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

Association between measurement and modeling results of roomacoustics in open-plan learning spaces
Joose TAKALA 1; Jesse LIETZÉN1; Saveli VALJAKKA1; Henry NIEMI1; Mikko KYLLIÄINEN1
1

AINS GROUP, Finland

ABSTRACTThe measurement methods presented in standard ISO 3382-3 have been suggested for evaluation of acoustics
in open-plan learning spaces. This means that there should also be a suitable design method to ensure theacoustics beforehand. The purpose of this study is to find out whether the results of the measurements carriedout in recently built open-plan learning spaces correspond to the results of room acoustic modelling
performed in design phase of the spaces. Measurements were done in five new open-plan schools. The results
show that room acoustic modeling can be used as a design method for open-plan learning spaces.
Keywords: Room acoustics, modelling, open-plan learning spaces

1. INTRODUCTION
Traditionally the room acoustic criteria for designing classrooms has been reverberation time T, [1]and in some cases also speech transmission index STI [2, 3]. The acoustic design criteria for open planschools has been proposed to be the spatial decay rate for speech D2,S and STI. [4, 5] To measure thesequantities one could use the open plan office measurement standard ISO 3382-3 [6].
In this paper, our aim is to find out whether it is possible to use room acoustic modelling as a tool
to meet the acoustical design criteria of D2,S and STI for open-plan learning spaces. This was studied
by collecting acoustical modelling data from the design phase of five different open plan schools andthis data was compared to the measurement data from the same schools once they were finished. AINSGroup has made both, the modelling and the measurements, therefore the measurements could be done
exactly at same positions as the modelling was carried out.
The five schools studied were all open plan schools with mineral wool ceiling and some dividing
elements such as curtains, large furniture or screens. The school 2 was different from the other four
schools because it was made for temporary use for the school. The temporary space was situated in alarge hall that was a former department store with a very high ceiling (~5 m) where the classes wereseparated from each other by a ~3 m high curtains and some cabinets. The other four schools followed
the same pattern of modern open plan schools: large space that has a lot of absorbing surfaces andspace dividers. The basic data of the schools is presented in table 1. An example of this kind of modern
open plan school is presented in figure 1.
Table 1 – The basic information of the studied open plan schools.
Area [m2]Flooringcurtains

soft furniture

Furniture and notes

School 1260

carpetyesyesscreens, furniture, curtains

School 2

1200

carpetyesnocabinets, curtains, high ceiling

School 3

380

soft flooring

yesyes"nests", sliding walls, high cabinets

School 4

1000

soft flooring

yesyes"nests", sliding walls, high cabinets

School 5400

carpetyesyes"nests", cabinets, low ceiling height15904

firstname.lastname@ains.fi

Figure 1 – 3D Layout of measured and modelled modern school (School 3).
The layout has a lot of curtains and other space dividing furniture.

2. MATERIALS AND METHODS
2.1 Room acoustic modelling
Room acoustic modelling was made with ODEON 14 software. First the 3D surface model was
made with Trimble SketchUp with the desks, curtains and bigger furniture in place. Then the 3D
surface model was exported to ODEON (figure 2) and all the surfaces were given absorption
coefficients depending on what material they were.

Figure 2 – The speaker position (red) and the microphone positions (blue) in a room acoustic model.

