Room acoustics in schools  Experiences using DIN...
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Room acoustics in schools ± Experiences using DIN 18041 W. Teuber IAB Institute for Acoustic and Building Physics, 61440 Oberursel, Germany, Email: [email protected] Introduction High standards for acoustical conditions in schools have to be fulfilled nowadays, especially in classrooms. During the last 10 years many efforts were taken by consultants, scientific institutes and the industry to expose all advantages of sound absorption, suppression of strong reflections and a low noise level. The author remembers consultancy work for an International School located in Germany; there suspended ceilings with high absorption coefficients in combination with carpet flooring were common for classrooms already in 1985. In Germany high standards exist since a long time for sound insulation to classrooms including airborne and structure borne / tapping noise (DIN 4109 [1]). In contrary requirement or recommendations existed such as DIN 18041:1968 [2], but these seldom were taken into account or fulfilled. No absorbing areas or sometimes thin plates at the middle of the ceiling could be found in many class rooms, resulting in long reverberation times of more than 1s. Room acoustic, especially in class rooms, interacts with numerous physical and psychological parameters as shown in Fig. 1. Many advantages of good acoustical conditions are obvious and described in publications. The American standard ANSI S12.60-2002 [3] gives guidelines for acoustical performance criteria and design requirements for schools. Purpose is to provide acoustical qualities for good speech communications between students and teachers in classrooms. Maximum reverberation time in rooms with enclosed volume less 283m³ should be < 0.6s in octave bands 500-2000Hz, unoccupied, furnished. Annex A1.4 indicates reduced speech intelligibility for reverberation times in excess of 0.4 to 0.6s. IEMB [4] (Institut für Erhaltung und Modernisierung von Bauwerken), a part of the Technical University Berlin, published measures for refurbishment of older schools. Maximum reverberation times should be less 0.5s (200m³) or 0.6s (300m³). In order to achieve good speech intelligibility first reflections 35 ± 50ms after direct sound are required. These can be achieved by a middle reflective part of the ceiling. Strong reflections >50ms should be suppressed, for instance by absorbent wall areas. In addition low frequency absorption is required in most rooms to fulfil a given tolerance field (+20% above 63Hz). A worldwide study at Heriot-Watt-University of Edinburgh indicated low speech intelligibility in most of 70 classrooms tested. Reasons were increased ambient noise and long reverberation times. Suggested were values from 0.3s to 0.5s. During the last years various DAGA publications were presented. [6, 7, 8] Requirements given in DIN 18041:2004 In this standard [9] titled ³Acoustical quality in small to medium-sized rooms´ two general types of rooms are defined: x A: audibility over medium and wider distances including classrooms, conference rooms, lecture halls, gymnasiums, indoor swimming pools x B: audibility over short distances including office rooms, open-plan offices, reading rooms, restaurants and corridors Next are described requirements to classrooms, concerning to type A, and a size of app. 200m³. x Noise level, distance speaker ± listener <8m 40 dB(A) x Same as before, hearing impaired persons 35 dB(A) or 30 dB(A) x Reverberation time (classroom) T soll = 0.32*logV ± 0.17 (s); V=volume (m³) e.g. for 200m³ T soll 0.57s x Tolerance zone for RT, see Fig. 2 Figure 2: Tolerance zone Figure 1: Interaction of acoustical parameters with psychological factors and quality of learning 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 63 125 250 500 1k 2k 4k 8k Frequency / Hz T / Tsoll Room Acoustics Sound Reflections Concentration Attention Disturbing Noise (traffic) Shorttime Reflections Hearing Impairment Quality of Learning Speech Intelligibility NAG/DAGA 2009 - Rotterdam 939
Transcript of Room acoustics in schools  Experiences using DIN...
