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BUILDING ACOUSTICS · Volume 20 · Number 1 · 2013 Pages 43–54 43
The Interrelationship Between Room Acoustics
Parameters as Measured in University Classrooms
Using Four Source Configurations
Christos Nestoras1 and Stephen Dance2
The Acoustics Group, FESBE, Department of Urban Engineering, London South Bank
University, Borough Road, London SE1 0AA, UK
nestorasc@gmail.com, dances@lsbu.ac.uk
(Received 20 August 2012 and accepted 30 April 2013)
ABSTRACT
This paper investigates the interrelation of room acoustics parameters as measured in lecture
theatres/classrooms using four sound source configurations. Ten typical rooms were selected as
representative of university premises and measured to ISO 3382 standards. The study focuses
initially on the type of sound source used, to establish the suitability of multi source based
measurements in assessing the acoustics of classrooms. Acoustic performance is then discussed
in the context of the relationship between room acoustics parameters with and without significant
background noise, with a particular focus on speech intelligibility. To facilitate a more efficient
discernment of results, EDT, T30, Clarity indices and MTI were considered, as they are commonly
included in general room acoustics assessments. Either of the source configurations was found to
be suitable for performing general purpose measurements in (small) rooms. Clarity and EDT
were found to be linearly related to the modulation transfer index in noiseless conditions, in line
with earlier findings, thus an excellent predictor of STI. Background noise could be ascertained
as of primary importance in the case of a non linear relation.
1. INTRODUCTION
Room acoustics of classrooms and lecture theatres has been extensively investigated
[1, 2, 3, 4, 5, 6]. Objectives of these investigations ranged from defining optimum RT’s
within classrooms to prospective interrelations of acoustic parameters that are typically
used in describing acoustic conditions, in particular speech intelligibility. Building on
this research, the performance of a multi source sound system as an alternative to the
traditional omni directional source type in room acoustics measurements is examined in
this paper. The potential advantage is in practicalities e.g. no setup time or access to
spaces otherwise unavailable for testing, as the effect of such a configuration on general
acoustic parameters e.g. RT has not been specifically addressed in comparison to
standard measurements using a single omni directional sound source.
The study is based on the room acoustics measurements for a combination of ten
lecture rooms that are considered typical within university premises; the latter
comprising building stock, as found within London South Bank University, sharing for
the most part characteristics such as a rectangular shape, hard (reflecting) walls in the
room perimeter and primarily a small volume with low overall RT. The measurement
methodology is described in detail for the experimental conditions and results are
analyzed in terms of the type of source to determine the level of usability for a
consistent room acoustics assessment. Acoustic performance is then discussed in the
context of the relationship between various parameters, with a particular focus on
speech intelligibility under noiseless and noisy conditions.
2. SPEECH INTELLIGIBILITY MEASURE INTERRELATIONS
The interrelation between measures of speech intelligibility and their individual relation
to STI has been extensively studied in the literature. These investigations revolved
around mathematical relations, to enable inter-comparison and/or discernment of
results, and to establish the potential correlation to speech intelligibility.
Bradley [1, 2] and Bistafa [3] have studied these interrelations for a range of
conditions and established a mathematical connection in a number of cases. Most
notably, the C50 ratio has been found to be linearly related to STI for simulated
controlled natural acoustics conditions, incorporating negligible background noise [1].
This work has further defined a just noticeable difference (JND) relating to the
subjective perception of level changes to a sound field.
For small rooms, such as classrooms, using either a 50 ms or 80 ms time limit for
defining early energy can be expected to result in similar trends when compared to STI
[2]. Thus, implying that for speech applications an efficient assessment can be made
using either limit. It was established by Bradley [5] that EDT and measures using the
50 ms limit (as opposed to 80 ms) could accurately (within ± 1 dB) be predicted from
the reverberation time in small rooms. This outcome was supported by Mapp [4] for
rooms with short reverberation times under sound system assisted conditions. In the
context of the current study, the impact of the latter is that either RT or EDT could be
used to describe a (small) room.
3. EXPERIMENTAL METHODS
3.1. Test rooms
The range of test rooms considered in this study consisted of fitted lecture theatres and
classrooms, typically found within university premises. The majority of the rooms can
be described as small to medium sized spaces, with two larger rooms also included in
the analysis, see table 1.
