## 1 Introduction
The knowhow relating to characterization of the acoustical conditions in public rooms has increased in recent years. Several investigations [22; 23; 24; 25] have pointed out the necessity of addressing several acoustic parameters to achieve a relevant characterization of the acoustic environment. As has been shown, parameters relating to noise levels and speech intelligibility are an important complement to reverberation time. In [26], the speech clarity parameter \(U_{50}\), i.e., \(C_{50}\), including the effect of background noise, is used for designing good speech conditions in classrooms. In [27; 28], [PERSON] presents a model for calculating clarity index and sound strength in rooms assuming linear sound decay. In [29], special effort was focused on explaining the non-diffusivity effect of the sound fields in public rooms with ceiling treatment and how these circumstances influence these parameters. Since [PERSON]'s [30] discovery and his classical formula, reverberation time has been the key parameter in room acoustics. In many standards and regulations, it is still the main parameter defining target values for good acoustics [31]. However, today, there are some new standards that have included measures, such as \(C_{50}\) and speech transmission index \(STI\)[32], as complements to reverberation time [33]. The idea of two rooms with approximately the same reverberation times being perceived as different is not a new finding and is mentioned in textbooks on acoustics [34; 35] as well. This is especially the case in public rooms with ceiling treatment. Many suggestions for improvement of the reverberation time formulas have been made. Several examples of such refinements are given in [34; 35]. The influence of different corrections to [PERSON]'s formula has been investigated by [PERSON] [36; 37]. In support of [PERSON]'s formula, [PERSON] shows that understated conditions of weak absorption and irregular reflections provides the correct answer. In [38] [PERSON] presents an empirically derived formula for the reverberation time in rooms with non-uniform distribution of absorption. A modified version of [PERSON]'s formula is presented by [PERSON] [39]. The non-uniform distribution of absorption is also dealt with by the formula of Arau-Puchades [40]. The effect of location of absorbent material in a mock-up of a classroom and in a reverberation chamber has recently been studied by [PERSON] et al. [41]. In [42], [PERSON] uses an image source method where the image sources are grouped as axial, tangential, and oblique groups corresponding to normal modes in wave acoustics. Scattering is taken into account by introducing the scattering coefficient. The non-linear decay in rooms with non-uniform distribution of absorption as well as the importance of scattering are apparent in the results. In [43], [PERSON] and [PERSON] compared experimental results with analytical and computer predictions of reverberation time in a simulated classroom. Their paper emphasizes the need to quantify the amount of scattering due to furniture and other objects in a room. The influence of scattering is also experimentally investigated by [PERSON] et al. [44]. A general problem in many reverberation time formulas is the use of a random absorption coefficients as input data. This is of course natural, as most manufacturers of absorbent products provide this data measured according to ISO 354 [45]. However, the non-isotropic properties in rooms with ceiling treatment differ from the almost diffuse conditions in reverberation chambers. In fact, even in reverberation chambers, the concept of a diffuse sound field is hard to achieve [46]. In [47; 48], [PERSON] presented a model particularly developed for rooms with suspended absorbent ceilings. The non-diffuse conditions were dealt with by introducing two sound fields related to grazing and non-grazing sound waves. The idea of subdividing the sound field into a grazing and non-grazing group were also adopted in [49].