CHAPTER 4 REVERBERATION - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/9572/33/12...142 CHAPTER 4 REVERBERATION 4.1 GENERAL Understanding speech is hindered by the combined

142

CHAPTER 4

REVERBERATION

4.1 GENERAL

Understanding speech is hindered by the combined effects of

excessive noise and reverberation in the classroom, which tends to interfere in

the learning process. The combination of noise and reverberation exerts a

stronger negative effect on speech recognition than the sum of their separate

effects (Bradley et al 1999, Crandell and Smaldino 2000). Having discussed

the background noise in the previous chapter, the effects of reverberation in

the classrooms would be discussed in this chapter. An acoustical phenomenon

that occurs in an enclosed space, such as a classroom, when sound waves

persist or prolong in that space as a result of repeated reflections or scattering

from hard surfaces enclosing the space or objects in the space, such as chairs

or cabinets is called reverberation (Siebein 1994, Siebein et al 1997). An

important variable that determines the acoustics of the classroom is

reverberation. If there is a high degree of reverberation, the sound reaching a

listener will be subjected to a prolonged number of reflections. Direct sound

will arrive at the listener first, followed by many reflections of the sound. For

a space designed for speech, late reflections can destroy Speech Intelligibility

as these reflections of earlier syllables of the speech mask subsequent

syllables, and thus the intelligibility of the speech is degraded. By contrast,

reflections which arrive at a listener soon after the direct sound strengthen the

direct signal and enhance intelligibility. Early (arriving up to 35 ms after the

direct sound) sound waves reaching the children from the teacher enhance the

143

hearing quality for children (Whitlock and Dodd 2008). It was also reported

by him that short Reverberation Times may actually be beneficial to the

listener if the speech signal is of insufficient intensity.

4.2 REVERBERATION TIME

The traditional parameter which has been studied in evaluating the

effect of room acoustics is Reverberation Time (RT). Reverberation Time is

defined as the time (in seconds) it takes for the sound from a source to

decrease in level by 60 dB (Sabine 1964) after the source has stopped. A

decrease of 60 dB represents a reduction of 1/1,000,000 of the original

intensity of the sound. A formula to calculate RT was described in 1964 by

Sabine and famously referred to as Sabine formula (Sabine 1964):

60

0.161VRT

S( )= S a (4.1)

where RT60 = RT in seconds (The suffix 60 indicates a decay of sound 60

dB), (0.161 is a constant if room volume is stated in meter3), V = room

volume in m3 and S = the surface area in m

2 of the various materials in the

room including the students absorption area; α = respective absorption

coefficients at a given frequency. From the RT formula described above, it

can be seen that there are two basic factors that affect the RT in a room. The

first is the room volume. The larger the room volume, the longer the RT will

be. The second variable is the amount of sound absorption in the room. The

greater the area of such materials, the shorter the RT. Room reverberation

varies as a function of frequency and, therefore, may need to be measured at

discrete frequencies. RT is often reported as the mean decay time at 500, 1000

and 2000 Hz. This average describes the characteristics of most rooms fairly

well, because most materials do not absorb low frequencies well, room

reverberation is shorter at higher frequencies and longer in lower frequency

144

regions. It is recommended that RT be measured at discrete frequencies from

125 to 8000 Hz, whenever excessive reverberation seems to interfere with

communication. Such information could significantly aid the audiologist in

determining the appropriate degree and type of absorptive materials needed

for a reduction of RT in that environment.

The Early Decay Time (EDT) parameter is the Reverberation Time,

measured over the decay of the first 10 dB. The EDT is also used instead of

RT in Speech Intelligibility predictions and it can be represented as RT10.

RT20 is the Reverberation Time of the room evaluated over a 20 dB decay

range (from -5 to -25 dB). It is the time distance between the -5 dB and the -

25 dB. RT30 is the reverberation time of the room evaluated over a 30 dB

decay range (from -5 to -35 dB). RT 20 and RT 30 can be evaluated from the

decay plot using regression analysis. The decay plot denotes the reduction in

sound level from 0 to - 60 and hence the decay ranges are shown as negative.

The RT for classrooms are mostly stipulated by standards in the

unoccupied conditions of classes as seen in Table 2.7, Chapter 2 and it

depends on the size of the classrooms, as RT is directly proportional to the

volume of the enclosures and inversely proportional to the absorbing areas.

The effects of reverberation can easily be felt in a completely empty room

when there is nothing to absorb the sound. Such rooms have long RT’s and

are often described as ‘echoy’ or ‘hollow’. The addition of appropriate

amount of absorbing material into a room removes this effect and lowers RT.

WHO (1999) and ANSI S12.6 (2002) recommend the RT of school

classrooms to be 0.6 s for classrooms having volumes less than 283 m3 and,

for volumes up to double this value, up to 0.7 s. The RT is for the unoccupied

condition of the class in most of the regulations. In the UK the Building

Bulletin (2003) has proposed RT less than 0.6 s for primary school

classrooms and, for secondary schools, up to 0.8 s, but for the hearing

145

impaired a lower RT of 0.4s is suggested. The American Speech Language

Hearing Association (1995) has proposed a value of less than 0.4 s and the

British Association of Teachers of the Deaf (BATOD 2001) has suggested a

RT of less than 0.4 s. The (AS/NZ 2000), the New Zealand standards suggest

RT of 0.4 s - 0.5 s for primary school teaching spaces. The Indian National

Building Code (NBC 2005) proposed a value of 0.75 s for an occupied class

and a higher value of 1.25 s for the empty classroom.

4.3 MEASUREMENT OF REVERBERATION TIME

A two channel Bruel and Kjaer (B&K) 2250 as seen in Figures 4.1a

and b, modular real time sound analyzer has been used to measure

Reverberation Time inside occupied and empty classrooms. Figure 4.1a

shows the sound level metre used in the hand held position without a tri-pod

in occupied class as the use of a tri-pod disturbs the normal class functioning.

Figure 4.1b shows the sound level metre placed on the tri-pod used for

measuring unoccupied class. It automatically calculates the mean

Reverberation Time for each frequency. The measurements were taken

following the specifications of the ISO 3382 (1997). RT measurements were

taken at three different points R1, R2 and R3 in the classroom as shown in

Figure 4.2. Three readings were taken at each point (Figures 4.3, 4.4 and 4.5)

and the mean was calculated.

The measurements were then transferred to a computer using

Qualifier 7830 software from Bruel and Kjaer, which calculated the mean

Reverberation Time for each frequency. This procedure was repeated for all

the classrooms in which RT was measured. The impact sound signal was

produced by clapping of hands and the decay was recorded which gave the

Reverberation Time.

