A basic protocol to characterize classroom acoustics of primary- schools Arianna Astolfi 1 , Greta Minelli 2 , Giuseppina Emma Puglisi 3 Dipartimento Energia Politecnico di Torino Corso Duca degli Abruzzi, 24 10129 Torino (Italy)

ABSTRACT With the aim to promote a fast and effective characterization of the sound environment in educational facilities and an adequate classroom acoustics design, this work provides a basic measurement pro- tocol which consists of a minimum number of parameters and positions to be considered. The present study involved 29 primary-school classrooms where background noise level during silent and group activities, reverberation time, speech clarity, useful-to-detrimental ratio and speech levels have been acquired in occupied condition along the main axis and in one or two offset positions. Two cut-off values of maximum reverberation time to ensure optimal acoustic conditions in the case of moderate and severe requirements, respectively, were assumed equal to 0.8 s and 0.6 s, according to literature and subjective data. For each cut-off value, classrooms were divided in two consistent groups either if they were compliant or non-compliant with such requirements, respectively. Given the strong cor- relation among the measured quantities, cut-off values were also identified for the other acoustical parameters. The main result of the work suggests that more convenient parameters, such as clarity in the central location of the classroom, can be used beyond reverberation time, which implies a more laborious measurement procedure.

1. INTRODUCTION

The scope of the present work has been to identify the basic parameters and their optimal values necessary to fast and effectively characterize the acoustic quality of parallelepipedal shaped primary school classrooms with small and medium volumes, i.e., between 100 and 290 m 3 . The study arises from the awareness that many of the available indexes and parameters used for classroom acoustic characterization are closely correlated. On the basis of this knowledge, which is well-supported by literature, there is a need to advance such characterization practices by simplifying the measurement protocols in order to save time and to allow wide experimental campaigns to be made to certify a higher number of classrooms. To this aim, 29 primary classrooms in Turin, Italy, which are hetero- geneous in terms of building typologies and acoustic conditions, have been considered in this study. The database we used in Astolfi et al. (2019) [1], has been enlarged in this work, although we have used the same protocol.

1 arianna.astolfi@polito.it

2 greta.minelli@polito.it

3 giuseppina.puglisi@polito.it

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2. MATERIALS AND METHODS

Measurement campaign were carried out in 29 first-grade classrooms belonging to 13 primary schools in Turin, where about 550 pupils aged from 6 to 7 years participated in this study over three scholastic years, that is, from 2016 to 2019.

2.1. Schools and classrooms The location, periods of construction and architectural features of the 29 schools involved in the pre- sent study differ. Moreover, they were built between 1846 and 1975, and are scattered over the city of Turin, in neighborhoods characterized by low or medium volumes of traffic [2]. The classroom volumes vary between 105 m 3 and 290 m 3 , and all the classrooms, except one (G1), have a rectangular shape. The average height of the classrooms ranges from 3.0 m to 5.3 m, and the ceilings are flat or vaulted. The floor finishes are venetian tiles, except for in one case (H1), which is instead in linoleum. The furniture consists of desks and chairs, bookshelves and blackboards. Some classrooms have un- dergone acoustic correction interventions.

2.2. Characterization of the classroom acoustics We performed the acoustic measurements in one-day in each school in the last two months of the school year, over three scholastic years, that is, from 2017 to 2019. We carried out the measurements under occupied conditions, with an average number of 19 children per class. We also measured the reverberation time under empty conditions, at the end of each session. The pupils were seated in the traditional teaching layout (in rows facing the teacher’s desk) in all the classrooms, with three or four rows of desks in each classroom, which were sometimes joined. The teacher’s desk was parallel to one of the shorter walls in each room, except for in E2, where the teacher’s desk and the blackboard were parallel to the longer side of the room, for teaching purposes. We identified the main axis in each classroom, starting from the center of the wall behind the teacher’s area. Details of the measurements, set-up and equipment can be found in Astolfi et al . (2019) [1]. The parameters that were measured are: T20, which is the reverberation time when children are in a class- room; T20 _e, which refers to the empty conditions; L N_sil, which is the noise acquired when the stu- dents are silent and L N_gr, which is the noise acquired while the students are performing group activ- ities. Moreover, the speech level was recorded at 1 m from the source, L S_ref , and then in the other positions, and its slope per double distance along the main axis, mL S , was evaluated; speech clarity and the useful-to-detrimental ratio referred to the mean distributions in the classroom and were la- beled with the subscript “M”, e.g., C50 _M and U50 _M , or referred to single values in the center of the classroom, which were labeled with the subscript “ctr” , e.g., C50 _ctr and U50 _ctr .

