PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Acoustical simulation of the sound quality of standardized
classrooms in higher education institutions in Natal / Brazil
Luciana ALVES1; Bianca ARAÚJO1
1

Federal University of Rio Grande do Norte, Brazil

ABSTRACTIn architectural design of schools, architects usually replicate one or more types of classrooms throughout thebuilding. However, few studies refer to the verification of acoustical quality of classrooms, which can cause
damages in the teaching-learning process, in case there is low quality, commonly found in Brazilian schools.
This study aimed to propose basic guidelines for room acoustics of classrooms in two universities in Rio
Grande do Norte, Brazil, considering the replication of solutions in standardized rooms. The methodology
was a simulation of acoustical quality of standardized classrooms with alterations on ceiling and posteriorwall through the software EASE® in comparison to the results of the measurements in situ, used as calibration
control of the computational model. The best solution, among the proposals, depended on the shape andproportion of each classroom, varying between different percentages of absorbent material applied in theceiling and in the posterior wall. The Definition (D50) was better distributed in all rooms with a 100%
absorbent ceiling, however, the other parameters of intelligibility presented variables according to the shapeof the room and the application of absorbent material. From the simulations, some guidelines for thearchitectural design of these classrooms were purposed in terms of acoustical quality.
Keywords: Sound quality, Classroom acoustics, Acoustical simulation

1. INTRODUCTION
The comprehension of the speech is fundamental for the achievement of a good teaching-learning
process. When the spoken message is not clearly listened by the students, the understanding and,
consequently, the learning process is impaired (1). It is common to find in Brazil classrooms with poor
acoustical quality (2), which implies in a bad intelligibility. According to (3), good acousticalconditions in classrooms improves the learning process, making it more fluid and less stressful.
It is important to consider that the cost of a renovation is still low in comparison to the social costs
derived of classrooms with bad acoustical quality (4). Studies (5–7) reveal the importance of an
acoustical design for classrooms, considering the external and internal noise as well as thearchitectural characteristics aiming for a good sound quality inside the room.
Besides the poor acoustical quality, it is also common in Brazil to replicate the same sort ofclassroom in the design of a school or university. So, the school or university is designed with the samekind of classroom with poor sound quality and taken as reference for other designs. This type of room
is called here as standardized classroom, what it is also common to identify in new building
extensions.
The objective of this study was to simulate the application of absorbent material in different
proportions in the ceiling of classrooms in two universities in Rio Grande do Norte, Brazil, aiming topropose basic guidelines for room acoustics considering the replication of solutions in standardized
rooms throughout the institutions.
To acoustically characterize a room the most important parameter is the reverberation time, whichis the interval after the sound source mute that the sound remains audible (8). Considering classrooms,
Brazilian standards do not have recommended values of reverberation time These standards should be
reviewed, since they are an important consultation document for architects in the design process.

1

luciana_ralves@hotmail.com

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2. METHODOLOGY
2.1 Characterization of the universities
Aiming to propose basic guidelines for architectural acoustics of classrooms in institutions with thereplication of the same pattern of classrooms, this study was applied in two federal and public
universities in the state of Rio Grande do Norte, located in Brazilian northeast. Both universities havecampus in many cities of the state, but this study was applied in the campus situated in Natal, the state
capital.
The first institution analyzed was the Federal Institute of Rio Grande do Norte (IFRN), founde d in1910 as a technical school which, after 1999, added higher education courses in their grade. IFRN has,
nowadays, 21 campuses throughout the state and 12 undergraduate and six postgraduate courses in thecampus analyzed, called Campus Natal-Central.
This campus is situated in an important region of the city in a 90.000m² lot since 1967. It has acentral building which one floor for administration rooms (called Floor A) and two others withclassrooms (called Floors B and C), which were used in this study. In the figure bellow, this building
has a tag highlighted in yellow. The academic centers also have classrooms, but, as this rooms wereadapted in building renovations, they do not follow a pattern of size or shape.

