Impedance Tube Experiments Research Articles

Band-gap tunable dielectric elastomer filter for low frequency noise

In the last decades, diverse materials and technologies for sound insulation have been widely applied in engineering. However, suppressing the noise radiation at low frequency still remains a challenge. In this work, a novel membrane-type smart filter, consisting of a pre-stretched dielectric elastomer membrane with two compliant electrodes coated on the both sides, is presented to control the low frequency noise. Since the stiffness of membrane dominates its acoustic properties, sound transmission band-gap of the membrane filter can be tuned by adjusting the voltage applied to the membrane. The impedance tube experiments have been carried out to measure the sound transmission loss (STL) of the filters with different electrodes, membrane thickness and pre-stretch conditions. The experimental results show that the center frequency of sound transmission band-gap mainly depends on the stress in the dielectric elastomer, and a large band-gap shift (more than 60 Hz) can be achieved by tuning the voltage applied to the 85 mm diameter VHB4910 specimen with pre-stretch Based on the experimental results and the assumption that applied electric field is independent of the membrane behavior, 3D finite element analysis has also been conducted to calculate the membrane stress variation. The sound filter proposed herein may provide a promising facility to control low frequency noise source with tonal characteristics.

Experimental study on sound absorption performance of microperforated panel with membrane cell

Abstract Microperforated panel (MPP) absorbers have been widely used in noise reduction and are regarded as a promising alternative to the traditional porous materials. However, the absorption bandwidth of a single-layer MPP is insufficient to compete with the porous materials. In order to improve the sound absorption ability of the single-layer MPP, a composite MPP sound absorber with membrane cells (MPPM) is introduced. Sound absorption properties of the MPPM are studied by the impedance tube experiment. Results show that the membranes have a significant influence on the sound impedance. The sound absorption performance of MPPM gradually increases with the increase of the membrane area. The single-layer MPP with some small area membrane cells may have the same effect and single large area membranes. By adjusting the size of the membrane cells, one can implement a sound absorber with a wider absorption bandwidth and higher absorption peaks than the single-layer MPP.

Tire cavity resonance mitigation using acoustic absorbent materials

In this study, the use of acoustic absorbent materials specifically felt to mitigate tire cavity resonance noise is presented. The inclusion of a trim in the tire cavity is represented by the addition of the acoustic damping loss factor into the sound pressure response function. In addition, the possible solution of using multilayer trim materials to mitigate the cavity mode effect is presented using the sound absorption coefficient values from the impedance tube experiments and by adopting other empirical models. Moreover, the sound absorption coefficient calculated from the method of electrical-analogy is compared with that from the experimental data and found to be reasonable. Experimental modal analysis was performed to show the effect of inserting an absorbent material (polyfelt) onto the inside surface of the tire where reduction in both the inside cavity sound pressure level and the wheel hub acceleration was observed. A Taguchi analysis is also done to rank the effectiveness of varying trim thickness and mass density as well as adding air gap to suppress tire cavity resonance noise.

Impedance tube experiments as a tool to teach acoustics of porous media

As a practical matter, sound propagation in many everyday situations includes interactions with porous materials. Such materials include textiles, foams, gravel, grass-covered ground, asphalt, and trees. Modeling sound propagation in complex environments, such as passenger cabins or urban spaces, with porous materials like these is essential to understanding realistic acoustics problems. The theory behind the sound absorption in porous material is not typically included in introductory acoustics courses. This is, at least in part, attributable to the mathematical complexity of the topic. Despite this barrier, acoustic properties of porous material can be easily introduced to typical undergraduate students using simple experiments to demonstrate the acoustic effects of different materials. This presentation describes acoustic property measurements of rigid-frame porous material with straight cylindrical pores as well as foam and soil included in an “Introduction to Acoustics” class at the Catholic University of America. The apparatus is composed of a LabVIEW measurement interface, a pair of microphones, and a simple aluminum impedance tube. The modular design allows students to adapt the software or hardware for other experiments. These hands-on experiences are intended to impart a conceptual understanding of the theory of sound absorption and the acoustic performance of sound absorbing (porous) media.

