Paper ICA2016-843

AA - Architectural Acoustics - Room and Building Acoustics:
Paper ICA2016-843
Room acoustic experiments inside the
Universidade Federal de Santa Maria Industrial College:
A case study with low-cost instrumentation
Jean Carlo Bernardi(a) , Bruno G. Knebel(b) , Bernardo H. Pereira Murta(c) ,
William D’A. Fonseca(d) , Paulo H. Mareze(e) , Eric Brandão(f)
(a - f) Federal
University of Santa Maria, Acoustical Engineering, Santa Maria, RS, Brazil,
[email protected], [email protected], [email protected],
[email protected], [email protected], [email protected]
Abstract:
When studying Room Acoustics, simulations simply are not enough to get the whole picture of the
subject. It is important to notice that, measurements on site are also a vital part of the learning
process, since it allows the estimation of relevant information concerning the rooms’ acoustic
functional efficiency. Later, this information can be used to evaluate acoustic parameters that will
play an important role in the acoustic quality for the room. It is very important for students to
get experience with rooms acoustic measurements. Nevertheless, it cannot always be carried
out easily. Among several reasons, one is the high price of the professional acoustic equipment
needed to properly do so. Facing this problem, this work aims to present a case study using lowcost equipment as an alternative method to grant more learning freedom for the students. The
case study takes place at Universidade Federal de Santa Maria’s Technical Industrial College
Auditorium, Brazil, where the students performed several binaural measurements to evaluate the
Auditorium’s performance related to speech and music.
Keywords: room acoustics, instrumentation, impulse response, low-cost, experiments.
Room acoustic experiments inside the Universidade
Federal de Santa Maria Industrial College: A case study
with low-cost instrumentation
1
Introduction
Over the recent years the use of simulations has dramatically increased in every field of science,
thus, room acoustics is no exception. Every year, algorithms have become more sophisticated
and improvements on computer processing power are developed. Hence, more complex simulations can be processed in less time, yielding more accurate results than ever before. The
pros of using simulation are not a mystery as well; it is possible to obtain several acoustical
parameters through it, for example, Reverberation Time (RT), Early Decay Time (EDT), among
more specific ones, such as D50 or C80 , for instance [1]. Such parameters are of great importance
in determining the acoustic quality of a room [2]. Having all this in mind, it becomes convenient
to make use of simulations as the main tool to teach room acoustics, however, while it holds
great importance, simulations simply are not enough to get the whole picture of the subject.
On place measurements are time-consuming activities since loads of requirements and plans
are needed to be resolved beforehand to optimize time spent in room and logistics of equipment
management and properly installation. In addition, one of the most challenging parts when
handling a relatively large student class is guaranteeing the proper use of the equipment in
order to keep it safe avoiding damage. Nonetheless, measurements on site are also a vital part
of the learning process allowing the students a more vivid experience on how the theory of
room acoustics was built and how it works.
Facing these challenges the use of low-cost instrumentation can have a significant impact on the
approach taken by a professor when teaching room acoustics. More freedom for both students
and the professor can be achieved, allowing measurements to take place without the risk of
damaging expensive equipment. Furthermore, this allows several measurements to be taken by
distinct students in different rooms simultaneously.
Aiming to provide a solution for the challenge, the present work makes use of low-cost instrumentation to perform room acoustic measurements at a lecture hall, the Universidade Federal
de Santa Maria Industrial College, in Brazil. The measurements were acquired by a MATLABcontrolled USB external soundcard as both monaural and binaural impulse responses, the latter
through the use of a customized head-torso simulator with electret microphones. These impulse
responses were later post processed using the ITA-Toolbox1 and self-written scripts. The results
obtained were compared to different measurement positions and the acoustic quality of the
lecture hall was evaluated accordingly to these parameters. The case study measurements were
carried out by a group of three senior undergraduate students of Acoustical Engineering through
the use of both omnidirectional sources as well as the lecture hall sound reinforcement system.
