CHAPTER 4 REVERBERATION - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/9572/33/12...142...
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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
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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):
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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
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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
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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.
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(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.
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* *
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
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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
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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
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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
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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’
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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’
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Figure 4.7b RT measured in occupied condition of classroom ‘b’
Figure 4.8a RT measurement in occupied classroom ‘c’
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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
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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
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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
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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.
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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.
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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
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Table 4.3 (Continued)
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
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Table 4.3 (Continued)
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.
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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
Table 4.4 (Continued)
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
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
Table 4.4 (Continued)
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
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
Table 4.4 (Continued)
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
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
Table 4.4 (Continued)
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
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.