Eric Brandão

Impedance measurement of non-locally reactive samples and the influence of the assumption of local reaction

In this paper, the measurement of the absorption coefficient of non-locally reactive sample layers of thickness d1 backed by a rigid wall is investigated. The investigation is carried out with the aid of real and theoretical experiments, which assume a monopole sound source radiating sound above an infinite non-locally reactive layer. A literature search revealed that the number of papers devoted to this matter is rather limited in comparison to those which address the measurement of locally reactive samples. Furthermore, the majority of papers published describe the use of two or more microphones whereas this paper focuses on the measurement with the pressure-particle velocity sensor (PU technique). For these reasons, the assumption that the sample is locally reactive is initially explored, so that the associated measurement errors can be quantified. Measurements in the impedance tube and in a semi-anechoic room are presented to validate the theoretical experiment. For samples with a high non-local reaction behavior, for which the measurement errors tend to be high, two different algorithms are proposed in order to minimize the associated errors.

A Comparison of Three Methods to Calculate the Surface Impedance and Absorption Coefficient from Measurements Under Free Field or in situ Conditions

Eric Brandão1), Emiel Tijs2), Arcanjo Lenzi3), Hans-Elias de Bree2) 1) Engenharia Acústica. Universidade Federal de Santa Maria, Departamento de Estruturas e Construção Civil, Av. Roraima, 1000, Cidade Universitária, Santa Maria, RS, 97105-900, Brasil. ericbacustica@gmail.com 2) Microflown Technologies, PO BOX 2205, 6802 CE Arnhem, The Netherlands. tijs@microflown.com, debree@microflown.com 3) Laboratório de Vibrações e Acústica, Departmento de Engenharia Mecânica, Universidade Federal de Santa Catarina, Campus Trindade, Florianópolis, SC, 88040-900, Brasil. arcanjo@lva.ufsc.br

A Review of the In Situ Impedance and Sound Absorption Measurement Techniques

This paper presents a review of the literature on the subject of in situ acoustic impedance measurement techniques. The review is intended to be as extensive enough, so that the reader can obtain a broad perspective of the historical evolution of the techniques. Thus, the results of several studies are discussed in order to connect the ideas and insights of many authors and provide an overview of the state of the art technologies in this particular field of acoustics and the path which has led the scientific community to this point. A total of 79 papers are reviewed, 64 of which are directly related to techniques for measuring the surface impedance in situ. Besides a thorough review, a classification scheme for the different methods according to their complexity is also proposed. Finally, some suggestions for future work are given aimed at achieving further advances in the area of in situ impedance measurements.

Estimation of pressure-particle velocity impedance measurement uncertainty using the Monte Carlo method

The pressure-particle velocity (PU) impedance measurement technique is an experimental method used to measure the surface impedance and the absorption coefficient of acoustic samples in situ or under free-field conditions. In this paper, the measurement uncertainty of the the absorption coefficient determined using the PU technique is explored applying the Monte Carlo method. It is shown that because of the uncertainty, it is particularly difficult to measure samples with low absorption and that difficulties associated with the localization of the acoustic centers of the sound source and the PU sensor affect the quality of the measurement roughly to the same extent as the errors in the transfer function between pressure and particle velocity do.

Boundary condition effects of porous material in absorption measurements: Comparison of two impedance tubes

Porous materials are widely used for acoustical treatment and insulation of various environments. Its main feature is acoustic absorption, yielding the acoustical absorption coefficient over frequency. One of the ways to extract this parameter experimentally is with impedance tube (or Kundts Tube) measurement via the transfer function method. However, the compression of the sample or the existence of gaps between the sample and the tube causes of uncertainty in the experiment. This research focuses upon the study of these effects. In order to analyze the influence of the boundary condition in the measurements, they were performed into two impedance tubes of slightly different internal diameters (both have the maximum frequency of analysis up to 6.4 kHz, approximately). Two different porous materials with samples of the same thickness were characterized using both tubes. In this situation, the procedure forces the influence of lateral leakage or compression of the samples against the tube to arise. Thus, ...