1 Free
MATLAB toolbox developed at the Institute of Technical Acoustics at RWTH Aachen University (Germany) for
acoustic measurements and signal processing.
2
2
Object of study: Industrial College Auditorium
This section presents a description of the Industrial College Auditorium, where measurements
took place. Originally built in 1960, the auditorium has gone through several refurbishments and
improvements untill this day. The auditorium has a total capacity of 106 persons in the audience
in addition to four seats for the stage. The auditorium plan is presented in Figure 1 to clarify the
audience distribution and its size.
Figure 1: Industrial College Auditorium’s Plan.
The auditorium’s main use is for seminars, lectures and conferences, therefore, requiring a
good acoustic quality for speech. Despite that, there is no element of acoustic treatment in the
auditorium, the seats are heavily padded chairs and the floor is light carpet covered by a rubber
material on the passageways. The walls are made of concrete, finished with cement and painted.
The ceiling is made of regular gypsum boards.
Auditorium’s height, floor to ceiling, is considered low for such an environment, reaching a
maximum of 3 meters at the front floor, just before the stage, and lowering to a minimum of
2.25 meters at the last row. The estimated room’s volume is, approximately, 300 m3 .
3
Equipment
The equipment used for the measurements is an important feature of this paper. Aiming to
fulfill the low-cost equipment premise proposed for such an activity, the students carried out the
measurements with the following set of equipment:
• Data acquisition system:
– M-Audio Ultra Track Pro (commercial USB eternal soundcard);
3
• Sources of sound:
– Auditorium’s Sound System (P.A.): four SKP Pro Audio Speakers;
– Omnidirectional source to validate measurements;
• Microphones:
– Two Sennheiser type KE4 [3];
– One Panasonic type WM61 [4];
• Measurement control and post-processing:
– Notebook controlling acquisition system through ITA-Toolbox via MATLAB Student
version.
To provide a better understanding of the whole system working together a flowchart of the
connections and equipment setup is shown at Figure 2. It is to be noted that the pair of KE4
microphones were positioned in the earplugs of a dummy head in order to impose a Head
Related Transfer Function (HRTF) to the binaural measurements [5, 6]. The WM61 microphone
was placed in the position of the center of the head with the head absent for the single channel
measurements. To make it possible, the dummy head and the WM61 were positioned in different
seats at each measurement chain, the whole measurement planning will be addressed with
more details in Section 4. Absolute pressure levels can also be achieved with this system. In
this case, acoustical and voltage calibrators must be included in the instrumentation list [7].
Figure 2: Flowchart presenting the connections of the measurement setup.
4
4
Measurement Plan
The studied auditorium is used daily for lectures and seminars. Thus, students were needed to
work during the night shift and spend as less time as possible inside it during the measurements
(there was also a concern about the background noise and vibration). The complete measurement was planned beforehand and all the logistics were pre-organized in order to measure 5
positions for each monaural and binaural measurements, using two different sound sources
(conventional omnidirectional and room’s existing P.A.) as fast as possible.
Measurements positions were selected to provide a relatively good representation of the sound
field in the auditorium. The chosen positions were: first and last row, near the sidewalls as well
as in the center of the room, as depicted in Figure 3. The idea is to cover variations in the
sound field that the audience may experiment.
Figure 3: Measurement map with the five seats, covering different sound field scenarios.
Due to the time constraints previously mentioned, the measurement logistic was carefully planned
in advance, variables such as cable lengths, measurement positions, number of measurements,
data storage requirements and side equipment as measuring tape, thermometer and barometer,
were all taken into the account. The planning allowed the students to optimize the measurement
process, every step was planned in a Test Matrix. Thus, on site, there was almost no decision
left to be taken, avoiding inconvenient situations and/or surprises. From previous experience,
the students have estimated that this previous planning has saved half of the time that it would
be spent without the Test Matrix developed. To achieve this optimal time, both monoaural and
binaural measurements were carried out simultaneously in different positions. It was needed a
total of 5 batch measurements to acquire all the data.
