Measurement of the ambient noise level, reverberation time and transmission loss for classrooms in a public school Paulo Henrique Trombetta Zannina) and Carmen Pezette Loro (Received 2006 November 23; revised 2007 March 19; accepted 2007 March 23)
The acoustical quality of classrooms in Brazilian public schools is evaluated. Reverberation time (RT) was measured in classrooms according to ISO 3382. Measurements have shown that empty classrooms display an RT of 1.6 s at 500 Hz. Ambient noise of the classrooms was also evaluated according to the Brazilian Standard for Acoustical Comfort NBR 10152 and the British Building Bulletin BB93. The walls separating the corridor and the classrooms have high level louvers, with fixed glass slats which provide a permanent opening for ventilation and illumination, and one door. The measured weighed apparent sound reduction index Rw⬘ (ISO 717-1) for these walls was Rw⬘ = 13 dB. This is a very low value, when confronted with those demanded, for example, by the German Standards DIN 4109—“Noise Control and Building”, and DIN 18041— “Acoustical quality in small to medium-sized rooms” „Rw⬘ = 32 dB…. © 2007 Institute of Noise Control Engineering. Primary subject classification: 51.1; Secondary subject classification: 72
1
INTRODUCTION
Human learning depends strongly on verbal communication. Some studies have shown that the quality of an acoustical environment is a significant element of verbal learning.1 The effects of chronic noise exposure at home and school on the health of children in the community have also been previously examined.2 The acoustical quality of model modular public school classrooms built in southern Brazil was evaluated. In order to evaluate the acoustical quality of these classrooms, three acoustical parameters have been investigated: 1) Ambient noise level; 2) Reverberation time inside the classrooms; 3) Sound isolation between corridor and classrooms. Ambient noise levels, reverberation times and sound isolation are parameters widely considered to significantly affect the acoustical quality of classrooms.1–11 The several studies by Hodgson and collaborators on the acoustics of classrooms, either in Universities or in preschools, must also be highlighted.12–14
a)
Laboratório de Acústica Ambiental-Industrial e Conforto Acústico, Universidade Federal do Paraná-Departamento de Engenharia Mecaˇnica, Centro Politécnico-Bairro Jardim das Américas, CEP: 81.531-900-Curitiba-PRBRAZIL; email:
[email protected].
Noise Control Eng. J. 55 (3), 2007 May-June
2
DESCRIPTION OF THE CLASSROOMS
The Standard Project developed by the Educational Secretary of Paraná State is called the Standard Project 023. Forty-two percent of the classrooms built in the last decade were built under this Standard Project 023.15 Classrooms of Standard Project 023 have a maximum capacity of 40 students. There are classes in such classrooms during two periods of the day, mornings and afternoons, summing up to approximately 142,800 students. Two classrooms separated by a central circulation corridor compose the Standard Project 023 (Fig. 1). The classroom dimensions are: 1) Width: 7 m, 2) Length: 7.1 m, 3) Height: 3.1 m. The height of the central circulation is 6 m, where each module has a ventilation shutter and zenithal skylight in polycarbonate. The walls separating the corridor and the classrooms have high level louvers with fixed glass slats which provide a permanent opening for ventilation and illumination.
