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State of the art in the measurement of sound absorption in a reverberation room (longish)

Hi all,

during some recent discussions on porous absorbers I discovered that the method (ISO 354) used to determine the random incident absorption coefficient in the reverberation room appears to be heavily flawed, which makes the results unreliable.

Below is what I could find on this issue so far, thought that it might be of some interest for some forum participants.

Klaus




ISO 354: Acoustics – Measurement of sound absorption in a reverberation room


1. Benedetto et al., “The effect of stationary diffusors in the measurement of sound absorption coefficients in a reverberation room: An experimental study”, Applied Acoustics 14 (1981), pp.49-63

The recent proposal to revise ISO 354:1963 recommends the use of diffusors when the decaying sound field is not insufficiently diffuse, without explaining the meaning of this assertion or the differences between using and not using diffusors in absorption measurements on materials of different types.


2. Halliwell, “Interlaboratory variability of sound absorption measurement”, J. of the Acoustical Society of America 72 (1983), pp.880-886


3. Kob, “Vergleichbarkeit von Absorptionsgradmessungen im Hallraum”, Fortschritte der Akustik DAGA ‚95, S.339

Der Absorptionsgrad von Materialien liefert dem Akustiker einen wichtigen Hinweis auf die Wirkung von z.B. Baustoffen oder Möbeln im Schallfeld. Er ist definiert als der Quotient aus nicht reflektierter und einfallender Schallintensität. Der Absorptionsgrad ist sowohl von der Frequenz als auch vom Einfallswinkel abhängig.

Bereits die ersten Vergleichsmessungen [1960] zeigten, daß die Vergleichbarkeit von Absorptionsgradmessungen in Hallräumen sehr unbefriedigend ist. Doch auch die nach den aktuellen Fassungen der Normen ISO 354:1985 bzw. DIN 52212 :1961 in verschiedenen Hallräumen durchgeführten Absorptionsgradmessungen liefern nicht genügend vergleichbare Resultate.


4.Cops et al., “Sound absorption in a reverberation room: Causes of discrepancies on measurement results”, Applied Acoustics 46 (1995), pp.215-232

The best-known technique to determine the sound absorption coefficient of acoustic materials at random sound incidence is the reverberant room method. Although the most important details of the measurements method are prescribed in ISO 354:1988, comparisons between different laboratories and even within the same laboratory give large discrepancies between the sound absorbing values obtained.

The measuring method is very simple and lack of care cannot be the cause of the large discrepancies at medium and low frequencies. In the last decade the measuring equipment has reached such a high accuracy that the reasons for discrepancies cannot be caused by the measuring devices currently in use. Therefore it is evident that the differences are due to the sound field to which the absorbing material is exposed during the measurements. This means that parameters such as the design of the test rooms, the position of the test material, the position of the sound source(s) and the microphone(s) are responsible for the reproducibility and repeatability of the results. Although the most essential details in this respect have been specified in ISO-354, with the intention of obtaining high accuracy, the shape of the room is not without influence on the measurement results. A prerequisite for the application of the reverberation room measurement technique is the presence of a diffuse sound field during the measurement.

One of the assumptions is that the sound field shall be diffuse. Even if this is true for the empty room, the introduction of the absorbing material into the room disturbs this diffusion and this disturbance is strongly dependent on the material used. A deficiency in the ideal diffusivity will influence measurement results.

From recent experience in this field it is clear that prescriptions for calculations of the absorption coefficient from reverberant room measurements have to be more severe in order to decrease the discrepancies of the measurement results within the rooms.


5. De Godoy et al., “A study of the influence of mounting conditions on the measured sound absorption in laboratory tests of suspended ceilings (L)”, J. of the Acoustical Society of America 119 (1), January 2006, pp.33-36


6. Sauro et al., Absorption coefficients – part 2 : is edge effect more important that expected ? », Internoise 2009, 23-26 August 2009, Ottawa, Canada

It was observed that the edges of the sample had a significant effect on the absorption coefficient. When people in the industry were asked about this [different results when using different standards] they indicated that it must be because of the edge effect. This was also the reason given when absorption coefficients were being measured that were above 1.00. When asked what edge effect was an answer was never received that made sense or was consistent from one person to another.

Again it can be seen in the prior data that edge effect is much more important than previously thought. It can introduce considerably more absorption to a specimen than just a surface area based calculation would indicate. The author now thinks that absorption coefficients that are calculated using the methods recommended in ASTM-C423 and ISO-354 may be inaccurate at best.


7. Andersson, “European Round Robin Test for sound absorption measurements”, Internoise 2010, 13-16 June 2010, Lisbon, Portugal

It is well known from earlier Round Robin tests that the uncertainty in sound absorption measurements lead to poor reproducibility between laboratories.


