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

Some comments on using a reference absorber for absorption
measurements in reverberation rooms
Volker WITTSTOCK1; Heinrich BIETZ1; Sylvia STANGE-KÖLLING
1

Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany

ABSTRACTThe uncertainty of absorption measurements in reverberation rooms is considered to be unacceptably large
for certain applications. A long-discussed possibility to reduce the uncertainty is to measure the absorption
relative to the known or defined absorption of a reference absorber. To check this approach, measured
absorption coefficients from a large data base with round robin results are explored. This check is possible
because several different specimens have been measured in the same set of reverberation rooms in some of
the round robins. Furthermore, a special reference absorber has been proposed recently. This material wasused for a comparison measurement at PTB. About 10 different laboratories measured the sound absorptionof the proposed reference absorber, using their own measurement equipment and data analysis, in one of
PTB’s reverberation rooms. Additional investigations involved measurements of the reference absorber andsome other specimens in two different reverberation rooms at PTB. Measurement and analysis results arepresented and discussed in view of the current proposals for the revision of ISO 354.
Keywords: Sound absorption, reverberation room

1. INTRODUCTION
The uncertainty of absorption measurements in reverberation rooms can be estimated according to(1 and 2). There, many round robins are statistically analysed, and the results are compressed into a
simple numerical estimate for the standard deviation of reproducibility and repeatability. It is planned
to incorporate this method in a possible future international standard ISO 12999-2, which is now underdevelopment. Nevertheless, this is a mere description of the current situation and there is no possibility
to reduce the uncertainty which could be deduced from (1 or 2).
One proposal for reducing the uncertainty is the application of an appropriate reference absorber.
This could be used for at least two different purposes. The first is to adjust a reverberation room in acertain manner. In the current ISO 354 (3), such a procedure is prescribed . A highly absorbing
specimen is to be placed in the reverberation room and its absorption is to be measured with anincreasing number of diffusors. The number of diffusors required for a maximum measured absorption
is then to be used for all other specimens. A current proposal for revising ISO 354 is to prescribe one
particular reference absorber, and to require that reverberation rooms are adjusted to yield a given
minimum absorption. This is a much stronger requirement than in the current standard which is hoped
to reduce the overall spread of measured sound absorptions.
Another application of a reference absorber would be to generally refer all measured absorption
coefficients to the measurement results obtained with the reference absorber individually in eachlaboratory. This would reduce the spread of the measurement results if the uncertainty of the correction
is sufficiently small. This is checked in this contribution in chapter 2 using existing round robin data.
To gain experience with the proposed reference absorber, PTB included sound absorption
measurements into the comparison measurements. The participation in these measurements iscompulsory for laboratories recognised by DIBt – the highest German building authority. A list ofthese laboratories is found under https://www.schall-pruefstellen.de. Usually, there are further
companies and institutions with own laboratories, which voluntarily take part in the comparison
measurements. So, different teams come to PTB’s reverberation room and measure the soundabsorption of the identical reference absorber using their own equipment. Results from thesemeasurements are reported in chapter 3.
1

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volker.wittstock@ptb.deFinally, absorption measurements in two different reverberation rooms at PTB were performed
with the proposed reference absorber and several other absorbers. Results from these measurements
are reported in chapter 4.

2. ANALYSIS OF EXISTING ROUND ROBIN DATA
2.1 Existing data sets
To check whether a reference absorber may be useful, only the round robin data where more than
one product was measured in the same set of reverberation rooms can be used. An overview on suchdata sets is given in table 1. It is a subset of data from earlier investigations (1 and 2). It is consideredthat all these measurement results were obtained applying the procedure of ISO 354 (3). Measurement
results from (7) were obtained following ASTM C 423 (8), nevertheless, in terms of uncertainty theseresults matched the ISO-results quite well (1). Therefore, this data set is also included in the analysis.
Table 1 – Round robins with more than one test object measured in the same set of reverberation
rooms
Year
2011