146

(a) (b)

Figure 4.1 (a) Bruel and Kjaer (BK 2250) equipment used for

measuring Reverberation Time (b) BK 2250 placed on a tri-

pod to measure RT in unoccupied classroom

Measured data have been compared with reference values found in

the standards ANS1 S12.60 (2002) and DIN 18041 (2004), NBC (2005). The

classrooms measured had the following internal finishes--floor and ceiling

were concrete, finished with cement plaster and painted. The four walls were

of brick with cement plaster and painted. The seats in the classroom were

mostly wood painted or varnished with some classrooms having metal painted

or plastic seats. Doors and windows provided were kept open and so the

absorption coefficients corresponding to the openings were provided.

147

* *

R3 R1

*R2

Figure 4.2 RT measured in three positions R1, R2 and R3 in both

occupied and unoccupied classrooms

Figure 4.3 Right Side Position of RT measurement at R1

Figure 4.4 RT measured in the centre position R2 of the classroom

148

RT was measured using BK 2250 by producing a sufficiently loud

impulse noise and as the sound dies away the trace on the level recorder will

show a distinct slope. Analysis of this slope reveals the measured

Reverberation Time. As BK 2250 is a modern digital sound level meter it can

carry out this analysis automatically. The loud impulse noise was created by

clapping of hands and a few seconds were given to record the trace of the

sound dying. The recorded value gave the RT in seconds for various frequencies.

Figure 4.5 Left side position of RT measurement at R3

4.3.1 Reverberation Time Measured in Unoccupied class

Reverberation Time was measured in furnished unoccupied

classrooms which had a volume and a seat capacity as ranging in the

Table 3.1, 3.2 and 3.3 of Chapter 3, for the three sites of Noisy, Housing and

Quiet. RT is a function of the volume and the internal characteristics of the

room and is not influenced by the zone in which the school is located, so the

discussion of RT is made in total and not as per the zones. In unoccupied

149

classroom the absorption is by the room interior surfaces and by the furniture

present in the class. The absorption of sound by the children is absent and so

the RT for unoccupied class would be more than the RT for occupied class.

4.3.2 Reverberation Time Measured in Occupied Class

When the students have occupied the class and class was ongoing

the RT was measured. The students were made to sit quietly so that the

impact sound created was heard clearly. The RT in occupied condition would

be less than that in unoccupied class due to student absorption. Table 4.1

shows the unoccupied and occupied values of RT measured using BK 2250

and as a sample, only 30 classrooms measured are shown.

Table 4.1 Measured RT in seconds for classrooms

Classrooms 125Hz 250Hz 500Hz 1000Hz 2000Hz 4000Hz 8000Hz

1 Unoccupied 1.7 1.8 1.7 1.7 1.6 1.6 1.5

Occupied 1.1 0.8 0.6 0.6 0.5 0.5 0.5

2 Unoccupied 1.5 1.7 1.8 1.7 1.7 1.7 1.7

Occupied 1.0 0.8 0.6 0.5 0.5 0.5 0.5

3 Unoccupied 1.6 1.6 1.6 1.6 1.5 1/4 1.4

Occupied 1.4 1.3 1.3 1.0 0.7 0.7 0.8

4 Unoccupied 1.5 1.6 1.5 1.4 1.4 1.3 1.1

Occupied 1.1 1.1 0.8 0.7 0.6 0.6 0.6

5 Unoccupied 1.6 1.6 1.5 1.4 1.3 1.2 1.2

Occupied 1.3 1.0 0.7 0.8 0.8 0.8 0.8

6 Unoccupied 1.6 1.6 1.7 1.6 1.6 1.5 1.2

Occupied 1.2 1.2 0.9 0.6 0.6 0.7 0.7

7 Unoccupied 1.5 1.7 1.6 1.6 1.7 1.7 1.7

Occupied 0.8 0.5 0.5 0.5 0.5 0.5 0.4

8 Unoccupied 1.1 1.1 1.1 1.1 1.0 0.9 0.9

Occupied 0.9 1.0 0.8 0.6 0.6 0.7 0.7

9 Unoccupied 1.6 1.6 1.6 1.5 1.4 1.3 1.2

Occupied 1.4 1.4 1.2 0.9 0.9 0.9 0.9

10 Unoccupied 1.5 1.4 1.3 1.2 1.1 1.1 1.1

Occupied 1.1 0.8 0.6 0.6 0.7 0.6 0.6

11 Unoccupied 1.8 1.8 1.8 1.6 1.5 1.4 1.2

Occupied 1.5 1.5 1.2 0.9 0,9 0.9 0.9

150

Table 4.1 (Continued)

