732

J. Eng. Technol. Sci., Vol. 52, No. 5, 2020, 732-744

Low-Cost and Portable Sound Reduction Box: Innovation
for Acoustic Material Performance Measurement
Budi Purwanto*, Melania H. Aryantie, Zulfachmi & Rina Aprishanty
Center for Research and Development of Quality and Environmental Laboratory
Research, Development, and Innovation Agency, Ministry of Environment and Forestry
of the Republic of Indonesia, Kawasan Puspiptek Gedung 210, South Tangerang 15310,
Indonesia
*E-mail: sipurman@gmail.com
Highlights:





Measurement of the sound reduction index with a sound intensity analyzer using the
proposed sound reduction box is an alternative low-cost and portable sound insulation
testing method.
The proposed sound reduction box is a miniature reverberation chamber for testing
wood board samples as soundproofing material.
The proposed sound reduction box is made from medium density fiberboard.
The combination of the proposed sound reduction box with a sound intensity analyzer
was able to produce similar reduction index measurement results as a standard sound
insulation testing system.

Abstract. A sound reduction index (RIc) is a laboratory measurement of the sound
insulating properties of a material or building element, commonly conducted using
a reverberation chamber and an anechoic chamber (SIC), which requires high
expenses. This study aimed to perform RIc analysis using a sound reduction box
(SRB) to assess the accuracy and precision of the associated result compared to an
SIC. The SRB is a miniature reverberation chamber innovation that is owned by
the Center for Research and Development of Quality and Environmental
Laboratory (P3KLL). The anechoic chamber is substituted by open space as freefield environment. The methods used in this study are based on ISO 15186-1 and
ISO 717-1. Measurement was executed using a sound intensity analyzer and data
interpretation was done by employing statistical analysis. The types of insulating
materials tested were wood boards made of Shorea sp., Swietenia sp. and
Dryobalanops sp. with a thickness of 2 cm and 4 cm. Test material measurement
was done using the same measuring instruments, sound generators, sound
amplifiers, and personnel. The results show that the RIc values were almost the
same for both methods (SIC and SRB). When the weighted sound reduction index
(Rw) rating calculated from the RIc was compared between the SIC and the SRB,
the results were not statistically different. It is interesting that an SRB can be
developed in the future as an alternative device for acoustic materials testing.
Keywords: portable sound reduction box; sound insulation test; sound reduction index;
anechoic chamber; sound intensity.
Received November 9th, 2019, 1st Revision January 29th, 2020, 2nd Revision April 3rd, 2020, 3rd Revision June
11th, 2020, Accepted for publication September 9th, 2020.
Copyright ©2020 Published by ITB Institute for Research and Community Services, ISSN: 2337-5779,
DOI: 10.5614/j.eng.technol.sci.2020.52.5.9

Low-Cost and Portable Sound Reduction Box

1

733

Introduction

Three different types environments in which noise sources are found in modern
laboratories are: anechoic chambers (free field), hemi-anechoic chambers (free
field over a reflecting plane) and reverberation chambers (diffused field). In an
anechoic chamber, all of the boundaries are highly absorbent and the free-field
region extends very nearly to the boundaries of the chamber, while in a
reverberation chamber all boundaries are acoustically hard and reflective. In an
anechoic chamber, the chamber surfaces are treated with acoustic material such
that surface absorption is practically 100% [1]. The reverberant field extends
throughout the volume of the chamber, except for a small region in the vicinity
of the source [2].
The accurate assessment of the sound insulation properties of panels and
partitions is an important area in acoustics [3]. The sound reduction index (RIc)
of partitions is used to qualify a considerable range of structures, from fuselage
panels to building elements [4]. It is a parameter for measuring acoustic materials
that is useful for determining the most appropriate material to use as noise
insulation. This measurement is generally done using a reverberation chamber
and an anechoic chamber, which requires high expenses. However, measurement
on samples of unattended materials is cheaper than installed materials in terms of
their application. The various methods for sound reduction index measurement
are based on sound pressure (ISO 10140:2010) [5] or sound intensity (ISO 151861:2003 and ISO 15186-3:2010) [6-8]. The major variations in the results between
the two methods are due to the fact that the pressure-to-pressure method measures
the sound reduction index of all the boundary walls inside the SIC, including the
wall, the sample under test, baffles and mountings. In contrast, the intensity
method only measures the transmission loss of the sample scanned by the
intensity probe, so it costs less time to do the measurement and it can also be used
on structures in situ [9]. Despite the different methods used, the results from both
methods should be comparative to one another [10].
Testing of specimens of noise reducing materials requires sound insulation
chamber (SIC) facilities consisting of a reverberation chamber (RC) and an
anechoic chamber (AC), as shown in Figure 1. Anechoic chambers (ACs) and
reverberation chambers (RCs) are two very different types of laboratory facilities
and are widely used in acoustics measurements as well as in electromagnetics
[11].
The functions of the two chambers are complementary in acoustic measurements.
On a large physical scale both of them require extensive space and the related
development costs are not low. The advantage is that testing materials with large
dimensions or at industrial scale, such as mining machines, can be carried out at