Figure 1 - The subdivision of IFRN Natal-Central campus (9).
The second institution analyzed was the Federal University of Rio Grande do Norte, founded in
1958. The Campus Central is situated at the current location since 1968 o n a plot of 123 hectares in acentral region of the city. There are 84 undergraduate and 86 postgraduate courses throughout theuniversity, divided into five academic centers, according to the related study areas.
Each of these academic centers has acentral academic building with classrooms,
called “sectors”. At the beginning of theuniversity, five buildings were constructedwith the same modular structure and, as theuniversity expanded, other sorts of building
were built, sometimes replicating the same
originalbuildingorstandardized
classrooms of the original sectors with nosignificant changes. In the figure bellow,
the sectors with classrooms for theoricalclasses are highlighted. This study wasapplied in the Sector IV, which houses
technological courses.
Figure 2 – The subdivision of UFRN Central campus (10).
2.2 Characterization of the standardized classrooms
In each institution some sorts of classrooms were chosen to this study, considering the dimensions,shape, proportion between width and length, volume, location in the building and representative
quantity of this classroom throughout each university, which characterizes the sample as astandardized classroom in the institute.

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At the IFRN, one sort of classroom has more significantly repetitions and it is called as Classroom
Type 01 for this study, which groups the classrooms B01, B06, C01, C06 and C20. The rooms named
as “B” are located at the second floor, called Floor B ( Figure 3), and the rooms “C” are located at thethird floor, called Floor C (Figure 4). For this paper the results presented are from the room C01. The
Type 02 was inconsiderate for this paper because at the time of the research only one room with thesecharacteristics (C12) was working as a classroom. The others have been used with other functions.

Figure 3 - Floor plan of "Floor B" with classrooms Type 01 highlighted in light green.

Figure 4 - Floor plan of "Floor C" with classrooms Type 01 highlighted in light and Type 02 in dark green.
At the UFRN, the academic buildings, called sectors, have a modulate structure, what allows somekinds of standardized classrooms with variation in lengths. The sector chosen for this study hasclassrooms with the same width and five different lengths following the structure modulation (Figure5), what also occurs in other sectors of UFRN. For the study, three kind of classrooms were chosen: 2
modules (G03 – Type 03) and 3 modules, without (A03 and B02 – Type 04) and with absorbent ceiling(I02 – Type 05).

Figure 5 - Classrooms studied in UFRN.
All classrooms studied in IFRN, as shown in Figure 6, have a brick wall with ceramic coating in thelower part and painted in the upper portion, student carts with polypropylene chair and wood table,
canvas blinds covering glass windows and a glass chalkboard. In UFRN, the classrooms have a brickwall painted, student carts with wood or polypropylene chair and wood table, glass windows and a
glass or white board, as shown in Figure 7.

Figure 6 - Standardized classroom in IFRN.
Figure 7 - Standardized classroom in UFRN.
The Erro! Autoreferência de indicador não válida. presents the properties of the classrooms4547

analyzed regarding the types, institutions, dimensions, volume and ratio. In the “observation” cells, it
is presented the most relevant motive for the study of each type of room.
Table 1 - Properties of the classrooms.
Type

Inst./RoomDimensions

Volume01

IFRN/C01

6,85 x 7,35 x 3,40 m

171 m³03

UFRN/G03

8,00 x 6,05 x 3,15 m

152 m³04

UFRN/A03

8,00 x 9,15 x 3,43 m

251 m³05

UFRN/I02

8,00 x 9,15 x 2,86 m

209 m³

Ratio

Observation

1:1

Classroom with the highest number of(1:1,07)

repetitions in IFRN

1,5:1

Classroom with length higher than

(1,32:1)