Classrooms 125Hz 250Hz 500Hz 1000Hz 2000Hz 4000Hz 8000Hz

12 Unoccupied 1.3 1,4 1,3 1.3 1.2 1,2 1.2

Occupied 1.0 1.0 0.7 0.5 0.5 0.6 0.5

13 Unoccupied 1.1 1.2 1.2 1.2 1.1 1.1 1.0

Occupied 0.9 0.9 0.9 0.7 0.6 0.5 0.5

14 Unoccupied 1.4 1.6 1.5 1.4 1.4 1.3 1.1

Occupied 1.1 1.1 0.9 0.5 0.5 0.6 0.5

15 Unoccupied 1.4 1.5 1.4 1.4 1.3 1.3 1.1

Occupied 1.2 1.2 0.9 0.7 0.7 0.8 0.7

16 Unoccupied 1.8 1.9 1.9 1.8 1.9 1.8 1.6

Occupied 1.0 0.6 1.5 0.4 0.4 0.4 0.3

17 Unoccupied 1.8 1.8 1.7 1.7 1.7 1.6 1.3

Occupied 1.1 0.8 0.5 0.5 0.5 0.6 0.5

18 Unoccupied 1.5 1.4 1.4 1.3 1.3 1.4 1.4

Occupied 1.1 0.9 0.6 0.6 0.7 0.6 0.6

19 Unoccupied 1.8 1.9 1.9 1.9 1.8 1.9 1.8

Occupied 1.0 0.6 0.6 0.5 0.4 0.4 0.4

20 Unoccupied 1.6 1.7 1.7 1.6 1.6 1.5 1.3

Occupied 1.2 1.2 0.9 0.6 0.6 0.6 0.6

21 Unoccupied 1.4 1.5 1.4 1.4 1.3 1.3 1.1

Occupied 1.1 1.1 1.8 0.6 0.6 0.7 0.6

22 Unoccupied 1.1 1.2 1.2 1.1 1.1 1.1 1.0.

Occupied 1.0 1.0 0.8 0.6 0.6 0.7 0.6

23 Unoccupied 1.6 1.6 1.5 1.4 1.4 1.3 1.2

Occupied 1.0 0.7 0.6 0.5 0.5 0.5 0.4

24 Unoccupied 1.0 1.0 1.0 0.9 0.9 0.9 0.8

Occupied 0.8 0.8 0.7 0.5 0.5 0.6 0.5

25 Unoccupied 1.7 1.6 1.5 1.4 1.4 1.2 1.2

Occupied 1.3 1.0 0.7 0.7 0.8 0.7 0.7

26 Unoccupied 0.7 0.8 0.8 0.7 0.7 0.7 0.7

Occupied 0.7 0.7 0.6 0.5 0.5 0.6 0.5

27 Unoccupied 1.1 1.2 1.1 1.1 1.1 1.0 1.0

Occupied 1.0 1.0 0.8 0.6 0.6 0.6 0.6

28 Unoccupied 1.0 1.0 1.0 1.0 0.9 0.9 0.8

Occupied 0.8 0.8 0.6 0.5 0.5 0.5 0.5

29 Unoccupied 0.9 0.9 0.9 0.9 0.9 0.8 0.7

Occupied 0.7 0.7 0.6 0.5 0.4 0.5 0.4

30 Unoccupied 1.0 1.1 1.1 1.0 1.0 0.9 0.8

Occupied 0.9 0.9 0.7 0.5 0.5 0.5 0.5

151

Some photographs showing the RT measured in unoccupied

condition of the classroom and also the same in the occupied condition are

shown below. The Figure 4.6a shows the unoccupied classroom ‘a’ measured

with the Sound level metre on a tri-pod. The classroom ‘a’ was measured

empty but fully furnished. The Figure 4.6b shows the classroom ‘a’ with

students measured using the Sound level meter in the hand held position as

the classroom was occupied. Similarly classroom ‘b’ in Figure 4.7a shows the

empty, furnished classroom measured and the same classroom ‘b’ measured

with students is shown in Figure 4.7b. Figure 4.8a shows the measurement

made and the impact sound created by clapping in occupied classroom ‘c’.

The classroom ‘c’ in Figure 4.8 b showed furnished unoccupied class

measured using tri-pod.

Figure 4.6a RT measured in unoccupied condition of classroom ‘a’

152

Figure 4.6b RT measured in occupied condition of classroom ‘a’ and the

impact noise created by clapping

Figure 4.7a RT measured in unoccupied condition of classroom ‘b’

153

Figure 4.7b RT measured in occupied condition of classroom ‘b’

Figure 4.8a RT measurement in occupied classroom ‘c’

154

Figure 4.8b RT measured in unoccupied classroom ‘c’

4.3.3 Calculation of Absorption Coefficient of Children

The Reverberation Time in a classroom is influenced by the

absorption offered by the surfaces. The absorption of the surfaces depends on

the texture hardness and other parameters of the surfaces. The absorption

coefficients (α) are important parameters in determining the Reverberation

Time of classrooms. The absorption coefficient of a material is a number

between 0 and 1 which indicates the proportion of sound which is absorbed

by the surface compared to the proportion which is reflected back into the

room. A large, fully open window would offer no reflection as any sound

reaching it would pass straight out and no sound would be reflected. This

would have an absorption coefficient of 1. Conversely, a thick, smooth

painted concrete ceiling would be the acoustic equivalent of a mirror and

would have an absorption coefficient very close to 0. Though the absorption

coefficients of various materials are known and are shown in, Table A1.1, the

absorption coefficients for the occupant children depends on the clothing

whether light or thick. Especially for the clothing used by children of tropical

155

warm-humid climates, the absorption coefficients given in the literature may

not suit the values available in literature. Therefore the absorption coefficients

of students were determined by the procedure followed by Sato and Bradley

(2008) and by Kousaie and Hodgson (2002) which used the Sabine’s

Equation (4.1). The occupied RT and the unoccupied RT measured for each

frequency were substituted in the equation to calculate absorption coefficients

and the calculated values were tabulated in Table 4.2.

Table 4.2 The Absorption coefficients of Children

No. 125Hz 250Hz 500Hz 1000Hz 2000Hz 4000Hz 8000Hz

1 0.05 0.08 0.22 0.49 0.39 0.2 0.34

2 0.06 0.13 0.2 0.47 0.38 0.29 0.32

3 0.07 0.14 0.2 0.48 0.43 0.3 0.27

4 0.08 0.09 0.25 0.45 0.42 0.3 0.34

5 0.09 0.07 0.24 0.46 0.41 0.29 0.32

6 0.07 0.11 0.25 0.47 0.40 0.30 0.31

7 0.08 0.12 0.23 0.46 0.42 0.31 0.32

Average 0.075 0.106 0.227 0.468 0.407 0.284 0.317

4.3.4 Discussion

From the Table 4.1 it is seen that the values of RT in occupied

classroom would be less than that in unoccupied classroom and is also shown

in Figure 4.9. This is due to the absorption of sound by the occupants in the

occupied class. For the initial frequencies of 125 Hz and 250 Hz the values of

RT are slightly higher and decrease towards higher frequencies. This is due to

the absorption coefficients of the materials used in the classroom being lesser

for lower frequencies. Many of the materials have higher absorption in higher

frequencies. Analysing the RT values at mid frequency (1K Hz) as shown in

156

Figure 4.10, unoccupied classes have RT more than 1 s in about 90 % of

classes and RT in occupied class was about 0.6 s.

Figure 4.9 RT in octave frequencies for a typical classroom

Figure 4.10 Measured RT at 1 kHz in occupied and unoccupied condition

The international standards (Table 2.7 of Chapter 2) specify a value

of 0.6 s for unoccupied conditions, in that way the RT in all classes measured

157

is above the stipulated value, the average value being 1.36 s. The average RT

in occupied condition is 0.6 s. Compared with the values stipulated in NBC

(2005), the RT value for 500 Hz in unoccupied class should be 1.25 s whereas

the average of measured values for the RT in 500 Hz is 1.42 s. For occupied

conditions value according to NBC (2005) is 0.75 s whereas the average of

the measured values is 0.78 s. Thus the RT values in the classrooms in

occupied and unoccupied conditions are more than the international standards

and national standard NBC (2005).

4.4 CALCULATION OF REVERBERATION TIME

The calculation of Reverberation Time is done using a software

ClassTalk (Hodgson and Graves 2009) which calculates and displays the RT

in the unoccupied (i.e., as designed by the architect) and occupied (i.e., as

experienced by the occupants) classroom and is capable of evaluating the

Speech Intelligibility parameters.

4.4.1 ClassTalk Software Description

ClassTalk (University of British Columbia,Vancouver,Canada:

www.flintbox.ca), a classroom Speech Intelligibility prediction tool,

applicable to typical classrooms is a novel simple, fast, accurate and

interactive hardware and software system used for modelling, predicting and

visualizing speech in noise in classrooms. Modelling involves defining the

classroom geometry, sources, sound absorbing features and receiver positions.

Empirical models, used to predict speech and noise levels and Reverberation

Times, are described in the software. Surface absorption coefficients were

assigned according to the material properties.

