## 5 Results
### Measurement Results
Curves shown are (red) diffuse calculation (Sabine), (blue) measurement, (dashed) non-diffuse calculation, where the number of non-grazing modes is estimated by Equation (31), and (dash-dotted) non-diffuse calculation, where the number of non-grazing modes is empirically estimated, see Table 1.  
Figure 10:0 Sparsely furnished room with dimensions 7.35 \(\times\) 7.50 \(\times\) 3.50 m. Ceiling treatment: 15 mm glass wool absorber at a mounting height (air cavity behind the absorber) of 185 mm. (**a**) Reverberation time \(\mathbf{T}_{20}\) in seconds, (**b**) speech clarity \(\mathbf{C}_{50}\) in dB, (**c**) sound strength \(\mathbf{G}\) in dB. Curves shown are (red) diffuse calculation (Sabine), (blue) measurement, (dashed) non-diffuse calculation, where the number of non-grazing modes is estimated by Equation (31), (dash-dotted) and non-diffuse calculation, where the number of non-grazing modes is empirically estimated, see Figure 10:1  
An important feature of the non-diffuse model is the reaction to wall panels. The effect of wall panels is the reduction in the energy in the grazing sound field. In sparsely furnished rooms, this largely influences the late reverberation time and the speech clarity. This is clearly shown in Figure 10:2 The correspondence between the non-diffuse calculation and measurement is good. For the diffuse model, a much smaller effect is noticed. The effect of wall panels on sound strength \(G\) is small. As \(G\) is a steady-state measurement, it is mainly related to the total absorption in the room, assuming that the sound field is fairly diffuse before the onset of the decay. During the decay, the degeneration of the sound field towards a grazing sound field will affect reverberation times and speech clarity to a great extent, as shown in the experimental results. Note that the results presented above refer to a sparsely furnished room which is very sensitive to the accuracy of the input data. It is notable that the case with the wall panels decrease the discrepancy between the theoretical and empirical model and also fits better with measurements. In Figure 16, a comparison of the two ceilings absorbers corresponding to case 1 and 2 in Table 2 is shown. The practical absorption coefficients for these absorbers, as given by the manufactures, is shown in Figure 10:3 Besides the large difference between the diffuse calculations on the one hand (red curves) and the measurements and non-diffuse calculations on the other (blue and green curves) some other remarks can be made. The practical absorption coefficients, based on ISO 354 measurements, show similar values at 125 Hz for the two absorbers. Accordingly, the diffuse calculations show the same reverberation time at this frequency. However, the non-diffuse calculations give a large difference at 125 Hz which also corresponds to the measurements. It is also noteworthy that, at high frequencies, the non-diffuse calculations and the measurements show contradictory behavior in comparison with the diffuse calculations. The diffuse calculations follow the difference in the practical absorption coefficients which is not the case for the measurements and the non-diffuse calculations. This emphasizes the fact that the absorption coefficients measured under reverberant conditions, as in ISO 354, do not comprise sufficient information for the acoustic design of rooms with ceiling treatment. Other information is needed and, in the model presented, the surface impedance of the ceiling absorber is necessary input data. Figure 10:4 Comparison of case 2 and 3 in Table 2, i.e., the cases with and without wall panels. (**a**) Reverberation time \(T_{20}\) in seconds, (**b**) speech clarity \(\mathbf{C}_{50}\) in dB, (**c**) sound strength \(\mathbf{G}\) in dB.