8. Vercammen, “Improving the accuracy of sound absorption measurement according to ISO 354”, International Symposium on Room Acoustics, 29-31 August 2010, Melbourne, Australia

The spread between data from different labs is also significant. And manufacturers may ‘shop’ for the laboratory with the highest values.

The main reason for the large spread in results is expected to be the lack of a diffuse field in the reverberation chamber.

So, although an attempt has been made with the qualification procedure in ISO 354:2003(E), the sound field in a reverberation chamber , with high absorptive sample, is not clearly defined so the conditions for application of Sabine’s equation are not met.


9. Lautenbach et al., “Volume Diffusors in the reverberation room”, 20th International Congress on Acoustics ICA 2010, 23/27 August 2010, Sydney, Australia

It is known that the interlaboratory reproducibility of these results is not very well, which leads to the undesired “shopping” phenomenon: material suppliers try to find the laboratories with the highest absorption coefficient of their material.

According to the mentioned standards [ISO 354:2003, ASTM C423-09a], the decaying sound field in the reverberation rooms shall be “sufficiently” diffuse. An “acceptable” diffusivity of a reverberation rooms is usually obtained with panel diffusors, as described in bothy standards.

The term “diffusivity” is not defined in one of these standards. Generally, a sound field is considered diffuse is the energy density is uniform at all positions. This definition does not give a criterion when it is “sufficiently” diffuse either.

The tests for diffusivity in both standards seem to imply that panel diffusors, rotating or not, are a necessity to gain sufficient diffusion, because only panel diffusors are incorporated in the procedure for the facility. The ISO 354 A2 method to check for diffusivity is based on adding panel diffusors until a maximum of absorption for an absorptive specimen is reached. Aiming for the maximum might not be the same as aiming for the right value.

Besides the fundamental problem that a reverberation room with a highly absorptive specimen of a certain size is not diffusive, these panel diffusors introduce a number of uncertainties. Due to all these panels, the acoustical behaviour in the room is much more complicated, and it is not easy to determine the real acoustical effective volume, nor the total boundary surface of the room. Either a panel shields a particular corner of the room, or it is a barrier in the room. It is therefore not easy to describe or predict the acoustical behaviour of a reverberation room, while in the meantime we use this same room to measure a material constant which is used for the prediction of the acoustical behaviour of not yet built rooms.


10. D’Antonio et al., “The state of the art in the measurement of acoustical coefficients”, 161st Meeting Acoustical Society of America, Seattle, Washington, 23-27 May 2011

After more than 100 years, we still do not know the actual random incidence absorption coefficient for an absorber and current standards are inadequate and under intense review!





Meetings of the Acoustical Society of America

Acoustics’08 Paris

4aAAc3. Measurement uncertainty of the sound absorption coefficient. Anna Izewska Building Research Institute, Filtrowa Str.1, 00-611 Warsaw, Poland

The standard ISO”IEC 17025:2005 on the competence of testing and calibration laboratories requires that these laboratories shall
apply procedures for estimating the uncertainty of their measurement results. One of the possibility is to evaluate the budget of uncertainty,
taking into account all components that contribute significant uncertainty to the final result. In case of the sound absorption
coefficient measurement, carried out according to the standard EN ISO 354:2003, the overall uncertainty is first of all influenced by the
reverberation times T1, T2 and the power attenuation coefficients m1 and m2, calculated according to the ISO 9613-1 standard and
representing the climatic conditions in the reverberation room. In spite of very little difference between the values m1 and m2 representing
the change of climatic conditions usually, it is the case in laboratory , exponential form of the coefficient’s function causes that
the uncertainty of measurement results increase with frequency very fast. Particularly for the high frequencies, the values of uncertainty
are so important that the evaluation of the sound absorption coefficient is practically not possible.


159th ASA Meeting/NOISE-CON 2010

1pAA2. Absorption coefficient error propagation into room acoustics parameters. Michael Vorländer Inst. of Tech. Acoust.,
RWTH Aachen Univ. D-52056 Aachen, Germany

It is well known that random-incidence absorption coefficients suffer from measurement uncertainties. Whether or not these uncertainties
affect the quality of results from room acoustic prediction models depends on the specific case. One might argue that the absorption
of the most relevant material must be known accurately. This, however, could be the material with highest absorption or the
material with the largest surface area. In this presentation, the true random-incidence absorption coefficient and measurement uncertainties
related to the current ISO 354 are briefly discussed. Then, in order to get a better view on the consequences of those uncertainties,
quantitative uncertainties of some predicted parameters such as reverberation time, strength, and clarity are calculated by using
an error propagation. Effects of uncertainties of absorption coefficients distributed in various room configurations are presented and
discussed.