Room acoustics in schools Experiences using DIN 18041W. Teuber
IAB Institute for Acoustic and Building Physics, 61440 Oberursel, Germany,Email: [email protected]
IntroductionHigh standards for acoustical conditions in schools have tobe fulfilled nowadays, especially in classrooms. During thelast 10 years many efforts were taken by consultants,scientific institutes and the industry to expose all advantagesof sound absorption, suppression of strong reflections and alow noise level. The author remembers consultancy work foran International School located in Germany; there suspendedceilings with high absorption coefficients in combinationwith carpet flooring were common for classrooms already in1985. In Germany high standards exist since a long time forsound insulation to classrooms including airborne andstructure borne / tapping noise (DIN 4109 [1]). In contraryrequirement or recommendations existed such as DIN18041:1968 [2], but these seldom were taken into account orfulfilled. No absorbing areas or sometimes thin plates at themiddle of the ceiling could be found in many class rooms,resulting in long reverberation times of more than 1s.
Room acoustic, especially in class rooms, interacts withnumerous physical and psychological parameters as shownin Fig. 1.
Many advantages of good acoustical conditions are obviousand described in publications. The American standard ANSIS12.60-2002 [3] gives guidelines for acoustical performancecriteria and design requirements for schools. Purpose is toprovide acoustical qualities for good speech communicationsbetween students and teachers in classrooms. Maximumreverberation time in rooms with enclosed volume less283m³ should be < 0.6s in octave bands 500-2000Hz,unoccupied, furnished. Annex A1.4 indicates reduced speechintelligibility for reverberation times in excess of 0.4 to 0.6s.IEMB [4] (Institut für Erhaltung und Modernisierung vonBauwerken), a part of the Technical University Berlin,published measures for refurbishment of older schools.
Maximum reverberation times should be less 0.5s (200m³) or0.6s (300m³). In order to achieve good speech intelligibilityfirst reflections 35 50ms after direct sound are required.These can be achieved by a middle reflective part of theceiling. Strong reflections >50ms should be suppressed, forinstance by absorbent wall areas. In addition low frequencyabsorption is required in most rooms to fulfil a giventolerance field (+20% above 63Hz). A worldwide study atHeriot-Watt-University of Edinburgh indicated low speechintelligibility in most of 70 classrooms tested. Reasons wereincreased ambient noise and long reverberation times.Suggested were values from 0.3s to 0.5s. During the lastyears various DAGA publications were presented. [6, 7, 8]
Requirements given in DIN 18041:2004In this standard [9] titled Acoustical quality in small tomedium-sized rooms two general types of rooms aredefined:
A: audibility over medium and wider distancesincluding classrooms, conference rooms,lecture halls, gymnasiums, indoor swimmingpools
B: audibility over short distances includingoffice rooms, open-plan offices, reading rooms,restaurants and corridors
Next are described requirements to classrooms, concerningto type A, and a size of app. 200m³.
Noise level, distance speaker listener <8m40 dB(A)
Same as before, hearing impaired persons35 dB(A) or 30 dB(A)
Reverberation time (classroom)
Tsoll = 0.32*logV 0.17 (s); V=volume (m³)
e.g. for 200m³ Tsoll 0.57s
Tolerance zone for RT, see Fig. 2
Figure 2: Tolerance zone
Figure 1: Interaction of acoustical parameters withpsychological factors and quality of learning
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
63 125 250 500 1k 2k 4k 8kFrequency / Hz
T/T
soll
RoomAcoustics
SoundReflections
ConcentrationAttention
DisturbingNoise (traffic)
ShorttimeReflections
HearingImpairment
Quality ofLearning
SpeechIntelligibility
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Hearing impaired persons: RT 20%
No risk of overdamping
Necessity of first reflections 30 50ms afterdirect sound
Experiences in room designBoth criteria reverberation time and reflections must beregarded. Direct sound can and should be improved by firstreflections especially from the ceiling. Otherwise laterreflections >30ms need to be suppressed. This meansabsorbing materials not only under the ceiling but also onwalls. Multiple reflections come up between parallel wallssuch as side-walls or front to back wall. These type ofreflections can also appear in nonrectangular spaces. Duringdesign phase, areas need to be found on the ceiling, one sidewall and on the back wall. In front of the classroom, besidethe blackboard, walls should be reflective. Here the teacher svoice is reflected back to the room when speaking indirection to the blackboard. Such a short-time reflection isdecided and will improve speech intelligibility. Thereforeabsorbent areas are preferred on the back wall. In mostrooms absorbing materials don t need to cover the completewall, panels at the height of a person s ear, standing orsitting, can provide sufficient damping.