4.1 Signal processing
After the measurements, the acquired data needs to be processed in order to be correlated with
relevant acoustic parameters. This signal processing was carried out using scripts via MATLAB
developed by the students. The data was batch processed in order to obtain the Impulse
Responses (IR) and Binaural Room Impulse Responses (BRIR), following the convolution and
signal processing theories.
5
Once the IR relative to each microphone at each position is obtained, the reverberation time
and other parameters are estimated employing ITA-Toolbox processing tools together with
the developed scripts. The analysis presented throughout this work is limited to detect some
characteristics of the room, such as its first resonant modes in the frequency domain analysis,
mean reverberation time for each position and the Early Decay Time (EDT).
5
Results
The results are presented in 3 parts. The first one shows the frequency response of the room,
comparing it to the room’s modes (see Figure 4). The second analysis shows the reverberation
times all positions measured with both sources (Section 5.2). Finally, the EDT is estimated for 3
positions (see Figure 8).
5.1 Frequency domain analysis - Room
An analysis of the impulse response in the frequency domain (or Frequency Response Function,
FRF) is carried out. The measured data with the omnidirectional source and the microphone at
the center of the head is compared to the estimated modal distribution of the room, per Figure
4. There are small variations concerning the modes frequency and FRF peaks. However, this
error may be related to the simplification of modal analysis. The auditorium was approached by
a rectangular room with dimensions (7.78 x 13.92 x 3.00) m, ignoring, therefore, the floor height
variation. Nevertheless, the modal distribution estimation can be considered valid except for the
small variations.
Modal distribution compared to the FRF position 1 - Omni source - Microphone on center of the head
1.0
0.9
normalized FRF
Room modes
0.8
Magnitude [-]
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
30
40
50
60
70
Frequency [Hz]
Figure 4: Modal distribution compared to the normalized FRF at Position 1
(by using the omnidirectional source and the microphone at the center of the head).
6
5.2 Reverberation Time (RT)
Figures 5 and 6 shows the results for the reverberation time in third octave bands, for all
positions and sources described in the measurement map shown in Figure 3, as well as the
average reverberation time. Error bars represent the standard deviation of the repetitions. In
Figure 5 it is possible to observe that in one position for the bands 400 Hz and 630 Hz the
reverberation time measured is clearly over-estimated. Therefore, those values were excluded in
the averaging process of the reverberation time for the P.A. system.
Reverberation time - Average of 5 positions - P.A. system source
3.0
Position 1
Position 2
Position 3
Position 4
Position 5
Average RT
Reverberation time [s]
2.5
2.0
1.5
1.0
0.5
0.0
100
160
250
400
630
1000
1600
2500
4000
6300
10000
Frequency [Hz]
Figure 5: Reverberation time in third octave bands relative to the measurement positions using
the omnidirectional microphone with the P.A. (as sound source).
Reverberation time - Average of 5 positions - Omni source
3.0
Position 1
Position 2
Position 3
Position 4
Position 5
Average RT
Reverberation time [s]
2.5
2.0
1.5
1.0
0.5
0.0
100
160
250
400
630
1000
1600
2500
4000
6300
10000
Frequency [Hz]
Figure 6: Reverberation time in third octave bands relative to the measurement positions using
the omnidirectional microphone with the omnidirectional source.
7
A comparison of the reverberation time measured with the two sound sources is shown in
Figure 7. It is possible to perceive good accordance between the measured values. This
reinforces the hypothesis that the electroacoustic P.A. system installed in the room is adequate
for its applications. It amplifies speech exciting the room (in the range of frequencies between
200 Hz and 10 kHz) similarly to the omnidirectional sound source (the RT nearly flat, about
0.6 s).
