Some notes about low frequency measurements in a reverberation room
Dario D’ORAZIO(1) , Luca BARBARESI(2) , Massimo GARAI(3)(1) DIN, Department of Industrial Engineering, University of Bologna, Italy, dario.dorazio@unibo.it(2) DIN, Department of Industrial Engineering, University of Bologna, Italy, luca.barbaresi@unibo.it(3) DIN, Department of Industrial Engineering, University of Bologna, Italy, massimo.garai@unibo.it

Abstract
It is well known that the modal field can influence reverberation time measurements below the Schroeder frequency, but reverberation room is an extreme environment that deserve some further considerations. Indeed theSchroeder frequency depends on the modal distribution, which in turn depends on the peculiar mode shapes,
due to the room shape. For a typical reverberation room the boundary frequency separating the modal region
from the statistical one may extend up to three times the Schroeder frequency. Moreover, the reverberation timein the modal region depends on the proper excitation of the sound field; this is the reason why some authors use
the ’modal reverberation time’ for each mode instead of an average over a frequency band. For these reasons,the recent ISO 354 draft recommends exciting the sound field by placing the sound sources in the corners. The
present work shows some practical experiences concerning the use of two kinds of sound sources, changing theloudspeakers size and the distance from the corners. The results show how a proper sound source, whose electroacoustic characteristics are optimised for the ISO 354 frequency range, may improve the sound absorption
measurements techniques and may influence room criteria.
Keywords: Reverberation room, Low frequencies, Sound sources

1INTRODUCTION

In order to qualify the reverberation room, the best possible uniform excitation of all room modes should begiven at low frequencies. Concerning this, the Annex H of the ISO 354 draft says [1]: The loudspeakers will bepositioned in the corners of the room. Possible arrangement suggested by the draft are:• a single loudspeaker mounted in a tetrahedral enclosure, placed in the corner of the room;• three loudspeaker mounted in an irregular enclosure, placed in the corner of the room, each face coveringa π/3 steradian.
The draft does not specify the technical characteristics of these loudspeakers, neither the acoustical load of thesesystems – e.g. if the loudspeaker should be loaded in a closed box or in a bass reflex system, etc... Moreover,the corners of a reverberation room often do not have a regular shape, so that the proposed enclosures could
not be placed in all the corners.
Furthermore, the dimension of the loudspeaker – from 4 to 6.5 inches, generally – may influence the emission
lobe at low frequencies: indeed the directivity of the single loudspeaker is omnidirectional below the 300 Hz
but this threshold may vary depending on dimension, load, etc... . In case of a multi-loudspeaker system, thewidth of the loudspeaker may influence the minimum distance from the acoustic centers and this may have an
influence - constructive or destructive - at certain frequencies.

2METHOD

In the frame of the Italian Round Robin test on ISO 354 draft [2] it has been proposed to qualify the rooms
by using both a dodecahedral sound source and other sound sources more powerful at low frequencies. In thepresent work both methods were tested in the reverberation room of the University of Bologna (V=280 m3 ):4075

1. Each sound source was placed in the corner of the reverberation room, facing the corner. Two corners
were chosen, labeled respectively SS6 and SS3 in fig. 1. The reverberation room was empty – without
any specimen;
2. Impulse responses were measured in six receiver points – respectively R1–R6 in fig. 1 – by using anexponential sine sweep (ESS) of about 20 s;

Figure 1. Sketch of the reverberation room at the University of Bologna

3. The measurements were replicated increasing the distance of the sound source from the corner, in stepsof 10 cm, for a total of three positions labeled, respectively, “30 cm”, “40 cm”, “50 cm”; the distances areintended between the front baffle of the loudspeaker and the corner;
4. The whole procedure from point 1 to 3 is repeated, turning the sound sources towards the room instead
than toward the corner.
Two sound sources were used: a custom high-SPL dodecahedron [3] and a custom subwoofer. This last one
mounts a custom 18-inches subwoofer – model 18NLW4500 made by 18sound – mounted in a closed box ofabout 80 liters. The resonance frequency of the loudspeaker in free field is about 28 Hz, instead of 100-150 Hzof the loudspeaker which used in the commercial dodecahedral sources (see frequency response in fig. 3). The
closed box load was chosen in order to avoid any delay due to reflex ducts.

