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WSEAS TRANSACTION ON ACOUSTICS AND MUSIC Issue 2, Vol. 1, April 2004 ISSN: 1109-9577

112

Evaluation of acoustical performance of enclosed-type schools

HANI S. OBEIDDepartment of Electrical & Computer Engineering

Applied Science University

P.O.Box 950674, Amman 11195JORDAN

[email protected]

Abstract- The paper presents the results of an acoustical study performed on an enclosed-typetwo-story school. The background noise levels were measured and the results evaluated bycalculating Transmission Loss and Sound Noise Reduction. Special Software SABIN is used tocalculate the reverberation time of internal spaces before and after treatment of ceilings to obtainthe recommended reverberation time for clear and intelligible speech.

Key Words: Reverberation time, Speech Interference Level, Noise Criteria, Transmission Loss

1. IntroductionThe architectural acoustics is vitallyimportant to the functionality of speech. Thesound is one of environmental conditionsthat staff and students notice most often.Since speech is a key element of effectiveteaching, poor acoustics can have a largeeffect on learning. In many classrooms,students cannot hear words clearly, and theirconcentration wanders. Many are strainingto hear rather than directing their energytowards understanding the lesson.

Nearly all schools have hard materials onwalls, ceilings and floors; when soundreflects off these surfaces it leads to highreverberation times and creates disturbingechoes. Requirement E4 from part E ofschedule 1 to The Building Regulations2000 states that: Each room or other spacein a school shall be designed and constructedin such a way that they have the acoustic

conditions and the insulation againstdisturbance by noise appropriate to itsintended use [1].

Background noise also can be a problem.Sound from outside traffic, adjacentclassrooms, corridors creates backgroundnoise that teachers have to raise their voicesin order to be heard. Combine this with normal

levels of noise from children, and it becomesdifficult to hear speech.

Excessive noise and reverberation interferewith speech intelligibility, resulting inreduced understanding and thereforereduced learning. In many classrooms in theUnited States, the speech intelligibilityrating is 75 percent or less. That means that,in speech intelligibility tests, listeners withnormal hearing can understand only 75percent of the words read from a list.

Imagine reading a textbook with everyfourth word missing, and being expected tounderstand the material and be tested on it.Sounds ridiculous? Well, that is exactly thesituation facing students every day inschools all across the United States [2].

It is common believe that noise generatesmore noise-that is, the poorer the acousticsand the noisier the environment, the louderand noisier the students will become.Therefore, It is important to have realistic

assessment of acoustical conditions prevailin schools in order to influence their existingprovision or new building regulations. Goodacoustics benefit both teachers and studentsand this is a major factor in improvinghearing environment.


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It is important to note that the overridingcriterion for speech is intelligibility. Sincespeech of short disconnected sounds of 30 to300 ms in length, among which are highfrequency, low energy phonemes, the idealroom must assure the ears undistorted

reception of these phonemes. This requireskeeping reverberation to a minimum. Wecan obtain a good approximation to thesubjective feeling of a room, for purposes ofspeech, from the relation:

10log3.060

VRT =

For a typical classroom of 154 m3, thereverberation time for mid frequencies isequal to 0.36 s, which is in line with what

we obtained after treatment of ceiling.

4. Noise Level Measurements

The background noise plays an importantrole in the presentation of acoustic signals.The background noise must be sufficientlylow in level so as to allow communication totake place. In the case of classroom thecommunication is speech.

Therefore, measurements of noise levels inthe schools are considered to be the first stepin evaluation room noise criteria.

Measurements were performed by usingBruel & Kjaer Precision Integration SoundLevel Meter SLM Type 2236.

Two criteria are used to analyze thebackground noise of the school. These are:

- Speech Interference Level SIL.-

Noise Criteria Curves NC.

The Speech Interference Level is thearithmetic mean of the sound pressure levelof a noise in the octave bands centered at500, 1000, 2000 and 4000 HZ. This measureis known as the four bands SIL. The SIL isused to characterize the overall backgroundnoise in the speech range of frequencies.

Once the SIL has been computed, SILcurves as defined by the ANSI Standard [6]is used to determine the speech levelrequired for just reliable speechcommunication between talker and receiverat a given distance.

SIL for various spaces in school werecomputed based on measurements of soundpressure levels. Table 6 shows the calculatedSIL and the expected voice level for a malespeaker.

The analysis of data shown in table 6indicate that the noise level in school is veryhigh and in order for speech to be heard it isrequired to raise the speakers voice to level

of shout. This is a straight violation ofstandards for intelligible speech.

In order to determine the required soundinsulation between spaces it is necessary tostudy the data obtained by measurementsand to apply the Noise Criteria NC Curves.For classrooms it is required to apply NC 35to define the background noise. Also, thesound reduction between rooms may beevaluated by calculating SoundTransmission Loss STL. Table 7 shows

Sound Transmission Loss of various typesof concrete blocks and walls.

The calculations indicated that the externaland internal walls provide good soundinsulation from background noise and fromnoise generated within classroom andmultipurpose rooms. The main path of noisewill be through corridors. The wallsseparating corridors from classroomscontain doors and windows, and the noise

reduction of such a composite wall will bemuch smaller than a wall of concrete block.The transmission Loss and the NoiseReduction of such composite wall at 1000Hz are 23 and 19 dB respectively, whichmean that a noise level of 86 dB in thecorridor will attenuate by 19 dB only and thenoise in the room will be 67. Therefore, themain sound will be transmitted with little


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attenuation from corridors to rooms via theseparating walls, and that is applicablepractically to all internal spaces.

