A review of variable-impedance acoustic liner concepts developed at NASA

This paper presents results attained in the NASA Langley Research Center test rigs using concepts for which the impedance varies over the surface of the liner. These liners are typically designed for significant sound absorption over a wide frequency range, but it is also possible to tune the design to achieve increased absorption at selected frequencies. A brief review is provided regarding a number of variable-impedance concepts. The first is a modified version of a conventional two-layer liner, in which the embedded septum location and acoustic properties are different for adjacent core chambers. Two concepts employ core chambers with different lengths, one with bent chambers to allow packaging within a limited volume, and the other with shared inlet ports to reduce the surface porosity. The last employs a perforated facesheet in which the hole diameter and porosity are varied over the surface of the liner. Data acquired in the NASA normal incidence and grazing flow impedance tubes are used to demonstrate the capabilities of these concepts. Impedance prediction models are also presented for comparison with these measured data.

Characterization of multi-layer porous media in an impedance tube

A porous material is the combination of a solid phase and a fluid phase, with interactions and energy exchanges between phases giving rise to the dissipation of waves traveling through the porous medium. In air, mostly viscous effects and thermal effects are responsible for dissipation, in a way that strongly depends on the pore microstructure. To evaluate the intrinsic properties pertaining to this microstructure, inverse acoustic methods have been used in the past, typically using impedance tubes to observe the way a porous sample interacts with an acoustic field. The impedance tube is a widespread tool in the acoustic community and has proven to be efficient in retrieving, via an inverse method, porous material intrinsic properties such as the porosity or the tortuosity of a sample. In this work, a Bayesian representation of knowledge is taken, where information on a material property is encoded in a probability density function. When multi-layer materials are considered, classical inverse methods become ill-posed and it might become impossible to retrieve exactly each layer's intrinsic properties. This work presents two straightforward improvements that can be used in order to lift this ill-posedness and increase the precision with which material properties are obtained.

High-frequency acoustic impedance tube based on MEMS microphones

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.

Acoustic characterization of Zea Mays culm fibers (corn) and Musa X Paradisiacal (banana) stem fibers in proportion 50% - 50%.

The following research arises as a proposal to the implementation of Zea Mays culm fibers and Musa X Paradisiaca stem fibers in proportion 50% - 50% for the development of a new eco-material with acoustic properties, the objective was to measure the absorption coefficient based on the international standard ISO 10534-1 Determination of the acoustic absorption coefficient and acoustic impedance in impedance tubes. Using the impedance tube to identify the minimums and maximums needed to perform the computational procedure in the MATLAB software tool, and finally obtain the sound absorption coefficients of the material. The measurement process is supported by the implementation of the impedance tube, having all its consideration and previous measures that support the veracity of the data taken through this process, in addition to the fact that background noise measurements were made in order to pass these values to ensure reliable results in the measurement, performed with a class 1 sound level meter; The fibers analyzed had a range between 0.8136 and 0.9225 absorption coefficient in the bands of 1000, 2000 and 4000 Hz, testing the effectiveness in their implementation as an acoustic barrier. Keywords: absorption, eco-material, fibers, barrier, acoustics.

ANALISIS KOEFISIEN ABSORPSI DARI MATERIAL BERBAHAN DASAR LIMBAH KAIN PERCA DAN PELEPAH PISANG

Title: Coefficient Absorpsi Analysis from Patchwork Material and Banana Midrib Preliminary research related composite waste material and nature has done. This research continues the previous research with a more accurate testing method using impedance tubes. Material samples made in this study are samples that have a greater NR (Noise Reduction) value in previous studies. Material samples were made as many as 4 samples with different substance variables, namely fine composite samples, rough composites, banana midrib, and combinations (fine composites and banana midrib). Three samples were mold using a tube with a diameter of 30 mm and a height of 70 mm while one sample combination was a material that was combined to reach a height of 50 mm. From the test results, the addition of cavity affects the absorption coefficient value, therefore the use of this material in buildings must use an air cavity so that the absorption coefficient value increases. Samples that have character as sound absorbing material are fine composite materials, banana fronds, and combination materials because they have an absorption coefficient value (?) of more than 0.30. The combination material has the highest NRC value among the other samples that is equal to 0.57.

The Process Development for Sound Absorption Design of Non-Woven Car Mat

Automotive interior material with consists of several material layers has the sound-absorbing function. It is difficult to predict sound absorbing coefficient because of several material layers. So, many experimental tuning is required to achieve the target of sound absorption. Therefore, while the car interior materials are developed, a lot of time and money is spent. In this study, we present the method to predict the sound absorbing performance of the material with multi-layer using physical properties of each material. The properties are predicted by foam-X software using sound absorption coefficient data measured by impedance tube. And we will compare and analyze the predicted sound absorption coefficient with the data measured by scaled reverberation chamber and impedance tubes for a prototype. If the method is used instead of experimental tuning in the development of car interior material, the time and money can be saved. And then, the development effort can be is reduced because it can be optimized by simulation.

An Investigation on the Sound Absorption Performance of Granular Molecular Sieves under Room Temperature and Pressure

The sound absorption of granular silica-aluminate molecular sieve pellets was investigated in this paper. The absorption coefficients of molecular sieve pellets with different pore sizes, pellet sizes, and layer thicknesses were measured through impedance tubes under room temperature and pressure conditions. The effects of pore size, pellet size, layer thickness were compared and explained. The comparisons show that at room temperature and pressure, the sound absorption of molecular sieve pellets is not a result of the crystalline structure, but rather it mainly changes with the pellet size and layer thickness. In addition, the five non-acoustical parameters of molecular sieve pellets were obtained by an inverse characterization method based on impedance tube measurements. The measurement by impedance tubes is in good agreement with the calculation of Johnson-Champoux-Allard (JCA) model, proving that the JCA model can be effectively used to predict the sound absorption of molecular sieve pellets.