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Martha C. Brown

Publications and source records attributed to Martha C. Brown.

9 records
All sourcesNASA NTRSDOE OSTI
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Comparing Acoustic Prediction Methods for Additively Manufactured Porous Strutures

While macroscale methods for predicting the acoustic properties of porous structures have been popular in the past, they often require time-consuming manufacturing and testing workflows. Meanwhile, microscale approaches allow the prediction of transport parameters based exclusively on a periodic structure’s unit cell geometry. Here, we compare these methods to predict the characteristic impedance of additively manufactured porous structures. We use the microscale approach to estimate the geometry’s transport parameters, then predict the characteristic properties using the Johnson-Champoux-Allard (JCA) model. We measure the acoustic properties of the printed structures using a normal incidence impedance tube and estimate the transport parameters using an inverse characterization approach. We use the two-thickness method as a macroscale approach to predict the characteristic properties from the measured surface impedances of two sample thicknesses. Finally, we compare these characteristic prediction methods. Our results show that the inverse characterization and two-thickness methods offer the closest match to the measured values at low frequencies.

impedance↗
NASA NTRSDate not supplied

Assessment of Acoustic Behavior for Perforate-Over-Large-Cell Liners

This study explores the effects of increasing the cell size for large-cell acoustic liners. Tests are conducted in the NASA Langley Grazing Flow Impedance Tube to evaluate liners with increasingly larger cell dimensions (up to 2” x 4” cross-section). Conventional impedance eduction confirms that liners with 2” x 3” cells (or larger) must be evaluated using nonlocally reacting assumptions. In addition, due to the sound propagation within cavities, the liners must be modeled using higher fidelity techniques. Thus, grazing flow duct and acoustic liner are modeled simultaneously using finite element methods. The facesheet is modeled using a transfer impedance, while the rest of the domain is modeled using the convected Helmholtz equation. The acoustic pressures predicted are shown to compare favorably with those measured in the Grazing Flow Impedance Tube.

large diameter↗
NASA NTRSDate not supplied

Preliminary Design of a Distributed Facesheet Acoustic Liner for Broadband Acoustic Attenuation

The purpose of this study is to investigate the acoustic performance of a liner with a distributed facesheet and a uniform depth core for broadband attenuation. The distributed facesheet is comprised of a cluster of three distinct cell resonators of varying hole diameter and porosity replicated over the active liner treatment area. A target frequency range of 1000 to 2000 Hz and an attenuation metric of 10 dB are chosen. An optimizer is used to determine the optimal facesheet designs for flow conditions of Mach 0.0 and Mach 0.3. The samples are tested in the Grazing Flow Impedance Tube at NASA Langley Research Center. The two liners are shown to achieve at least 10 dB attenuation over frequency ranges of 700 and 400 Hz, respectively, when tested at their respective designed flow speeds. This study demonstrate that a distributed facesheet with a uniform depth core can be successfully used to achieve broadband sound absorption.

acoustic liners↗
NASA NTRSDate not supplied

The Two-Cavity Method for Characterizing Acoustical Materials—an Interlaboratory Study

This study focuses on understanding the sound absorption characteristics of porous acoustic materials, which are determined by two key parameters independent of material thickness: characteristic impedance and propagation constant. These parameters can be characterized by testing a porous sample concept in a normal incidence impedance tube using either the two-thickness or the two-cavity method. In the two-thickness method, two samples of varying thicknesses are required. In contrast, the two-cavity method requires testing one sample at two different air cavity depths behind the porous material. This method is particularly advantageous for materials that are costly or challenging to fabricate. This interlaboratory study evaluates the variability of characteristic properties determined using the two-cavity method. Porous acoustic materials were additively manufactured and tested in the Liner Technology Facility(LTF)at NASA Langley Research Center and the Mechanics, Acoustics and Dynamics Lab (MADLab) at Michigan Technological University. The characteristic properties derived from various cavity depth combinations were used to predict the surface impedance of the sample with a rigid backing. The prediction was then compared the measured impedance spectrum. It was also found that the combinations of cavity depths can significantly influence the accuracy of the deduced properties.

acoustic liners↗