3. Cost AnalysisA cost analysis is performed for oneclassroom of the school to estimate the extracost that should be paid to eliminate theecho effect only and to obtain the requiredreverberation time for speech intelligibility.The dimension of the classroom is 7.45x6.4m and the ceiling height is 3.2 m. we willuse the figure for ministry of Education inJordan about the actual cost of such aclassroom in urban areas, which is 135JD/m2. the actual cost of such a classroom is6437 JD and if we deduct the cost of ceiling

paint, which is 48 JD, then the total costafter ceiling paint cutoff will be 6389 JD. Tothat figure we will add the cost of acousticspray (as an example we apply acousticspray K13 and the cost per meter square is17 JD), so the total cost of classroom aftertreatment of the ceiling will be 7205 JD.Therefore, the mark up on the total cost afterceiling treatment is 11%.

4. ConclusionsAn enclosed-type two floors school wasselected as an example to be studied fromacoustical point of view. The acousticalfeatures of the school are evaluated in termsof parameters that affect the clear andintelligible speech. The values of thecalculated reverberation time of all internalspaces indicate that the school sufferslargely from echo problems. Therefore, theceiling of internal spaces should be treatedto obtain the recommended values ofreverberation time.

The measurements of noise levels in allinternal spaces show that the main path ofsound is through the walls separating therooms from the corridors. The attenuationthrough these walls is very limited due to the

fact that these walls contain doors andwindows, and the Transmission Loss of sucha composite wall is poor.

Because the school is not provided with A/Csystems, so it is impossible to make the

internal spaces very well insulated from theacoustical point of view. It was found thatduring the teaching process the backgroundnoise in classrooms in not so annoying toteachers and students. The level of the noisewill be so high during the breaks and whenthe students enter or leave the school.Therefore, it is not recommended to useacoustical doors or double glaze windows,besides the cost of such doors are so high ($2500 to $ 3500) compared to the cost ofother building elements in the school.

5. The References1. The Education (School Premises)

Regulations. (1999) SI 1999 No. 2.The Stationary Office, 1999, ISBN.

2. Classroom acoustics. (2000). Apublication of the technicalcommittee on architectural acousticsof the Acoustical Society ofAmerica.

3. Statistical Acoustic BasedInvestigator. SABIN 3.0 (2002).Acoustical Engineering.Netherlands.

4. Guidelines for environmental designin schools. (1997). Building Bulletin87, (Revision on design note 17),Architects and Building Branch, DfEE,The Stationary Office, ISBN 0 112710131.

5. Classroom acoustics-RecommendedStandards. Information sheet,BATOD magazine, Jan. 2001, page

13.6. American National StandardInstitute (1977). ANSI S3.14-1977.Rating noise with respect to speechinterference.


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Table 1Reverberation time of school spaces at mid frequencies

No Space Reverberation Time in seconds

1 Typical classroom 1.16

2 Computer room 1.38

3 Library 1.38

4 Multipurpose room 1.38

5 Corridor 1.17

Table 2Recommended ranges of reverberation time at mid frequencies.

No Space Reverberation Time in seconds

1 Classroom 0.4 0.8

2 IT and Computer room 0.5 0.83 Library 0.5 1.0

4 Multipurpose room 0.5 0.8

5 Corridor 0.6

Table 3Absorption coefficient of material for ceiling treatment

Frequency, Hz 125 250 500 1000 2000 4000

Absorption coefficient 0.08 0.16 0.46 0.87 1.07 1.12

Table 4Values of reverberation time before and after treatment

Table 5Reverberation time of treated ceiling over a frequency range 125 Hz 4000 Hz

Reverberation TimeFrequency, HzType of space

125 250 500 1000 2000 4000

Typical classroom 1.33 1.04 0.55 0.33 0.25 0.22

Computer room 1.64 1.21 0.59 0.34 0.26 0.23

Library 1.63 1.2 0.59 0.34 0.26 0.23

Multipurpose room 1.64 1.21 0.59 0.34 0.26 0.23Corridor 1.51 1.19 0.64 0.39 0.30 0.27

Reverberation Time in secondsNo Space type

Before treatment After treatment

1 Typical classroom 1.18 0.44

2 Computer room 1.38 0.47

3 Library 1.38 0.46

4 Multipurpose room 1.38 0.46

5 Corridor 1.17 0.52


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Table 6SIL and expected voice level for various spaces in the school

Table 7Sound Transmission Loss for various types of concrete blocks and walls

Mass Sound Transmission Loss, dB

Frequency, HzType of wall Kg/m2 125 250 500 1000 2000 4000

Hallow block 15 cm with plastering 314 44 50 56 62 86 74

Hallow block 20 cm with plastering 377 45 51 58 64 70 76

Block 15 cm with plastering 396 46 52 58 64 70 76

Block 20 cm with plastering 497 48 54 60 66 72 78Concrete 15 cm without reinforcing withplastering

441 47 52.8 58.8 64.9 70.9 76.9

Concrete 20 cm without reinforcing withplastering

559 48.8 54.9 60.9 66.9 73 79

Reinforced concrete 15 cm with plastering 457 47 53.1 59.1 65.1 71.2 77.2

Reinforced concrete 20 cm with plastering 579 49.2 55.2 61.2 67.2 73.2 79.3

Space Sound Interference Level SIL Expected voice level

Classroom 62 Raised to very load

Computer 67 Very load

Multipurpose room 94 Shout at less than 0.5 m

Corridor 65 Raised to very loadLanding 81 Shout

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