2009

2006

1974

ObjectNumber of rev. rooms

15 mm wetfelt panel (x)22

50 mm glass wool panel

22

12,5 mm perforated plaster board panel

22

100 mm mineral wool (x)13„Dam board“ plywood/mineral wool

13

18 mm plywood on 100 mm mineral wool

13

33 mm Foam

13

Ceiling system, Mounting Type A (x)16

Ceiling system, Mounting Type E400

12

10 Baffles of mineral wool

11

50 mm mineral wool (x)1250 mm mineral wool on battens

12

6 mm hardboard on battens

11

perforated gypsum board

12

Organisation
CEN TC 126
WG 11 (4)PEUTZ (5)ASTM (6)Nord-Test (7)2.2 Correlation analysis
For a first check, possible correlations were explored by plotting the measured sound absorption
for one specimen as a function of the sound absorption for another specimen. Some examples areshown in Figure 1 and 2. In these graphs, each data point represents one specific laboratory which
performed measurements with the two mentioned specimens. When all points would be in a straight
line, strong correlation would occur between the measured absorption coefficients for different
specimens. This would mean that each laboratory would always measure with a certain offset which
could be compensated for e.g. by using a reference specimen.

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Figure 1 – Scatter plot between absorption coefficients of the wetfelt and the perforated plasterboard from [4], measured in different reverberation rooms, left graph: 125 Hz, right graph: 1 kHz

Figure 2 – Scatter plot between absorption coefficients of the 50 mm mineral wool and themineral wool baffles from [7], measured in different reverberation rooms, left g raph: 125 Hz, rightgraph: 1 kHz
Measurement results are available in one-third octave bands. The visual correlation analysis was
performed in octave and one-third octave bands where the former was calculated by linearly averaging
the three one-third octave band sound absorption coefficients which belong to the correspondingoctave band.
Observed correlations cover a large spread. Many graphs show a considerable degree of correlation
like presented in Figure 1. There are other examples where there is no correlation at all like presented
in Figure 2. Correlations are detected for the octave band and for the one-third octave band values.
A statistical descriptor for the correlation is the correlation coefficientσሺ‫ ݔ‬െ ‫ݔ‬ҧ ሻሺ‫ ݕ‬െ ‫ݕ‬തሻ‫ݎ‬ൌ(1)ඥσሺ‫ ݔ‬െ ‫ݔ‬ҧ ሻଶ σሺ‫ ݕ‬െ ‫ݕ‬തሻଶwhich was calculated for all cases of table 1. The test specimen chosen as the independent variable
x is marked in table 1 by (x). Observed correlation coefficients are sometimes negative or close tozero (Figure 3), but many of them indicate a significant degree of correlation. Performing the analysis
in octave bands smoothens the curves considerably and it increases the correlation coefficient in mostcases.

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Figure 3 – Correlation coefficients calculated from measured absorption coefficients for all datafrom table 1, left hand side: one-third octave band values, right hand side: octave band values
2.3 Application of a reference absorber correction
A correction of the measured absorption coefficients would re duce the spread of the results if thecorrelation as described in clause 2.2 would be sufficiently large. This is tested by applying thecorrectionߙୱ୮ୣୡǡ௜ߙୱ୮ୣୡǡ௜ǡ୰ୣ୤ ൌ
ߙ
തതതതത
(2)
ߙ୰ୣ୤ǡ௜ ୰ୣ୤
where ߙୱ୮ୣୡǡ௜ is the absorption coefficient of a certain specimen measured in laboratory i, ߙ୰ୣ୤ǡ௜ is theabsorption coefficient of the reference specimen measured in laboratory i and ߙതതതതത୰ୣ୤ is the reference value forthe absorption coefficient. For the analysis, the latter is chosen to be the mean value from all laboratories in
the round robin. The reference specimens are those marked by (x) in table 1.