3RESULTS

The impulse responses were measured by playing the ESS with one sound source at the time and recording insix fixed receivers positions (see fig. 1). A custom software was used for windowing and deconvolving the ESS[4]. IRs were then filtered in third-octave bands and analyzed using Dirac v.6, which has filters compliant withIEC 1260 [5]. Reverberation time values were finally averaged over all the receivers for each sound source
position. The results, plotted in the range 50-630 Hz, are shown in figs. 4–7.

4DISCUSSION AND CONCLUSIONS

The results show significative differences by varying the sound source type and its position. The decays measured with the subwoofer are generally longer the ones measured with the dodecahedral sound source. It may4076

Figure 2. Sound sources deployment: dodecahedron in SS6, subwoofer in SS3 turned towards the corner,subwoofer in SS3 turned towards the room.

depends on the following reasons:• The subwoofer loudspeaker is loaded by the coupling with the sound field in the space starting from alower frequency than the one of the dodecahedral sound source, so that the radiation impedance of thesubwoofer is more “matched” with the lowest modes of the rooms.
• The multiple emissions from the loudspeakers of dodecahedral source may result in a mutual negativeinterference, decreasing the excitation in the modal region. It is interesting to note that this effect ispresent below and above the Schroeder frequency, see e.g. the red curves in figure 5. This may be dueto the peculiar “frequency peakness” of the modal behavior in the reverberation room.
• The corner shape does have an effect. In fact the differences are more evident when the sound sources
are placed in the right-angle corner (position SS6) and less pronounced when the sound sources are in
the irregular corner (position SS3), see for example figs. 6 and 7.
• The low sound energy emitted by the dodecahedral source may add some uncertainty at very low frequencies. In fact the low signal to noise ratio of the IRs and the high level of background noise maylead to overestimating the reverberation time: see the blue curve in figs. 4 and 5.
On the basis of these empirical results, the placement and arrangement of the sound sources should be carefully
evaluated and discussed in order to increase the measurements accuracy. Further developments of this work
could include the matching of the experimental data with numerical simulation of single and multiple sources[6].

ACKNOWLEDGEMENTS
The authors wold like to thank Fabio Blasizzo of 18sound, Matteo Cingolani and Chiara Scrosati.

REFERENCES[1] ISO/TC43/SC2/WG26/N60 (Version July 3rd 2018) Acoustics – Measurement of sound absorption in a reverberation room.

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Figure 3. Frequency response in free air of the subwoofer 18sound model 18NLW4500 used in the test.

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Third-Octave bands (Hz)Figure 4. Sound source in the right-angle corner SS6 (see fig. 1), loudspeaker turned toward the corner[2] Scrosati, C. et al.; Design Principles of the Italian Round Robin Test on Reverberation Rooms, Proc. 23rdInternational Congress on Acoustics, 9–13 September 2019, Aachen, Germany.
[3] D’Orazio, D.; De Cesaris, S.; Guidorzi, P.; Barbaresi, L.; Garai, M.; Magalotti, R. Room acoustic measurements using a high SPL. In Proceedings of the 140th AES Convention, Paris, France, 4–7 June 2016.
[4] Guidorzi, P. Barbaresi, L., D’Orazio, D., Garai, M., Impulse responses measured with MLS or Swept–Sinesignals applied to architectural acoustics: an in-depth analysis of the two methods and some case studies ofmeasurements inside theaters, Energy Procedia, 2015, 78, 1611–1616.
[5] IEC 1260:1995, Electroacoustics – Octave-band and fractional-octave-band filters.
[6] N.B. Roozen; E.A. Piana; E. Deckers; C. Scrosati, On the numerical modelling of reverberation rooms,
including a comparison with experiments, 26th ICSV, 1–11 July 2019, Montreal.

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Third-Octave bands (Hz)Figure 6. Sound source in the convex corner SS3 (see fig. 1), loudspeaker turned toward the corner4079

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Third-Octave bands (Hz)Figure 7. Sound source in the Convex corner SS3 (see fig. 1), loudspeaker turned toward the corner4080