3
AMBIENT NOISE LEVELS FOR SCHOOLROOMS
Ambient noise is one of the main parameters affecting the acoustical quality of classrooms. Recommendations in countries such as Brazil, Japan, United Kingdom and the United States refer to a limiting level for indoor ambient noise. Several countries have revised the legislation on the acoustical quality of school buildings in recent years. One such example is 327
Fig. 1—The School Standard Project 023 with four classrooms. the United Kingdom, where “Building Bulletin 93—Acoustic Design of Schools—A Design Guide” has been published in 2003.16 Limiting levels are given according to the type and usage of the rooms (Table 1). For Japan,17 noise levels shown in Table 1 are to be measured in a furnished and unoccupied room, with doors/windows closed and HVAC systems operating. In the UK, Building Bulletin 93 proposes that indoor ambient noise levels should be the sum of the noise from all sources outside the school premises, including noise from the ventilation system and other plants serving the room or area in question. Ambient noise levels should not include noise from other activities within the school premises, and the room is assumed to be furnished but unoccupied. The standard ANSI S12.6018 from the USA states that background noise
level (ambient noise level) is the sound in a furnished, unoccupied space, including sounds from outdoors, building services and utilities operating at their maximum levels. For the purposes of this Standard, this excludes sound generated by people within the building or sound generated by temporary or permanent instructional equipment. The Brazilian Standard for Acoustical Comfort in Buildings NBR 1015219 does not impose restrictions to the presence of people or teaching activities within the school. The only assumption is that sound measurements are performed under the regular conditions of use of that space. For example, with windows and doors open or closed. The room must be unoccupied but furnished. An ambiguity is perceived in the Brazilian Standard, when compared to the Building Bulletin BB 9316 and standard ANSI S12.60.18 These last Standards impose the absence of people and/or school activities inside the school building during the measurements. In order to follow the indications of Building Bulletin BB 93, ambient noise was measured inside the classrooms depicted in Fig. 1, in the following manner: 1) without ongoing activities in any of the classrooms of the school, 2) without students in any classroom and 3) with doors closed and windows open. That is, under the regular conditions of use of these classrooms. BB 9316 also recommends that if a room is naturally ventilated, fans or windows should be assumed to be open as required to provide adequate ventilation. Public schools in Curitiba, southern Brazil, are only naturally ventilated. Average annual temperature in Curitiba is of 23 ° C.20
4
REVERBERATION TIME FOR SCHOOLROOMS
Another important parameter affecting the acoustical quality of classrooms is the reverberation time, RT.21 It is important that RT suits the intended use of
Table 1—Ambient noise limits for different rooms, according to the Standards for several countries. Country Brazil Japan UK USA
Noise descriptor LAeqa LAeq LAeq,30 min LAeq
Classroom 40 40 35 35b 40c
a
Library 35 35 35 35b 40c
Music classroom 35 35 35 35b 40c
Gymnasium 45 45 40 35b 40c
In Brazil there is no set value for the duration of the LAeq measurement. The duration of the measurement is established by the investigator according to his/her own experience. b Maximum one hour-average A-weighed steady background noise level. c The limits for A-weighed steady background noise levels shall be increased by 5 dB when the noisiest hour is dominated by transportation noise (unsteady background noise). Values for ambient noise in the USA refer to rooms with a maximum volume of 566 m3. 328
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Table 2—Recommended values for Reverberation Time RT in several countries. Country Brazil Japan UK USA
Reverberation Time (seconds) RT= 0.6-Classrooms with V ⬃ 200 m3 RT= 0.6-Classrooms with V ⬃ 200 m3 RT= 0.7-Classrooms with V ⬃ 300 m3 RT= Tmfa ⬍0.6 RT= Tmfb ⬍0.8 RT= 0.6-Classrooms with V ⬍ 283 m3 RT= 0.7-Classrooms with 283 m3 ⬍ V 艋 566 m3 Classrooms with V ⬎ 566 m3-see ANSI S12.60 for special recommendations on the control of RT in these spaces
a
For primary schools: classrooms, class bases, general teaching areas, small group rooms; b For secondary schools: classrooms, general teaching areas, seminar rooms, tutorial rooms, language laboratories.
the room. The standards or guidelines of the countries cited in Table 2 define the RT for the following conditions: a) furnished and unoccupied rooms: Brazil, Japan and the USA; b) unfurnished and unoccupied: United Kingdom. In Japan,17 RT values are the mean in the two octave bands centered at 500 and 1000 Hz. In the United Kingdom the Building Bulletin BB-9316 indicates that RT is given in terms of the maximum mid frequency reverberation time Tmf, the average RT in the 500 Hz, 1000 Hz, and 2000 Hz octave bands. In the USA RT18 is given as the maximum RT for sound pressure levels in octave bands with midband frequencies of 500 Hz, 1000 Hz and 2000 Hz. In Brazil there is no standard for RT in classrooms. The standard NBR 12179— Acoustic Treatment of Closed Rooms22—defines RT at the frequency at 500 Hz as a function of volume, for various internal spaces. Among these spaces, the one closest to a classroom is a conference room. Thus, the RT value in Table 2 for Brazil refers to conference rooms, considering the volume of the classrooms studied here, V = 154 m3.
5
MEASUREMENT OF THE AMBIENT NOISE, REVERBERATION TIME AND SOUND ISOLATION
In order to evaluate the acoustical performance of a classroom model standard 023, a school located in Curitiba was chosen, and four classrooms were evaluated in this school.