2.3. Statistical analysis The statistical analysis was carried out with SPSS (IBM Statistics 20, IBM, Armonk, NY, the USA). Once we had checked that the distribution of the database was non-normal, non-parametric methods were used to analyze the data. We investigated any correlations between the acoustic parameters by means of Spearman’s rho [3], and further analyzed those with a p-value < 0.01 through linear regres- sion analysis. We then divided the classrooms into two groups, starting from a reference T20 value chosen arbitrar- ily from among the ones identified as thresholds for evaluating the compliance or non-compliance of the classroom acoustic conditions with the literature or with the considered standards. In this way, the classroom acoustics were classified as compliant (C) or non-compliant (NC) with the requirement. The Mann–Whitney U Test (MWU) was used to assess the significance of the differences between the

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values of the parameters in the two groups. Only the parameters with a p-value below 0.05 were considered significantly different between the two groups. Once subdivided the classrooms between the two groups on the basis of the reverberation time thresh- olds, we resort to the receiver operating characteristic (ROC) approach three times. Firstly, the area under the curve (AUC) of each of the other acoustical parameter was inspected to assess its overall summary accuracy to classify cases between the groups of classrooms compliant (C) or non-compli- ant (NC) with the threshold. Secondly, the analysis of the ROC curves allowed for the identification of the most suitable threshold value for the other investigated acoustical parameters, based on the Minimum Squared Distance (MSqD) [4]. Thirdly, the accuracy and precision of the thresholds iden- tified through the MSqD from the ROC curve were then tested to assess their ability to classify the acoustic quality of the classrooms as either C or NC [5]. In order to perform these analyses, the values of the parameters were indicated as follows:

• True Positive (TP), when the classification was C and the value of the specific parameter was

correctly identified in the C group; • True Negative (TN), when the classification was NC and the value of the specific parameter

was correctly identified in the NC group; • False Positive (FP), when the classification was C and the value of the specific parameter was

identified in the NC group; • False Negative (FN), when the classification was NC and the value of the specific parameter

was identified in the C group. Accuracy,

𝑇𝑃+𝑇𝑁

𝑇𝑃+𝐹𝑃+𝑇𝑁+𝐹𝑁 , is the percentage of positive and negative predictions that were correct and responded to the question: “how many classrooms have been correctly labeled out of all the 29 classrooms?”. Precision ,

𝑇𝑃

𝑇𝑃+𝐹𝑃 , is the percentage of positive predictions that were correct and responded to the following question: “how many of those classrooms that have been labeled as compliant classrooms, i.e., C, actually respect the threshold?”. 3. RESULTS

3.1. Measurement results Table 1 shows the results of the acoustic measurements in the primary school classrooms. The meas- ured acoustical parameter range covers a wide span of values for all the considered parameters and represents most of the classroom environment typologies [6].

3.2. Relationship between the acoustical parameters The two-tailed significant correlations between the acoustical parameters measured in the classrooms corroborate the outcomes that had already been found in Astolfi et al. (2019) [1]. In particular, it was found: • a strong negative relationship between the reverberation time under occupied conditions T20, and

the C50 and U50 speech intelligibility indexes, which seems to suggest that it is necessary to use only one quantity to characterize classroom acoustics; • a positive and significant correlation between T20 and T20 _e , which suggests the necessity of only

characterizing classrooms under empty conditions; • a very close connection between the central and mean values of both the C50 and U50 quantities,

which indicates that only one measurement in the center of the room need to represent the behav- ior of the whole classroom, in terms of speech intelligibility;

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Table 1: The acoustic parameters measured for each classroom. Standard deviations are indicated in brackets, when available; n.a. stands for not available. It is also indicated whether the classrooms belonged to the compliant group, i.e., C, or to the non-compliant group, i.e., NC, in the case of group- ings (a) and (b). The values that do not comply with the threshold, i.e., False Positive (FP) and False Negative (FN), based on the different groups, are highlighted in bold when considering (a), in italics wh en considering (b), and in bold it ali cs i f the value does not comply with the (a) or (b) subdivisio ns.

