PROCEEDINGS of the24th International Congress on Acoustics
October 24 to 28, 2022 in Gyeongju, Korea

ABS-0664

Acoustical design of the Nishiwaki Community Hall “Orinas” –Study of the absorption characteristics of its interior perforatedblocks
Kimiko NAKAGAWA1; Hideo MIYAZAKI2; Masahiro TOYODA3

3

1

Spatial Audio Group, Yamaha Corporation, Japan2

Spatial Audio Group, Yamaha Corporation, Japan

Department of Architecture, Faculty of Environmental and Urban Engineering, Kansai University, JapanABSTRACT
The Nishiwaki Community Hall “Orinas” opened in May 2021. The main space in this facility is themultipurpose hall with 602 seats. One of the acoustic features of this hall is the use of perforated blocks as
the interior material for the side walls. In the design stage, we were concerned about possible acoustic
phenomena caused by the line grid arrangement of uniformly sized and equidistant holes. Therefore, in theconstruction stage, the sound absorption coefficients of the perforated blocks were measured under variousconditions while changing the depth of the air space behind the perforated blocks to the walls in thereverberation chamber. The measured results indicated that (1) with no or very small air space, there are large
peaks of absorption coefficient at certain frequencies, and (2) with 150–400 mm air space, the frequencies
and degrees of the peaks of absorption coefficient depend on the depth of air space. Finally, different airspaces in the range of 150–300 mm were selected by tilting the walls behind the perforated blocks. Acoustic
measurements after construction showed no significant acoustic phenomena. In addition, to reveal themechanism of the sound absorption, we conducted computer simulation based on the finite element method.
The results of the simulation are also introduced in this paper.
Keywords: Acoustical design, Absorption, Wave acoustic analysis

1. INTRODUCTION
The Nishiwaki Community Hall “Orinas” is a new community space in the central city area that
opened in May 2021 along with the new City Hall building. This facility was designed for replacing
the old civic center, which had problems such as deterioration and lack of the earthquake-resistantperformance. The facility includes a multipurpose hall, "Orinas Hall," which has 602 movable seats,
10 studios for various purposes such as music, dance, exercise, and cooking, a lounge, a coffee shop,and a rooftop terrace that can be used freely by anyone. The functions of this facility are not only torelated to the arts and cultural activities, but also to health and welfare, and to tourism.
Table 1 gives the outline of the facility. This paper introduces the acoustical design of Orinas Hall
which is the main space of this facility. One of the acoustic features of this hall is use of perforated
blocks as interior material for the side walls. In the design stage, we were concerned about possible
acoustic phenomena caused by the uniformly sized and equidistant holes forming a line grid. Therefore,
in the construction stage, the acoustical properties of the blocks were measured to determine theoptimum installation configuration. Upon completion of construction, we performed reflected soundmeasurement on site and confirmed that proper acoustic conditions were realized. In addition, toreveal the mechanism of the acoustic absorption, we conducted the computer simulation based on thefinite element method. The results of the simulation are also introduced in this paper.

1kimiko.nakagawa@music.yamaha.com
hideo.miyazaki@music.yamaha.com
3
toyoda@kansai-u.ac.jp
2

Table 1 – Outline of the facility
Name

The Nishiwaki Community Hall “Orinas”

Location

128-1 Shimotoda, Nishiwaki, Hyogo, Japan
Multipurpose hall, studios (10), lounge,Rooms

coffee shop, terrace, etc.

ClientNishiwaki City

DesignShowa Sekkei Inc.

Acoustical Design

Yamaha Spatial Audio Group

Constructor

Dai Nippon Construction

Structure

Steel structure (partially, RC and SRC)