In this way many classrooms were designed by the authorand described. A common room size in german schools is ofabout 200m³, giving space for about 25 children. DIN18041:2004 requires a reverberation time of 0.56s. Duringplanning stage reverberation was calculated based onSabine's equation T = 0.16 V / (S1 1 + S2 2 + . +Sn n)In addition room simulation software is a helpful tool andallows computation of ETC Energy-Time-Curve and STI.Fig. 3 shows the surrounding suspended ceiling, highabsorbing panels mounted at distance to the concrete ceiling.The space in between should be filled with mineral wool inorder to increase low frequency absorption. In theschoolhouse described here the thermal capacity of concreteceilings was needed saving energy for heating and aircondition. The middle was let free and can provide
additional first reflections. For children / teacher sittingcentral in the room there is a risk of multiple reflectionsbetween floor and ceiling. Here in the room described tablesand chairs led to sufficient sound diffusion, otherwise somepanels or better wings should be placed under the ceiling.The suspended ceiling described above gives a reverberationof 0.85s, too much compared with DIN 18041:2004, even ifthe whole area would be covered. Absorbing panels on wallsare absolutely necessary to fulfill the required reverberationtime on one hand and to reduce disturbing late reflections /multiple reflections on the other. One of these wall-absorbers can be seen in Fig. 4. Papers and posters reducethe acoustic efficiency, this should be included when thepanel-size is defined. In total a reverberation time of 0.6sresults, with 25 children 0.55s.
Further reductions of the reverberation time are in design forclassrooms in a new planned school. Intention is to achieve0.45s, DIN 18041:2004 is fulfilled, even strongerrequirements for hearing impaired persons or foreignlanguages. The ceiling is suspended with high absorbingpanels, size 27m². Low frequency absorbers are integratedas surrounding package of compressed mineral wool (21m²).Perforated wooden panels, Top-Akustik (27m²), will beplaced on 3 walls.
Improvements by absorbent wall panelsResults described next were measured in a classroom wherethe acoustical environment was complained by all: children,teacher, parents. (measured RT 1.0s; size 226m³; required0.58s) The question was: could this room be improved withadditional wall panels sufficient? Two office screens at theside wall, see photo in Fig. 6, and two at rear wall with atotal area of 7.6m² were brought in. Energy-Time-CurvesETC were measured by using a small loudspeaker (Bose101) at the teacher's place and the omnidirectionalmicrophone at ear-height at rear seats. The sound field infirst 100ms is of primary interest. In this time direct soundplus first reflections arrive up to 30ms. In many publications50ms can be found but in classrooms and similar smallspaces the value 30ms seems to be adequate. The limit 50msshould be applied for larger rooms such as auditoriums,
Figure 3: ceiling, partly absorption, in the middle reflectivearea
Figure 4: wall panel, sound absorbing
reflective
absorption
wall panel
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multi-purpose-halls etc. Fig. 5 shows test results: left firstsituation (complaints), middle test with office screens andright after renovation when app. 20m² ceiling panels and 7m²wall panels were mounted.
Maximum levels of direct sound and first reflections0....30ms are compared to 30....100ms. Level differencesshould be compared with the measured Speech TransmissionIndex STI and clarity / Deutlichkeit D [10]. D is calculatedfrom ETC results, ratio of sound energy 0...50ms to totalenergy, written in percent (%). [11, 12]
The subjective room quality and speech intelligibilityseemed better with office screens. Even after renovation thesituation was not optimal and should be further improved.Records were made by head-microphones in combinationwith digital recording and postprocessing. Such dummy-head recordings allow comparisons of different rooms, sizesand other test results, for example STI / STIPA, loss ofconsonants or listening tests / questionings.
In other good classrooms level differences 0..30 / 30..100msof 12 dB or more can be found, see fig. 7. To compare firstand later reflections is a good tool to estimate the acousticalquality of classrooms or equivalent spaces. This criteria canhelp to find out whether a classrooms is acceptable withoutcomplaints or acoustical improvements are needed. Thedistance and path-length can be interpreted from ETC. Thestrongest and most disturbing reflections can be found fromtheir delay to direct sound and comparing path-lengths.Replacing the omnidirectional microphone to a cordiod-typehelps to locate the direction of most reflective walls.