734

Budi Purwanto, et al.

these facilities [12]. However, for research/laboratory purposes, a small sample
size is sufficient, where the facilities are accessible and the cost is relatively low.
Also, it is often not possible to produce large quantities of the materials [13]. In
addition, not many noise research institutions have RC and AC testing facilities.

Figure 1 RIc test in SIC.

Through this research, a miniature RC for testing wood board specimens is
proposed. The miniature RC in the shape of a sound reduction box (SRB) has
dimensions of 1 meter in width, 1 meter in length and 1 meter in height, and its
constituent material is medium density fiberboard (MDF), as shown in Figure 2.
With an SRB, the cost of RC construction can be reduced and the space needed
is less extensive. Test results of an SRB or a small-scale reverberation chamber
can be considered intermediate results before being verified with a real RC [1,14].
The function of the AC is substituted with open space to represent a free-field
environment, with the background noise level difference higher than 10 dB and
no large reflective objects.
Research on making small-scale reverberation chambers is still limited because
of issues with low frequency cut-off. Based on the Garuda portal
Kemenristekdikti and Ebscohost, in the last 10 years such research has only been
conducted by Kim, et al. and Rajaram, et al. [14,15]. The difference between this
research and the two previous researches is that this study focused on testing
several types of wood boards, which were judged to be feasible as noise
mitigating materials. RIc and Rw are used as parameters in the legal regulations
for protection against noise in several European countries concerning the sound
insulation of partitions between dwellings [16] and are considered in the design
of road, rail, marine and airborne vehicles, where acoustic comfort of passengers
matters [17]. Consequently, the acoustic properties of a material are of great
importance [18]. The test results from SRB were compared with the test results
conducted at the full-scale SIC facilities of the Bandung Institute of Technology.

Low-Cost and Portable Sound Reduction Box

735

The purpose of this study was to develop a miniature reverberation chamber and
perform an analysis of the accuracy and precision of the associated test results
through comparison with an SIC as a reference. The benefits of this research are
that the proposed method provides a solution for testing small-size/laboratory
scale soundproofing materials, specifically made from wood. The SRB can be
further developed for other types of specimens, such as metals, composites, and
others. Another limitation is that this miniature RC still needs improvement
related to the verification and validation of the methods compared to the
requirements for reverberation chambers according to acoustic standards [5].

2

Materials and Methods

The research was conducted in Serpong and Bandung, Indonesia in 2017. We
used the SRB as an RC substitute (Figure 2) and open space as an AC substitute.
Utilizing the research facility in the Center for Research and Development of
Quality and Environmental Laboratory (P3KLL) Serpong, the SRB was lifted 1.5
meter from the ground, positioned at more than 5 meter away from a reflective
wall and was tested with low background noise. In comparison, we used a fullscale sound insulation chamber that consisted of a full-scale AC and RC in the
Center for Advanced Sciences (CAS) Bandung Institute of Technology (Figure
3). The scope of this research was to develop a portable sound reduction box for
conducting RIc tests and Rw rating of soundproofing materials using sound
intensity.

Figure 2 SRB with wood board testing material.

736

Budi Purwanto, et al.

(a)

(b)

Figure 3 Sound insulation chamber (SIC) located in the Center for Advanced
Science (CAS), Bandung Institute of Technology (ITB): (a) reverberation
chamber, (b) anechoic chamber.