158

ClassTalk visualizes the floor plan on the monitor along with a 1m

receiver grid, the position of speech source that is the human talker indicated

by a stick-figure icon and an ear indicating the receiver position. When the

input characteristics of a classroom, its description, average dimensions,

sound absorbing features, the number of occupants, and the source (human

talker or speaker) characteristics are defined, occupied and unoccupied

Reverberation Times (RT) and other quantities for Speech Intelligibility are

calculated. The input data required for ClassTalk are given in Table 4.3. The

inputs are to be entered as the case may be; if a particular case is not

applicable for a classroom then the value entered can be zero. A detailed input

specifying each line is given in Table A1.2.

4.4.1.1 Input data

The input characteristics of the classroom that must be entered are

its description, average dimensions, sound-absorbing features, the number of

occupants, the co-ordinates and output levels of the sources, and a constant

background-noise level. The constant background-noise level denoted as BN

is referred as BGNA (A-weighted background-noise) in ClassTalk. The

description is entered as three, 40-character lines of text. Sound-absorbing

features include hard (i.e., concrete with cement plaster) floor and ceiling

surfaces, cement plaster brick walls, and window openings, hard (wooden,

steel or plastic) seats - as well as the occupants, which absorb sound. Here the

walls, floors, ceiling do not have any acoustical treatment. Human-talker male

or female speakers should be specified with their speech levels. Some classes

were treated with carpet on the floor and curtain on the wall and the RT and

SI were evaluated.

159

Table 4.3 Input data for ClassTalk

Dimensions of classroom: length, width, height in meters

Carpeted Floor Area (if carpeted then area to be entered) in sq m

Surface Area of Hard floor (if floor not carpeted then the concrete floor area

and ceiling area should be given together) in sq m

The total opening area (window and door area ) in sq m

Surface area of panelled surface of the wall ( if the wall is covered with

panels of wood or any other material like porous absorber or acoustic tiles )

in sq m .

Volume of the class should be calculated and given in m3

Ceiling area ( tiles or suspended acoustic ceiling or any other material used)

in sq m

User defined area ( any material which is preferred to be used for the walls

or ceiling can be defined) in sq m

Number of occupants in the class

Number of seats in the class

Constant background-noise level

Occupant Absorption Co-efficient

125Hz .075

250Hz .106

500Hz .227

1000Hz .468

2000Hz .407

4000Hz .284

8000Hz .317

Carpeted Absorption Co-efficient

125Hz .2

250Hz .45

500Hz .67

1000Hz .81

2000Hz .82

4000Hz .83

8000Hz 1.14

160


Hard Surface (Concrete) Absorption Co-efficient

125Hz .059

250Hz .067

500Hz .087

1000Hz .087

2000Hz .083

4000Hz .074

8000Hz .071

Window Absorption Co-efficient (absorption of openings)

125Hz 1.0

250Hz 1.0

500Hz 1.0

1000Hz 1.0

2000Hz 1.0

4000Hz 1.0

8000Hz 1.0

Wall Absorption Co-efficient( cement plaster on brick)

125Hz .013

250Hz .015

500Hz .02

1000Hz .03

2000Hz .04

4000Hz .05

8000Hz .07

Upholstered / Non-upholstered seat absorption

Wooden seat absorption Co-efficient( all the classrooms had non-

upholstered seat)

125Hz .118

250Hz .093

500Hz .083

1000Hz .085

2000Hz .081

4000Hz .086

8000Hz .131

161


Speech source: male or female talker, output levels defined in four ranges

as quiet, normal, raised and loud voice levels

Other noise sources: fan, slide projector and overhead projector was defined

and values specified

Source direction has to be specified

Source position has to be defined by giving x, y, z co-ordinates

The quantity to be plotted as contour has to be selected- SI, STI , SNA,

SLA, BGNA, at a particular time one quantity can be selected and the

contour line width, contour interval width, contour offset distance has to be

specified

Position of student where speech-intelligibility is required

4.4.1.2 Calculated values of RT

When the input for ClassTalk is given the Reverberation Time is

calculated and displayed for each of the classroom. The calculated

Reverberation Time for 120 classrooms are tabulated and given in Table 4.4

for the occupied condition and unoccupied condition of the classroom. As it

was discussed earlier, in unoccupied condition the RT would be more as the

absorption of sound by the children are absent in the unoccupied classroom.

The RT is calculated for different frequencies. In Table 4.4, the RT for each

frequency is shown side by side to compare the values in the unoccupied and

occupied conditions of the classrooms.

162

Table 4.4 Calculated RT in seconds for 120 classrooms

No.125 Hz 250 Hz 500 Hz 1000Hz 2000Hz 4000 Hz 8000 Hz

Unoccupied Occupied Unoccupied occupied Unoccupied occupied unoccupied occupied unoccupied occupied unoccupied occupied unoccupied occupied