Construction period

September 2019 – March 2021

2. DESIGN CONCEPTThe hall was requested by the client, Nishiwaki City, to fulfill various civic activities, not onlyactivities that were held at the old civic center, such as music concerts, plays, and lectures, but also
calligraphy exhibitions, art exhibitions, and so on. Therefore, movable seats were installed toaccommodate various audiences and stage configurations according to performance styles, which
include two main formats: a stage viewing format with a stepped floor and seats, and a flat floor
format without seats. In addition, the configuration of the various equipment has been simplified withconsideration of a use case by a small number of residents. Thus, the hall can be used for multiple
purposes and it is designed to be easy to use by residents.
A plan and section of the hall and specifications are shown in Figure 1. A summary of the acousticaldesign concepts is shown in Figure 2.
Among the various uses, design for acoustical instruments was considered to be most important.
In order to achieve sufficient air volume to create a rich resonance for classical music performances,a general ceiling interior was not installed and a large volume above the ceiling was secured, resulting
in air volume of 13.1 m3 per seat. In addition, for sufficient loudness and clarity of sound, reflectors
above the front audience area were installed to provide early reflections continuously from the stage
ceiling reflectors. Other reflectors under the catwalks exposed above the audience area were also
installed to provide the early reflections from above.
These exposed catwalks can be utilized for various performances by hanging various equipment.
Sufficient reverberation time range is achieved due to large absorption inside the stage fly. For thesereasons, this configuration can be used for multiple purposes.

14.4 m(Drama mode)

26.3 m18.0 m
23.0 mV=7,910 m3
S=3,210 m2
N= 602 seats
V/N=13.1 m3/seat

9.7 m(Concert mode)

13.2 mFigure 1 – Plan and section of the hall

8.4 m

Provide early reflections to
all audience members byreflectors continuously fromthe stage ceiling reflectors

Provide early reflections
from above to the audience
by reflectors under the
exposed catwalks

Create rich resonance by large air
volume (13.1 m3/seat) byintegrating the space above theceiling with the audience space

Provide lateral reflections to all
audience members by zig-zag
diffuse reflection surfaces andperforated blocks with diffusion
effectFigure 2 – Summary of the acoustical design
Securing the spatial effect required for classical music performance was also an important
requirement. To ensure lateral reflections for all audience members, the lower part of the side walls
have zig-zag diffuse reflection surfaces and acoustical eaves. Further, the upper part of the side walls
was expected to have a diffusion effect in the design stage. On the other hand, the architecturaldesigner had as the design concept the creation of a hall imbued with calmness and warmth and aspecial place where audiences can feel a sense of elation. To achieve both the diffusion effect and thedesign concept, it was decided to install stacking perforated blocks, which is a characteristic design(the part shown in yellow in Figure 2). Out of concern about acoustic phenomena arising from theuniformly sized and equidistant holes forming a line grid, we conducted detailed studies in theconstruction stage. The findings are introduced in detail in the next chapter.

3. STUDY OF THE ACOUSTIC CHARACTERISTICS OF PERFORATED BLOCKS
3.1 Summary
The details of a “perforated block” (hereafter, block) are shown in Figure 3. By stacking theseblocks, 100 mm square holes are configured in vertical and horizontal lines at equidistant intervals,
the perforation ratio of the surface is about 25%. Due to the weight and perforated shape of the block
itself, it was expected to be effective as a diffusing element. On the other hand, we were concernedthat it would become a Helmholtz resonator that absorbs specific frequencies like a perforated panel.
Thus, in the design stage, the blocks were placed close to the walls.
However, as we had never used such shape elements, there was the risk of unexpected acoustic
characteristics. Therefore, to clarify the acoustic characteristics, we made a mockup and studied its
acoustic characteristics in the construction stage.
3.2 Acoustical Study in the Construction StageIn the construction stage, we made a side wall mockup of 10 m2 by arranging 126 blocks on gypsum
boards, and we measured the sound absorption coefficients in a reverberation chamber. Measurement
was done in three patterns, (1) Design specification: “with no air space” between blocks and boards,(2) Considering construction accuracy: “with 10 mm air space”, and (3) Large air space: “with 300mm air space”. The experiment photo is shown in Figure 4, the measurement results are shown in
Figure 5. Only in patterns (1) and (2), large peaks of sound absorption coefficient (about 0.6) were120 mm
390 mm
190 mm
100 mm
100 mmWeight: 13 kg
Material: concrete
Figure 3 – Detail of “perforated block”

Figure 5 – Measurement results: patterns (1) – (3)Figure 4 – Experiment photo (pattern (3))

