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09.06 Sound Absorption Measurements, Part 1

How small-scale variation in mineral wool products effect random incidence sound absorption
Time: 12:20 pm

Author: Mads Bolberg

Abstract ID: 2103

Mineral wool products are produced by creating a spray of fibres, that is collected and made into slabs. This randomized spray can lead to small variations across a slab. Nevertheless, mineral wool slabs are often treated in acoustics as locally reacting perfectly homogeneous, isotropic materials. This means that small-scale characterisations are extrapolated to large-scale without considering the impact from possible variations in a large-scale setup. The question is how the small-scale characterisations should be used for large-scale setups with this in mind. Three products with the same thickness and density, but with significantly different specific airflow resistances were selected for random incidence sound absorption tests. The products were all specially made ceiling tiles and measurements were conducted in E200 setup according to ISO 354:2003. The tiles were gradually exchanged in a random fashion, so measurement results were obtained using a combination of tiles with different specific airflow resistances. Results showed a surprisingly linear relation between the sound absorption and the average specific airflow resistance of tiles used in the measurements. The results point to that variations in products must be observed, but also that small variations in specific airflow resistance in standardized products are insignificant.

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An optimization method for reverberation room design
Time: 11:20 am

Author: Paul Didier

Abstract ID: 2118

The ISO 354:2003 standard relating to sound absorption measurements is currently under revision to improve the reproducibility of the procedure it describes. Round robin tests conducted across various reverberation rooms indeed revealed significant disparities between sound absorption measurements of the same sample. One of the reasons is that, at low frequencies, the sound field in a single laboratory cannot be considered fully diffuse. However, the average sound field across different laboratories may be considered diffuse if the interaction between the finite sample and the diffuse field is duly accounted for and the direct field close to the absorber is disregarded. In this work, a method is developed for optimizing reverberation room design such that measured absorption values are as close as possible to ensemble average diffuse values. The reverberation room is modelled using the finite element method and standardized measurements of an absorptive sample are simulated. The distance between resulting absorption coefficients and diffuse target values is minimized in an optimization procedure having the geometrical characteristics of the model as input parameters. The results are anticipated to participate to the revised ISO 354 as guidelines for the construction of new reverberation rooms or the improvement of existing ones.

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Numerical study of the impact of reverberation room design and test parameters on sound absorption measurements
Time: 11:00 am

Author: Paul Didier

Abstract ID: 2120

The measurement of sound absorption in reverberation rooms following the ISO 354:2003 standard relies on Sabine’s equation to derive absorption coefficients from reverberation times. This equation assumes perfect diffusivity, i.e. the sound field is composed of many statistically independent plane waves with uniformly distributed spatial phases, themselves uncorrelated to the corresponding amplitudes. In this work, both existing and fictitious reverberation rooms are numerically modelled using the finite element method. Finite porous absorbers are introduced in the rooms as equivalent fluid models. Standardized sound absorption measurement are simulated in the rooms through the determination of reverberation times. The respective effects of the sample size, sample placement, source positioning, and presence of finite panel diffusers are investigated. The resulting absorption coefficients are then confronted to the theoretical values in a perfectly diffuse sound field, that interacts with a baffled, finite-sized absorber, as obtained with a hybrid deterministic-statistical energy analysis model. The process notably underlines the strong, yet often disregarded, beneficial effect of panel diffusers at low frequencies in highly regularly-shaped rooms. Another conclusion of this work is that reverberation room design represents a crucial factor that can influence sound absorption measurements at low frequencies.

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Measurement of sound absorption coefficient in a reverberation room using probability density function of damping constant
Time: 8:40 pm

Author: Kosuke Goto

Abstract ID: 2262

The measurement method of the sound absorption coefficient in a reverberation room is standardized in ISO 354. However, the measurement accuracy often deteriorates at low frequencies. This paper proposes a method that improves the measurement accuracy of the sound absorption coefficient at low frequencies. It calculates the sound absorption coefficient using reverberation time (RT) that is derived from the distribution of a damping constant for a sinusoidal input. The measured values by the proposed method were compared with those by the ISO 354 method. As a result, the proposed method reduces the spatial variability of RT and gives a better agreement with the statistical absorption coefficient that is calculated by a transfer matrix model at low frequencies.

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A flow and acoustic facility for characterization of liner and meta-acoustic surfaces under grazing flow condition
Time: 8:20 pm

Author: Wei Yi

Abstract ID: 2324

The Hong Kong University of Science and Technology (HKUST) has designed and assembled a new facility, a grazing flow tube, for aeroacoustic characteristics measurement of acoustic liners, e.g. transmission loss, impedance, etc., under a high-speed grazing flow. The cross-section of the test section of the tube has a dimension of 50 mm × 50 mm, and the grazing flow speed can be up to 0.3 Ma. A settling chamber, a long-enough flow development section and a multi-stage anechoic termination are adopted to ensure the high-quality flow field and acoustic field. This paper presents the detailed designs of the key components of the facility, as well as the calibrations of the velocity profile in a series of cross-section surfaces of the duct along the streamwise direction and sound pressure distributions in the axial and circumferential directions. Pitot tube, Hotwire and PIV are used to obtain the flow field measurement results. The overall performance of the diagnostic facility is verified by comparing the impedance results of acoustic liners acquired from an impedance tube under the static condition and the theoretical variation of axial wavenumber with Ma number under the grazing flow.