Acoustical parameters Group

T20  0.8 s (a) T20  0.6 s

ID

T20

T20 _e

L N_sil [dBA]

L N_gr [dBA]

L s_ref [dBA]

mL S [dBA/dd]

C50 _M

C50 _ctr

U50 _M

U50 _ctr

(b)

[s]

[s]

[dB]

[dB]

[dB]

[dB]

A1 0.9 (0.02) 0.9 (0.04) 51.7 n.a. 61.3 -1.9 1.3 (1.2) 1.0 -1.2 (1.0) -1.1 NC NC A2 1.0 (0.06) 0.8 (0.06) 49.0 64.7 61.2 -2.4 2.2 (1.8) 0.0 0.2 (1.1) -1.3 NC NC A3 0.8 (0.06) 0.8 (0.05) 38.4 61.8 60.3 -2.0 4.1 (0.9) 5.1 3.8 (0.9) 4.8 C NC A4 0.7 (0.03) 0.7 (0.06) 47.1 69.2 61.3 -1.6 4.7 (1.4) 4.4 3.4 (1.4) 3.4 C NC A5 0.7 (0.04) 0.8 (0.06) 46.3 78.4 61.0 -2.3 5.4 (0.4) 4.8 3.9 (0.6) 3.5 C NC B1 0.5 (0.01) 0.6 (0.07) 49.3 66.3 60.8 -2.1 7.6 (1.5) 7.3 4.0 (1.9) 2.9 C C B2 0.5 (0.09) 0.5 (0.02) 39.9 66.3 61.7 -2.6 7.0 (1.0) 8.1 6.5 (0.9) 7.3 C C C1 0.7 (0.03) 0.9 (0.03) 49.3 62.2 62.8 -1.6 3.3 (0.8) 2.8 2.2 (0.6) 1.9 C NC D1 1.3 (0.05) 1.4 (0.06) 51.2 68.0 63.0 -1.8 -0.1 (0.6) -0.6 -1.6 (0.8) -2.1 NC NC D2 1.3 (0.10) 1.4 (0.01) 52.0 n.a. 62.7 -2.1 -0.1 (1.0) 0.0 -1.6 (1.3) -1.6 NC NC E1 1.2 (0.06) 1.3 (0.06) 54.0 66.6 62.1 -1.4 1.1 (0.9) 0.7 -1.2 (0.6) -1.5 NC NC E2 1.0 (0.08) 1.0 (0.15) 54.3 73.7 61.5 -1.9 2.7 (1.0) 3.8 -0.9 (0.8) 0.0 NC NC F1 1.2 (0.03) 1.5 (0.01) 52.0 75.1 62.1 -1.7 -0.3 (1.8) 1.1 -2.2 (1.6) -0.9 NC NC F2 1.4 (0.13) 1.7 (0.03) 52.0 73.8 62.9 -1.8 -0.1 (1.2) -1.1 -1.8 (1.3) -2.7 NC NC G1 0.9 (0.05) 1.2 (0.07) 51.5 72.2 62.3 -2.1 2.6 (1.0) 3.3 0.9 (0.9) 1.3 NC NC G2 0.6 (0.01) 0.9 (0.01) 51.9 65.3 60.7 -0.8 2.9 (0.9) 3.3 0.8 (0.6) 1.4 C C G3 0.7 (0.05) 0.8 (0.04) 52.5 63.5 n.a. 0.0 4.4 (0.3) 4.7 n.a. n.a. C NC H1 0.7 (0.03) 0.8 (0.03) 51.6 71.9 61.5 -1.1 3.6 (0.2) 3.9 1.5 (0.1) 1.4 C NC H2 0.6 (0.08) 0.9 (0.04) 55.9 68.1 62.4 -2.2 5.3 (0.3) 5.9 -0.7 (0.5) -1.0 C C H3 0.7 (0.04) 1.0 (0.19) 45.5 63.9 62.9 -1.8 3.8 (0.3) 3.8 3.2 (0.5) 3.0 C NC H4 0.7 (0.03) 0.8 (0.04) 53.1 65.5 62.9 -2.1 4.1 (0.6) 3.6 0.7 (0.6) -0.1 C NC I1 1.3 (0.10) 1.3 (0.07) 45.7 59.9 61.9 -1.6 -2.2 (0.2) -1.1 -1.7 (0.3) -2.6 NC NC I2 1.1 (0.03) 1.3 (0.03) 42.3 71.1 63.3 -2.3 0.0 (0.9) -0.3 -0.2 (0.9) -0.5 NC NC L1 0.9 (0.05) n.a. 47.9 67.4 61.1 -1.9 1.6 (1.0) 1.0 0.3 (1.1) 0.0 NC NC L2 1.0 (0.04) 1.2 (0.02) 46.0 71.6 62.0 -2.2 0.5 (2.0) 0.6 -0.2 (2.1) -0.1 NC NC L3 1.0 (0.05) 1.3 (0.04) 43.0 81.3 62.6 -2.3 0.8 (0.7) 0.7 0.5 (0.7) 0.3 NC NC M1 0.9 (0.04) 1.1 (0.06) 52.0 81.9 63.0 -1.7 2.1 (1.3) 2.3 0.4 (0.9) 0.5 NC NC N1 1.1 (0.05) 1.1 (0.04) 54.3 76.1 63.9 -1.4 1.4 (0.7) 0.9 -1.0 (1.0) -1.4 NC NC O1 1.0 (0.04) 1.1 (0.04) 49.6 76.7 62.3 -1.9 2.3 (1.1) 2.4 0.5 (0.7) 0.7 NC NC Starting from the significant correlations, regressions analyses were carried out. High R 2 values were found for all the significant correlations, except for L S_ref and T20, for which the relationship was weak. From correlations and regression analyses it is possible to assume that classroom acoustics can be fully characterized from just a single measure, e.g., T20 or C50 _ctr , since it would be possible to esti- mate the other parameters through robust equations. C50 in the central position in occupied settings, i.e., C50 _ctr , can be considered as one of the most effective quantities for measurements inside a class- room to investigate classroom acoustics. It is preferable to T20 and U50, because of its easier meas- urement procedure. As far as the relationship between T20 and T20 _e is concerned, a close correlation emerged from the regression analyses, which suggests that only the empty condition is needed to be considered for an experimental survey.

