Estimating the diffuseness level of the acoustic field - reverberation chamber under study Bartłomiej Chojnacki, Adam Pilch, Tadeusz Kamisiński, Artur Flach AGH University of Science and Technology Department of Mechanics and Vibroacoustics al. Mickiewicza 30, 30-059 Kraków, Poland
[email protected]
Background
Reverberation chamber modeling
Current ISO 354 and ISO 3741 standards do not contain restrictive requirements, which causes grand dispersion in measurements performed in different laboratories. This poster introduces the basis of the sound field diffuseness level estimation for rating the quality of the reverberation chamber by using some modern parameters and comparing them with old, suggested in mentioned standards.
Room diffuseness level indicators Despite ISO 354 requrement (maximum equvalent sound absorption area) and ISO 3741 (maximum standard deviation of the SPL measure), it is suggested to use some additional coefficients for diffuseness measure, like kurtosis of the impulse response, described by equation: Figure 4 - parametrized model of the geometry Kurtosis is aplied on normalised room impulse response (NIR). It is required to use 10-100 ms time interval to avoid direct sound and late reflections [1].
Reverberation chamber h(t)
1 0 -1 0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
0.04
0.05
0.06
0.07
0.08
0.09
0.1
Anechoic chamber h(t)
1 0 -1 0.01
h(t)
1
0.02
0.03
Living room
By using ray-tracing method algorithms, example reverberation chamber with dimensons 6 x 6 x 6 [m.] had been considered. Geometry model was parametrized with variables dx and dy as the functions of the wall rotation in searching for the best angle with highest diffussion. Modeling in parameters grid 6 x 6 in range from 0 to 2 [m] were used. Results are presented in tables 1 and 2. In both ways, the lower value coefficient reached, the more diffused field is.
Figure 5 - examples of used geometry
0
Table 1 - Kurtosis coefficient modeling
-1 0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
Time [s] Figure 1 - NIRs calculated for verification of computation algorithms 250
Reverberation chamber Anechoic chamber Living room
Kurtosis 10-100 ms
200
150
Table 2 - DFF coefficient modeling
100
50
0
0
0.5
1
1.5
2
2.5 10-3
Time [s]
Figure 2 - kurtosis calculated for the test rooms Second proposed coefficient is so-called diffuse field factor, described by the ratio of standard deviation obtained by the measurements of reverberation time to the theoretical one [2].
3
Minimum and maximum values of the parameters are presented in the tables 1,2 and 3. The parameters do not correspond - each parameter reaches extremes for different variants of the geometry. Further studies are suggested in this field to designate the optimal parameters for estimating the diffuseness level of the acoustic field. Currently, by considering all taken parameters credible, the rating by the weighted average is proposed with weights established in the way of preliminary tests
2
Bibliography
6 Reverberation chamber Anechoic chamber Living room
5
DFF
4
1 0 125
250
500
1000
2000
4000
8000
Frequency [Hz]
Katedra Mechaniki
KMiW i Wibroakustyki
Discussion
Figure 3 - DFF calculated for the test rooms
[1] C-H Jeong, Kurtosis of room impulse responses as a diffuseness measure for reverberation chambers, Journal Acoustical Society of America no. 139(5) [2] M. Nolan, M. Vercammen, C-H. Jeong, Effects of different diffusers types on diffusivity in reverberation chamber, Proceedings of EuroNoise 2015
142nd International AES Convention, Berlin