Figure 6 – Measurement results: pattern (4) – (6)seen around 315 Hz. These results indicate that the specification of boards closely behind the blocks
entails the risk of absorption at a certain frequency. In addition, the result of pattern (3) suggested thatthe peak of the absorption may become smaller and the peak frequency may shift by large air space.
There is a possibility that the sound absorption coefficient of the high frequencies above 1 kHz isaffected by the absorption itself of the elements (interlocking blocks) used for making air space.
Based on these results, in order to study the effect of the depth of air space, we performed
measurements in three additional patterns, (4) “with 150 mm air space”, (5) “with 250 mm air space”,and (6) “with 400 mm air space”. The elements used for making air space were changed to wood withlow absorption. The measurement results are shown in Figure 6. In patterns (4) and (5), no large peaksof absorption were seen, and sound absorption coefficients ranged from 0.1 to 0.3 at all frequencies.
In pattern (6), a peak of sound absorption coefficient (about 0.4) was seen around 500 Hz, and thepeak was smaller than the peaks in patterns (1) and (2). At above 1 kHz, there is a possibility that thesound absorption coefficient of the block seems to be slightly larger due to the effect of the absorption
itself of the elements (wood) used for making air space.
The results of this series of experiments indicated that with no or very small air space, there arelarge peaks of absorption coefficient at certain frequencies, and with 150–400 mm air space, thefrequencies and degrees of the peaks of sound absorption coefficient depend on the depth of air space.
Therefore, we decided to change the depth of the air space in order to eliminate the bias of soundabsorption coefficient by frequency. Finally, different air spaces in the range of 150–300 mm wereselected by tilting the walls behind the perforated blocks (Figure 7). In addition, after reviewing thesound absorption coefficient, the reverberation time calculation was performed again. Based on thecalculation results, we adjusted reverberation by removing the GW on the rear wall and not installing
acoustic curtains around the catwalks area.
3.3 Measurement of a Reflection Sound on Site
In order to confirm the acoustic characteristics on site, the primary reflection sound from the blockwall was measured after the completion of construction. The positions of the sound source and themeasurement points are shown in Figure 7. The sound source was placed at a position without ceiling1.2 m
2.4 m

0.3 m

Gypsum board“Perforated block”↑Measurement points(omni-directional mic.)←Sound source (confronting speaker)

Figure 7 – Positions of the sound source andmeasurement points

Figure 8 – Frequency characteristics ofprimary reflection

reflectors to eliminate the influence of reflection from the ceiling, and a total of 10 measurement
points were set at intervals of 0.3 m from the front of the sound source. The impulse response wasmeasured at each measurement point, and the primary reflection from the block wall was cut out andanalyzed for frequency characteristics in the range from 200 Hz to 2 kHz.
The frequency characteristics of each point and the average of 10 points are shown in Figure 8.
Although peaks and dips are seen at each measurement point, each frequency is different, and noparticular peaks or dips are seen from the results of average energy.
The results of these measurements indicated that the peculiar characteristics of sound absorption
at certain frequencies can be avoided, and the blocks provide proper sound diffusion.
3.4 Verification of Acoustic Characteristics by Wave Acoustic Analysis
The presence of large peaks of sound absorption coefficient around 315 Hz with no or very small
air space cannot be explained because the frequency does not match with the air column resonanceabsorption by the block holes. In the future, in order to avoid the risk of interior materials having suchspecial acoustical characteristics, we thought that it is necessary to elucidate the mechanism of thissound absorption. To this end, we conducted computer simulation based on the finite element method.
Commercial analysis software (COMSOL) was used as the simulation software.
First, we made a 3D model of half of the block, with a hole of 100 mm square and 120 mm depth,
and calculated the normal incidence absorption coefficient and the random incidence absorption
coefficient, with no air space behind the block. An overview of the 3D model 1 is shown in Figure 9,
and the results are shown in Figure 10. In both incidence patterns, the peaks of sound absorption
coefficient were seen around 600 Hz. The frequency of the peak did not match the experiment results.
Since the air column resonance frequency is 709 Hz without considering the end correction, the peaks
around 600 Hz seen in these simulations are considered to be due to the air column resonance.
Next, we calculated the same patterns as the experiment, with 10 mm, 150 mm, 250 mm, 300 mm,and 400 mm air space behind the block. The results are shown in Figure 11. The large peak of thesound absorption coefficient was seen around 500 Hz with 10 mm air space, but the peak frequency
did not match the experiment results. In addition, it was indicated that the peak of the sound absorption
coefficient becomes smaller and the peak frequency shifts to lower frequencies as the depth of the airspace becomes larger. These results of simulation indicate that the cause of the peak of absorption
around 315 Hz is not a hole of 100 mm square and 120 mm depth.
Then, we observed the shape of the block again and considered the possibility that a Helmholtz
resonator is formed by the gaps between the blocks and the hollows on the side of the blocks (Figure12). We made a 3D model with a gap of 1 mm between the blocks, and calculated the normal incidence
absorption coefficient and the random incidence absorption coefficient, with no air space behind theblock. Thermoviscous acoustics was used to consider sound absorption by viscous friction ofHelmholtz resonance. An overview of the 3D model 2 is shown in Figure 13, and the results are shownin Figure 14. The large peaks of sound absorption coefficient were seen around 350 Hz in the normal
incidence pattern, and around 250 Hz in the random incidence pattern. These frequencies are close to
the experiment results (around 315 Hz). Therefore, the cause of the large peak of sound absorption
observed in the experiment is considered to be the absorption by the Helmholtz resonator formed by
the gaps and hollows between the blocks. In the experiment, the gaps between the blocks were covered
with masking tape to prevent air from entering and exiting, but it is considered that the effect wasinsufficient. On the other hand, since the gaps and hollows between the blocks are actually filled withmortar, it is considered that Helmholtz resonance does not occur on site.