Figure 4 – Ratio between standard deviations of the corrected sound absorption coefficients tothe standard deviation of the direct measurement results, left hand side: correction with one-thirdoctave bands, right hand side: correction with octave band values for th e reference absorber
The correction according to eq. (2) was applied, and the standard deviation of the corrected
absorption coefficients was calculated. This standard deviation was divided by the standard deviationof the uncorrected absorption coefficients to see the change. When the whole calculation is performed
in one-third octave bands, there are many cases where the standard deviation is significantly reduced(Figure 4, left hand side). But there are other cases where it increases. Omitting the w orst cases, theratio of standard deviations is roughly between 0.5 and 2.0. Correcting the measured one -third octave
band absorption coefficients ߙୱ୮ୣୡǡ௜ with octave band values for the reference absorber ߙ୰ୣ୤ǡ௜ and ߙ
തതതതത
୰ୣ୤
gives a slightly smaller ratio of standard deviation, i.e. it improves the effect of the correction.
Since there is no general dependency on frequency observed, neither for the correlation coefficients(Figure 3) nor for the standard deviation ratio (Figure 4), it is rather interesting to explore which correlation
coefficient is required for a reduction of the standard deviation. The corresponding graph (right hand side of
Figure 5) shows that correlation coefficients above 0.8 guarantee a reduction of the standard deviation. For
smaller correlation coefficients, increased or reduced standard deviations are observed.

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Another interesting question is whether the reference absorber should have a similar absorption as the test
specimen. From the right-hand graph of Figure 5, it can be seen that the largest reductions of the standard
deviation indeed occur for an D-ratio of 1. Nevertheless, a similar absorption coefficient between thespecimen under test and the reference absorber does not guarantee a reduction of the standard deviation.

Figure 5 – Ratio between standard deviations of the corrected sound absorption coefficients tothe standard deviation of the direct measurement results, left hand side: as a function of thecorrelation coefficient, right hand side: as a function of the ratio between the absorption coefficientand the absorption coefficient of the reference absorber

3. Comparison measurements at PTB
Measurements are performed in PTB's non-rectangular reverberation room. The room volume is
204 m³, the surface area is 210 m². The room is equipped with altogether 7 diffusors with a size of
2,4 m² (one-sided) which were arranged according to the procedure of ISO 354 (3). Furthermore,
additional low-frequency damping was added to achieve sufficient modal overlap at low frequencies.
Each measurement team had the task to determine the absorption coefficient of a given test
specimen according to ISO 354 (3). The arrangement of diffusors and additional absorption was not
to be changed by the participants. Nevertheless, loudspeaker and microphone positions as well as the
position of the test specimen had to be chosen by the participating acousticians. Each team used its
own measurement equipment, i.e. loudspeaker, microphones and sound analysers. For test purposes,
the frequency range of measurements was extended to one-third octave bands from 50 Hz to 10 kHz.
The test object is the currently proposed reference absorber. It consists of mineral wool panels of
1.2 m x 1.2 m x 0.1 m. These panels are to be installed by each measurement team on the floor of thereverberation room to give an absorbing element of 3.6 m x 3.0 m x 0.2 m. To reach these dimensions,some panels had to be cut which was done by PTB. All teams used the same set of panels. A 19 mmcore board frame is provided by PTB so that the participants can cover the edges of the absorber.
The absorber is stored in the hall where the reverberation room is situated. Climatic conditions inthis hall and in the reverberation room are very similar since there is no air-conditioning installed. All
participants performed their own calculations based on their own measurement results. Only thevolume of the reverberation room was provided since it is difficult to determine.
During the measurements, the static pressure was between 996 hPa and 1025 hPa, the temperature
between 291 K and 296 °C and the relative humidity between 35 % and 49 %.
In general, measured absorption coefficients are within a relatively narrow range (Figure 6, lefthand). At the lowest frequencies from 50 to 80 Hz, the scatter is surprisingly small whereas it issignificantly larger at 100 Hz and 200 Hz. At 100 Hz, measured absorption coefficients cover the verylarge range between 0.35 and 0.85. For medium frequencies, the scatter range becomes narrower.
Starting from about 4 kHz, it increases again, probably due to the air damping effect.
With the exception of 100 Hz and 200 Hz, the standard deviation of the measured absorption
coefficients is much smaller than the standard deviation of reproducibility calculated by (1), see Figure
6, right hand. This result was expected, since the reproducibility standard deviation is considered tobe caused mainly by the differences of the reverberation rooms. It is furthermore interesting to noticethat the standard deviation from PTB's comparison measurement is very close to the repeatability
standard deviation estimated from (1) with the mentioned exceptions. This indicates that the exact
choice of microphone and loudspeaker positions as well as the measurement equipment and evaluation
algorithms for the calculation of reverberation time are only of minor importance for the overall
uncertainty. The main influence seems to be the reverberation room itself.