5.1
Indoor Ambient Noise in Classrooms
As previously explained in Sec. 3, the Brazilian Standard NBR 10152 establishes that the evaluated Noise Control Eng. J. 55 (3), 2007 May-June
classroom must be unoccupied but furnished, not imposing any other restriction with respect to the presence of people or learning activities inside the building during the measurements. This Standard also does not establish the duration of the measurements. NBR 10152 indicates only that the continuous equivalent sound level Leq should not exceed 40 dBA (Table 1). Following international Standards, such as the ones adopted in the UK by Building Bulletin BB 93, the evaluation of ambient noise was performed in empty but furnished classrooms, with doors closed but open windows. Measurements were performed in the absence of other people in the school, and without any ongoing learning activities inside the building. Measurements were performed using the sound level meter Brüel and Kjaer BK 2238,23 operating at the datalog mode. Measurement time was of 30 min, thus characterizing the Leq,30 min, as prescribed by Building Bulletin BB93.
5.2
External Ambient Noise
Ambient noise Leq expressed in dBA, was measured in 10 positions around the school; measurement time was 5 minutes for each position. The school is located in a peripheral residential area of the city of Curitiba. According to the noise immission legislation of Curitiba, the allowed equivalent continuous sound level Leq during daytime (7:00 am—7:00 pm) for this zone is of 55 dBA.24–26 All measurements were carried out under ideal meteorological conditions: no wind and no rain.
5.3
Reverberation Time
Reverberation time RT was measured under the following conditions: 1) furnished but unoccupied room, and 2) furnished and occupied room. RT was measured inside each of the classrooms following the international Standard ISO 3382.27 Equipments used for the measurement of RT were: 1) Sound Analyzer BK 2260, 2) Sound Amplifier BK 2716, 3) Sound Source BK 4296, 4) building acoustics Software BZ 7204 and 5) building acoustics Software Qualifier BK 7830.
5.4
Sound Isolation
The weighed apparent sound reduction index (or transmission loss) Rw⬘ was measured for the wall separating the corridor and the classroom. The walls separating the corridor and the classrooms have high level louvers with fixed glass slats, providing permanent openings for ventilation and illumination, and one door (Fig. 2). Measurements were performed according to ISO 140-4.28 Sound measurements were performed 329
room 01. The same measurements were performed for classrooms 02, 03, and 04. For classroom 01, LAeq,30 min = 36 dB; for classroom 02, LAeq,30 min = 37 dB; for classroom 03, LAeq,30 min = 38 dB; and for classroom 04, LAeq,30 min = 38 dB. All classrooms thus met the reference value set by the Brazilian Standard NBR10152, of 40 dB. On the other hand, the reference established by BB 93, of LAeq,30 min = 35 dB, was not met by any of the evaluated classrooms.
6.3
Fig. 2—The wall separating the corridor and the classrooms have high level louvers with fixed glass slats which provide a permanent opening for ventilation and illumination. Rw⬘ = 13 dB—between the classroom wall with a door, and the corridor. using two microphones; one was located at the source side (corridor), and the other was located at the receiver site (classroom). The determination of the weighed apparent sound reduction index Rw⬘ was performed in accordance with ISO 717-1.29 In Brazil there are no Standards imposing reference values for acoustical insulation between a classroom and another classroom with a door, or between a classroom and a corridor with a door. For this reason, the Standards used as reference for this study were the German Standards DIN 410930,31 and DIN 18041.32,33 Those Standards establish Rw⬘ = 32 dB for the acoustical insulation by a wall with a door separating a classroom and the corridor.
6 6.1
RESULTS AND DISCUSSION External Ambient Noise
The school is located in the suburban area of Curitiba. The area where the school is located is classified as a residential area according to the local legislation (quiet urban zone). External ambient noise for such areas during daytime should not exceed 55 dBA.24–26 The mean value of equivalent continuous sound level measured was of 54 dBA, meaning that the daytime limit of sound immission of 55 dBA, is met.
6.2
Indoor Ambient Noise
Indoor ambient noise of the schools was measured in datalog, as the continuous equivalent sound level Leq for a period of 30 min. Figure 3 shows the result of the measurement of the indoor ambient noise for class330
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Reverberation Time
Reverberation time RT was measured for three different situations: 1) empty classrooms; 2) classrooms with 20 students; and 3) classrooms with their maximum student load, i.e. 40 students. The volume of the classrooms is 151.4 m3. RT values measured (Table 3) indicate that this classroom does not satisfy either the Brazilian Standard NBR 12179, or any of the other standards considered in this study (Table 2). As it can be observed in Table 3, even with the classroom fully occupied, that is, with 40 students, the RT for the frequency of 500 Hz is of 0.8 s. The Brazilian Standard NBR 12179 recommends an RT of 0.6 s for the empty room at 500 Hz. The high reverberation times measured are a result of the lack of absorbing materials inside the classrooms. Reverberation times above 1 s produce loss in speech discrimination and make speech perception more difficult and strained.34 RTs measured in Classrooms Standard 023 also do not satisfy the recommendation from the World Health Organization34 which states: “For vulnerable groups even lower background levels are needed, and a reverberation time below 0.6 s is desirable for adequate speech intelligibility, even in a quiet environment. The inability to understand speech results in a large number of personal handicaps and behavioral changes”.