1 1.9 1.6 2 1.6 1.9 1.2 1.8 0.8 1.7 0.8 1.6 0.9 1.4 0.8

2 1.7 1.1 1.8 0.8 1.7 0.6 1.7 0.5 1.8 0.5 1.8 0.5 1.5 0.4

3 1.5 1 1.7 0.8 1.8 0.6 1.7 0.5 1.7 0.5 1.7 0.5 1.4 0.4

4 1.5 1.3 1 0.8 0.7 0.6 0.8 0.5 1 0.6 1.3 0.8 1.1 0.7

5 1.2 1.1 0.6 0.6 0.4 0.4 0.5 0.4 0.7 0.5 1 0.7 0.9 0.6

6 1.6 1.4 1.6 1.3 1.6 1.1 1.4 0.8 1.3 0.8 1.2 0.8 1.1 0.8

7 1.6 1.4 1.6 1.3 1.6 1.1 1.4 0.8 1.3 0.8 1.2 0.8 1.1 0.8

8 1.5 1.1 1.6 1.1 1.5 0.8 1.4 0.5 1.4 0.6 1.3 0.6 1.1 0.6

9 1.5 1.3 1.6 1.3 1.6 1 1.5 0.7 1.4 0.8 1.3 0.8 1.2 0.8

10 1.5 1.2 1.6 1.2 1.5 0.8 1.4 0.6 1.4 0.6 1.3 0.7 1.1 0.6

11 1.6 1.3 1.6 1.3 1.6 1 1.5 0.7 1.4 0.7 1.4 0.8 1.2 0.7

12 1.5 0.8 1.7 0.5 1.6 0.4 1.6 0.3 1.7 0.3 1.7 0.3 1.3 0.3

13 1.6 1.3 1.8 1.3 1.7 0.9 1.6 0.6 1.6 0.6 1.5 0.7 1.2 0.6

14 1 0.8 1.1 0.8 1 0.6 1 0.4 1 0.5 0.9 0.5 0.8 0.5

15 1.6 1.2 1.7 1 1.6 0.8 1.6 0.7 1.7 0.7 1.7 0.7 1.4 0.5

16 1 0.8 1.1 0.8 1 0.7 1 0.5 1 0.5 0.9 0.5 0.8 0.5

17 1.8 1.6 1.9 1.6 1.8 1.3 1.6 0.9 1.6 1 1.5 1 1.3 0.9

18 1.8 1.6 1.9 1.6 1.8 1.3 1.6 0.9 1.6 1 1.5 1 1.3 0.9

19 1.5 1.4 1.6 1.4 1.5 1.2 1.4 0.9 1.3 0.9 1.2 0.9 1 0.8

20 1.5 1.3 1.6 1.4 1.6 1.1 1.4 0.9 1.4 0.9 1.2 0.9 1.1 0.8

21 1.1 0.9 1.1 1 1.1 0.8 1.1 0.6 1 0.6 0.9 0.7 0.8 0.6

22 1.6 1.4 1.6 1.4 1.6 1.2 1.5 0.9 1.4 0.9 1.3 0.9 1.2 0.9

23 1.6 1.4 1.6 1.4 1.6 1.2 1.5 0.9 1.4 0.9 1.3 1 1.2 0.9

24 1.5 1.2 1.5 1.1 1.4 0.8 1.3 0.6 1.2 0.6 1.1 0.7 1.1 0.6

163




25 1.2 1 1.2 0.9 1.2 0.7 1.1 0.5 1 0.5 1 0.6 0.9 0.5

26 1.8 1.5 1.8 1.5 1.8 1.2 1.6 0.9 1.5 0.9 1.4 0.9 1.2 0.9

27 1.3 1 1.4 1 1.3 0.7 1.3 0.5 1.2 0.5 1.2 0.6 1 0.5

28 1.5 1.1 1.5 1 1.4 0.7 1.3 0.5 1.2 0.5 1.1 0.6 1.1 0.6

29 1.1 0.9 1.2 0.9 1.2 0.7 1.2 0.5 1.2 0.5 1.1 0.6 0.9 0.5

30 1.2 0.9 1.3 0.9 1.2 0.7 1.2 0.5 1.2 0.5 1.1 0.6 0.9 0.5

31 1.2 0.9 1.3 0.9 1.2 0.7 1.2 0.5 1.2 0.5 1.1 0.6 0.9 0.5

32 1.4 1.1 1.6 1.1 1.5 0.7 1.4 0.5 1.4 0.5 1.3 0.6 1.1 0.5

33 1.4 1.1 1.5 1.1 1.5 0.8 1.4 0.5 1.3 0.5 1.2 0.6 1.1 0.6

34 1.4 1.2 1.5 1.2 1.4 0.9 1.4 0.7 1.3 0.7 1.3 0.8 1.1 0.7

35 1.2 1 1.3 1.1 1.3 0.9 1.3 0.6 1.3 0.7 1.2 0.7 1 0.7

36 1.2 1.1 1.3 1.1 1.3 0.9 1.3 0.7 1.3 0.7 1.2 0.8 1 0.7

37 1.4 1.2 1.5 1.3 1.4 1.1 1.4 0.8 1.3 0.8 1.3 0.9 1.1 0.8

38 1.4 1.1 1.5 1.1 1.4 0.9 1.4 0.6 1.3 0.6 1.3 0.7 1.1 0.6

39 1.8 1 1.9 0.6 1.9 0.5 1.8 0.4 1.9 0.4 1.8 0.4 1.6 0.3

40 1.6 1.2 1.7 1.2 1.7 0.9 1.6 0.6 1.6 0.6 1.5 1.7 1.3 0.6

41 1.6 1.1 1.8 1.1 1.8 0.8 1.7 0.5 1.7 0.5 1.6 0.6 1.3 0.5

42 1.6 1.2 1.7 1.2 1.7 0.9 1.6 0.6 1.6 0.6 1.5 0.7 1.3 0.7

43 2.1 1.7 2.2 1.7 2.2 1.3 2 0.8 1.8 0.9 1.6 0.9 1.4 0.8

44 1.6 1.4 1.8 1.4 1.7 1.1 1.6 0.8 1.6 0.8 1.4 0.9 1.2 0.8

45 1.6 1.4 1.8 1.5 1.7 1.2 1.6 0.9 1.6 0.9 1.5 0.9 1.2 0.8

46 1.4 1.1 1.5 1 1.4 0.7 1.4 0.5 1.3 0.5 1.3 0.6 1.1 0.5

47 1.4 1.1 1.5 1.1 1.4 0.8 1.4 0.6 1.3 0.6 1.3 0.7 1.1 0.6

48 1.8 1.4 1.9 1.4 1.8 1 1.7 0.7 1.6 0.7 1.5 0.8 1.3 0.7

164




49 1.8 1.4 1.9 1.4 1.8 1 1.7 0.7 1.6 0.7 1.5 0.8 1.3 0.8

50 1.8 1.4 1.9 1.4 1.8 1.1 1.7 0.7 1.6 0.8 1.5 0.9 1.3 0.8

51 1.4 1.1 1.5 1.1 1.5 0.8 1.4 0.5 1.3 0.6 1.3 0.6 1.1 0.6

52 1.8 1.5 1.9 1.5 1.8 1.1 1.7 0.8 1.6 0.8 1.5 0.9 1.3 0.8

53 1.1 1 1.2 1 1.2 0.8 1.1 0.6 1.1 0.6 1.1 0.7 0.9 0.6

54 0.6 0.5 0.6 0.6 0.6 0.5 0.6 0.4 0.6 0.4 0.6 0.5 0.6 0.4

55 1.6 1 1.6 0.7 1.5 0.6 1.4 0.5 1.4 0.5 1.3 0.5 1.2 0.4

56 1.2 1.1 1.3 1.1 1.3 0.9 1.2 0.6 1.1 0.6 1.1 0.7 1 0.6

57 1 0.8 1 0.8 1 0.7 0.9 0.5 0.9 0.5 0.9 0.6 0.8 0.5

58 1.6 1.3 1.7 1.3 1.6 1 1.5 0.7 1.4 0.7 1.4 0.8 1.2 0.7

59 1.5 1.3 1.6 1.3 1.6 0.9 1.5 0.6 1.4 0.7 1.4 0.8 1.2 0.7

60 1.6 1.3 1.7 1.3 1.7 0.9 1.6 0.6 1.5 0.7 1.4 0.8 1.2 0.7

61 1.6 1.4 1.7 1.4 1.6 1.1 1.5 0.8 1.4 0.9 1.4 0.9 1.2 0.8

62 1.7 1.5 1.8 1.5 1.7 1.2 1.6 0.9 1.5 0.9 1.4 1 1.3 0.9

63 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.6 0.7 0.6 0.7 0.6 0.6 0.6