3.2. Measurement instrumentation
Background noise measurements were taken using a Norsonic 140 sound level meter
calibrated using a B & K Type 4230 calibrator. For the room acoustics measurements a
Dell Latitude PC D610 fitted with a Digigram VXpocket v2 PCMCIA sound card was
used. This was routed through an Audio SR707 power amplifier to a Rion
44 The Interrelationship Between Room Acoustics Parameters as Measured in
University Classrooms Using Four Source Configurations
dodecahedron loudspeaker. For the multi-source configurations an audio splitter (1 in,
4 out) and two or four active Yamaha HS50M studio monitor loudspeakers on tripod
stands were used. Impulse responses were recorded using an Earthworks M30BX omni
directional measurement microphone.
3.3. Measurement methodology
The room acoustics measurements aimed at a general assessment of the spaces
considered, enabling nonetheless further post processing where necessary, as for
example accounting for absolute speech and background noise levels. Natural acoustics
were assessed with the use of a dodecahedron (omni directional) loudspeaker, while the
sound system assisted conditions were based on a portable sound system set up (SS)
common in all rooms in order to eliminate inconsistencies due to different system
characteristics. Both source types approximated a flat frequency response while the
directivity pattern and aiming of the distributed system were not considered at this
stage. The portable sound system consisted of a combination of two or four monitor
loudspeakers, positioned respectively at the two front or main four corners of the
audience area. Overall, a total of four source configurations were used: two omni
directional source positions (S1, S2) and two sound system formations (SS2, SS4).
Room acoustics measurements were based on the WinMLS 2004 [7] platform and a
combination of a sound source (single omni directional or distributed configuration)
with an omni directional receiver. The receiver positions and omni directional source
positions were set at a height of 1.1 m–1.2 m and 1.7–1.8 m, respectively. Positioning
of the distributed system in terms of height depended largely on the distance from the
ceiling in each case, given an angled audience area for a number of rooms; a height
setting in the range of 1.8 m–2.35 m was used, as appropriate. More information on the
measurement setup can be found in [8].
A10 second exponentially swept sine was used to take multiple impulse response
measurements in line with BS ISO 3382-2: 2008 [9]. Divergence from the standard
procedure was necessary for a number of receiver positions due to the relative position
BUILDING ACOUSTICS · Volume 20 · Number 1 · 2013 45
Table 1: Classrooms and lecture rooms used in the study
Room Capacity (seating) Volume (m3)Size category
Room 1 30 138 Small
Room 2 30 156 Small
Room 3 50 260 Small
Room 4 30 148 Small
Room 5 40 218 Small
Room 6 80 242 Medium
Room 7 140 250 Medium
Room 8 110 267 Medium
Room 9 62 356 Large
Room 10 240 753 Large
of reflective surfaces i.e. desks. The aim of this variation was to assess a more realistic
environment; however, alternative positioning was chosen were possible. Samples of
unoccupied background noise levels (LAeq, 1min) were recorded throughout the sessions,
typically 5 samples in every room for each 4 hour measurement session.
4. RESULTS AND DISCUSSION
Measurement results are presented for the different analysis approaches of the study. In
view of the base condition, the use of four sound source configurations and the acoustic
parameter interrelationships are discussed.
4.1. Base measurements
Table 2 presents the averaged background noise levels over the ten test rooms. The
overall linear and A-weighted levels varied from 39.7 dB–57.8 dB and 34.1 dBA–48.4 dBA,
respectively, a number of rooms having at times increased exposure to low frequency
noise, as also supported by the large 125 Hz octave band standard deviation.
Astatistical summary of the results for each of the test rooms and the average T30,
EDT, C50, C80, D50, Ts, MTI and STI values are given in table 3 to establish the general
character of the rooms considered. T30 as such varied from 0.49–0.92 while EDT was
measured within the range of 0.42–0.81. STI varied from 0.66–0.79 with an average of
0.74, translating to a prospective ‘good’ speech intelligibility rating for all rooms.