The methods used in this study are based on ISO 15186-1 and ISO 717-1, using
a sound intensity analyzer, statistical data processing, statistical analysis, and data
interpretation. A literature study was conducted to find out the novelty of the
research and the research preparedness.
Determination of the wood species was based on availability, modulus of
elasticity, durability, and especially specific gravity to meet the criteria as a sound
proofing material of 20 kg/m2 [19]. Test material measurement was done using
the same measuring instruments, sound generators, and sound amplifiers. The RIc
measurements was done using the same measuring instruments, sound
generators, sound amplifiers, and personnel. The specimens that were tested
using the SRB were made from wood board with square-rectangle sides of the
rectangle of 1 meter, with two different thicknesses, i.e. 2 cm and 4 cm.
As a rule of thumb, the minimum density of an effective soundproofing material
is 20 kg/m2 [19]. In addition, for selecting the test material to be used, also the
cost and availability of the materials were considered. After considering all the
requirements, wood from Shorea sp., Swietenia sp., and Dryobalanops sp. were
selected as materials to be tested, as shown in Figure 4.
The samples tested in the SIC were rectangular, with size 140 cm x 140 cm and
2 cm and 4 cm thickness for each rectangle, while those measured in the SRB
were rectangular, with size 100 cm x 100 cm and the same thicknesses (2 and 4
cm). The test used Brüel & Kjær 2734 as a signal generator and also as an
amplifier.

Low-Cost and Portable Sound Reduction Box

(a)

737

(b)

Figure 4 Picture of several test materials used in the research: (a) front view, (b)
side view.

Furthermore, a Brüel & Kjær 2270 sound intensity analyzer, a Brüel & Kjær 2250
sound pressure level analyzer and a Brüel & Kjær 4231 sound calibrator [20]
were also used. In general, RIc measurements consist of three working steps:
sound pressure level measurement, sound intensity measurement and calculation
of the RIc using formula below [5].
𝑆
𝑆

RIC = Lp − 6 − {LI + 10log ( 𝑚 )} + 10 𝑙𝑜𝑔 (1 +

𝑆𝑏2 λ
)
8𝑉2

(1)

where:
Lp
LI
Sm
S
Sb2
V2


: the average sound pressure level in the source chamber
: the measured intensity level normal to the measurement surface
: the area of the measurement surface
: the area of the test specimen
: the area of all the boundary walls in the receiving chamber
: the volume of the receiving chamber
 the wavelength of the mid-band frequency

The sound pressure level in the reverberation chamber (RC) was measured as a
source chamber, and at least 3 measurement points were measured in the RC
located in the SIC and the SRB. Furthermore, sound intensity level measurements
were done in the AC located in the SIC and on top of the SRB as receiving
chamber.

738

Budi Purwanto, et al.

Each material tested was divided into 4 imaginary segments with scanning
methods [6], as shown in Figure 5, after which scanning was applied at 20
seconds of duration to all 4 segments. The RIc values were calculated in a onethird octave band frequency, from 100 Hz to 3150 Hz.

Figure 5 Scanning direction in sound intensity level measurement [21].

The RIc value were then compared with reference values from ISO 717-1:2013
[22] as listed in Table 1 at the measurement frequencies within the range 100 Hz
to 3150 Hz to get the Rw rating values (concluded from 500 Hz) for each of the
tested materials [23]. This frequency range was also chosen to measure the
potential impact on people, considering speech, music and cars as noise sources
[24].
Table 1

Reference values for rw rating of airborne sound.
Frequency
(Hz)
100
125
160
200
250
315
400
500
630
800
1000
1250
1600
2000
2500
3150

Reference values
(dB)
33
36
39
42
45
48
51
52
53
54
55
56
56
56
56
56

Low-Cost and Portable Sound Reduction Box

739

The data measurements obtained from the two facilities (SIC and SRB) were then
processed with Measurement Partner Suite BZ 5503 [20] and the data were made
available in Microsoft Excel format. Finally, all collected data were analyzed
using analysis of variance and data comparison and a means separation test was
done using the paired t-test.

3

Results and Discussion

All RIc data obtained from this research, concerning two chamber facilities, three
different wood materials, two different thicknesses, recorded at 16 different
frequencies, are presented in Table 2, indicating a compromising value at 250 Hz.
Theoretically, the SRB cut-off frequency is under 630 Hz [25], but looking at the
trend indicated by Figures 6, 7 and 8 we can still be confident about 500 Hz.
Table 2

RIc of the test materials in one-third octave band.

Figure 6 RIc values of Shorea sp. tested with SRB and SIC.

740

Budi Purwanto, et al.

Figure 7 RIc values of Swietenia sp. tested with SRB and SIC.