64 0.8 0.7 0.8 0.7 0.8 0.6 0.8 0.5 0.8 0.5 0.8 0.6 0.7 0.5

65 0.8 0.7 0.8 0.7 0.8 0.6 0.8 0.5 0.8 0.5 0.8 0.6 0.7 0.5

66 0.7 0.7 0.8 0.7 0.8 0.6 0.7 0.5 0.7 0.5 0.7 0.6 0.7 0.5

67 0.9 0.7 0.9 0.7 0.9 0.6 0.9 0.4 0.9 0.4 0.8 0.5 0.7 0.4

68 1.1 0.9 1.2 0.9 1.1 0.7 1.1 0.5 1.1 0.5 1 0.6 0.9 0.5

69 1 0.9 1.1 0.9 1.1 0.7 1 0.5 1 0.5 0.9 0.6 0.8 0.5

70 0.7 0.7 0.8 0.7 0.7 0.6 0.7 0.5 0.7 0.5 0.7 0.6 0.6 0.5

71 1.1 1 1.2 1 1.1 0.8 1.1 0.6 1 0.6 1 0.6 0.9 0.6

72 1.1 1 1.1 1 1.1 0.8 1 0.6 1 0.6 0.9 0.7 0.8 0.6

165




73 1.1 0.9 1.1 0.9 1.1 0.7 1 0.6 1 0.6 0.9 0.6 0.8 0.6

74 1 0.8 1 0.8 1 0.6 1 0.5 0.9 0.5 0.9 0.5 0.8 0.5

75 1.1 0.9 1.1 0.9 1.1 0.7 1 0.5 1 0.6 0.9 0.6 0.8 0.6

76 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

77 0.4 0.3 0.4 0.3 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

78 0.4 0.4 0.4 0.4 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

79 0.4 0.3 0.4 0.3 0.4 0.3 0.3 0.3 0.3 0.3 0.4 0.3 0.3 0.3

80 1.1 0.9 1.2 0.9 1.2 0.7 1.1 0.5 1.1 0.5 1 0.6 0.9 0.5

81 1.6 1.3 1.6 0.3 1.6 0.1 1.4 0.8 1.4 0.8 1.3 0.9 1.2 0.8

82 1.5 1.2 1.5 1.2 1.5 0.9 1.4 0.6 1.3 0.7 1.3 0.7 1.1 0.7

83 1.5 1.2 1.5 1.2 1.5 0.9 1.4 0.7 1.3 0.7 1.3 0.7 1.1 0.7

84 1.5 1.4 1.6 1.4 1.5 1.2 1.4 0.9 1.3 0.9 1.2 0.9 1.1 0.9

85 1.5 1.4 1.6 1.4 1.5 1.2 1.4 0.9 1.3 0.9 1.3 1 1.1 0.9

86 1.3 1.1 1.5 1.2 1.5 0.9 1.4 0.7 1.4 0.7 1.4 0.8 1.1 0.7

87 1.9 1.7 2 1.7 1.9 1.3 1.8 1 1.7 1 1.6 1.1 1.4 1

88 1.5 1.2 1.6 1.2 1.6 0.9 1.5 0.6 1.5 0.6 1.4 0.7 1.2 0.6

89 1.3 1.1 1.5 1.2 1.5 1 1.4 0.7 1.4 0.7 1.3 0.8 1.1 0.7

90 1.9 1.7 2 1.7 1.9 1.4 1.8 1.1 1.7 1.1 1.6 1.1 1.4 1

91 1.9 1.6 2 1.7 1.9 1.4 1.8 1 1.7 1.1 1.5 1.1 1.3 0.9

92 0.7 0.7 0.8 0.7 0.7 0.6 0.7 0.5 0.7 0.5 0.7 0.5 0.6 0.5

93 0.9 0.9 1 0.9 0.9 0.8 0.9 0.7 0.8 0.6 0.8 0.7 0.7 0.6

94 1 0.8 1.1 0.9 1.1 0.7 1 0.5 1 0.5 1 0.6 0.8 0.5

95 1.9 1.7 2 1.7 2 1.5 1.8 1.1 1.7 1.1 1.5 1.1 1.3 1

96 1.8 1.4 1.9 1.4 1.9 1 1.7 0.6 1.7 0.7 1.6 0.8 1.3 0.7

166




97 1.8 1.4 1.9 1.4 1.9 1 1.7 0.6 1.7 0.7 1.6 0.8 1.3 0.7

98 1.9 1.5 2 1.5 1.9 1.1 1.8 0.8 1.7 0.8 1.6 0.9 1.3 0.8

99 1.8 1.4 2 1.4 1.9 1 1.7 0.6 1.7 0.7 1.6 0.8 1.3 0.7

100 1 0.9 1 0.9 1 0.8 0.9 0.6 0.9 0.6 0.9 0.7 0.8 0.6

101 1.6 1.4 1.6 1.4 1.6 1.2 1.5 0.9 1.4 0.9 1.3 0.9 1.2 0.9

102 1.9 1.7 2.1 1.7 1.9 1.3 1.8 1 1.7 1 1.6 1.1 1.4 0.9

103 1.2 1.1 1.3 1.1 1.2 0.9 1.2 0.7 1.1 0.7 1.1 0.7 1 0.7

104 1.2 1 1.3 1 1.2 0.8 1.2 0.6 1.1 0.6 1.1 0.7 1 0.6

105 0.4 0.4 0.4 0.4 0.4 0.3 0.4 0.3 0.3 0.3 0.4 0.3 0.3 0.3

106 0.4 0.4 0.4 0.4 0.4 0.3 0.4 0.3 0.3 0.3 0.4 0.3 0.3 0.3

107 1.8 1.7 1.8 1.7 1.7 1.5 1.6 1.3 1.5 1.3 1.5 1.3 1.4 1.2

108 1.2 1.1 1.2 1.1 1.2 0.9 1.1 0.7 1.1 0.7 1 0.8 1 0.7

109 1.5 1.3 1.5 1.3 1.5 1.1 1.4 0.8 1.3 0.9 1.2 0.9 1.1 0.8

110 1.5 1.3 1.5 1.3 1.4 1.1 1.4 0.8 1.3 0.8 1.2 0.9 1.1 0.8

111 1.2 1 1.2 1 1.2 0.8 1.1 0.6 1.1 0.6 1 0.7 0.9 0.6

112 1.2 1 1.3 1 1.2 0.8 1.2 0.6 1.1 0.6 1.1 0.7 0.9 0.6

113 1.6 1.4 1.7 1.4 1.6 1.1 1.5 0.8 1.5 0.9 1.4 0.9 1.3 0.9

114 1.6 1.4 1.7 1.5 1.7 1.2 1.6 0.9 1.5 0.9 1.5 1 1.3 0.9

115 1.3 1.1 1.4 1.2 1.3 0.9 1.3 0.7 1.3 0.7 1.2 0.8 1.1 0.7

116 1.7 1.5 1.8 1.5 1.8 1.2 1.7 0.9 1.6 1.1 1.6 1.1 1.4 1

117 0.9 0.9 0.9 0.9 0.9 0.8 0.9 0.7 0.8 0.7 0.8 0.7 0.8 0.7

118 1.6 1.4 1.7 1.4 1.6 1.2 1.5 0.9 1.4 0.9 1.3 1 1.2 0.9

119 1.6 1.4 1.7 1.4 1.6 1.1 1.5 0.8 1.4 0.9 1.3 0.9 1.2 0.8

120 1.6 1.4 1.7 1.4 1.6 1.1 1.5 0.8 1.4 0.8 1.3 0.9 1.2 0.8

167

From the Table 4.4, the calculated values of unoccupied and

occupied RT for the 120 classrooms are graphically represented in Figure

4.11a and Figure 4.11b. Figure 4.11a gives RT’s for occupied classrooms at

mid-frequency 1000 Hz, and shows that in 45 % of classrooms the RT values

are 0.5 s and 0.6 s and in another 40 % of classrooms the RT values are 0.7 to

0.9 s. In the rest of the classrooms it is 0.3 s and 0.4 s, and in very few it is

