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09.00 Acoustic Materials – Aerospace Applications

Development of acoustic mufflers for cabin noise reduction in Orion spacecraft
Time: 5:00 pm

Author: Indranil Dandaroy

Abstract ID: 1568

Controlling cabin acoustic noise levels in the Crew Module (CM) of the Orion spacecraft is critical for adequate speech intelligibility, avoid fatigue, and prevent any possibility of temporary and permanent hearing loss to the crew. The primary source of cabin noise for the on-orbit phase of the mission is from the Environmental Control and Life Support System (ECLSS) which recycles and conditions breathing air and maintains cabin pressurization through its ducting network and components. Unfortunately, as a side effect, noise from the ECLSS fans propagates through theses ducts and emanate into the cabin habitable volume via the ECLSS inlet and outlets. To mitigate excessive duct-borne noise, two ECLSS mufflers have been designed to provide significant acoustic transmission loss (TL) so that the cabin noise requirements can be met. Each muffler is meant to be installed in the ducting of the ECLSS air inlet and outlet sides, respectively. Packaging constraints and tight volume requirements necessitated the mufflers to be of complex geometry and compatible with the bends of the ECLSS duct layout. To design and characterize the acoustic performance of the inlet and outlet mufflers, computational acoustic models were developed using the Finite Element Method (FEM) with  software. Characterization of the acoustic material and perforations in the mufflers were addressed with poro-elastic theory. Once the mufflers were designed on paper and its TL predicted, prototypes of these mufflers were created using additive manufacturing. The muffler prototypes were subsequently tested for acoustic TL in the laboratory with various configurations of acoustic materials. Comparing the analytical predictions to the test performance yielded excellent correlation for acoustic TL and demonstrated significant broadband noise attenuation. The ECLSS mufflers are currently scheduled to be installed on the Artemis II CM of the Orion spacecraft and will provide significant cabin comfort to crew during the mission.

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Low frequency vibration suppression of a moderate thick cabin structure by multiple piezoelectric patches shunted with RL-double negative capacitance circuits
Time: 6:40 pm

Author: Zhiwei Zheng

Abstract ID: 1807

Multiple piezoelectric patches shunted with RL-double negative capacitances circuits, which are bonded on the bulkhead, are proposed to control the resonant response of multiple low frequency modes of a moderate thick cabin structure. Dynamic modeling of the electromechanical coupling system of the cabin structure and the piezoelectric shunt circuit is established by employing the three-dimensional finite element. Optimum tuning strategy is based on the trial and error method. It is shown that the proposed approach is effective in enhancing the generalized electromechanical coupling coefficient and controlling the low frequency modes that exhibits coupled deformation of the bulkhead and cabin structure.

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Structural Dynamics, Noise and Vibration: Buildings Adjacent to Train Lines
Time: 8:20 am

Author: Pablo Reboredo Gasalla

Abstract ID: 2078

In architecture and civil engineering there is a growing interest in the study of the dynamics of structures. The dynamic effects are due to various actions, but their influence is widened or reduced according to the design of the structure. Structures can be highly susceptible to the action of dynamic loads produced by the action of wind, earthquake, industrial activities, blasting activities, construction sites, road vehicles, trains, building services and human activities. Noise and vibration can have significant environmental impact on buildings and structures and can damage track components, crack roadways, unsettle foundations, affect sensitive equipment, impact human comfort and damage structures. Structure-borne noise and vibration can be a major and often overlooked consideration in the planning, design and operation of existing or new developments. A reliable, considered approach to assessing noise and vibration is needed to achieve outcomes that meet client expectations while maintaining on-going compliance with relevant standards and regulatory and planning requirements. The proposed paper will focus on a general overview of the structure-borne noise and vibration risks, associated with new developments that are adjacent to existing rail lines during the early design stages, from commercial and residential buildings to sensitive research facilities. It will further explain the general description of the design process, including problem descriptions, measurements of ground and structural vibration levels at the proposed site, vibration criteria, finite element analysis and provisions of the isolation system required to be considered at the building to achieve the relevant criteria.