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3.2. Compliant and non-compliant classrooms In order to categorize classrooms compliant (C) or non-compliant (NC), the classrooms were subdi- vided into two groups twice, i.e., assuming arbitrary T20 cut-off values of 0.8 s (a) and of 0.6 s (b), respectively. The objective was to ascertain the threshold that divides C from NC for each acoustical parameter in both of these cases. The T20 cut-off value of 0.8 s was chosen as being representative of moderate requirements for class- room acoustics since: (i) it is toward the upper range of admittable reverberation times in comfortable classrooms used for learning [7, 8] , (ii) it is the optimal value advised in the case of the refurbishment of primary school classrooms and it is recommended for older pupils [9], and (iii) it is also recom- mended for speaking to older pupils [10]. The same T20 threshold of 0.8 s was used in Astolfi et al. (2019) [1] to discriminate between good acoustics and bad acoustics classrooms. In [1], the pupils’ perception of noise disturbance and their sense of fitting in at school differed significantly for the two groups, and the average subjective score was higher in bad acoustics than in good acoustics for the former and lower for the latter, respectively. On the other hand, a T20 value equal to 0.6 s is the cut-off value that is required in the case of more severe classroom acoustics requirements. It was chosen since: (i) it is recommended in the most recent standards on classroom acoustics, such as the DIN 18041:2016 [11] and the UNI 11532-2:2020 stand- ards [12], (ii) there is evidence that 0.6 s is the optimal value to guarantee a better learning perfor- mance for primary school students [13]. A reverberation time (T soll ) of 0.6±0.1 s, from 0.250 kHz to 4 kHz, is considered optimal, according to the DIN 18041:2016 [11] and UNI 11532-2:2020 stand- ards [12], for rooms dedicated to communication with the simultaneous presence of several people speaking in the classroom, as is the case during primary school lessons. Table 2 reports the descriptive statistics of grouping (a) together with the information if the two groups differ significantly, i.e., if p-value is below 0.05. As can be seen from Table 2, there is no significant difference between the C and NC groups for the L N_sil , L S_ref or mL S parameters, since the results of the Mann–Whitney U Test (MWU) are greater than 0.05. The same procedure was done for grouping (b) that was made by imposing T20 equal to or below 0.6 s and then higher than 0.6 s to attribute classrooms to the C and NC groups, respectively. Once the classrooms had been assigned to the NC or C group on the basis of the T20 criterion, the descriptive statistics were calculated for each parameter as shown in Table 3. Table 4 shows the AUC and the threshold values of the acoustical parameters obtained for the two groupings (a) and (b), which were used to subdivide the classrooms into C and NC. By comparing the thresholds in Table 4 with the optimal values of the acoustical parameters reported in literatureit is possible to label the two groups of classrooms as compliant (C) or non-compliant (NC), respec- tively, considering the upper limit of the admissible values in the case of T20 for C. The close corre- lation between the parameters determines the pairing of the thresholds for the two groups with the optimal values. Table 4 also reports the accuracy and precision of the thresholds identified through the MSqD from the ROC curve for each acoustical parameter in labeling the classrooms in the two categories. Table 1 shows the values that do not comply with the threshold of the specific parameters, i.e., False Positive (FP) and False Negative (FN), which are in bold, in italics or in bold italics, on the basis of a group belonging to (a) or (b).