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After importing the needed absorption coefficients, the sound source and the microphone positions
was put in place. The height of the sound source and microphone positions were put as ISO 3382-3
suggests: sound source 1,5 m above the floor and the microphone positions 1,2 m above from the floor.
Also, the sound source was determined to have the sound power of speech as an omnidirectional soundsource according to the standard ISO 3382-3 [6].
The sound source was positioned to be on the place of teacher and th e microphone positions werepositioned in a straight line away from the teacher. The distances of the microphone positions weretypically 2 m from each other, but if a curtain or other obstacle was too close then this position waschanged to meet the requirements of the ISO 3382-3. An example of the room acoustic model and themicrophone and sound source positioning can be seen in figure 2. The curved “walls” in the middle of
the room are curtains that can be used to divide the room.
2.2 Room acoustic measurements
The room acoustic measurements were made according to the standard ISO 3383-3. The describedmeasurement method in the standard is based on measuring acoustic parameters at different distancesfrom the speaker (figure 3). The spatial decay rate for speech D2,S is calculated from the measured
sound levels Lp,A,S of different distances from a speaker that has the sound power of a speech according
to the standard. The distraction distance rD and the privacy distance rP is determined by measuring STI
from different distances from the speaker. The distraction distance is the distance from the speaker
where STI is 0,5 and the privacy distance is the distance from the speaker where STI is 0,2. To
determine the STI the background noise levels Lp,A,B were also measured.
The speech transmission index STI measurements were made using Odeon software, a laptop, adigital sound card and an omnidirectional microphone. To measure the STI correctly with Odeon, thewhole measurement setup (laptop, sound card, cords and microphones) was calibrated in an anechoic
chamber according to Odeon manual. The distances between the microphone and the speaker wasmeasured using a laser distance meter. From these measures the distraction and privacy distances werecalculated.

Figure 3 – An example of the measurement setup in an open plan school. The microphone positions are to beon a straight line from the speaker to the right behind the cabinets.

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The spatial decay rate for speech D2,S was measured using a omnidirectional speaker with a soundpower level calibrated in anechoic chamber. The sound levels at different distances were measured
using class I sound level meter. The same sound level meter was used to measure the background noiselevels.
2.3 Comparison
The comparison of the spatial decay rate of speech is easily comparable for the modelled andsimulated results because the D2,S parameter is only dependent on the measured sound level of thespeaker and the distance between the microphone and the speaker. The comparison was done bysubtracting the measured values from the modelled values individually for each school . Also, theaverage subtractions of all the results were calculated. The modelled and measured STI as a function of
distance is shown in figure 4. The modelled and measured sound levels of speech as a function of
distance is shown in figure 5.

Figure 4 – The modelled and measured STI as of results distance of five schools.
Background noise 40 dB, -5 dB per octave

Figure 5 – The modelled and measured spatial decay rates of speech as of results of five schools.

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The distraction distance and privacy distance results are not comparable by subtracting themodelled results from the measured results because the measured background noise levels weredifferent on each of the measurement sites. The measured background noises between the schools
varied from 25 dB to 34 dB. In the Odeon software, the background noise cannot be measured.
Therefore, it is mandatory to measure the background noise with calibrated sound level meter and put
the result in the software by hand. This allows the changing the background noise of the measurements
and the modelled results with ease afterwards. Both the measured and the modelled results weremodified to have the same background noise of 40 dB with -5 dB drop in every octave from 63 Hz to8000 Hz. Background noise of 40 dB was chosen to show how the space would act with sound masking[4]. The measured and modelled STI results as a function of distance is shown in figure 4 with themodified background noise of 40 dB.

3. RESULTS
The measured spatial decay rate of speech varied from 6,2 dB to 9,2 dB and the modelled from
6,7 dB to 12 dB. The measured distraction distances varied from 4,7 m to 9,2 m and the modelled from
3,3 m to 7,9 dB. The measured privacy distances varied from 10,1 m to 15,6 dB. The acoustic
measurement and modelling results are presented in table 2 in detail for all five schools.
Table 2 – The modelled and measured room acoustic parameters for open plan schools.
Modelled values

Measured values

D2,S [dB]rD [m]

rP [m]

D2,S [dB]rD [m]

rP [m]

School1

9,74,910,16,2

5,9

15,6

School2

7,4

7,8

13,46,9

6,211,6

School3

6,7

7,9

15,6

6,6

6,4

14,0

School4

8,3

3,8

12,9

7,2

4,7

10,1

School5

12

3,3

6,6

9,2

6,4

12,8

Average

8,8

5,5

11,7

7,2

5,9

12,8

Standard deviation

1,9

2,0

3,1

1,0

0,7

1,9

From the data of table 2, the subtraction of the modelled and the measured values was made. Afterthat the average and the standard deviation of the subtractions was calculated. These results are shownin detail in table 3.
Table 3 – The subtraction of table 1 results: modelled values – measured values.
On the bottom line is the standard deviation of the subtractions.

