Treble simulation of large auditorium (CR4 from BRAS)

Treble simulation of large auditorium (CR4 from BRAS)

ABSTRACT

Treble outperforms any conventional geometrical acoustics (GA) software in simulating single reflections (BRAS - RS1), diffraction from a finite object (BRAS - RS6), diffraction from a large barrier (BRAS – RS5) by directly solving the wave equation This study shows Treble’s performance in a large namely, BRAS – CR4.

1. Introduction

Accurate room acoustic simulations fundamentally rely on accurate simulation of individual reflections off the surfaces, diffraction and scattering due to obstacles and furniture. It has been shown that Treble accurately captures the reflections and diffraction for various conditions, for example, BRAS RS1, RS5, RS6, RS7, CR2, CR3, etc. To experimentally validate a Treble simulation of a large room, BRAS scene CR4 is chosen. The room is basically a huge auditorium with a volume of 8700 m3. The surfaces are rather reflective as shown in Table 1.

Figure 1. CR2 and Treble simulation snapshot [3].

2. Measurement setup

A 3-way dodecahedron sound source was used during the measurement and an omni-directional microphones B&K 4134 was used in the seminar room. Similarly, the Treble simulation uses an omni-directional source and omni-directional microphone in the simulation. The transition frequency is 355 Hz, meaning up to the 250 Hz octave band DGFEM is used. The locations of the mid-frequency sources are LS1(0, 4.5, 1.38) m and LS2(-2.8, -4.5, 1.38) m. The high-frequency source has a height of 1.68 m while the low source has a height of 1.12 m. 5 microphones are spread over the audience plan as can be seen in Fig. 1 at left.

The absorption coefficients in the BRAS database (fitted absorption) are shown as follows:

Table 1. The absorption coefficient from BRAS CR4 database

The ISO 3382 parameters are compared in Fig. 2. The ISO 3382 parameters are calculated from 2 sources and 5 receiver locations (thus 10 combinations) and then their averages and standard deviations are plotted. Frist, EDT is a reverberation parameter which is really sensitive to early reflections. EDT is a more difficult parameter to make a good agreement between simulations and measurements – the low frequency bands up to 125 Hz, a slight overestimation is observed. Typically the spatial deviation of EDT is larger at lower frequencies, which is not well captured by GA simulations. Treble results shown in Fig. 2 agree well with the measurement not only the average values but also the spatial standard deviations. Center time, Ts, is also a sensitive parameter to the temporal structure of the impulse responses and known to be more difficult to match than C50, which has a fixed demarcation point separating the early and late reflections. The average values of Ts from Treble are reasonable as well as the standard deviation. C50 is a typical parameter that quantifies the speech clarity. C50 agrees well, particularly the average values. It is quite interesting to see that the overestimation of Treble in EDT at the low frequencies is not seen in the clarity parameters. D50 is another clarity parameter that uses 50 ms as the transition time from early to late reflections. We can observe good agreements of D50 parameter between the simulation and BRAS measurement.

Figure 2. ISO 3382 parameter comparisons. EDT, Ts, C50 and D50.

3. Conclusions

This study demonstrates accurate Treble simulations of a large room with a volume over 8000 m3. Treble’s calculation of the acoustic parameters agrees well with the measurement data thanks to its accurate hybrid simulation solver. A source of the errors are uncertainties in the source and receiver locations in the measurement, particularly the measurement used a 3-way dodecahedron source. Despite all the uncertainties, the Treble simulation results are convincing over a large frequency range from 63 Hz to 4 kHz.

References

[1] Benchmark of Room Acoustical Simulation (BRAS) database CR4: https://depositonce.tu-berlin.de/items/38410727-febb-4769-8002-9c710ba393c4