PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Practical aspects related to the measurement of the diffuse fieldabsorption coefficient in scaled reverberation rooms
Louena SHTREPI1; Francesca LATORELLA1; Andrea PRATO2; Alessandro SCHIAVI2; Marco
MASOERO1; Arianna ASTOLFI1
1
2

Politecnico di Torino, Italy

National Institute of Metrological Research, Italy

ABSTRACTThe scaled reverberation room has proven to be an excellent tool for acoustic consultants and professionals
alike to carry out comparison tests between acoustic materials and 3D systems or structures. The suitabilityof a scaled reverberation room is currently under investigation for the evaluation of the frequency-dependent
sound absorption. At present, no standard is available on the methods that could be used to derive acoustic
absorption coefficients from scaled measurements. In this work practical aspects of ISO 354 have been
investigated within a 1:5 scaled reverberation room: a balance has been sought between reducing sample size,
to reduce the manufacturing costs of materials, and finding the appropriate sample area, thickness, orientationand edges treatment, to obtain reliable values at mid and high frequencies. Four different materials have beentested. The paper discusses some of the findings of the measurements conducted on the same materials in afull-scale reverberation room according to ISO 354, in a 1:5 scaled reverberation room and in an impedance
tube according to ISO 10534-2. The absorption coefficients data collected have been effective in proving that
small reverberation room tests can provide compatible results compared to standard ones in 400-5000 Hz
frequency range.
Keywords: Sound, Absorption coefficient, Reverberation room, Scaled measurements

1. INTRODUCTION
The characterization of the absorption coefficient has been the focus of continuous research evenafter the consolidation of the reverberation room method described in ISO 354 (1) and ASTM Standard
C423(2). However, it has been shown that large discrepancies over the full frequency range of interest
may occur between different laboratories even though they fulfill the standard requirements (3).
Moreover, a prerequisite for the good reliability of this method is the presence of a diffuse field , which
is usually improved by equipping the room with sound diffusers of different types (4,5). However, it
should be considered that the efficiency of the diffusers is reduced when the frequency of the soundto be measured decreases (6). This basic assumption, i.e. the diffuse field conditions differences
between laboratories has been questioned lately aiming at new requirements to be defined in terms ofdiffusivity for the laboratories (7).
Given the continuous research on several critical aspects within the full-scale reverberation roommeasurements, this paper intends to explore also the acoustic conditions and reliability of the random incidence absorption coefficient within a scaled reverberation room. The need for scale measurementsof the absorption coefficient arises for several practical reasons related to the economical and timeefforts due to the preparation of the samples. The main advantage of a scaled room is that of being
able to analyze samples that are much smaller than the 12 m 2 required by the real scale roommeasurements (V>200 m 3 ). Two main difficulties appear when dealing with small reverberation
rooms: the lack of a sufficient degree of diffusivity of the acoustic field, and the diffraction due to thefinite size of the tested material (edge effects) especially at the low frequencies (8-10).
In particular, the scaled rooms have been of interest in the automotive sector (11), which usuallyrequires absorption data at medium-high frequencies. This interest has led to a SAE (Society ofAutomotive Engineers) standard (12) on the use of the small rooms for absorption coefficients
1louena.shtrepi@polito.it