Enhancing rigid frame porous layer absorption with three-dimensional periodic irregularities

This papers reports a three-dimensional (3D) extension of the model proposed by Groby et al. [J. Acoust. Soc. Am. 127, 2865-2874 (2010)]. The acoustic properties of a porous layer backed by a rigid plate with periodic rectangular irregularities are investigated. The Johnson-Champoux-Allard model is used to predict the complex bulk modulus and density of the equivalent fluid in the porous material. The method of variable separation is used together with the radiation conditions and Floquet theorem to derive the analytical expression for the acoustic reflection coefficient from the porous layer with 3D inhomogeneities. Finite element method is also used to validate the proposed analytical solution. The theoretical and numerical predictions agree well with the experimental data obtained from an impedance tube experiment. It is shown that the measured acoustic absorption coefficient spectrum exhibits a quasi-total absorption peak at the predicted frequency of the mode trapped in the porous layer. When more than one irregularity per spatial period is considered, additional absorption peaks are observed.

Impedance Eduction Based on Microphone Measurements of Liners Under Grazing Flow Conditions

This paper presents the results of insertion loss measurements and numerical impedance eduction of three different liner samples. An overview of the test rig and methodology is given, and preprocessed results in terms of reflection and transmission coefficients as well as the energy dissipation are discussed. these coefficients are calculated for discrete frequencies within the ivestigated frequency range. Subsequently, a numerical postprocessing is performed in the time domain, and the educed impedance function for each sample and flow Mach number is presented. This postprocessing in the time domain uses an impedance model based on the extended Helmholtz resonator with five free parameters. the parameters of the model are fitted via an optimization, which determines the whole frequency response in one optimization process. The comparison of measured and numerically evaluated energy coefficients proves the reliability of the tools for impedance evaluation under flow conditions. Finally, the impendance results of the different samples are discussed, including a comparative study with Aermacchi data of the National Aerospace Laboratory (The Netherlands) flow tube and Aermacchi impedance tube experiments

Experimental and numerical investigations on melamine wedges

Melamine wedges are often used as acoustic lining material for anechoic chambers. It was proposed here to study the effects of the mounting conditions on the acoustic properties of the melamine wedges used in the large anechoic chamber at the LMA. The results of the impedance tube measurements carried out show that the mounting conditions must be taken into account when assessing the quality of an acoustic lining. As it can be difficult to simulate these mounting conditions in impedance tube experiments, a numerical method was developed, which can be used to complete the experiments or for parametric studies. By combining the finite and the boundary element method, it is possible to investigate acoustic linings with almost no restrictions as to the geometry, material behavior, or mounting conditions. The numerical method presented here was used to study the acoustic properties of the acoustic lining installed in the anechoic chamber at the LMA. Further experiments showed that the behavior of the melamine foam is anisotropic. Numerical simulations showed that this anisotropy can be used to advantage when designing an acoustic lining.

Comparison of Time-Domain Impedance Boundary Conditions for Lined Duct Flows

impedance of liners. This part of the model, however, is limited to single frequency sound source in its present form. The EHR is based on the z‐transformation of the corresponding impedance function for the Extended Helmholtz Resonator in the frequency‐domain. The implementations of the two models in the numerical simulation tool with the DRP scheme are discussed in details. The results from the both models in general agree notably well with each other. However, in one case, the results from the two models dier significantly due to the presence of strongly attenuated modes, which renders the approximations of the EFI invalid. The validation and verification of both models are carried out for the latest NASA impedance tube experiment and the generic aero‐engine frequency domain results of Rienstra and Eversman. In addition, an impedance eduction technique based on the EHR and the optimization process has been developed. This further extends applications of the time‐domain models. The simultaneous optimization of all frequencies considered in the NASA’s impedance tube experiments results in a physically reasonable set of parameters for the EHR. The fitted terminal impedances and the measured mean flow profiles are used for the impedance eduction. The numerical results of both models for the generic aero‐engine inlet test case with higher azimuthal modes also compare well if the same flow assumptions and eigenvalues are used as for the frequency domain methods. Overall both models give a similar physical behavior, but the EHR requires smaller time steps in the case of small face sheet reactances.