Characterization of the acoustic quality of classrooms in a music education facility

Classrooms of a music education facility were characterized by the user by means of a questionnaire study and by acoustic impulse response measurements and subsequent calculation of reverberation time, definition, and other room acoustics parameters. The questionnaire study enabled to understand the preferences of students and music teachers regarding the acoustic quality of a total of twenty rooms for music practice and teaching. It was observed that preference for a certain room also depends on the instrument mostly played by the user. Three study rooms and three collective classrooms were the most cited ones and these have been evaluated according to ISO 3382. Opinions of the musicians showed to be coherent with the measurement data as the rooms considered to be dry had reverberation times around 0.3 seconds, and the rooms considered to be reverberant had reverberation times around 1.5 seconds. The six classrooms that were characterized as clear and well defined rooms, showed values for Clarity around ...

Prediction of sound absorption in rigid porous media with the lattice Boltzmann method

In this work, sound absorption phenomena associated with the viscous shear stress within rigid porous media is investigated with a simple isothermal lattice Boltzmann BGK model. Simulations are conducted for different macroscopic material properties such as sample thickness and porosity and the results are compared with the exact analytical solution for materials with slit-like structure in terms of acoustic impedance and sound absorption coefficient. The numerical results agree very well with the exact solution, particularly for the sound absorption coefficient. The small deviations found in the low frequency limit for the real part of the acoustic impedance are attributed to the ratio between the thicknesses of the slit and the viscous boundary layer. The results suggest that the lattice Boltzmann method can be a very compelling numerical tool for simulating viscous sound absorption phenomena in the time domain, particularly due to its computational simplicity when compared to traditional continuum based techniques.

Simulating sound absorption in porous material with the lattice Boltzmann method

The development of porous materials that are able to absorb sound in specific frequency bands has been an important challenge in the acoustic research. Thus, the development new numerical techniques that allow one to correctly capture the mechanisms of sound absorption can be seen as an important step to developing new materials. In this work, the lattice Boltzmann method is used to predict the sound absorption coefficient in porous material with straight porous structure. Six configurations of porous material were investigated, involving different thickness and porosity values. A very good agreement was found between the numerical results and those obtained by the analytical model provided in the literature. The results suggest that the lattice Boltzmann model can be a powerful alternative to simulating viscous sound absorption, particularly due to its reduced computational effort when compared to traditional numerical methods.

Black box measurements − Using a family of electrical circuits as a tool for self-guided learning in acoustical engineering

A partnership between Brazils first undergraduate program in Acoustical Engineering and the Institute of Technical Acoustics of RWTH Aachen University yielded a didactic project that uses the engineering software MATLAB with the ITA-Toolbox to teach acoustic measurements. Simple electrical circuits are used to mimic typical behavior of acoustical systems. This low-cost solution has proven to be didactically very effective since it helps students to identify themselves with the measurement tasks. Two hardware solutions were developed-a simple oscillator circuit integrated into connectors of audio cables and a desktop box containing seven different transfer characteristics ranging from ideal linear and time-invariant to nonlinear and time-varying behavior. Undergraduate students of Acoustical Engineering used both devices in classroom experiments for self-guided learning by comparing their results to published results. Students were able to learn the fundamental concepts of acoustical measurements and to h...

Error estimation due to sample size effects of in situ surface impedance measurements.

The boundary element method (BEM) is used to predict the effect of the sample size on in situ impedance measurements on a locally reactive sample at normal incidence. The BEM model is based on a direct measurement of the acoustic impedance above the sample (with a combined sound pressure and particle velocity sensor). As the sound source is placed close to the sensor, the acoustic waves cannot be considered plane anymore. Because of that, two models to obtain the surface impedance based on the acoustic field are presented and compared. It is also shown that the sample size, the thickness of the sample, and the positions of the source and receiver affect the measured surface impedance and the calculated absorption coefficient. Those effects are parametrized in a way that the user of the in situ measurement setup may have an estimate of the required sample size for a desired frequency range with a given error allowance.