width

1:1

Classroom with the highest number of(1:1,14)repetitions in UFRN

1:1
(1:1,14)Classroom with absorbent ceiling

2.3 Simulation method
The first step to obtain an accurate simulation was to measure the room dimensions and to estimate
the corresponding material for the determination of the coefficient of absorption according to theliterature or software database. All sorts of classrooms were acoustically measured using Brüel & Kjærequipment. The measurements result of reverberation time were used as a reference value to calibrate
the simulated rooms. The current situation of each classroom was modeled using Trimble Sketchup
2016 (11) and imported into EASE version 4.3 (12), acoustic simulation software, in which thematerials were associated to each surface and the audience and listener seats were positioned
according to the measurements. The simulations used the AURA module of EASE.
The audience area considered a distance of 0,10 m from the side and rear walls, 1,50 m from thefront wall and 1,20 m from the floor. The listener seats were positioned according to the microphone
position in the measurements, as well as the height of 1,20 m. The simulations considered thebackground noise measured in each frequency and a split time of 10 ms.
The current situation of each type of classroom was simulated and calibrated with themeasurements results. After the validation of the model, a mineral ceiling was added into the model on
EASE in different proportions (50, 75 or 100% of the ceiling area, as shown in the table below). For all
the propositions the mineral ceiling used was the model Janus from the manufacturer OWA. This
commercial ceiling was chosen based on a real proposition for the enhancement of the classrooms andon the material properties, but it could be exchanged for other material with similar acousticalcharacteristics. The ceiling height for all simulations was fixed in 2,60 m to compare the results
according to the shape of the room, not considering height variations. The simulations encompassed
the Classroom Types 01, 03, 04 and 05. The Type 02 was inconsiderate because o f the low
representativity of the sample. The Types 04 and 05 were grouped, being called as Type 04-05, because
the addition of a plaster ceiling turns the two kinds of classrooms in one, considering that the majordifference between them was the ceiling height and material. The simulations, thus, considered foursituations, as described in the following table.
Table 2 - Simulation scenarios.
C.S. (Current

AC50% (Absorbent

AC75% (Absorbent

AC100% (Absorbent

situation)ceiling 50%)

ceiling 75%)

ceiling 100%)

Absorbent ceiling

0%50% rear

75% rear

100%

Reflector ceiling

100%

50% frontal

25% frontal

0%

Nomenclature
Position of theabsorbent ceiling(in purple)4548

2.4 Analyzed parameters
The simulations resulted in graphs, images and tables. The main objective of these simulations wasto compare and analyze the improvement of the intelligibility in the classrooms studied. For that, theparameters reverberation time (T 20), early decay time (EDT), definition (D 50) and speech transmission
index (STI) were analyzed. The reverberation time is the parameter that most characterize the acoustic
response of a room, being the time that the sound remains in the room after the source mutes. When thelate reflections carry too much energy, it is harmful for the understanding of the receivers in places ofspeech (8). The early decay time (EDT) is considered an important parameter for the evaluation ofrooms for speech because it concerns about the early decay, which is more evidenced in the speechsince the reverberant tail is often merged with the next words or syllables (13). Usually, the humanbeings react better to these first reflections, giving a subjective sensation of sound quality (14). The
definition (D50) is related to the ability of distinguish subsequent sounds, promoting a sensation of“clear sound”. The greater energy transmitted by the first reflections, the greater the sound definition(13). And, finally, the speech transmission index (STI) that refers to the modulation of sound,considering the difference between the background noise and the sound pressure leve l of the source.
The STI decreases in places with a high background noise and reverberation time (13).
In order to evaluate these results, it was important to considerer reference values. The table below
presents the ideal and acceptable values of four parameters related to intelligibility.
Table 3 - Recommended values for the analyzed parameters.
Analyzed Parameters

Ideal value

Acceptable values

References

Reverberation time (T20)≤ 0,6 s in 500 Hz≤ 0,8 s in 500 Hz(14–16)

Early Decay Time (EDT)≤ 0,05 s≤ 0,05 s(17)

Definition (D50)

≥ 5,0

≥ 5,0

(13,14)

Speech Transmission Index (STI)≥ 0,75

≥ 0,6

(13)