46 The Interrelationship Between Room Acoustics Parameters as Measured in
University Classrooms Using Four Source Configurations
Table 2: Averaged unoccupied background noise levels over ten rooms (Leq, 1min)
with standard deviations
Frequency (Hz) 125 250 500 1000 2000 4000 8000 Overall level
Linear 37.3 34.8 34.1 33.9 32.6 27.8 24.1 42.1 dB
A-weighted 21.2 26.2 30.9 33.9 33.8 28.8 23.0 38.8 dBA
σ9.9 5.6 4.3 4.3 3.9 5.2 6.7
Table 3: Acoustic parameters measured in ten test rooms (broadband average
over receiver positions and source configurations) and statistical summary
Room 1 2 3 4 5 6 7 8 9 10 Min Average Max STD
EDT (s) 0.45 0.49 0.42 0.81 0.73 0.53 0.54 0.50 0.71 0.45 0.42 0.56 0.81 0.14
T30 (s) 0.50 0.56 0.49 0.85 0.89 0.56 0.62 0.59 0.92 0.56 0.49 0.65 0.92 0.17
Ts (ms) 31.6 34.6 38.4 58.8 51.7 38.2 47.1 40.3 51.6 32.0 31.6 42.4 58.8 9.34
C50 (dB) 6.5 6.0 5.7 1.8 3.6 4.9 3.7 5.1 3.5 7.0 1.8 4.8 7.0 1.61
C80 (dB) 11.3 10.1 10.8 5.5 7.1 8.8 8.0 9.4 6.8 11.2 5.5 8.9 11.3 2.01
D50 (%) 80.9 79.7 77.0 65.2 68.1 76.3 68.1 73.7 67.8 81.3 65.2 73.8 81.3 6.09
MTI 0.77 0.75 0.79 0.65 0.69 0.74 0.74 0.76 0.70 0.77 0.65 0.74 0.79 0.042
STI 0.77 0.75 0.79 0.66 0.70 0.74 0.74 0.76 0.70 0.78 0.66 0.74 0.79 0.041
4.2. Effect of source type on base measurements (data comparison)
Results were compared in terms of reverberation times to establish the effect of the
source type on the room response and assess the feasibility of substituting source types
with alternatives when necessary, or simply when more practical to do so (also see
[10,11]). Considering the measured data it can be deduced that the majority of the test
rooms produced a primarily diffuse sound field for the higher frequency range, with the
partial exception of room 10.
Table 4 shows the standard deviation for the T30 and EDT variation among the four
source configurations as an average over all receiver positions in a room. For
comparison purposes, the standard deviations are also interpreted as an equivalent
percentage (%) relating in each case to the mean value at the particular data point.
Low standard deviations were established for the T30 case over all rooms with values
well below 5% error for the majority of the experimental data. A notable exception can
be seen at higher frequencies in room 10 where errors reached 23% at 8 kHz. However,
with the latter room being the largest in the investigation, a quasi diffuse sound field
was considered responsible for the discrepancies observed, differentiating in character
from typical lecture rooms.
In the EDT case, larger differences were found given the different source
configurations and related positioning within the room. Averaged results in table 4
suggest that a reasonably accurate assessment can be made with an error margin below
10% (for larger rooms an EDT JND of 5% has been defined in ISO 3382-1:2009 [12]).
The smaller rooms in the investigation gave confidence in the data consistency,
supporting analogous examinations (further room design characteristics can be found
in [8]).
Accordingly, a room assessment on the basis of T30 and EDT can be performed
using either of the source types to characterize a room on general performance,
considering when necessary the related error margins. For computer modelling
purposes where measurement results are used at a post processing stage, e.g. for
model calibration, the output via either of the source configurations is further
directly applicable in this respect, facilitating consistency in computer simulations
for the prediction of speech intelligibility parameters. Considering a direct
assessment within the rooms, the design approach should nonetheless be further
addressed to account for particular characteristics, such as the provision for early
reflections or BGNL variance, which could significantly affect intelligibility values
for particular positions within the rooms.
4.3. Parameter interrelations
Measured room acoustics parameters for the ten rooms were analysed to address their
prospective interrelationships.
