PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Estimation of the background noise levels in large atria with known
room acoustic properties and function
Richard ŠIMEK1; Vojtech CHMELÍK1; Jarmila HÚSENICOVÁ1 Monika RYCHTÁRIKOVÁ2
1

Slovak University of Technology in Bratislava, Faculty of Civil Engineering, Department of Architecture,Radlinského 11, 81005, Bratislava, Slovakia

2KU Leuven, Faculty of Architecture, Hoogstraat 51, 9000 Gent/ Paleizenstraat 65, 1030 Brussel, BelgiumABSTRACT
One of the major identified acoustic problems in large halls such as atria or shopping malls is continuous
unpleasant background noise, based on typically long reverberation of sound. Levels of background noisecaused by internal sound sources depend on activity performed in the place on one hand and on room acoustic
conditions on the other hand. Present noise can be measured relatively easily, by means of sound level metersand evaluated through statistical noise analysis, expressed e.g. by histogram of noise levels. However, theprediction of statistical noise levels, such as L95, L90 or even Leq, in rooms is very difficult, as it doesn’t
depend only on the overall sound power spectrum of sound source (typical sound source definition in
ray-based algorithms), but also on the time varying character of sound. Function and thus activity performed
in an atrium plays therefore an important role in terms of acoustic comfort analysis and without the
information on time variation in source signal, parameters such as L90 or L5 cannot be successfully predicted.
In this paper several typical signal sounds (such as human steps, speech, music, ventilation etc.) are
synthesized and auralized in Odeon software and assessed in parametric study of an atrium, followed bystatistical analysis of predicted auralized wave files. Correlation between the activity and room acoustic
parameters is calculated and estimation of background noise levels L99 is shown together with equivalentsound pressure level LA,eq.
Keywords: Room acoustics, background noise, statistical noise levels, large halls

1. INTRODUCTION
Acoustic comfort in large halls, where many people gather, is rather complex issue which depends
on several objective and subjective factors. The challenge is to find correlation between the two inorder to propose a suitable acoustic prediction parameter and to summar ize the knowledge into
guidelines that would help architects in their design. In spite of many already existing acoustic
parameters, an adequate acoustic comfort descriptor does not exist yet. Many questions typically arrise
in renovation projects of historical atria that belong to national or regional monuments (1,2).
Common practice in preparation of guidelines for room acoustics is stereotypically based onreverberation time measurement, calculation or simulation. The reverberation time is a good measurein the assessment of classrooms, auditoria or aulas, thanks to their high correlation with the Speechtransmission index (STI) and the Clarity (C 50 ) of the space, which takes into account the ratio between
early and late reflections of sound) and thus the speech intelligibility. Combined with the otherparameters that can be found in ISO 3382, the value of the reverberation time, gives an adequate
description of the acoustics, suitable for designing concert halls, operas or theatres (3). However, whenit comes to functional places, often high or continous background noise levels are identified as aproblem as these can be influenced not only by global reverberation time, but also by the shape o f theroom and the distribution of sound absorption over the interior surfaces .
12richard.simek@stuba.sk; vojtech.chmelik@stuba.sk; jarmila.husenicova@stuba.sk;
monika.rychtarikova@kuleuven.be;6067

Furthermore, unpleasant acoustic situations are not always associated with extremely high noiselevels. Sound reflections in rooms with large volumes arrive from surrounding su rfaces from relatively
far distances are not strong enough to significantly increase the maximal noise levels directly.
However, due to Lombard effect (4), reflections increase noise levels indirectly as shown in (5). As amatter of fact, typically reported problems are pointing at the presence of continuously present
background sound (6,7).
This article is therefore based on a paramteric study that shows, how do the statistical noise levels
such as L90, L95 and L99 change with different activity, volume and sound absorbing properties ofinterior surfaces.

2. DESCRIPTION OF THE CASE STUDY
2.1 Description of the atrium
For this study the atrium of the manor house in the village of Halič has been taken as a starting point.
Given atrium has been built in a Baroque style. It has been originally an opened exterior space, withgrassy vegetation and stone walkways on the ground. In 2016, the purpose of the space was changed
and atrium was covered by glassed roof and the grass on the ground was replaced by marble stones.
Doing so, an interior space was created, suitable for hotel dining room and transition space.