1.0 s. The RT values in the unoccupied condition, as is expected, are higher.

The absorption by the children would contribute to the increase in the total

absorption area and thus the resulting RT values in the occupied classrooms

are always lower than those in unoccupied conditions. Figure 4.11b shows

that the RT in unoccupied conditions in 64% of classrooms varies from 1.4 s

to 1.8 s, in 20 % of classrooms from 1.0 to 1.3 s and in 16 % of classrooms,

from 0.5 to 0.9 s. In one classroom alone RT is 2.0 s. All the classrooms are

with walls of hard surfaces and are acoustically untreated. Figure 4.12 shows

RT for all 120 occupied and unoccupied classrooms at mid-frequency (1000

Hz). However, the variation of RT for octave range is shown in Figure 4.13

for a typical classroom.

Figure 4.11a Reverberation Time at mid frequency (1000 Hz) in

occupied classrooms

168

Figure 4.11b Reverberation Time at mid frequency (1000 Hz) in

unoccupied classrooms

Figure 4.12 RT in 120 classrooms in occupied and unoccupied conditions

169

Figure 4.13 Calculated RT for a typical classroom

Table 4.4a Distribution of RT at 1 kHz in occupied classrooms

RT values % of Classrooms

1.25 and above 62.5%

0.8 to 1.25 23.2%

0.6 to 0.8 6.8%

0.6 and below 7.5%

Table 4.4b Distribution of RT at 1 kHz in unoccupied classrooms

RT values % of Classrooms

More than 1.0 5.0%

0.76 to 1.00 21.7%

0.61 to 0.75 16.7%

0.41 to 0.6 46.6%

0.4 and below 10.0%

The stipulated value of RT according to NBC 2005 in unoccupied

classrooms should be 1.25s or less at the frequency of 500 Hz. On comparison

170

with the calculated values from Table 4.4, it is seen that only in 36 % of

classes, the RT is less than 1.25 s. Similarly in the occupied classes the

permitted value of RT by NBC 2005 is 0.75. It is seen that only in 36 % of

classes the RT values are within this value. Observing the distribution of

calculated values of RT at the mid frequency 1k Hz the value in occupied and

unoccupied classes are as shown in Table 4.4 a and Table 4.4 b. For the

unoccupied class the international standards specify a mean value of RT as

0.6s for mid frequency of 1 kHz. From the calculated values as in Table 4.4 it

is seen that only 7.5 % classes come under this value. DIN standard stipulates

the RT values in occupied class can be 0.2 s below the unoccupied values. In

that case 0.4 s is the permitted value and only 10 % of the classes will come

under this stipulation.

4.4.1.3 Reverberation time (RT) measured by other researchers

In a recent study, Sato and Bradley (2008) reported that

measurements of classroom Reverberation Times were mostly in the range

between 0.4 and 1.2 s. Bradley (1986a) reported that the mean measured

Reverberation Time in 10 classrooms was 0.7 s at 1 kHz. Measurements made

in Brazilian classrooms by Losso et al (2004) reported that Reverberation

Times ranged from 1.1 to 1.7 s. However optimal values may in fact be

impossible to achieve in well occupied classrooms, since the absorption

provided by the occupants may exceed, that required for optimal

Reverberation Times. Reverberation Time measured in 32 classrooms by

Knecht et al (2002) ranged from 0.2 s to 1.27 s. Only four of these classrooms

had RT’s less than the desired value of 0.6 s. In a recent study in Brazil

(Zannin and Zwirtes 2009) RT was measured in furnished unoccupied and

occupied classrooms, which had a volume of about 139 m3 and 156 m

3 and

seat up to 40 students each and the mean RT values at 1000 Hz varied from

171

0.89 s in occupied classrooms to 2.2 s in unoccupied classrooms and the RT

values do not satisfy ANSI S12.60. However RT measurements for

classrooms with wall coverings have not been reported by researchers.

In the 120 classrooms measured/calculated in the present study, the

RT values were in the similar range as above.

4.4.1.4 Early arrival time

Several studies have been carried out to emphasize that if the early

arrival or early reflections reach the students sufficiently early, they enforce

the speech intelligibility, before the reverberation starts masking the clarity of

speech. It is established by Sato and Bradley (2008) that early arriving

reflections of speech sounds reaching the listener within 50 ms after the

arrival of direct sound are useful because they can help to increase the

effective signal to noise ratio and hence the intelligibility of the speech. A

room with shorter Reverberation Time will be lacking in early reflection

energy at positions farther from the teacher where the early reflections energy

would be most helpful to add to the weaker direct speech sound. Based on the

analysis of the measurements in the 30 classrooms, it is clear that very short

Reverberation Times should be avoided so that the room can usefully enhance

teacher voice levels and help to reduce voice strain for teachers.

Whitlock and Dodd studied (2004, 2008) the effect of early

reflection and established, by calculating the integration time of speech, that

early reflections reaching the young students within 35 ms would enhance the

Speech Intelligibility, whereas it would be 50 ms for adults. Reflections

arriving outside the integration time contribute less and less usefully and will

interfere with speech perception and hence reduce intelligibility. It is verified

that an integration time of 35 ms corresponds to a distance of 12 m for the

sound wave to travel at its speed. This is the ideal maximum path length

172

difference for any receiver between a direct sound and a fully useful

reflection, and the classroom geometry should ideally be constrained

accordingly (Whitlock and Dodd 2008). The dimensions of the classrooms

under study are in the range of 6 to 7 m in width and length, as shown in

Table 3.4. Figure 4.14 shows that in a typical classroom under study, the

sound path is less than 12 m, as stipulated in (Whitlock and Dodd 2008).