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Loudspeaker configuration in reverberation rooms for sound absorption measurement using room mode determination
Time: 11:40 am

Author: Niels Peter Moos

Abstract ID: 2575

For Sabine absorption coefficient measurements in reverberation rooms, it is preferable to excite as many room modes as possible. But the number of excited room modes is significantly affected by many factors, among which one dominating factor is the loudspeaker location particularly at frequencies below the Schroeder frequency. In turn, this could significantly influence the statistical absorption coefficient obtained according to ISO 354. Therefore, this study aims to investigate the impact of loudspeaker configurations on the excitation of room modes. Frequency response functions are measured using a broad band steady state noise signal using four sound sources at various locations, and the numbers of excited room modes in the one-third octave bands are quantified and compared against those by Green’s function simulations. A procedure for determining favorable loudspeaker positions based on the excited room modes is proposed, which can be a useful input to the working group of the ISO 354 standard.

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Trial applications at gymnasiums of in-situ sound absorption measurement method by ensemble averaging technique
Time: 7:20 pm

Author: Toru Otsuru

Abstract ID: 2729

The authors have been published a series of papers on a measurement method for sound absorption characteristics of materials using ensemble averaging technique, i.e., EA method. The papers’ results included measurement mechanisms, measurement uncertainty, and so on. Herein, to examine adaptability, especially in in-situ conditions, the EA method is applied to measure absorption characteristics of materials installed in two gymnasiums. A glass-wool panel with the dimension of 0.5 m by 0.5 m by 0.05 m and with the density of 32 kg m^-3 was brought around and measured to check the measurement consistency. Several measurements were conducted during badminton plays were undergoing. Measured sound absorption coefficients revealed that most results agree well with those measured in reverberation rooms. Certain improvement is necessary for the specimen brought to the in-situ measurement to keep the consistency. The inconsistency is considered to originate from unstable conditions between the specimen and floor.

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High-frequency acoustic impedance tube based on MEMS microphones
Time: 12:00 pm

Author: Roman Schlieper

Abstract ID: 2810

Acoustic impedance tubes are commonly used to measure a test specimen's acoustic characteristics, such as reflection factor, absorption coefficient, or acoustic impedance, in combination with one or two condenser measurement microphones according to associated standards. In the development process of an impedance tube, the microphone diaphragm's size has an important role in the measurement quality. On the one hand, the microphone diameter has to be large enough to ensure the possibility of measuring at low sound pressure levels (SPLs), but on the other hand, the size of the microphone diaphragm should be small in order not to influence the sound propagation through the impedance tube due to the microphone coupling. Micro-Electro-Mechanical Systems (MEMS) microphones are recently widely applied in various acoustic applications due to their small size and high sensitivity. This paper proposes the development of an acoustic impedance tube equipped with 16 MEMS microphones and an inner diameter of 8 mm with an operating frequency range between 60 Hz and 16 kHz. The bottom port MEMS microphones are connected via a 1 mm hole to the tube. The system evaluation is based on standard test specimens like empty probe adapters, rigid termination, and porous absorbers.

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Finite element sound field analysis on measurement of absorption coefficient in a reverberation room -Relationships between inclination of walls and measurement results-
Time: 7:40 pm

Author: Reiji Tomiku

Abstract ID: 3158

The absorption coefficients in a reverberation room are most representative measure for evaluating absorption performance of architectural materials. However, it is well known that measurement results of the coefficient vary according to a room shape of the measurement and area of the specimen. Numerical analyses based on wave acoustics are effective tools to investigate these factors on absorption coefficient measurement in reverberation room.  In this study, sound fields for the measurement of absorption coefficient in reverberation room are analyzed by time domain finite element method (TDFEM). This study shows effectiveness of the analysis for investigation on causes of variation in the measurement results and improvement methods of the measurement. First, some measurement sound fields for absorption coefficient in reverberation rooms the walls of which are incline or decline are analyzed by the TDFEM. Next, reverberation times in each sound fields are calculated from the results obtained by TDFEM and the absorption coefficients are evaluated from the reverberation time of the room with and without specimen. Finally, the relationships among room shape, degree of inclination of the wall, the sound absorption coefficient of the specimen, frequencies and the measurement absorption coefficient are investigated.

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Calculation of statistical absorption coefficient using ensemble averaged surface normal impedance of material
Time: 8:00 pm

Author: Noriko Okamoto

Abstract ID: 3163

To predict and control the indoor sound field, it is important to comprehend sound absorption characteristics of building materials. In the previous studies, the authors have proposed an in-situ sound absorption measurement method of materials using ensemble averaging technique, namely EA method. The method yields a simple and efficient in-situ measurement of surface normal impedance of materials at random-incidence. In this paper, the authors calculate the statistical absorption coefficient using the surface normal impedance of material by the EA method to obtain random incidence absorption coefficient. At first, the procedure of calculating the statistical absorption coefficient from the normal impedance by EA method is described. Next, the sound absorption characteristics for five kinds of materials are measured by the EA method and the reverberation room method. Finally, the statistical absorption coefficients are calculated from results obtained by the EA method and are compared with absorption coefficients by the reverberation room method.

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Diffusion equation modelling for energy flow analysis in reverberation chambers
Time: 12:40 pm

Author: Ryan Hao

Abstract ID: 3197

Noise is a growing concern in the built environment. Sound absorbers are a viable option for noise treatment. However, the characterization of their absorption coefficient in standardized measurement chambers still show challenges for high accuracy as required in practice. In recent years, experimental analysis has shown that assumptions of diffuse sound fields made in well-known reverberation chambers are unfulfilled. Specifically, that sound intensities in chamber-based measurement methods are presumed to be isotropic or diffuse. Diffusion equation models have shown dramatic changes in energy flow in the presence of highly absorptive materials under test. This has been attributed to well-documented inconsistencies reported from reverberation chamber measurements across different laboratories. This work will demonstrate that the diffusion equation model is proving to be a computationally efficient and viable method for predicting sound energy flows, garnering an increasing amount of interest from the acoustical community.

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