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The challenges and successes of passive acoustic treatments on the International Space Station
Time: 4:40 pm

Author: Holly Smith-Dalenberg

Abstract ID: 2126

Habitable space environments pose unique challenges to the selection of passive acoustic treatments used to mitigate noise. On the International Space Station, strict regulations regarding flammability, particulate release, and off-gassing must be considered during material selection, resulting in the exclusion of many common acoustic treatments used in ground-based applications. The Johnson Space Center Acoustics Office has identified a small subset of acoustic absorption and barrier materials that meet these stringent requirements, and has developed numerous treatments for noise mitigation, including duct-wrapping and liners, acoustic absorption and barrier blankets, and mufflers. The Acoustics Office utilizes impedance and transmission loss tubes to optimize the layering of acoustic materials for these treatments while observing restrictive volume and mass limits. Future acoustic mitigation development will focus on moisture and microbial-resistant materials and treatments that can be utilized in enclosed spaces that require higher scrutiny in regards to cleanliness, such as waste management bays and surfaces surrounding exercise equipment or galleys.

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Tunable frequency bandgaps in elastic metamaterials with internal contacts
Time: 6:40 pm

Author: Eunho Kim

Abstract ID: 2162

We design an elastic metamaterial with internal contacts and study the tunable frequency band structure of the metamaterial. It is well-known that the frequency band of granule structures consisting of particles changes depending on the system's compression because of the nonlinearity of the contact between particles. We adopt this efficient tunning mechanism, i.e., contact, in the design of continuum type elastic metamaterials. We first design a unit cell structure showing internal contacts under compression and fabricate it using a 3D printer. We numerically and experimentally identify that the unit cell's stiffness suddenly increases when the internal contact happens. This sudden change of the stiffness induces a change of frequency characteristics of the structure. Here, we demonstrate that internal contacts are useful for designing various frequency bandgaps and tuning them efficiently.

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Sound absorption of polydisperse heterogeneous porous composites
Time: 3:00 pm

Author: Gabriel Núñez

Abstract ID: 2217

Sound absorption of polydisperse heterogeneous porous composites is investigated in this paper. The wave equation in polydisperse heterogeneous porous composites is upscaled by using the two-scale method of homogenisation, which allows the material to be modeled as an equivalent fluid with atypical effective parameters. This upscaled model is numerically validated and demonstrates that the dissipation of sound in polydisperse heterogeneous porous composites is due to visco-thermal dissipation in the composite constituents and multiple pressure diffusion in the polydisperse heterogeneous inclusions.  Analytical and semi-analytical models are developed for the acoustical effective parameters of polydisperse heterogeneous porous composites with canonical geometry (e.g. porous matrix with cylindrical and spherical inclusions) and with complex geometries. Furthermore, by comparing the sound absorption coefficient of a hard-backed composite layer with that of layers made from the composite constituents alone, it is demonstrated that embedding polydisperse heterogeneous inclusions in a porous matrix can provide a practical way for significantly increasing low frequency sound absorption. The results of this work are expected to serve as a model for the rational design of novel acoustic materials with enhanced sound absorption properties.

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Using noise control principles when evaluating the acoustic impacts of face coverings during the coronavirus pandemic
Time: 7:40 pm

Author: Richard Ruhala

Abstract ID: 2451

Several different combinations of face masks and shields are evaluated for their acoustic performance using a head and torso simulator (HATS).  The HATS is used as a controlled and repeatable artificial sound source using white noise in a classroom environment.  Sound pressure levels at octave band frequencies due to the face coverings are evaluated at a location of 2.0 meters from the HATS which is within the direct field to reduce the room acoustical effects.  The problem is modeled as a barrier separating a source and receiver using fundamental noise control principles.  Fabric material properties are used such as thickness, density, stiffness, and damping.  The results are compared with experimental tests.  The face shield with clear plastic barrier produces a resonance in the 1000 Hz octave band.  Analytical models of cavity resonances, standing wave resonances, or plate resonances are calculated and compared with the experimental resonance. The speech interference level is used to determine the frequency content that is most likely to cause hearing difficulties and compared with A-weighted differences between the unmasked condition and masked.