6.4
Sound Isolation
The walls between the classrooms and the corridor have permanent ventilation openings through glass slats (Fig. 2). The measured weighed apparent sound reduction index (or transmission loss) Rw⬘ for those walls was Rw⬘ = 13 dB. This is a very low value, when confronted with those demanded, for example, by the German Standards DIN 410930,31—“Noise Control and Building” and DIN 1804132,33—“Acoustical quality in small to medium-sized rooms”. These Standards require a minimum Rw⬘ = 32 dB between a classroom and another classroom with a door, or between a classroom and corridor with a door.
Fig. 3—Measurement in datalog mode of the ambient noise level in classroom 01. Vertical (Y) axis displays LAeq. 7
CONCLUSIONS
The present survey evaluated the acoustical characteristics of one type of public classroom built in Brazil, and has focused on the reverberation time (RT), ambient noise and sound isolation parameters. This set of classrooms is denominate in Standard Project 023. Classrooms built according to the Standard Project 023 did not satisfy either the Brazilian recommendations or any of the other International recommendations concerning reverberation times, as shown in Table 2. Standard 023 classrooms simply lack absorbing materials (ceiling of concrete, floor of ceramics and walls of masonry brick on both sides), which resulted in the high RT measured.
Table 3—Reverberation Time measured in seconds for classroom 01 from Standard Project 023 (furnished classroom). Classroom 01 Empty 20 students 40 students
Reverberation Time in seconds Octave band center frequency [Hz] 500 1000 2000 1.6 1.7 1.4 1.1 0.9 0.9 0.8 0.7 0.6
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Measurements of ambient noise levels were compared with the recommendations from Building Bulletin BB 93 of the UK, which sets the limit at LAeq,30 min = 35 dB for the ambient noise inside the classroom, and those of the Brazilian Standard NBR 10152, which sets the limit at LAeq = 40 dB. Of the four evaluated classrooms, none has satisfied the British Standard, as their measured LAeq,30 min were above 35 dB, ranging between 36 and 38 dB. On the other hand, all the evaluated classrooms met the Brazilian Standard, that is, LAeq was not above 40 dB. Building Bulletin 93 from the UK and ANSI S12.60 from the USA, both specified that during ambient noise measurements the school building must be empty, without any school activity within the school premises. That requirement is not posed by the Brazilian Standard NBR 10152, which also does not precisely set the duration of the measurements of ambient noise. This lack of precision can certainly lead to false estimates of ambient noise inside the classrooms. Thus, NBR 10152 needs urgent revision by the Brazilian Association of Technical Standards, in order to precisely set measurement conditions, such as in Building Bulletin BB 93 and ANSI S12.60. Sound isolation by the wall separating the classrooms and the corridor had a weighed apparent sound 331
reduction index (or transmission loss) of Rw⬘ = 13 dB. This value is well below the value of Rw⬘ = 32 dB established by DIN 4109 and DIN 18041. This low value is certainly a problem when the school is under full activity, as the sound generated in each classroom is easily transmitted to the adjacent classrooms. This is another serious problem about the acoustical quality of the Brazilian built environment, the non-existence of a standard setting precise measurement procedures and reference values for acoustical insulation in several rooms of different use. The heart of the problem, however, is not only in the establishment of technical standards setting more precise limits for the acoustical quality of the buildings, but also in the proper education of architects and civil engineers, public administrators and legislators. The inclusion of a course on luminic, thermal and acoustic comfort has been a very recent improvement for architecture undergraduates.
8
ACKNOWLEDGMENTS
The authors would like to thank the government of the Federal Republic of Germany by means of the German Academic Exchange Service—DAAD (Deutscher Akademischer Austauschdienst) and the government of the Federal Republic of Brazil by means of CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) for financial support, allowing the purchase of the sound level meters and the software used in this work. The authors would also like to acknowledge the support by the Principal of the School evaluated, allowing this study to be conducted. The authors thank Dr. Carolina A. Freire for revising the text.
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