Considering speech intelligibility parameters in particular, it is noted that different
elements of the acoustic conditions are used to attain a result. For example, clarity
energy ratios make use of the room effect on acoustic behaviour while ignoring
background noise, effectively the S/N. In contrast, parameters such as the STI comprise
a more elaborate approach in an attempt to account for all the variables that affect
BUILDING ACOUSTICS · Volume 20 · Number 1 · 2013 47
48 The Interrelationship Between Room Acoustics Parameters as Measured in
University Classrooms Using Four Source Configurations
Table 4: Standard deviation for T30 and EDT among the four source types in
Rooms 1–10 (over all receivers)
Octave band 125 250 500 1000 2000 4000 8000
EDT σ0.069 0.052 0.022 0.019 0.019 0.011 0.021 Room 1
T30 σ0.055 0.02 0.02 0.006 0.006 0.006 0.041
% EDT 11.7 9.3 5.6 4.8 4.2 2.6 6.2
% T30 7.7 3.5 4.7 1.5 1.3 1.3 10.3
EDT σ0.087 0.041 0.027 0.029 0.013 0.022 0.045 Room 2
T30 σ0.071 0.044 0.014 0.002 0.003 0.008 0.047
% EDT 12.8 7.0 6.3 7.2 2.8 4.8 11.8
% T30 8.7 7.1 3.0 0.4 0.6 1.5 9.9
EDT σ0.046 0.024 0.051 0.027 0.018 0.022 0.021 Room 3
T30 σ0.068 0.007 0.009 0.023 0.032 0.038 0.008
% EDT 7.8 4.8 13.1 7.4 4.1 5.7 7.0
% T30 11.5 1.3 2.2 5.1 5.6 7.6 2.2
EDT σ0.258 0.053 0.042 0.038 0.019 0.008 0.062 Room 4
T30 σ0.293 0.157 0.028 0.011 0.009 0.014 0.029
% EDT 26.6 4.6 4.2 5.0 2.9 1.2 11.9
% T30 33.8 14.0 2.8 1.3 1.2 1.9 4.9
EDT σ0.179 0.081 0.039 0.016 0.056 0.042 0.059 Room 5
T30 σ0.107 0.048 0.019 0.006 0.008 0.01 0.049
% EDT 12.2 9.0 6.5 3.0 10.3 7.7 12.2
% T30 5.9 4.5 2.9 1.0 1.2 1.4 7.1
EDT σ0.052 0.061 0.017 0.026 0.012 0.052 0.025 Room 6
T30 σ0.028 0.072 0.013 0.01 0.02 0.065 0.055
% EDT 7.7 9.5 3.3 5.7 2.2 10.1 6.3
% T30 4.8 11.1 2.5 2.0 3.3 10.8 11.3
EDT σ0.093 0.044 0.053 0.03 0.033 0.048 0.047 Room 7
T30 σ0.067 0.021 0.007 0.01 0.007 0.012 0.026
% EDT 14.8 7.3 9.9 6.0 6.2 8.6 10.7
% T30 7.3 3.3 1.3 1.9 1.2 2.0 5.2
EDT σ0.109 0.083 0.038 0.042 0.042 0.046 0.06 Room 8
T30 σ0.089 0.023 0.003 0.009 0.02 0.007 0.035
% EDT 17.0 15.4 7.6 9.1 8.6 9.4 16.5
% T30 10.1 3.6 0.6 1.7 3.4 1.3 8.2
EDT σ0.073 0.031 0.044 0.04 0.108 0.233 0.089 Room 9
T30 σ0.035 0.014 0.018 0.012 0.007 0.007 0.05
% EDT 7.2 3.7 7.4 6.8 15.3 30.6 19.1
% T30 3.1 1.5 2.8 1.5 0.7 0.7 5.9
EDT σ0.056 0.036 0.019 0.024 0.045 0.08 0.082 Room 10
T30 σ0.031 0.032 0.001 0.003 0.037 0.098 0.107
% EDT 11.5 6.4 3.6 5.2 10.4 20.6 25.6
% T30 4.9 5.3 0.2 0.6 6.5 16.7 22.9
acoustic performance. The conditions present in a space during a measuring session will
thus unavoidably affect the output in different ways for different measures. As such,
care needs to be taken when comparing dissimilar parameters or making an assumptive
assessment, based on a particular methodology.