Figure 9 – Overview of the 3D model 1 (Hole of 100 mm square and 120 mm depth)

Figure 10 – Simulation results with no air space: 3D model 1

Figure 11 – Simulation results with air space: 3D model 1

GapHollow

Figure 12 – Gaps and hollows between the blocks in the experiments

Figure 13 – Overview of the 3D model 2 (Gaps and hollows between the blocks)

Figure 14 – Simulation results with no air space: 3D model 2

4. MEASUREMENT RESULTS OF THE HALL
The measurement results of the hall are shown in Figures 15, 16, 17.
In the concert mode, the reverberation time was 1.8 seconds (average in the midrange (250–2 kHz),
unoccupied, same hereafter), and the average absorption coefficient was 0.20. These results satisfy
the design target values, and sufficient reverberation is achieved. The frequency characteristics areflat in the midrange, resulting in a well-balanced acoustic condition. The value of J LF (23%, averagein the range of 500–2 kHz) is large, and a sufficient spatial impression is achieved.
In the drama mode, the reverberation time was 1.4 seconds, and the average absorption coefficient
was 0.24. These results satisfy the design target values, and sufficient reverberation time range isachieved. The value of D50 (59%, average in the range of 500–2 kHz) is large enough, and speech
intelligibility without using the sound system is achieved sufficiently. The value of STI (0.63) using
the sound system is “good”, and it is possible to amplify speech clearly.
After the completion of construction, we confirmed the acoustic of the hall by playing instruments(violin and cello). Both instruments produced a clear and rich resonance, and thus a proper acoustic
condition for playing acoustical instruments was realized. In addition, the lateral reflection soundgave a soft impression without glare, which is attributed to the diffusion effect of the perforated blocks.

5. CONCLUSIONS
Through the acoustical design of Orinas Hall, we confirmed the risks of using special interiormaterial in the hall, as well as the importance of conducting a series of acoustic studies from thedesign stage to the construction stage. The acoustic characteristics of perforated blocks were revealed
by experiments using mockups in the construction stage, measurements of the reflection sound on site,
and wave acoustic analysis.
Finally, a good acoustic condition was achieved by acoustic adjustment in the construction stage.

Figure 15 – Reverberation time and average absorption coefficient of the hall*: Values of 80% occupied are calculated

D50JLF

Figure 16 – Distribution of JLF(average in the range of 500–2 kHz)

STI

Figure 17 – Distribution of D50 (average in therange of 500–2 kHz) and STIACKNOWLEDGEMENTS
The authors wish to express their sincere thanks to all the designers and all the constructors
involved in this project.