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measurements. The common dimensions of these rooms are in the range 3-9.68m3 and a sample areaof 0.4-1.5m2 is usually sufficient (13). The most well-known one is the Alpha Cabin built by the Swisscompany Rieter, with a volume of 6.5m3, which is largely used in the automotive industry allowing
to measure 1.2m 2 of flat samples or trimmed parts. The design and size of the first Alpha Cabin is 1:3of the large reverberation room located in the Swiss Federal Laboratory of Material Testing andResearch Institute (EMPA). It provides accurate measurements in the frequency range of 400-10000Hz.
Further smaller scaled rooms are reported in Rey et al. (14) with a volume of 1.12 m3 and test sample
area of 0.3 m2, and Pacheco et al. (15) with a volume of 0.96 m3and and test sample area of 0.3 m2 .
A few studies have also compared scaled reverberation room measurements with those performed
in a full-scale reverberation room. A good match of the results has been shown in the range of
frequencies (above 400Hz) in which the small sized reverberation room fulfils the diffusion conditions,i.e. where the degree of diffusion is close to 1 (14, 16-18). However, these studies highlight also larger
discrepancies at low frequencies due to the reduced dimensions of the room. This seems to be morecritical as the size of the reverberation room becomes significantly smaller, as the resulting smaller
sample area produces a larger edge effect (8, 19). These effects are particularly high at low frequencies
in those cases when highly absorbing materials with high thicknesses are tested.
Further investigation is needed to clarify the compromise between reducing the sample size toreduce the manufacturing costs and finding the appropriate room conditions to obtain reliable results.
This study examines the measurements over four different materials in a 1:5 scale reverberation
room in the laboratories of the Energy Department (DENERG) of Politecnico di Torino, with the aimof evaluating the validity of the sound absorption coefficient meas urements. Three different
dimensions, three orientations and two conditions of the sample edges have been investigated. The
work involves a comparison with two other different measurements made on the same materials , thatis, in a full-scale reverberation room (ISO 354) and in the impedance tube or Kundt’s tube (ISO 105342).

2. METHODS
2.1 Tested Materials
A set of four materials has been tested (Figure 1) in scaled reverberation room. These materials
were available at INRiM (National Institute for Metrological Research), where have been also
performed the measurements in a full-scale reverberation room according to ISO 354 (1) and in an
impedance tube (or Kundt’s tube) according to ISO 10534-2 (20).
Materials A and B are made of glass wool panels with a density of 80 kg/m3 and an overall thicknessof 40mm and 50mm, respectively. Material C is a 15mm thick panel with a layer of 12.5 mm ofplasterboard. These three panels have a layer of 12.5mm of finish made of glass spheres and a marble
powder. Material D is composed of two superimposed layers of polyester fiber with a density of
80kg/m3 and a thickness of 30mm each. Also, this material has a cellular glass finish of 12.5mm overthe upper layer, and a mixture of rubber and concrete layer of 12.5mm at the bottom.

A

B
C
D
Figure 1 – A) e B) Glass wool panels with a finish of glass spheres and a marble powder (40 and50mm). C) Panel with a layer of plasterboard and one of glass spheres and a marble powder (15mm). D)
Double layered Polyester fiber panel (75mm).
Since the sound field in the reverberation room is strongly dependent on the configuration of themeasured material, the following parameters have been tested in order to evaluate the optimal
configuration of the scaled reverberation room. Measurements have been performed considering:- three different sample sizes for all the materials (0.6x0.4m; 0.6x0.6m; and 0.6x0.8m);- three different orientations on the floor (Fig.2) for the 0.6x0.4m and 0.6x0.8m sample sizes andeach material. It should be noted that the ISO standard recommends an oblique orientation(orientation 2).

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- two conditions with and without isolated borders (for A and B material only). An aluminumadhesive tape has been used seal the edges of each sample. It should be noted that the ISO standard
recommends to seal the edges of the sample with reflective material.
Three repetitions have been performed for each condition in order to evaluate the measurements
repeatability.