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Influence of the Poisson’s ratio on the efficiency of viscoelastic damping treatments
Time: 2:20 pm

Author: Lucie Rouleau

Abstract ID: 2521

An efficient way of mitigating noise and vibration is to embed viscoelastic patches into the host structure. Viscoelastic properties are of significant importance in determining the performance of the passive damping treatment. The behaviour of homogeneous isotropic materials is described by two elastic constants (generally the Young modulus and the Poisson ratio, or the shear and bulk moduli), which are frequency- and temperature-dependent in the case of viscoelastic materials. In practice, the Poisson's ratio is often considered as independent of temperature and frequency. One goal of this work is to numerically evaluate the validity of this assumption and its limitations (frequency range, thickness of the viscoelastic layer). To this end, a thermo-mechanical characterization of a viscoelastic material is carried out by dynamic measurements of the complex shear and bulk moduli, allowing the indirect measurement of the frequency- and temperature-dependent Poisson's ratio. Moreover, the measurements of the Poisson's ratio (direct or indirect) can lead to considerable uncertainties. For instance, large discrepancies have been observed when characterizing the Poisson's ratio of polymer foams. Another goal of this work is to investigate the influence of those uncertainties on the dynamic response of a damped structure.

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Controlling the amount of acoustic absorption by using clusters of hard cylinders
Time: 12:20 pm

Author: Vicente Cutanda Henriquez

Abstract ID: 2759

The viscothermal absorption of a cluster of hard cylinders periodically arranged in air is directly related with the filling fraction of the underlying lattice. In this work, we present a comprehensive study of the viscous absorption of clusters with circular external shape. The study has been performed by using a homogenization theory in which the clusters have been represented by a single fluid-like cylinder with effective parameters. The validity of the homogenization approach has been supported with numerical experiments in which the viscosity of the actual cluster is calculated with an improved version of the boundary element method. The simulations have been performed by embedding the clusters in a multimode impedance tube. For example, for a circular cluster containing 817 hard cylinders distributed in a hexagonal lattice with filling ratio of 0.836, the absorptive factor calculated with the homogenization approach is 41.5%, which underestimates by about 1% the value obtained with the complete cluster.

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Investigation of frequency dependent mechanical properties of porous materials using dynamic mechanical analyzer and frequency-temperature superposition theory
Time: 4:00 pm

Author: Attila Schweighardt

Abstract ID: 2859

In acoustic design of engineering applications – such as in the acoustic analysis of passenger vehicles – poroelastic materials are of great importance. One of the most influencing properties in determining their noise-reduction potential is the storage modulus. The purpose of this study is to examine the frequency dependence of storage modulus of selected porous acoustic materials at least up to 1000 Hz. This is executed by using the combined use of dynamic mechanical analyzer and frequency-temperature superposition theory. All other methods for measuring the storage modulus fall short in determining frequency-dependence above 100 Hz: quasi-static mechanical analyzer is mostly used for determining an averaged constant value deduced from low-frequency measurements, while the usage of an electromagnetic shaker capable for high-frequency excitation may include effects of fluid motion inside the pores, thus significantly modifying the results. Frequency-temperature superposition enables to determine the storage modulus values in a wide frequency range, based on low-frequency measurements, where fluid-structure interaction is negligible. It was found that the modulus varied significantly up to and beyond 1000 Hz, and thus, acoustical characterization of these materials can be significantly improved using the proposed method. The work concludes with recommendations to improve the accuracy of the results.

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Inverse metacluster design using generative modeling for minimal scattering response
Time: 11:00 am

Author: Peter Lai

Abstract ID: 2913

Metamaterials are subwavelength-sized artificial structures with the ability to manipulate incident waves in such a way that affects how the energy propagates throughout the medium. In acoustics, particularly placed scattering elements can reduce the total scattering cross section (TSCS) response. We propose a method to inversely design acoustic metamaterial configurations using deep learning and generative modeling. Using our proprietary multiple scattering solver with MATLAB optimization toolbox, we generate a dataset of optimal configurations with minimized TSCS within a discrete range of wavenumbers. We use this dataset to train a Conditional Wasserstein Generative Adversarial Network (cWGAN) to generate similar metacluster designs corresponding to specified input TSCS. To improve the coordinate recognition ability of the cWGAN, we include the novel CoordConv layer in the generator and critic. After training, the cWGAN can produce a variety of optimal configurations given an expected TSCS. Evaluating TSCS of generated configurations shows that the model is capable of proposing scatterer configurations that are comparable or better than the dataset within the optimized range.

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A review of variable-impedance acoustic liner concepts developed at NASA
Time: 5:20 pm

Author: Michael Jones

Abstract ID: 1886

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.

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