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D2,S [dB]rD [m]

rP [m]

School1

3,5-1,1-5,5School2

0,5

1,6

1,8School30,1

1,5

1,5School4

1,1-0,92,8School5

2,8-3,1-6,3Modelled average - measured average

1,6-0,4-1,1Standard deviation of the subtractions

1,3

1,8

3,9

Table 3 results show that on average the difference between measured and modelled spatial decayrate of speech D2,S is 1,6 dB, for distraction distance rD 0,4 m and for privacy distance rP 1,1 m. The
standard deviation of the subtractions are 1,3 dB for D2,S and 1,8 m dB for r D and 3,9 m dB for rP. The
biggest difference in modelling versus measurements of spatial decay rate of speech was 3,5 dB, for rD
3,1 m and for rP -6,3 m.

4. DISCUSSION
Table 2 shows that the modelled values for D2,S were bigger than the measured values and the rD andrP were not always accurate, especially in the case of schools 1 and 5. The comparison of schools 2,3and 4 showed, however, quite a good agreement between the modelled and measured values.
The modelled version of the rooms were the design versions. From the time of the design to theready space, many things can affect the outcome since acoustic engineer is only one designer in the bigconstruction project. Sometimes the construction work might not have gone in budget and many times
it is the furniture, acoustic treatments and surfacing where saving money is most likely. In some cases,
the furniture design was changed in the design process and the changed furniture was not modelled .
But it is also the nature of the open plan schools to have maximum mobility of the space.
In most of the cases, the furniture designed for the space is something that doesn’t ha ve measured
absorption data to use in the model. When this occurs, the acoustic modeler must make an estimation
about the absorption coefficients. Of course, it is not especially exact to estimate the absorption databy looking a brochure of a furniture. However, the architects usually want the furniture to help with theacoustics and therefore the furniture are covered with a 2–4 cm thick soft materials thus making theevaluation of the absorption coefficients somewhat easier. This is a known problem with any type ofacoustical modelling, but in open plan schools there are a lot of different furniture to model andtherefore the absorption coefficients can make the model inaccurate if the estimations are wrong.
The modelling and measurements were both done with teacher to student situation. In the reference[5] it is proposed to use three different situations: teacher to student, student to teacher and student tostudent. It is not possible to say how the different situations change the STI from the data of this study,
but it would be a natural next step to study in the future.
The receiver height in this study was the same as in ISO 3382-3 [6], but in the reference [5] theheights vary from 0,8 m to 1,65 m (ISO 3382-3 1,5 m and 1,2 m) and therefore it is rather complex touse in consultant work for the everyday measurements in-situ. The more variation the bigger risk thereis for mistakes. From this perspective it would be more convenient to use the heights of the standard[6]. The differences of the heights in [5] should be compared to ISO 3383-3 and study the differences
they create.

5. CONCLUSIONS
The results indicate that room acoustic modelling can provide useful data to design open planschools. When comparing the modelled and the measured results one should keep in mind that theoriginally designed space might be different from the plans due to many decision makers in the projector possible cost reductions.

REFERENCES
1. SFS 5907, Rakennusten akustinen luokitus, Suomen Standardisoimisliitto SFS ry, Helsinki, 2004.
(in Finnish).
2. Ministry of The Environment, Ääniympäristö, Ympäristöministeriön ohje rakennuksenääniympäristöstä. Helsinki, 2018. (in Finnish).
3. Petersen C M & Rasmussen B, Acoustic design of open plan schools and comparison ofrequirements, Joint Baltic-Nordic Acoustics Meeting, Odense, June 18–20, 2012.
4. M. Kylliäinen, R. Pääkkönen, Ääniolosuhteet avoimissa oppimisympäristöissä, Akustiikkapäivät,
Espoo, August 24-25, 2017. (in Finnish).
5. D. Canning, N. Cogger, E. Greenland, J. Harvie-Clark, A. James, D. Oeters, R. Orlowski, A. Parkin,
R. Richardson, B.Shield, Acoustics of Schools: a design guide, Insitute of Acoustics & Associationof Noise Consultants, London, 2015.
6. ISO 3382-3, Acoustics – Measurement of room acoustic parameters – Part 3: Open plan offices,
International Organization for Standardization, Genéve, 2012.

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