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Table 2: Descriptive statistics of the acoustical parameters with a subdivision into C and NC for grouping (a). Standard deviations are indicated in brackets, while n.a. stands for “not available”.

The p-values that were obtained with the MWU test are also shown.

Average 25 th Percentile 50 th Percentile 75 th Percentile p-value

T20 [s] C 0.6 (0.1) 0.6 0.7 0.7 0.00 NC 1.1 (0.2) 1.0 1.0 1.2

T20 _e [s] C 0.8 (0.1) 0.7 0.8 0.9 0.00 NC 1.2 (0.2) 1.1 1.2 1.3

L N_sil [dBA] C 48.0 (5.3) 45.9 49.3 51.8 0.48 NC 49.7 (4.3) 46.0 51.3 52.0

L N_gr [dBA] C 67.2 (4.7) 64.6 66.3 68.6 0.02 NC 72.3 (6.1) 68.0 73.0 76.1

Ls_ref [dBA] C 61.7 (1.0) 60.9 61.5 62.6 0.09 NC 62.4 (0.7) 62.0 62.3 63.0

mL s [dBA/dd] C -1.7 (0.8) -2.2 -2.0 -1.6 0.85 NC -1.9 (0.3) -2.2 -1.9 -1.7

C50_ M [dB] C 4.7 (1.5) 3.7 4.1 5.4 0.00 NC 1 (1.4) -0.1 1.0 2.2

C50_ ctr [dB] C 4.8 (1.7) 3.7 4.4 5.5 0.00 NC 0.9 (1.5) -0.3 0.7 2.3

U50_ M [dB] C 2.6 (2.0) 1.1 3.2 3.8 0.00 NC -0.6 (1.0) -1.7 -0.6 0.4

U50_ ctr [dB] C 2.6 (2.3) 1.4 2.9 3.5 0.00 NC -0.7 (1.3) -1.5 -0.7 0.4 Table 3: Descriptive statistics of the acoustical parameters with a subdivision into C and NC for grouping (b). Standard deviations are indicated in brackets, while n.a. stands for “not available”.

The p-values that were obtained with the MWU test are also shown.