Further perspectivesAdditional measurements and listening tests will be carriedout to compare subjective impressions with measured resultsand the subjective quality / acoustical comfort of classrooms.
Figure 5: ETC of direct sound and first reflections, 3 room situations, frequency range 100 4100Hz, time 0 100ms
Figure 7: classroom with good acoustical characteristics
Figure 6: Office screens to examine attenuation of wallreflections
0 - 30ms
12dB
> 30ms
time (ms) 10 20 30 40 50 60 70 80 90
6dB
0 - 30ms
12dB
> 30ms
time (ms) 10 20 30 40 50 60 70 80 90
6dB
0 - 30ms
9dB
> 30ms
STI = 0.57; D = 58% STI = 0.69; D = 74%STI = 0.61; D = 70%
0 - 30ms0 - 30ms
> 30ms> 30ms11dB
7dB
time (ms) 10 20 30 40 50 60 70 800 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80
6dB
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SummaryLow reverberation times and regarding existing standards,such as DIN 18041 is one part in the way to good acousticalconditions in classrooms. Further parameters must belong toan adequate design; beside a low level of ambient noise thesuppression of reflections and multiple reflections isessential. The right placement of absorbing panels is aprecondition to suppress later reflections. Therefore besideambient noise level 3 parameters of room acoustics must betaken into account:
Reverberation Time
Energy Time - Curve
Speech Intelligibility
A room design with an absorbing ceiling, even highabsorbing materials, but without any absorption or diffusionat walls is not acceptable; strong reflections with delay timeslonger than 30ms after direct sound will occur and speechintelligibility is reduced. Design criteria are:
Reverberation time equal or shorter thanrequirements in DIN 18041: 2004:classroom e.g. 200m³ T<0.6s
Hearing impaired children T 20% (0.45s)
Absorption on ceiling and walls, min. 2walls
Wall panels at height of ear, standing or sittingperson
No strong reflections >30ms after direct sound
Reflections >30ms: level difference to direct soundand first reflections <30ms: more than 12dB
Noise level <40 dB(A); hearing impairedpersons or foreign language <35 dB(A)
References[1] DIN 4109:1989: Schallschutz im Hochbau, Beuth-
Verlag, Berlin, 1989
[2] DIN 18041:1968: Hörsamkeit in kleinen bismittelgroßen Räumen, Beuth-Verlag, Berlin, 1968
[3] ANSI S12.60-2002: Acoustical performance criteria,design requirements and guidelines for schools; Acoust.Society of America, Melville, NY; 2002
[4] IEMB; Institut für Erhaltung und Modernisierung vonBauwerken e.V. an der TU Berlin: Planungshilfen fürdie Modernisierung von Schulbauten der Baualters-gruppe von 1860 1920; Zentralstelle für Normungs-fragen und Wirtschaftlichkeit im Bildungswesen, Berlin
[5] McKenzie, D.; Airey, Sh.: Classroom acoustics; Heriot-Watt University, Edinburgh; 1999
[6] Mommerz, E.; Drescher, K.; Engel, G.: Untersuchungenzur Anordnung schallabsorbierender Oberflächen inKlassenräumen; DAGA 2002
[7] Seidel, J.; Weber L.; Leistner P.: AkustischeBedingungen in Grundschulen und ihre Wirkung auf dieLernleistung der Kinder; DAGA 2006
[8] Klatte, M.; Wegner, M.; Hellbrück, J.: Feldstudie zurAkustik in Schulen und ihrer Wirkungen auf Kinder;DAGA 2006
[9] DIN 18041:2004: Hörsamkeit in kleinen bismittelgroßen Räumen (Acoustical quality in small tomedium-sized rooms), Beuth-Verlag, Berlin, 2004
[10]Thiele, R.: Richtungsverteilung und Zeitfolge derSchallrückwürfe in Räumen; Acustica 3, 1952
[11]Teuber, W; Völker, E.J.: Improved speech intelligibilityby use of time delays in sound systems, 84 AESconvention Paris, preprint 2562, 1988
[12]Völker, E.J.: The importance of early sound forrecording and reproduction is the quality of digitalsound transmission sufficient? 103 AES conventionNew York, preprint 4579, 1997
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