Figure 2 – Measurements in the scaled reverberation room of one of the samples with three different
orientations, and with sealed (BI) and unsealed (BNI) edges.
2.2 Reverberation Room Measurements (1:5 scale)
The scaled reverberation room (Figure 2) is a laboratory at DENERG (Department of Energy,
Politecnico di Torino, Italy). It is a 1:5 scale reproduction of the reverberation room of INRiM, which
has a volume of 296 m 3 . The room has been primarily built for random-incidence scattering coefficientmeasurements according to ISO 17497-1 (21). It is an oblique angled room with pairs of nonparallelwalls. The average room dimensions are 1.53 x 1.56 x 1.20 m, which lead to a volume of 2.86 m 3.
One of the sides consist of two movable parts that allow to have a large opening to ease the positioningof the sample. The construction material is self-supporting lightweight partitions of MDF (Medium
Density Fibreboard) with a thickness of 3.8 cm, which has been further covered by a layer of adhesive
film in order to maximize its reflective properties. The structure is raised from the ground on a wooden
structure and damping layers have been used along the joints and openings. In order to assure a highdiffusivity of the sound field (22), 8 diffusers (13.5% of the total room area) have been hanged overthe ceiling, which is considered as a more economical solution compared to boundary diffusers leading
to an almost equivalent effect on the diffusion of the sound field (5). A systematic study of the soundfield diffusivity evaluation of the room has been shown in (23). The diffusivity check has been
performed in accordance with ISO 354 based on the measurements of the equivalent absorption areaof a highly sound absorptive panel made of polyester fiber.
The measurement chain is composed of six BSWA Tech MPA45 microphones and two ITA
dodecahedral sources in order to perform 12 measurements (the minimum number required by ISO
354). The software used for the measurements is Matlab, combined with the functions of the ITA Toolbox (an opensource toolbox from RWTH-Aachen, Germany).
The set-up and the samples of each material have been produced in accordance with therecommendations of the ISO 354 standard:- Microphones should be positioned at a minimum distance of 1.5m from each other, 1m fromthe room surfaces and 2m from the sources (0.3m; 0.2m and 0.4m in 1:5 scale).
- The two sources must be at least 3m apart (0.6m in 1:5 scale) between them. A spatial averaging
is performed considering all the 12 source and microphones combination.
- The interval frequencies of interest is reported as third-octave-bands in the range 100-5000Hz.
- Controlled conditions of temperature (> 15 ° C) and humidity (between 30 -90%). A sensor has
been installed within the room.
- The sample must be rectangular with a ratio between width and length of 0.7-1.

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- The sides of the sample must be distant from the walls of the room by at least 1m (0.2m in 1:5
scale).
2.3 Reverberation Room (full-scale) and Kundt’s Tube Measurements
All the materials have been tested in the full-scale reverberation room at INRiM in accordancewith ISO 354 considering samples of 12m 2 . In this way the random-incidence absorption coefficient
has been obtained for each material in the frequency range 100-5000Hz.
Further measurements have been performed in the Kundt’s tube in accordance with ISO 10534-2
in order to measure the normal-incidence absorption coefficient. These values have been corrected for
diffuse incidence based on the approach proposed in Spagnolo and Benedetto (24), which is shown inall graphs as “Kundt’s tube S-B”. Two different tubes with 30mm and 50mm diameters have beenused in order to assure a higher accuracy at different frequency ranges. The 30mm tube allows tomeasure with a high accuracy in the frequency range of 1075-5733Hz and the 50mm in the frequency
range of 661-3440Hz.

Figure 3 – a) Full-scale reverberation room set-up of one of the samples, b) measurements set-up in the
Kundt’s tube with a diameter of 30mm (upper image) and 50mm (bottom image), and c) two circular samplesof the same material with a diameter of 30 and 50mm.

3. RESULTS
For the analysis of the results, the SPSS Statistics software has been used to perform the ANOVA(ANalysis Of VAriance). Specifically, the evaluated factors, i.e. the independent variables, are sample
size, sample orientation, and edge condition. The data have been first analyzed with a normality test
(Kolmogorov-Smirnov test), which showed a skewness and kurtosis within the range of -2 to +2 (25).
The uncertainty of the measurements has been assessed according to GUM (ISO / IEC Guide 98-3:
2008). The absorption coefficients obtained, together with the standard deviations of the results, areshown in Figures 4-6. These graphs show also the data from the full-scale reverberation room andKundt’s tube measurements for an easier comparison. Moreover, the graphs include also the Kundt’s
tube measurements corrected for the diffuse incidence condition (Kundt’s tube S-B).
3.1 Effect of the Sample Dimensions
The effect of the sample size has been analyzed for all the materials considering the recommendedorientation and edges condition, i.e. oblique orientation (Orientation 2) and sealed edges (BI). The
ANOVA showed that the variable related to the sample dimension (Fig. 4), is statistically significant
for all the samples except for sample C (panel with a thickness of 15 mm). Specifically the results
show: for Sample A → F (2, 162) = 30.351; p = 0.000; Sample B → F (2, 162) = 25.351; p = 0.000;
Sample C → F (2, 108) = 1.354; p = 0.263; and Sample D → F (4, 180) = 62.153; p = 0.000.
This exception for the sample C is probably due to its reduced thickness , which entails less edge
diffraction. Conversely, the edge effect becomes critical for panels with higher thickness (i.e. A, B,
D) when the panel reaches the smallest dimensions 0.6x0.4m. In these cases are evident irregularpeaks at mid and high frequencies for panels A and B, and also at low and mid frequencies for panel
D. It can be noticed that the sound absorption increases at 160-400Hz and above 800Hz withdecreasing samples size. The same result has been highlighted also in full -scale rooms by Jain et al.
(26). This can also be observed for the three different orientations as shown in Figure 5.
The correct scaling of the sample dimensions with respect to the room volume has been
investigated in Veen et al. (9). This study shows that a sample of 1.12m 2 could be considered in orderto have reliable results in a small reverberation room with a volume of about 6.4m3 . The ratio between