Average 25 th Percentile 50 th Percentile 75 th Percentile p-value

T20 [s] C 0.6 (0.1) 0.5 0.6 0.6 0.001 NC 1.0 (0.2) 0.7 1.0 1.1

T20 _e [s] C 0.7 (0.2) 0.6 0.8 0.9 0.029 NC 1.1 (0.3) 0.8 1.1 1.3

L N_sil [dBA] C 49.2 (6.8) 44.6 50.6 53.9 0.874 NC 49.3 (4.1) 46.3 51.2 52.0

L N_gr [dBA] C 66.5 (1.2) 65.8 66.3 67.2 0.275 NC 70.3 (6.3) 65.1 71.1 74.5

Ls_ref [dBA] C 61.4 (0.8) 60.8 61.3 62.1 0.115 NC 62.1 (0.9) 61.4 62.2 62.9

mL s [dBA/dd] C -1.4 (1.4) -2.2 -2.2 -0.8 0.575 NC -1.9 (0.3) -2.1 -1.9 -1.7

C50_ M [dB] C 5.7 (2.1) 4.1 6.2 7.3 0.009 NC 2.0 (1.9) 0.5 2.1 3.6

C50_ ctr [dB] C 6.2 (2.1) 4.6 6.6 7.7 0.009 NC 1.9 (2.0) 0.6 1.1 3.8

U50_ M [dB] C 2.6 (3.2) 0.1 2.4 5.3 0.088 NC 0.3 (1.9) -1.2 0.3 1.2

U50_ ctr [dB] C 2.7 (3.4) 0.3 2.2 5.1 0.123 NC 0.2 (2.0) -1.4 -0.1 1.4

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In short, Table 4 shows, in bold, the thresholds of the parameters that can be used alternatively to characterize classroom acoustics, according to the different level of performance required in the class- room, that is, moderate performance, as with grouping (a), or severe performance, as with grouping (b). In the case of moderate requirements, such as for the refurbishment of existing classrooms or in the case of older pupils [9], as well as in the case of specific requirements for voice support [10], where a slightly higher reverberation time than 0.6 s is required, thresholds (a) are preferable. In this case, the parameters that are preferable to use alternatively for classroom acoustic characterization are T20, T20 _e , C50 _M , C50 _ctr and U50 _M . In the case of a severe performance requirement, that is, in the case of the respect of standard requirements, thresholds (b) are advised. T20, C50 _M and C50 _ctr result to be the most accurate and precise parameters for an alternative use to fully characterize class- room acoustics for both moderate and severe requirements. Table 4 also shows the paucity of cases belonging to the C group for the second grouping (b). A similar amount of data is needed for both groups to improve the statistical analysis and to have more robust results, like those in the case of grouping (a).This will be one of the aims of future research and it will be achieved by boosting the sharing of acoustical measures across Europe, since Italian schools are generally hosted in typical Southern European buildings, which have high ceilings and plaster walls, and are thus associated with higher reverberation time values.

Table 4 – Area under the curve (AUC), thresholds identified through the minimum squared distance

from the ROC curve and their accuracy and precision for groupings (a) and (b). The parameter data

that can be used alternatively to characterize classroom acoustics, according to the different levels

of performance required in the classroom, are shown in bold. Grouping T20 _e

L N_gr [dBA]

C50 _M

C50 _ctr

U50 _M

U50 _ctr

[s]

[dB]

[dB]

[dB]

[dB] (a) T20≤0.8 s

AUC 0.95 0.77 1.00 0.98 0.95 0.90 Threshold for C

classrooms ≤0.9 ≤67 ≥3 ≥3 ≥1 ≥1

Accuracy 89% (25/28)

74% (20/27)

100% (29/29)

93% (27/29)

100% (28/28)

86% (24/28)

Precision 92% (11/12)

67% (8/12)

100% (12/12)

100% (12/12)

100% (11/11)

82% (9/11) (b) T20≤0.6 s

AUC 0.85

0.91 0.92

Threshold for C

classrooms ≤0.9 ≥5 ≥6

n.a.

n.a. n.a.

Accuracy 68% (19/28)

93% (27/29)

100% (29/29)

Precision 100%

75% (3/4)

100%

(4/4)

(4/4)

4. CONCLUSIONS

The obtained results show that most of the usually considered parameters are closely correlated. Re- verberation time, T20, or speech clarity in the central point, C50 _ctr , or average across measurement positions, C50 _M , can all be used as the most representative parameters to characterize classroom acoustics. Therefore, using either T20 or C50, it is possible to estimate the useful-to-detrimental ratio, U50, which is the parameter that is most closely related with speech intelligibility, as it accounts for both noise and room acoustic defects. In order to reduce the measurement points to a minimum number, it is advisable to first characterize classrooms by means of the speech clarity parameter in the central position, C50 _ctr .

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Since the reverberation in occupied and unoccupied conditions resulted to be positively and signifi- cantly correlated, it is possible to state that it is sufficient to measure T20 in unoccupied conditions, using a procedure that gradually becomes smoother and faster, and from which the other related pa- rameters can be estimated. New thresholds for classroom acoustics parameters which discriminate the acoustic quality of pri- mary school classrooms between compliant and non-compliant are provided for moderate and severe requirements. For moderate requirements the thresholds are 0.8 s for T20, 0.9 s for T20 _e , 67 dB(A) for L N_gr , 3 dB for C50 _M and C50 _ctr , 1 dB for U50 _M and U50 _ctr. For severe requirements the thresholds are 0.6 s for T20, 0.9 s for T20 _e , 5 dB for C50 _M and 6 dB for C50 _ctr . Moderate limits are intended for older pupils and for refurbishment, whereas severe limits are intended for younger pupils and in the case of new schools. Both limits also support the teacher’s voice in the classrooms. However, the new thresholds are not intended to be definitive since they are not based on studies testing performance or subjective perception of pupils. 5. REFERENCES

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