3. RESULTS AND DISCUSSION
The results obtained with the simulations were compared with the measurements results for eachtype of classroom analyzed in the following graphs. Each graph shows the ideal value (line),
acceptable values (filling) and the data from the measurement and the three scenarios of simulation: 50,
75 or 100% of absorbent ceiling.
All graphs have the same color legend, presented in
Figure 8. The graphs of Classrooms Type 04-05
present two “current situation”, being the yellow linewith circle bullets representing the room withoutabsorbent ceiling (UFRN A03) and the yellow linewith X bullets representing the room with absorbent
Figure 8 - Color legend for the graphs.
ceiling (UFRN I02).
3.1 Comparison between measurements and simulations
The Classroom Type 01, which is an approximately square, 1: 1 ratio, located in IFRN, presents
reverberation times above 1,5 seconds in most of the frequencies analyzed and the EDT follows thesame tendency, decreasing only in 500 and 1000 Hz. Regarding the Definition, none of t he frequencies
reach the ideal range of the parameter. Considering the simulations, all results are satisfactory, being
the A.C 100% the one with the best values. The application of absorbent material in only 75% of theceiling (A.C. 75%) also shows good results, very close to the A.C. 100% in T20 and EDT, with a little
bit lower result in D50. This type of classroom presents a bad STI in the current situation and a good
STI, between 0,67 and 0,74, in all the simulated scenarios.

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Type 01 - T20 [s]Type 01 - EDT [s]125 250 500

1k

2k

4k

Type 01 - D50

5,0
4,5
4,0
3,5
3,0
2,5
2,0
1,5
1,0
0,5
0,0

D50

5,0
4,5
4,0
3,5
3,0
2,5
2,0
1,5
1,0
0,5
0,0

EDT [s]T20 [s]Graphs 1 - Results of Classroom Type 01.

125 250 500

8k

1k

2k

4k

1
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0

8k

125 250 500

Frequency [Hz]Frequency [Hz]

1k

2k

4k

8k

Frequency [Hz]Type 03 - T20 [s]

125 250 500

1k

2k

4k

5,0
4,5
4,0
3,5
3,0
2,5
2,0
1,5
1,0
0,5
0,0

8k

Type 03 - EDT [s]

D50

5,0
4,5
4,0
3,5
3,0
2,5
2,0
1,5
1,0
0,5
0,0

EDT [s]T20 [s]As for the Classroom Type 03, which is rectangular classroom located in UFRN and has a ratio
1,5:1, being larger than deeper, the reverberation time has the lower values among the classrooms
studied. Although, the T20 is still high in comparison to the ide al values. The same happens with the
EDT, which presents a different behavior in relation to T20 throughout the frequencies. Considering
the simulations, all scenarios present similar values of T20 and EDT. Moreover, the T20 founded in all
simulations is considerably near of the recommended values. The D50 is better with in the A.C. 100%
scenario and distributed more evenly between the frequencies, although the other scenarios also reachthe ideal values. Regarding the STI, the current situation is evaluated as acceptable, bordering the badband, and the all simulated scenarios as good (values between 0,67 and 0,74).
Graphs 2 - Results of Classroom Type 03.

125 250 500

Frequency [Hz]

1k

2k

4k

1
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0

8k

Type 03 - D50

125 250 500

Frequency [Hz]

1k

2k

4k

8k

Frequency [Hz]The Classroom Type 04-05 is located in UFRN and has a square shape with ratio 1:1. The room A03(yellow line with circle bullets) presents the worst results between all classrooms analyzed and is themost repeated standard in the university. The room I02 (yellow line with X bullets), that is the sameroom but with absorbent ceiling, presents better results, although they are not satisfactory yet. The
highest difference is in EDT, which determines the subjective impression of the sound. Thus, this sortof classroom promotes a better intelligibility of speech than the A03. The d efinition is also better,
mainly in 500 Hz, but still out of the ideal values.
Graphs 3 - Results of the Classroom Type 04-05.
Type 04-05 - EDT [s]125 250 5001k2kFrequency [Hz]4k8kType 04-05 - D50

5,0
4,5
4,0
3,5
3,0
2,5
2,0
1,5
1,0
0,5
0,0

D50

EDT [s]T20 [s]Type 04-05 - T20 [s]
5,0
4,5
4,0
3,5
3,0
2,5
2,0
1,5
1,0
0,5
0,0

125 250 5001k2kFrequency [Hz]

4k

8k

1
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0
125 250 500

1k

2k

4k

8k

Frequency [Hz]Considering the simulations, all scenarios present similar values in T20 and EDT, being the A.C.