Figure 8 RIc values of Dryobalanops sp. tested with SRB and SIC.

All RIc readings were then converted using reference values provided by ISO
717-1:2013 [18]. The results of the converted values are expressed as weighted
sound reduction index (Rw) and are summarized in Table 3. In most cases the
RIc values increased with an increase of the reading frequency. Note also that the
sound reduction, indicated by the RIc values, also increased with an increase of
wood thickness. It has been reported that RIc measurement using a sound
intensity analyzer conducted in a laboratory is considered to be more precise than
field measurements [26].
Statistical inference in terms of effect of treatment, in this case focused only on
two different chambers, is presented in Table 4. The other parameters observed
will be discussed in a separate report. Based on Table 3, it is apparent that the
level of the weighted sound reduction index in the SIC was somewhat higher than
that of the SRB. However, based on a mean separation test employing the paired

Low-Cost and Portable Sound Reduction Box

741

t-test, shown in the analysis of variance table (Table 4), it can be concluded that
there was no statistically significant difference between the two chambers.
The average value of the different values of Rw between the two measurement
procedures was only 1.7 dB. This research also indicates that all the wood
materials used as covering, Shorea sp., Swietenia sp., and Dryobalanops sp., were
comparably the same in terms of their physical quality. The average magnitude
of Rw for Shorea sp. was 34 dB, while the average magnitude of Rw for Swietenia
sp. was 33 dB, and the average magnitude of Rw for Dryobalanops sp. was also
33 dB.
It is interesting to note that there was no significant statistical difference between
the SIC in terms of the weighted sound reduction index (Rw) and the SRB. This
strongly suggests that an SRB can be a substitute for an SIC facility. Bearing in
mind that the cost of an SRB is much lower than that of an SIC facility, it is
important to verify this result in the future.
Table 3

Rw of the test materials.

Test materials
Swietenia sp. 4cm
Shorea sp. 4cm
Dryobalanops sp. 4cm
Swietenia sp. 2cm
Shorea sp. 2cm
Dryobalanops sp. 2cm
Table 4

SIC
(dB)
37
37
36
31
33
32

SRB
(dB)
33
34
34
31
33
31

Difference
(dB)
4
3
2
0
0
1

Analysis of variance of T-test between SIC and SRB.

Test materials
Mean
Variance
Observations
Pooled variance
Hypothesized mean difference
df
t Stat
P(T <= t) one-tail
t Critical one-tail
P(T <= t) two-tail
t Critical two-tail

SIC
34.33333333
7.066666667
6
4.466666667
0
10
1.365895912
0.100951102
1.812461123
0.201902204
2.228138852

SRB
32.66666667
1.866666667
6

742

4

Budi Purwanto, et al.

Conclusion

This research showed that the Rw tests carried out with the SIC in Bandung
Institute of Technology and and an SRB owned by the Center for Research and
Development of Quality and Environmental Laboratory were not significantly
different. The types of wood boards tested were Shorea sp., Swietenia sp. and
Dryobalanops sp. at 2 cm and 4 cm wood thickness, so for now SRB is proposed
as a miniature RC for testing wood board samples as soundproofing materials.
However, the results of this research still require further validation and
verification before being designated a prototype. The advantage of using an SRB
are the much lower cost to build one and the small size of the test specimens
required. Still, it is important to note that it require investment in a sound intensity
analyzer, which is costly and does not cover low frequencies.

Acknowledgements
The author wishes to thank all personnel involved in the research process and also
Iwan Prasetyo, Ph.D., Faculty of Industrial Technology, Bandung Institute of
Technology. This research was funded by the Center for Research and
Development of Quality and Environmental Laboratory, Research, Development
and Innovation Agency, Ministry of Environment and Forestry of the Republic
of Indonesia.

Nomenclature
AC
=
CAS =
ISO
=
ITB
=
MDF =
P3KLL =

RC
RIc
Rw
SIC
SRB

=
=
=
=
=

anechoic chamber
Center for Advanced Sciences
International Organization for Standardization
Institut Teknologi Bandung (Bandung Institute of Technology)
medium density fiberboard
Pusat Penelitian dan Pengembangan Kualitas dan
Laboratorium Lingkungan (Center for Research and
Development of Quality and Environmental Laboratory)
reverberation chamber
sound reduction index
weighted sound reduction index
sound insulation chamber
sound reduction box

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