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Another interesting question is how important the correction for air absorption is. To test this, theabsorption coefficient was calculated from the directly reported reverberation times assuming thesame environmental conditions for the measurement with and without the specimen in place. This is
identical to the assumption of a constant air damping. The standard deviation of these absorption
coefficients is identical to the one with air damping correction included (Figure 6, right graph).

Figure 6 – Sound absorption coefficients measured by 12 independent teams in PTB’s nonrectangular reverberation room (left hand side) and standard deviation of the results compared to thestandard deviation of reproducibility and repeatability calculated by (1)Figure 7 shows the total absorption area calculated from the mean reverberation times measured
by all participants with and without the test specimen. Furthermore, the absorption areas corrected for
air absorption A1 (without specimen) and A2 (with specimen) are shown as reported by theparticipants. The error bars indicate the standard deviation of the reported absorption areas. Since thedifference of these two values has to be calculated, it is obvious that the result has a large uncertainty
at high frequencies due to the large error bars in this range. The maximum absorption mentioned in
ISO 354 (3) is also displayed. The additional low-frequency damping is adjusted to be close to thatvalue to increase the modal overlap at low frequencies for the measurements without the specimen.
Nevertheless, the equivalent absorption area A1 is below the maximum absorption area at all
frequencies.

Figure 7 – Total equivalent absorption areas with and without specimen, equivalent absorption
areas corrected for air absorption A1 (without specimen) and A2 (with specimen): mean values andstandard deviation calculated from participant's data and maximum equivalent absorption area given
in ISO 354 (3)4009

4. Additional measurements in two reverberation rooms at PTB
The measurements were performed in two different reverberation rooms at PTB. One is the non rectangular reverberation room (NRR) described in clause 3.1. The other room is the rectangularreverberation room (RRR) with a volume of 237 m³ and a surface area of 246 m². The room is equippedwith a rotating diffusor which rotates at 20 rpm during the measurements and has a surface area of
24 m² (both sides).
Test specimens are the reference absorber and additional specimens. One further specimen
consisted of commercially available high density mineral wool panels. The nominal size of one panel
is 600 mm x 600 mm x 15 mm. An array of 5 x 6 panels with a total surface of 10,6 m² was used. For
a next set of tests, 12 specially designed absorbers were employed. These consist of a 1000 mm x 700
mm x 125 mm metal frame filled with a special polyester fleece material. One side is covered with awooden panel of 16 mm (5 Absorbers) or 8 mm (7 Absorbers) thickness. In a first series, theseabsorbers were installed in a 4 x 3 array to create a plane absorber with a surface area of 8,6 m². The
absorbers were arranged in three different configurations: 1.) All open sides facing upward; 2.) As achequerboard pattern with 6 covered sides (all 8 mm panel) facing upward; 3.) All covered sides facing
upward. In a second series, always two absorbers were placed together on edge (long side) to create
six individual objects, which were distributed randomly in the reverberation room. Again, threedifferent configurations were employed: 1.) all uncovered sides fac ing outward; 2.) Six covered sides(all 8 mm panels) facing outward; 3.) All covered sides facing out ward. For this series, th e equivalent
absorption area A and the absorption coefficient Dobj according to ISO 20189 (9) were calculated.