4550

100% the one with the best D50. Comparing the A.C. 75% and A.C. 100% it is possi ble to observe thatboth have a similar behavior. Considering the financial resources to buy and install an absorbent
material in all ceiling area of the most repeated standard of classroom of the university, the A.C. 75%
is considered as the best solution, giving a satisfactory intelligibility. The A03 presents bad STI values,
while the I02 has acceptable values of STI. The simulated situations present good STI from 0,65 to
0,74.
In all cases studied, the addition of an absorbent ceiling improves the intelligibility of theclassrooms in any proportion: 50, 75 or 100%. All classrooms present the best result with a 100%
absorbent ceiling, but considering the proximity between the values and the achievement of a good
intelligibility in all simulated scenarios, the addition of a lower area of absorbent material may
indicate savings on financial resources and use of the existing ceiling in part of the classroom.
3.2 Basic Guidelines
Considering the results found, it was possible to determine some basic guidelines for the design ofclassrooms in these institutions or with similar conditions. The first one is that a room larger thandeeper presents a better distribution of the parameters throughout the room. Thus, a classroom with aratio until 2:1 has a better acoustical behavior than a room 1:1 (square), as long as the dimensions arecompatible with classrooms (between 6 and 8 m).
The second guideline is to use a plan ceiling without the interventions of beams or other elements.
Also, the ceiling in the frontal area (reflective material) of the situations A.C. 50% and A.C. 75% can
be inclined for a better distribution of the early reflections. Each case should be studied to deter mine
the better inclination of the reflective ceiling and the reinforcement area.
In addition, the lower the room volume, the better the reverberation time, regardless of the material
used in the ceiling. However, if the volume change is a result of changi ng the ceiling material of theroom, provided that at least 50% ceiling area is in absorbent material, the design or constructive
transformation will bring benefits to the acoustic quality of the room. If possible, the greater theabsorbent area in the ceiling, the better the result obtained.
To avoid the formation of a reverberant field at the back of the room, one can also apply absorbent
materials or diffusers, causing absorption or reflections on the back wall to be scattered around theroom, reaching more students. An alternative is to widen the angles between the side walls and thebottom wall, making it an oblique angle in order to promote diffusion. This solution, however,
becomes more difficult to apply in classrooms already built, and can be adapte d with the application ofpanels to avoid the right angle.

4. CONCLUSIONSIn order to promote classroom improvements and considering that speech intelligibility parameters
are generally dependent on reverberation time, changes were proposed that involved the two mostinfluential factors in RT: volume and area of absorption. A standard height ceiling was inserted for all
the rooms studied in the simulations, while three situations of increase of absorbent area were
simulated. These proposals indicated that the greater the area of absorption in the ceiling, the better the
acoustic response of the room. The parameter that suffered the greatest variation according to thesimulated proposals was the Definition, being better distributed in the case of a larger ar ea ofabsorption. However, in all cases proposed, all rooms achieved good results for speech intelligibility.
Analyzing the relation of the shape of the room with the distribution of the values of the parametersin the classroom, it can be seen that the room with a ratio of 1.5:1 presents a more uniform distribution,
while the room of proportion 1:1 shows the worst distribution. That is, the smaller the depth of theroom, considering dimensions commonly found in classrooms, the better the sound distribu tion, whichmakes students sitting in any position have similar sound perception and understanding.
Finally, it was possible to perceive similarities in the acoustic behavior between classrooms
according to the modifications made, which determined some design guidelines for classrooms with
better acoustic conditioning. Some modifications in existing classrooms are easy to implement andreplicate and contribute greatly to improving speech intelligibility for students. These improvements
foster better interaction between teachers and students, encourage the teaching process and support thebetter training of students and, consequently, future professionals.

4551

ACKNOWLEDGEMENTS
We acknowledge gratefully the Coordination of Improvement of Higher-Level Personnel(Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES) for the scholarship granted
and the Post-Graduation Program in Architecture and Urban Planning for the financial support.

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