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Abstract

This paper presents and compares microphone calibration methods for the simultaneous calibration of small electret microphones in a wave guide. The microphones are simultaneously calibrated to a reference microphone both in amplitude and phase. The calibration procedure is formulated on the basis of the damped plane wave propagation equation, from which the acoustics field along the wave guide is predicted, using several reference measurements. Different calibration models are presented and the methods were found to be sensitive to the formulation, as well as to the number of free parameters used during the reconstruction of the wave-field. The wave guide model based on five free parameters was found to be the preferred method for this type of calibration procedure.

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Authors and Affiliations

Péter Tóth
Christophe Schram
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Abstract

This paper presents an investigation about acoustic absorption of mortars with partial replacement of sand by waste (plywood formwork, rice husk, and thermoplastic shoe counters), examining different levels of replacement (0%, 5%, 10%, 25%, and 50%). The measurement of acoustic absorption was performed using a plane wave impedance tube with 100 mm diameter, using mortar samples of 20 mm, in frequency range 200-2000 Hz. Results demonstrated that some composite with waste presented noise reduction coefficient (NRC) above the reference mortar (NRC = 0.0343), such as a composite with 50% rice husk (NRC = 0.2757) and other with 50% of plywood waste (NRC = 0.2052). Since there is virtually no cost or difficulty to use these residuals, it may be concluded that it is a sustainable alternative to improve the acoustic comfort and reduce the impact of the waste on the environment.
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Authors and Affiliations

Marco Aurélio Stumpf González
Fernanda Flach
Josiane Reschke Pires
Marlova Piva Kulakowski
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Abstract

Numerical models allow structural characteristics to be obtained by solving mathematical formulations. The sound absorption capacity of a material can be acquired by numerically simulating an impedance tube and using the method governed by ISO 10534-2. This study presents a procedure of obtaining sound pressure using two microphones and as outline condition, at one end of the tube, the impedance of fiber samples extracted from the pseudostem of banana plants. The numerical methodology was conducted in the ANSYS® Workbench software. The sound absorption coefficient was obtained in the MATLAB® software using as input data the sound pressure captured in the microphones and applying the mathematical formulations exposed in this study. For the validation of the numerical model, the results were compared with the sound absorption coefficients of the fiber sample collected from an experimental procedure and also with the results of a microperforated panel developed by Maa (1998). According to the results, the methodology presented in this study showed effective results, since the largest absolute and relative errors were 0.001 and 3.162%, respectively.
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Bibliography

1. ASTM E1050:2019, Standard test method for impedance and absorption of acoustical materials using a tube, two microphones and a digital frequency analysis system.
2. ASTM E354:2003, Acoustics – measurement of sound absorption in a reverberation room.
3. Bóden H., Abom M. (1986), Influence of errors on the two-microphone method for measuring acoustic properties in ducts, The Journal of the Acoustical Society of America, 79(2): 541–549, doi: 10.1121/1.393542.
4. Ming-hui G., Qing-quan H., Jin-man W., Haipeng Y. (2010), The modeling and simulation analysis of wooden perforated panel absorption structure, Noise & Vibration Wordwide, 41(10): 72–75, doi: 10.1260/0957-4565.41.10.72.
5. Howard C.Q., Cazzolato B.S. (2014), Acoustic Analyses using MATLAB® and ANSYS®, Boca Raton: CRC Press, Taylor & Francis Group.
6. ISO 10534-1:1996, Acoustic – Determination of sound absorption coefficient and impedance in impedance tubes – Part 1: Method using standing wave ratio.
7. ISO 10534-2:1998, Acoustics – Determination of sound absorption coefficient and impedance in impedance tubes. Part 2: Transfer-function method.
8. ISO 354:2003, Measurement of sound absorption in a reverberant room.
9. Kinsler L.E., Frey A.R., Coppens A.B., Sanders J.V. (2000), Fundamentals of Acoustics, Hoboken: John Wiley & Sons, New York.
10. Lara L.T., Boaventura W.C., Pasqual A.M. (2016), Improving the estimated acoustic absorption curves in impedance tubes by using wavelet-based denoising methods, Congresso Iberoamericano de Acústica, Buenos Aires, Argentina, 22, 1–10.
11. Maa D.Y. (1998), Potential of microperforated panel absorber, The Journal of the Acoustical Society of America, 104(5): 2861–2866, doi: 10.1121/1.423870.
12. Rienstra S.W., Hirschberg A. (2014), An Introduction to Acoustics, Eindhoven University of Technology, Netherlands.
13. Silva G.C.C., Nunes M.A.A., Almeida Jr A.B., Lopes R.V. (2013), Acoustic design and construction of an impedance tube for experimental characterization of sound absorbed materials [in Portuguese: Projeto Acústico e Construção de um Tubo de Impedância para Caracterização Experimental de Materiais com Absorção Sonora], [in:] XVIII Congresso de Iniciação Científica da UnB, Brasília, Brazil.
14. Soriano H.L. (2009), Finite Elements – Formulation and Application in Static and Dynamic Structures [in Portuguese: Elementos Finitos – Formulação e Aplicação na Estática e Dinâmica das Estruturas], Rio de Janeiro: Editora Ciência Moderna Ltda.
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Authors and Affiliations

Cláudia Ohana Borges Mendes
1
Maria Alzira De Araújo Nunes
1

  1. Graduate Program in Engineering Materials Integrity, University of Brasília-UnB, College UnB Gama-FGA Área Especial de Indústria Projeção A, Setor Leste, CEP:72.444-240, Gama, Distrito Federal, Brazil
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Abstract

Micro perforated panel (MPP) absorber is a new form of acoustic absorbing material in comparison with porous ones. These absorbers are considered as next generation ones and the best alternative for traditional porous materials like foams. MPP combined with a uniform air gap constructs an absorber which has high absorption but in a narrow bandwidth of frequency. This characteristic makes MPPAs insufficient for practical purposes in comparison with porous materials. In this study instead of using a uniform air gap behind the MPP, the cavity is divided into several partitions with different depth arrangement which have parallel faces. This method improves the absorption bandwidth to reach the looked for goal. To achieve theoretical absorption of this absorber, equivalent electro-acoustic circuit and Maa’s theory (Maa, 1998) are employed. Maa suggested formulas to calculate MPP’s impedance which show good match with experimental results carried out in previous studies. Electro-acoustic analogy is used to combine MPP’s impedance with acoustic impedances of complex partitioned cavity. To verify the theoretical analyses, constructed samples are experimentally tested via impedance tube. To establish the test, a multi-depth setup facing a MPP is inserted into impedance tube and the absorption coefficient is examined in the 63–1600 Hz frequency range. Theoretical results show good agreement compared to measured data, by which a conclusion can be made that partitioning the cavity behind MPP into different depths will improve absorption bandwidth and the electro-acoustic analogy is an appropriate theoretical method for absorption enhancement research, although an optimisation process is needed to achieve best results to prove the capability of this absorber. The optimisation process provides maximum possible absorption in a desired frequency range for a specified cavity configuration by giving the proper cavity depths. In this article numerical optimisation has been done to find cavity depths for a unique MPP.
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Authors and Affiliations

Falsafi Iman
Ohadi Abdolreza

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