Figure 1. Left picture (Variant A): original atrium before renovation; middle picture (Variant B): renovatedatrium covered by ETFE foils; right picture (Variant C): atrium after renovation covered by glass.
First, the impact of renovation (glass roof and hard floor) is investigated, on the known acoustic
parameters (such as reverberation time, sound pressure level distrubution and clarity of sound)
predicted by means of simulation model in Odeon software. Later, third alternative was created inwhich ETFE is used as roof material instead of glass and porous plaster is placed on the walls, and wascompared with the first two cases (Figure 1). However, the main scope of this paper concerns theacoustic comfort issues under different boundary conditions such as volume and sound absorption
properties of interior surfaces.
2.2 Description of the parametric study
In order to investigate the acoustic comfort issues in large rooms vs small rooms, a parametric study
was performed that consisted of 3 different volumes and 3 different sound absorbing properties ofinterior surfaces, and thus 9 different alternatives. The volume 1, corresponds to the original atrium,
the second volume is modeled with doubled dimensions in comparison with the original atrium and so
8x larger volume and 4x larger area of interior surfaces. The volume 3 is modeled with half of thedimensions of the original volume, e.g. 8x smaller volume and 4x smaller area of interior surfaces. All
simulated cases are sumamrized in the Table 1.
3

Volume (m )Table 1 – Summary of simulated variants
Variant A (original)
Variant B (ETFE)

Variant C (glass)8767

Alt.1A

Alt.1B

Alt.1C

70100

Alt.2A

Alt.2B

Alt.2C

1097
Alt.3A
Alt.3B
Alt.3CThe three variants (A, B and C) have different sound absorbing properties of interior surfaces.
Variant A represents the situation before renovation, it is without roof and contains grass on the groundand stone walkways. Variant B contains roof made out of ETFE foil roof system, slightly porous

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plaster on the wall surfaces and marble floor. In the variant C, glass roof, marble floor and painted
plaster is presumed.

3. DESCRIPTION OF EXPERIMENTS
The two main experiments (based on Odeon simulations) were performed for the 9 given cases(alternatives). The 3D simulation model is shown in the Figure 3.
Experiment 1, was performed for one sound source and 20 recivers on a grid with dimensions 3x3m,
across the atrium part. It was done to have a detailed information about the sound presure level
distribution G(dB) in each alternative. Position of sound source and receivers in three different
volumes is shown in the Figure 4.
In the experiment 2, one receiver position was chosen together with a set of sound sources
simulating walking person, through the middle part of atrium in two different speeds. Sound sources
simulated positions of impact sound sources (discrete steps of person, later synthethised into continous
walking sound). Distribution position between the receiver and sources was the same in all variants.
(Figure 5)
3.1

Simulation models
Simulation model for this experiment was prepared in Google Sketchup software and simplified
for the needs of the Odeon acoustic model to include only surfaces coming into contact with acoustic
waves represented by rays in simulation software. Software used to simulate room acoustics is basedon a hybrid calculation method. This method combines the use of image source method for early soundreflections, and a special raytracing method with advanced scattering algorithm for simulations ofhigher order reflections (8).

Figure 3. Simulation model 3D perspecive view

Figure 4. Topview of the 3 atria (Vol.1,2 and 3) with indication of the position of the omnidirectional pointsound source (black triangle) and distribution of receivers (blue dots), in Experiment 1.

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Figure 5. Topview of the 3 atria (Vol.1, 2 and 3) with indication of the position of the receiver (blue dot) andsound sources (steps), indicated on red line, in Experiment 24. RESULTS AND ANALYSIS
4.1 Experiment 1
Results of the reverberation time and mean sound absorption coefficient  (-) is shown in the Figure
6. Figure 6 – left shows large differences between different alternatives caused by differnces in volumeand absorption. In the Figure 6-right we can see the mean absorption alpha, calculated as weightedaverage. In theory, the mean alpha should be the same for each variant (A, B or C) because it isindependent on volume, except of high frequencies where the air sound absorption contributes toreverberation time. These trends can be observed.
Reverberation time is clearly longest in large room with least absorption and the shortest
reverberation time is observed logically in the small volume (Vol.3) in situation without roof. It can bealso observed that within one volume the reverberation time at 4000 Hz is almost the same, e.g. at highfrequencies the influence of volume was larger than actual mean  (-) of interior surfaces.