Figure 4.14 Example of ray tracing of first order7 reflection

The dimensions of the classrooms both in length and breadth are

around 6 m except for a very few out of 120 classrooms, thereby satisfying

the room dimension to satisfy the early reflection sound criteria to reach the

listeners. For a classroom in a Housing site having 5.2 m x 4.8 m plan

dimensions and 3 m ceiling height, a direct ray of 3.94 m and a reflected ray

of 5.5 m small enough for early reflections to reach the listeners well within

35 ms. Even if the dimensions are 8 m x 8 m as shown in Whitlock and Dodd

(2008), the students in the classrooms in this study will hear the teacher’s

voice enhanced by the early reflections. The RT ’s for many of the classrooms

in the occupied condition are also about 0.5 to 0.7 s. The implications for

classroom design are predominantly with regard to Reverberation Time and

the early arrival time (Whitlock and Dodd 2008). A room with a long

3.94

4.30

0.5

Source

5.20

3.44

3.94

3.01

5.20

PLAN

SECTION

173

Reverberation Time will cause the individual phonemes to become masked by

the persistence of previous phonemes and intelligibility will be degraded.

If the time over which the reflections occurs is within the

integration time of speech (ie early reflections), then the clarity of speech will

be maximised. However if the room were to have no reflections beyond 35 ms

it would subjectively have a zero RT and this may not be the optimum for

comfort or intelligibility. A room with low RT will obviously have less

unwanted reverberant sound energy which is occurring outside the integration

time. The design feature for a primary school classroom should therefore be a

low RT around 0.4 s (Whitlock and Dodd 2008). This provides the neutral

basis upon which the acoustical environment can be developed by way of

harnessing early reflections.

4.5 COMPARISON OF RT MEASURED AND CALCULATED

The measured and calculated values of RT were compared and it

was found to be in close range as shown in Table 4.5.

Table 4.5 Comparison of RT measured and calculated in seconds

Unoccupied

Occupied

Classroom

Measured/

Calculated125Hz 250Hz 500Hz 1000Hz 2000Hz 4000Hz 8000Hz

Unoccupied

Measured 1.7 1.8 1.8 1.5 1.4 1.4 1.3

Calculated 1.5 1.6 1.6 1.5 1.4 1.4 1.2

Measured 1.7 1.8 1.8 1.7 1.6 1.4 1.3

Calculated 1.7 1.8 1.7 1.6 1.5 1.4 1.2

Measured 2.6 2.7 2.5 2.3 2.2 2.1 2.0

Calculated 2.5 2.4 2.3 2.0 2.0 2.0 1.9

Occupied

Measured 1.3 1.3 1.0 0.8 0.8 0.8 0.7

Calculated 1.1 1.1 0.9 0.7 0.7 0.7 0.7

Measured 1.5 1.6 1.5 1.3 1.4 1.4 1.4

Calculated 1.4 1.5 1.2 0.9 0.9 0.9 1.0

Measured 2.2 2.2 1.8 1.4 1.5 1.4 1.4

Calculated 2.1 2.1 1.7 1.3 1.4 1.4 1.3

174

The values decreased towards higher frequencies, due to the

absorption coefficients being higher at higher frequencies. The calculated

values were slightly lower than the measured values. This can be due to the

values of absorption of the materials in reality which may not be the exact

with the input values of ClassTalk. The comparison is also shown in

Figure 4.15.

Figure 4.15 Comparison of measured and calculated RT

4.6 IMPROVEMENT IN RT WITH FLOORMAT AND WALL

COVERING

To reduce the ‘student generated background noise’, floor mat in

the form of coir mat was laid on the floor in some of the classrooms and the

RT was measured. As RT is influenced by the absorption of the interior

surface, it improved the RT to some extent. In the same classrooms walls

were covered in certain portions with wall coverings of cotton cloth fabric

without obstructing light to increase the absorption and the RT of the

classrooms were measured. Cotton cloth fabric was used as it was cost

effective. The RT in such classrooms was tabulated in Table 4.6. The

specification of coir mat is given below. The RT was measured in 10

175

classrooms and the values are shown in the Table 4.6. Figure 4.16a shows RT

measured in classroom- 1 with floor mat, wall covering and with occupancy

using a hand held sound level metre. Figure 4.16b shows the same classroom

- 1 measured with children and coir mat and wall covering seen clearly.

Figure 4.17a shows another classroom - 2, measured with floor mat and wall

covering and the clapping of hands seen to create impact noise. Figure 4.17b

shows the same classroom - 2 measured in the unoccupied furnished

condition using a tri-pod for the sound level metre. Figure 4.18a shows the

children sitting in silence during RT measurements in the classroom -2

without carpet and wall covering. Figure 4.18 b shows the furnished

unoccupied measurement in the classroom -2 without floor mat and wall

covering using tri-pod. With the BN measured as shown in Table 3.13 of

Chapter 3 and RT shown in Table 4.6, the STI values get improved for better

Speech Intelligibility as discussed in the next chapter for classrooms with

carpet and wall covering.

Table 4.6 RT with coir mat and wall covering

Class

rooms

Classroom without

Coir mat and wall

covering

Classroom with

Coir mat and wall

covering

RT for 1 k Hz

seconds

RT for 1 kHz

seconds

1 0.6 0.5

2 0.9 0.8

3 0.8 0.7

4 0.7 0.6

5 0.6 0.5

6 0.7 0.6

7 0.8 0.7

8 0.6 0.5

9 0.8 0.7

10 0.9 0.8

176

Figure 4.16a Occupied classroom - 1 measured with carpet and curtain

Figure 4.16b Hand held Sound level meter used for measuring occupied

classroom - 1

177

Figure 4.17a Another classroom - 2 measured with floor mat and curtain

in the occupied condition with impact noise created by

clapping

Figure 4.17b Waiting for the RT recording after the impact noise has

been created in classroom- 2

178

Figure 4.18a Children in silence during the RT measured in the same

classroom - 2 without floor mat and curtain

Figure 4.18b The RT measured in the same unoccupied classroom - 2

using sound level metre placed on tri-pod

179

4.7 SUMMARY

Reverberation being an important parameter in determining the

Speech Intelligibility, RT in classrooms both in unoccupied condition and

occupied conditions were measured. The RT in classrooms were also

calculated using the software ClassTalk. The measured and calculated RT

were compared and found that they were in good agreement within about 10

%. RT was calculated for all 120 classrooms and the values are presented. It is

seen that in unoccupied condition, the international standards stipulate a value

of 0.6 s and only 7.5% of classes in unoccupied condition satisfy this

requirement. Similarly NBC 2005 stipulates a value of 1.25s for RT in

unoccupied condition and only 36 % of classes fulfilled this requirement,

though NBC stipulated values are almost double of the value stipulated in

international standards. In a few classrooms coir mat was spread and wall

coverings were hung and the RT was measured. It was found that RT was

reduced by about 0.1 s. By providing more wall coverings the RT can be still

reduced.

CHAPTER 4 REVERBERATION - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/9572/33/12...142 CHAPTER 4 REVERBERATION 4.1 GENERAL Understanding speech is hindered by the combined

Documents