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the room volume and the sample area is comparable to that obtained with the room volume of 2.86m 3
and the sample dimensions 0.6x0.8m (0.48m 2 ) used in the present study (i.e. ≈6).

Figure 4 – The absorption coefficients obtained, together with the standard deviations of the results foreach material and sample dimension. Orientation 2 and sealed edges (BI) have been considered for samples
A, B, D, and unsealed edges (BNI) for sample C.

Figure 5 –The absorption coefficients obtained, together with the standard deviations of the results foreach material and sample orientation. Sealed edges (BI) have been considered for samples A, B, D, andunsealed edges (BNI) for sample C for the different dimensions.
3.2 Effect of the Sample Orientation
The effect of the sample orientation has been analyzed for all the materials and all the sample sizes.
The ANOVA shows that the differences due to the different orientations of the sample are notstatistically significant for all the materials considered. Specifically, for sample A → F (2.162) = 1.086

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and p = 0.340; sample B → F (2, 162) = 0.738 and p = 0.480; Sample C → F (2, 108) = 0.137; p =0.872; and Sample D → F (2, 216) = 0.368; p = 0.692. It is therefore possible to choose to keep thepanel obliquely oriented (Orientation 2), as suggested in the standard for full -scale measurements, orto calculate the absorption coefficients for the three orientations and perform an average of the thre edata. In this way, it is possible to take into account the small variations of the sound field due to thesample orientation.
Previous research (27) has shown that different orientations may cause discrepancies at lower
frequencies (below 400Hz) and that the smoothest curve is obtained for the oblique orientation, whichis the most asymmetric one. This study also highlighted that the other two orientations cause strong
peaks in the absorption coefficient, which were unrealistic for the tested porous mater ials. The authors
argued that the reason for this might be the parallel orientation of two edges of the material versus
two of the side walls of the reverberation room.
However, this is not observed in the study presented in this paper. Some differences b etween thethree orientations are observed at specific frequencies for the smallest sample dimensions, i.e.
0.6x0.4m. It can be observed that discrepancies at lower frequencies become lower when the material
has lower thickness, i.e. these differences are more evident in the case of panel D, which has athickness of 75mm. This finding is coherent with the results of Cops et al. (27), which showed thesame discrepancies between different orientations for samples with thickness higher than 100mm infull-scale.
3.3 Effect of the Sample Edges
The effect of the samples edges has been analyzed for sample A and B only (all corresponding
sample sizes). The results are shown in Figure 6. This factor did not result statistically significant for
sample A with F (1, 216) = 2.621 and p = 0.107. Conversely, it resulted statistically significant for
sample B with F (1, 216) = 127.806 and p = 0.000.
It should be noted that the absorption due to the edges of the sa mple is of little significance only
for sample A, which is 40 mm. This might be considered as a threshold that allows to avoid further
efforts when performing measurements below this thickness. This finding is coherent with what isreported in (10), i.e. values of the absorption coefficient obtained with the sealed and unsealed edges
may differ by more than 10% for noninotropic materials of thickness >50mm. Therefore, themeasurements with sample C (15mm) have been performed without any edge sealing, and co nversely,
for sample D (75mm) only with sealed edges.
As it is shown from Figure 6, the main discrepancies between sealed (BI) and unsealed (BNI) edges
occur at frequencies below 630Hz (maximized around 250 Hz) for the smallest sample dimensions,i.e. 0.6x0.4m.