4.3.1. Discerning a comparison of Clarity (Cx) energy ratios versus MTI
STI comprises a measure describing speech intelligibility using a single number for
seven octave bands, subsequently corresponding to more than a single Clarity value. In
order to enable a comparison in octave band level detail, the modulation transfer index
(MTI) is considered as the equivalent octave band ‘STI’, nonetheless the benefit of
octave band weighting and redundancy corrections is not considered and therefore
results could underestimate the potential relationship. In utilizing the relation between
the Cxenergy ratios and STI it should be reminded that the former does not account for
the influence of background noise. Thus, the particular interrelation is subject to change
in every environment, depending on the noise character. It is worth noting that while Ux
is a measure that can be used as an alternative to Cx(in order to account for S/N), clarity
is commonly used to quantify general acoustic quality in rooms. For this reason, its
design and specific purpose is often overlooked, with comparisons likely to take place
without regard to any limitations. A comparison of fundamentally different measures on
this basis can be constructively utilized to discern the acoustic conditions.
Figure 1 shows the relation of C50 and C80 to MTI for two conditions, with and
without background noise. For noiseless conditions the relation of the two measures was
approximately linear, agreeing with earlier results by Bradley [1], while C80 appeared to
be better related to MTI. The associated correlation coefficients were nonetheless
comparable with values of 0.91 and 0.96, respectively for the pairs C50-MTI and
C80-MTI, see figure 1(I-II), with analogous performance for all four source configurations.
In the conditions accounting for background noise the particular associations break
down, as the measures compared are effectively modified into two fundamentally
different measures. Considering that the particular relation, see figure 1(III-IV) could be
altered even within the same room under different noise conditions, a comparison of C
to STI when accounting for background noise would appear as of minor significance
unless some level of consistency in the noise character can be achieved.
When the S/N is high enough to render the effect of BGNL negligible in a practical
application, it would be possible to predict the speech intelligibility in terms of STI
from the C50 or C80 datasets with a high level of accuracy, see [2]. Therefore, for a high
signal level condition Cxmight also be used as a direct descriptor of speech
intelligibility.
This relationship would be invalidated to a large extent when considering marginal
conditions and thus could be used, if established, to ascertain BGNL as a significant
factor in the acoustical conditions.
4.3.2. Room reverberance (EDT, T30) versus STI
The relation of room reverberance to STI followed a similar trend as regards the effect
of background noise. Considering EDT and T30, an evident relation of reverberance to
the MTI was found for noiseless (or adequate S/N) conditions, see figure 2, comparable
BUILDING ACOUSTICS · Volume 20 · Number 1 · 2013 49
to steady state BGNL conditions. EDT was more closely related to MTI with a
correlation coefficient of 0.98 (0.85 for T30) having a near linear relationship,
particularly for shorter reverberation times. A similar degree of agreement was further
50 The Interrelationship Between Room Acoustics Parameters as Measured in
University Classrooms Using Four Source Configurations
Figure 1. Relation of Cxto MTI in ten test rooms (all data points, S1), I) C50 to MTI
without background noise, II) C80 to MTI without background noise, III)
C50 to MTI with background noise, IV) C80 to MTI with background noise.
Figure 2. MTI relation to space reverberance in ten test rooms (no noise) for S1.
found for all four source configurations. The relationship between the measures became
less evident with the incorporation of background noise for all experimental setups (e.g.
correlation coefficient 0.67 for both reverberation indices with S1). The resulting
relationship would again be subject to the character of noise, being the only altered
variable between the two conditions.
Accordingly, for adequate S/N, a general speech intelligibility evaluation of
reasonable accuracy could be made in typical classrooms based on RT (primarily EDT)
alone.
4.3.3. EDT versus T30
The results for T30 and EDT value interrelations partially supported the findings of
earlier studies [5]. T30 could not be unconditionally used as a baseline to predict EDT
and C50, among other measures, within university classrooms and lecture theatres.