RT by position - Microphone on center of the head
Time [s]
3.6
3.0
2.4
1.8
1.2
0.6
0.0
100
160
250
400
630
1000
1600
2500
4000
10000
P.A.
Omni
Frequency [Hz]
Figure 7: Comparison betwen the calculated TR in third octave bands using as sound source
the P.A. system and the B&K dodecahedron (averaging between positions).
According to Long [8], rooms that the main objective is to use for speech should have a flat
reverberation time at all frequencies. The analyzed room meets the RT requirements specified by
the Brazilian standard NBR 12179 [9]. Since its estimated volume is approximately 300 m3 , the
RT is close to the recommended by Long for environments intended to speak with this volume,
approximately 0.5 seconds. Furthermore, the room have shown a nearly flat reverberation time
up above the band 250 Hz.
The EDT is a parameter related to the intelligibility in a room [1]. Thus, it is important that this
parameter does not present significant variations among different seats. Figure 8 depicts the
mean EDT and its standard deviation over the three repeats for three distinct positions.
8
EDT - Average of 3 positions - P.A. system source
3.0
Position 2
Position 3
Position 4
2.5
EDT [s]
2.0
1.5
1.0
0.5
0.0
160
250
400
630
1000
1600
2500
4000
6300
10000
Frequency [Hz]
Figure 8: Objective parameter EDT, measured for 3 positions with the P.A. system.
6
Conclusion
From the student’s perspective, the activities carried out from planning to post-processing
comprehends an important role in the final preparation aiming the inclusion in the job market.
Besides applying the room acoustics theory and getting experience in handling measurement
equipment and in post-processing, other skills such as logistic management, team work and
optimization of time were improved after the activities.
The results of reverberation times were regarded consistent, given the Long [8] recommendations
for the omnidirectional and P.A. sound sources (for all measurement positions). Moreover, small
variations of the EDT along the seats reinforces the good quality of the studied auditorium.
The students were also able to apply auralization techniques using the BRIRs to listen at the
different seats. Although no subjective study was applied, the experience to process such signals
and the understanding of the different conditions of each BRIR have contributed to the overall
development of practical knowledge in room acoustics.
Acknowledgements
Data acquisition and also some analysis and post processing have been carried out by using
the ITA-Toolbox for MATLAB. It is developed by the Institute of Technical Acoustics at RWTH
Aachen University [7] (available at www.ita-toolbox.org).
References
[1] Eric Brandão. Acústica de salas: projeto e modelagem. Blucher, Brasil, Jun. 2016.
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[2] H. Kuttruff. Room Acoustics. CRC Press, 5 edition, Jun. 2009.
[3] Martin Guski. Influences of external error sources on measurements of room acoustic parameters. PhD thesis, RWTH Aachen, Aachen, Germany, 2015.
[4] William D’A. Fonseca. Development and Application of an Acoustic Imaging System using
Beamforming Technique for Moving Sources (original: Desenvolvimento e Aplicação de
Sistema para Obtenção de Imagens Acústicas pelo Método do Beamforming para Fontes em
Movimento). Master’s thesis, Federal University of Santa Catarina, Florianópolis, SC, Brazil,
Feb. 2009.
[5] Michael Vorländer. Auralization: Fundamentals of Acoustics, Modelling, Simulation, Algorithms
and Acoustic Virtual Reality. Springer, Berlin, Germany, 2007.
[6] Henrik Møller. Fundamentals of binaural technology. Applied Acoustics, 36(3):171 – 218,
1992.
[7] Pascal Dietrich, Bruno Masiero, Martin Pollow, Roman Scharrer, and M Muller-Trapet. Matlab
toolbox for the comprehension of acoustic measurement and signal processing. Fortschritte
der Akustik–DAGA, Berlin, 2010.
[8] Marshall Long. Architectural acoustics. Elsevier, 2005.
[9] ABNT Associação Brasileira de Normas Técnicas. NBR 12179 - 1992 - Tratamento acústico
em recintos fechados, Abril 1992.
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