Figure 6 – The absorption coefficients obtained, together with the standard deviations of the results for eachmaterial and edge condition, i.e. sealed (BI) and unsealed (BNI). Orientation 2 has been considered for all
samples and dimensions.

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3.4 Comparisons with full-scale reverberation room and Kundt’s tube measurements
The comparisons of the absorption coefficient with the full-scale reverberation room and Kundt’s
tube measurements show that these differences are minimized when the sample size is 0.6x0.8m withan oblique orientation (O2) and with sealed edges (BI). Moreover, the differences are reduced when
the sample thickness is below 40mm (i.e. Sample A and C). The differences are higher for frequencies
above 800 Hz for samples A, B and D and above 2000Hz for sample C. One of the causes is that theabsorption coefficient tends to a value close to 1 at these frequency ranges and influences the
diffusivity of the sound field generated within the scaled room. This has been observed also in Veenet al. (9), that showed higher discrepancies around 1000Hz for samples with thickness of 25mm.
Conversely, the phenomenon did not appear for smaller thickness of about 6mm. Moreover, this effect,
i.e. the effect of the room volume differences, has been highlighted in Jain et al. (26). In this study
both full-scale and small-scale reverberation rooms showed a good match in sound absorption
coefficient at mid frequencies from 400 Hz - 1000 Hz. However, the small room overestimates soundabsorption values above 1000 Hz. This is attributed to the use of Sabine’s formulas instead of Eyring’s
as highlighted by Vercammen (3).
Moreover, it can be highlighted that the differences obtained here between the scaled and full-scaleroom or Kundt’s tube measurements (sample A and C) are comparable with based the reproducibilityof the absorption coefficient measurements in 13 laboratories. This value is around 0.2 based on
Vercammen (3).

4. CONCLUSIONSThis paper aimed to explore the acoustic conditions and reliability of the random-incidence
absorption coefficient measured within a scaled reverberation room based on its main advantage isthat of being able to analyze samples of very reduced dimensions. The main difficulties that appear
when dealing with small reverberation rooms are the lack of a sufficient degree of diffusivity of theacoustic field, especially in the low frequencies due to the small size of the room and diffraction due
to the finite size of the tested material (edge effects). Moreover, the absorption coefficient isoverestimated at high frequencies even for thinner panels, which might be due to the use of Sabine’s
formula. Future comparisons will also test the results using Eyring’s formulas.
However, the small reverberation room demonstrated to be a reliable tool for mid and highfrequency acoustic measurements for panels with limited thickness, which showed differencescomparable to the reproducibility in full scale. However, it should be highlighted that due to limited
diffusivity of the sound field, the range of useful frequencies is to be considered above 400Hz formaterials that do not present an extremely high absorption (≈1). For very absorptive materials
overestimated values can occur also at high frequencies.
The measurements carried out over four different materials show that the use of a scaled room
might be an alternative to the measurements in full-scale or to the measurements in the Kundt’s tube.
Overall the results showed that in order to have reliable data:- the sample size should be of about 0,6x0,8m;- the sample thickness should be not higher than 50mm;- an oblique orientation of the sample is preferred;- the edges of the sample should be sealed with reflective material.
It should be noted that the main aim of the reduced-size room is that of being a very useful tool for
acoustic consultants, engineers and architects to carry out comparison tests between acoustic materials
and 3D systems or structures of limited dimensions. This results of great importance at the earlyphases of a design project, when a great number of alternatives are compared.

ACKNOWLEDGEMENTS
The authors thank the architect Chiara Devecchi, the engineer Paolo Onali and the engineer Davide
Squarciapino for having provided some of the materials used in these measurements.

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