Figure 6 Sumary of average T30 over the all simulated impulse responses and mean sound absorption
coefficient  (-) of interior surfaces for all of simulated variants. NOTE 1* The colours represent theabsorptive properties of surfaces. Blue is used to show the orginal situations before renovation, without roof;in green are alternatives with ETFE foils and porous plaster; and the red colour represents variants withglassed roof and painted plaster. Volume 1 (current room volume) is shown in full line; dashed line represents
cases with Volume 2 (big volume); and the dotted line is showning results for small volume 3.
Another important parameter in general room acoustics is the sound pressure level distribution over6070the room case. In this paper we show the results through a parameter sound strength G (dB) for two
octave bands, 125 Hz and 1000 Hz (Figure 7). The most noisy rooms according to the predicted soundlevel should be small room (Vol.3) with roof, where the glazing + painted plaster is ca 5 dB louder incomparison with ETFE situation and porous plaster. The density of reflection plays here a strong role.
Interestingly, sound levels in small room without roof (at the distance of 14m from the sound source)
are less “noisy” than Sound levels in Volume 1 (original volume) when roof is present. The lowest
sound pressure level is observed in large volume (Vol.2), where in Alt.2A (situation without roof) isalmost following the free field theory. The large dimensions of the atrium don’t bring significant
contribution to the sound strength G increase at further distances.
G (dB) at 125 Hz

G (dB) at 1000 Hz
25202015

15

G (dB)G (dB)

25

10105

5000

51015051015

Source - receiver distance d (m)Source - receiver distance d (m)Alt.1A
Alt.1B
Alt.1C

Alt.1A
Alt.1B
Alt.1C

Alt.2A
Alt.2B
Alt.3C

Alt.3A
Alt.3B
Alt.3C

Alt.2A
Alt.2B
Alt.2C

Alt.3A
Alt.3B
Alt.3C

Figure 7. Graphical representation of the decrease of sound pressure level, expressed through the parametersound strength G (dB) with increasing distance between omnidirectional sound source and receivers. Data
are described as following: circles are showing data for Volume 1 (original volume), squares are used to show
results of Volume 2 (big) and triangles represent the samal volume (Volume 3). The same logic of colours andline representation is used for trendlines as in the Figure 6 (see NOTE 1).

4.2 Experiment 2
Experiment 2 was conducted for sake of better understanding the acoustic comfort issues. Here wefocuse on the research question: “How does the human steps in an atrium influence the overall
equivalent sound pressure level (LA,eq ) on one hand, and the quasi continous background noise caused
by long reverberation defined (in our experiment) by statistical noise level parameter L99 on the otherhand.”
Simulations of person walking at the distance of ca 10m from the receiver was performed for 9alternatives (3 volume cases), shown in the Figure 5. Discrete steps were simulated and auralized at
two different speeds as (quick and slow). The statistical noise analysis was performed on auralizedsamples. Results given in the Figure 8 are averaged values between the left and right ear.
On a first sight some inconsistencies could be observed when comparing LA,eq with reverberation
time and sound pressure level distribution, however this can be explained easily. Parameters LA,eq andLA,99 were calculated based on binaural auralized sound, whereas the T30 and G values were calculated
from omnidirectional receiver. The binaural simulation in which the sound reflections are arrivingfrom walls are for real listening person stronger than those from ground or ceiling, due to shadowing
effects of head (defined by HRTF – Head related transfer function). Therefore, they might influence
the final auralized results especially if the wall properties and distance of receiver differ from6071

alternative to alternative.

Figure 8. Results LA,eq and L99 as calculated from auralized samples.

5. CONCLUSIONS
Based on the performed experiment 1 and 2 it can be concluded, that there might be difference
between the objective assessment (according to the ISO 3382) using omnidirectional speaker andomnidirectional receiver, in comparison with finally binauraly simulated sounds. HRTF might haveinfluence on accuracy of results once the comfort issues in rooms are adressed as they represent thelistening person better.
Sometimes the same backgroun noise levels are produced in rooms with different sizes and interior
surfaces. The question arrises, wheter these would be judged by people as similar (in relation to nosieannoyance).
In the proposal/development of a suitable paramter or criterion for acoustic comfort in atria, thatwhould express the comfort issues related to quasi continous background noise caused by internalsources (such as walking people), therefore requires suitable listening test.

ACKNOWLEDGEMENTS
This work was supported by the European Commission, H2020-MSCA-RISE-2015 project 690970,“PAPABUILD” and Erasmus+ for students exchange”.

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