162nd Meeting: Acoustical Society of America 2011

2aAAb2. The sound absorption measurement according to ISO 354. Martijn Vercammen and Margriet Lautenbach (Peut, P.O. Box 66, Mook ZH-6585, Netherlands)

Sound absorption measurements of building materials such as sound absorbing ceilings and other products are performed in a reverberation
chamber according to ISO 354. It is known that the interlaboratory reproducibility of these measurements is not very well. At this moment, the differences of results between laboratories are much larger than can be accepted, from a practical point of view for predictions as well as from a jurisdictional point of view. An ISO working group has started to investigate possibilities to improve the method. Due to the insufficient diffuse sound field in a reverberation chamber with test sample, the shape of the reverberation room and the placing of diffusers will influence the result. A round robin research containing ten laboratories is performed to get information on the spread and if it is possible to reduce this by correcting for the mean free path or by application of a reference material. Additional measurements are performed to improve the measurement conditions such as measurements with volume diffusers. Possible improvements of ISO 354 will be presented. These consist of a procedure to qualify laboratories based on the statistical variation of the reverberation time and based on the results of a reference absorber.


2aAAb3. Diffusivity of diffusers in the reverberation room. Margriet Lautenbach (Peutz BV, Paletsingel 2, P.O. Box 696, 2700 AR
Zoetermeer)

The random incidence absorption coefficient is measured in a reverberation room according to the ISO354 or ASTM C423-09.
According to these standards, the diffusivity of a reverberation room is usually obtained with panel diffusers. Besides the fundamental problem that a reverberation room with a highly absorptive specimen is not diffuse, these panel diffusers introduce a number of uncertainties
like the acoustical effective volume and the total boundary surface of the reverberation room. This might be one of the causes that some laboratories are structurally able to measure absorption coefficients larger than 1, even if the volume of the specimen, edge absorption, and the absorption of the surface covered by the specimen are taken into account. To reduce the difference in measurement results between different laboratories, the possible use of volume diffusers instead of panel diffusers is investigated. The following criteria are investigated to substantiate the hypothesis that volume diffusers lead to better results: (1) Deviation between microphone-source positions. (2) Comparison to maximum relative standard deviation (ASTM). (3) Comparison to theoretical variance. (4) Influence of place of specimen. The investigations have been performed in a 1:10 scale model. The results are presented in this paper.


2aAAb4. Measuring absorption: Bad methods and worse assumptions. Ronald Sauro (NWAA Labs, Inc., 90 Tower Blvd., SDP\#8, Elma, WA 98541)

Ever since Sabine described the use of absorptive materials to affect reverberation time, the acoustic community has been trying to quantify this effect. Standards like C-423 and ISO-354 have been developed to aid in that process. These standards, describing coefficients ranging from 0.0 to 1.0, have been inadequate to fully describe the actual absorption of the tested materials. It is not uncommon to find actual measurements that result in coefficients exceeding the number 1.0. When this happens, assumptions are made that other properties of the material are not taken into account by the coefficient. The present standards also suffer from inadequacies of methodology. They do not measure the full range of absorptive qualities. This paper describes the incorrect assumptions about absorption and measuring it as well as illustrates the incorrect methodologies that are used in these standards. Because of the flaws in the assumptions and the inherent defects in the methodologies, this author believes the current standards need to be replaced by new standards. Replacement instead of correction is preferable because of the extremely large database of materials already measured and the difficulty explaining how new measurements based upon corrections would be comparable to old measurements.


2aAAb5. On the reproducibility of measuring random incidence sound absorption. Anthony Nash (Charles M. Salter Assoc., 130
Sutter St., San Francisco, CA 94104)

For over 50 years, the American Society for Testing and Materials (ASTM International) has promulgated a method for the laboratory testing of random-incidence sound absorption coefficients (Test Method C423). This test method falls under the purview of ASTM Committee E33 (Environmental Acoustics). In 1999, the protocols in this test method became significantly more stringent with the goal of improving inter-laboratory “reproducibility” (i.e., quantitative differences among laboratories when testing the same specimen).

ASTM calls such an inter-laboratory test a “round robin”; its outcome is an array of computed precision (i.e., uncertainty) values. This paper presents results from several “round robin” evaluations and discusses some possible causes for the range of values. If time permits, the fine points of the test protocols in C423 will be described and compared to those in ISO 354.


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Topic - State of the art in the measurement of sound absorption in a reverberation room (longish) - KlausR. 04:40:55 02/14/13 (8)

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