However, considering alternate source configurations appeared to influence the T30-EDT
relationship producing, on a relative basis, a better defined trend. Figure 3 illustrates the
closer connection between the reverberation indices when the multi-source sound system
is used, particularly in the SS2 case. While all four configurations produced a relatively
small deviation in terms of the correlation between parameters, it should be noted that
BUILDING ACOUSTICS · Volume 20 · Number 1 · 2013 51
Figure 3. Relation of EDT to T30 for four source configurations in ten test rooms
(S1, S2, SS2, SS4).
excluding the two larger rooms of the study (rooms 9–10) from the statistical analysis
resulted in a closer association for all conditions, primarily enhanced for the single
source case, see figure 4. Accordingly, results suggest better uniformity among the
different conditions with added confidence when assessing smaller sized rooms.
5. CONCLUSIONS
This investigation has considered ten test rooms covering a range of acoustic conditions
found within typical university classrooms. For a consistent room assessment in terms
of T30, either of the four source configurations in test could be used. Larger differences
were found for EDT between source types, given the relative source-receiver
positioning in each case. Averaged values over the measuring positions suggested the
feasibility of a reasonably accurate assessment on this basis. Smaller sized rooms
enhanced confidence in the assessment method.
Good correlation was established between Clarity and STI (0.91 and 0.96, for C50
and C80 respectively) for noiseless or adequate S/N measurement conditions, with
analogous outcomes for all four source configurations. Speech intelligibility in terms of
STI can thus be predicted with confidence from clarity datasets under these conditions,
52 The Interrelationship Between Room Acoustics Parameters as Measured in
University Classrooms Using Four Source Configurations
Figure 4. Relation of EDT to T30 for four source configurations after excluding
larger rooms (S1, S2, SS2, SS4).
marginally more relevant for the C80 case. Accordingly, for high S/N in an actual case,
clarity ratios can be used as a direct descriptor of intelligibility, in line with earlier
research.
Given that the particular interrelation differentiates from ‘near linear’ when
background noise is considerable, the latter can be identified as a significant factor in
the acoustical conditions of a space in such a case and should be accounted for in the
room description.
The relation between room reverberance (EDT, T30) and STI, primarily for the EDT
case, can be similarly discerned to attain an indication of the impact of background
noise.
REFERENCES
[1] Bradley J.S., A just noticeable difference in C50 for speech, Applied Acoustics 58
(1999) p.99–108
[2] Bradley J. S., Relationships among Measures of Speech Intelligibility in Rooms,
J. Audio Eng. Soc., Vol. 46, No. 5, May 1998
[3] Bistafa S. R., Bradley J. S., Reverberation time and maximum background-noise
level for classrooms from a comparative study of speech intelligibility metrics,
J. Acoust. Soc. Am. 107 (2), February 2000
[4] Mapp P., Relationships between Speech Intelligibility Measures for Sound
Systems, Presented at AES 112th Convention , Munich, Germany, 2002 May
10–13, Convention paper 5604
[5] Bradley J. S., Speech intelligibility studies in classrooms, J. Acoust. Soc. Am. 80
(3), September 1986
[6] Hodgson M., Experimental investigation of the acoustical characteristics of
university classrooms, J. Acoust. Soc. Am. 106 (4), Pt. 1, October 1999
[7] Morset L., Morset development, WinMLS 2004, Professional Measurement
Software for PC and Soundcard, User’s Manual, www.winmls.com
[8] Nestoras C., The assessment of speech intelligibility in room acoustics for efficient
application in computer modelling and improved enclosed spaces, PhD Thesis,
London South Bank University, 2009
[9] BS EN ISO 3382-2: 2008, Acoustics - Measurement of room acoustic parameters
- Reverberation time in ordinary rooms
[10] Nestoras C., Gomez L., Dance S., Murano S., Speech intelligibility measurements
in a diffuse space using open and closed loop systems, Proceedings of the 19th
International Congress on Acoustics, Madrid, 2007
[11] Gomez L., Nestoras C., Dance S., Murano S., Speech intelligibility measurements
in a non-diffuse space using open and closed loop systems, Proceedings of the
19th International Congress on Acoustics, Madrid, 2007
[12] BS EN ISO 3382-1:2009, Acoustics - Measurement of room acoustic parameters,
Performance spaces
BUILDING ACOUSTICS · Volume 20 · Number 1 · 2013 53