Figure 8 – Left-hand side: measured absorption coefficients, solid curves: non -rectangularreverberation room, dashed curves: rectangular reverberation room, colour code in the right graph,right-handside: difference related to uncertaintyy for directlyy measured absorption
coefficientsgp

Figure 9 – Left-hand side: difference related to uncertainty for directly measured absorption
coefficients, right-hand side: probability density functions of the differences related to uncertainty
Due to the variety of absorbers used, measurement results cover a large range of absorption
coefficients (left graph of Figure 8). The difference of the measured absorption coefficients from bothrooms was related to the uncertainties byߙ୒ୖୖ  െ  ߙୖୖୖ
] ൌ

(3)
ඥ‫ݑ‬ଶ ሺߙ୒ୖୖ ሻ ൅ ‫ݑ‬ଶ ሺߙୖୖୖ ሻ
where the uncertainties were estimated by the standard deviation of reproducibility from (1).
The results from both rooms clearly exhibit significant systematic deviations below 2 kHz (rightgraph of Figure 8). The rectangular reverberation room usually gives the larger results with the4010

exception of the 125 Hz one-third octave band. Nevertheless, measured absorption coefficients are
not really discrepant in view of the uncertainties. With the approach from eq. (3), about 95 % of themeasurement results should be in the interval of ± 2,0. The observed percentage is 94 %.
Referring measured absorption coefficients to the absorption coefficients of the reference absorberin one-third octave bands introduces a positive shift to the results (left graph of Figure 9). An ideal
result is obtained at 125 Hz. The systematic deviation between both rooms is eliminated by thecorrection at that frequency. At other frequencies, e.g. at 200 Hz, the systematic difference becomeslarger by the correction. Since eq. (3) removes the main influences of frequency and magnitude of theabsorption coefficient, results from all frequencies and test specimens were compressed into
probability density functions (pdfs) for ] (right graph of Figure 9). It can be seen from the pdfs thatthe mean value is shifted by referring the measured absorption coefficients to the reference absorberdata in one-third octave bands from -0,6 to 0,2 whereas the standard deviation is reduced from 0,8 to
0,7. The effect of a correction with the octave band results of the reference absorber is nearly identical(also shown in the right graph of Figure 9). This demonstrates that the systematic difference between
both rooms is significantly reduced whereas the statistical scatter is not changed significan tly.

5. CONCLUSIONS
Round robin test results as well as additional measurements from two reverberation rooms at PTB
suggest that the application of a reference absorber for correcting measured absorption coefficients
may reduce systematic differences between laboratories. Further research has to show to which extent
the uncertainty is decreased and whether the additional effort is justified.
Comparison measurements at PTB furthermore revealed that the spread of the measured absorption
coefficients is relatively small when different measurement teams come to the same reverberation
room and perform measurements without changing the room. This indicates that the exact choice of
microphone and loudspeaker position as well as the algorithm for the calculation of reverberation time
are only of minor importance.

ACKNOWLEDGEMENTS
The provision of the reference absorber by Ecophon is gratefully acknowledged. Furthermore, theauthors thank the laboratories which participated in PTB's comparison measurement for the abs orption
coefficient in 2019.

REFERENCES
1. Wittstock, V.: Determination of measurement uncertainties in building acoustics by interlaboratory tests
Part 2: Sound absorption measured in reverberation rooms. Acta Acustica united with Acustica: 104(2018), 999 - 1008
2. Wittstock, V.: Uncertainty of sound absorption measurement in reverberation rooms: practical absorption
coefficient and and single number value according to EN 1793-1. DAGA 2019, Rostock, 18.-21. März
2019
3. ISO 354, Acoustics -- Measurement of sound absorption in a reverberation room, 2003
4. A.-C. Thysell: Test Codes for suspended ceilings. – Sound absorption RRT. Tyrens AB project no.
224628, 2011
5. D. Tompson, M. Vercammen: Round robin project. PEUTZ B.V., 2009
6. Interlaboratory Study to establish precision statements for ASTM C423, Research Report E331010, ASTM, 2006
7. Internordisk Jämförelsemätning av Ljudabsorptionsfaktorn, SP-Rapp. 1974: 30
8. ASTM C423 - 09a Standard Test Method for Sound Absorption and Sound Absorption Coefficients by
the Reverberation Room Method
9. ISO 20189:2018 Acoustics -- Screens, furniture and single objects intended for interior use -- Rating ofsound absorption and sound reduction of elements based on laboratory measurements

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