diffuse. The spatially averaged sound pressure level in the source and receiving room are measured by a microphone, and the contribution of the reverberant sound field to the total measured sound field in the receiving room is accounted for. The less popular and alternative laboratory technique follows the standard ISO 15186-1 [40] where the intensity sound reduction index \(R_{I}\) is determined based on averaged sound pressure level in the source room, and the sound intensity level radiated from specimen surface in the receiving room. In this approach, the radiated intensity levels are determined by means of a sound intensity probe, and the receiving room is preferably an anechoic room. For in situ measurements, the standards ISO 16283-1 [41] and ISO 16283-3 [42] for airborne sound insulation of walls, small elements and facades, respectively, were established. Similar to the case of laboratory measurement, the intensity method exists for in situ cases [43]. To cope with the high measurement uncertainty of the sound reduction index measurements of building elements in the laboratory according to standards, at low frequencies, two alternative measurement approaches were developed. The first one is a measurement procedure in which a diffuse field is created in the source room through positioning an array of loudspeakers close to the specimen to be excited in the near field [44]. The structural response measurement was done by means of vibrometry (laser Doppler vibrometer, LDV). The second one is a hybrid experimental numerical approach to determine airborne sound insulation by using mobility measurement combined with a numerical procedure [45]. In this approach the device under test is mechanically excited by means of a shaker, and the response is measured by LDV. An alternative approach is to excite the structure by means of a Nd:YAG pump laser [46]. This measurement method has the advantage that the excitation is done contact-less, yielding a contact-less approach for both excitation and response measurement, which would have advantages for application in situ. The sound absorption coefficient as well as sound transmission loss can be determined by different methods relating to the type of acoustic field and source. The application of the vibrometry technique has already proved its usefulness in building acoustics several times. Vibrometry generally helps for better understanding the structure's behaviour in the low frequency range. In the literature, one can find already the scanning laser Doppler vibrometer application for a normal incidence sound absorption determination (for high frequency range--above 3 kHz) [47].