Measuring and Simulating Acoustic Absorption of Open‐Celled Metals

Abstract

Due to increasing air traffic and the extension of airports, many people are exposed to significant noise loads causing insomnia and other reactions, which impair health and productivity. [1,2] To change this development, various approaches have been developed to reduce airframe noise, jet noise, and engine noise. This work focuses on the approach to reduce noise by inserting open-celled metals in the hot gas path. Polymers are not able to bear the heat of around 614°C in this section of the aero engine and brittle ceramic structures would crack under the high fatigue loading. [3] Open-celled metals are metal sponges or sintered fiber felts which also resemble geometrically conventional passive absorbers.The most important material property in regard to acoustic absorption is the air flow resistivity J resulting from pore morphology and porosity. Depending on the production processes, e.g. sintering of metal fibers, metal flakes, or hollow spheres, coating of polymer foams, or powder metallurgy, the characteristic dimensions of the pores are usually in a range of 0.01 to 10 mm and the porosity in a range of 20-95%. [4][5][6][7] In this broad spectrum of materials, the air flow resistivity varies several orders of magnitude. To choose the correct material, it is necessary to characterize the different materials, measure, and simulate their acoustic absorption at room temperature. Finally, the simulation is adjusted by changing the properties of air meaning temperature and pressure to simulate the acoustic absorption at conditions of an aero engine. Note, that fluid flow effects are still neglected.

Numerical modelling of the sound absorption spectra for bottleneck dominated porous metallic structures

“…The structural morphology of melamine foam is similar to that of commercially available porous metallic structures made by replication of open cell foams, for example Recemat TM , Porvair TM and Alantum TM foams [15]. It is not surprising, therefore, that modelling the sound absorption behaviour for these foams is also quite accurate using the Delany-Bazley-Miki (DBM) model [7]. The JCA model gives a better fit to the experimental data recorded for porous sintered fiber metal [8]; the DBM model was not able to accurately predict the behaviour characterized by a dip in the absorption curve for frequencies beyond 3000 Hz.…”

“…The predicted characteristic sound absorption spectra fit well to the experimental data observed for both the glass wool fiber (Fig 4a) and melamine foam (Fig 4b) structures, for both models, with an overall better fit to the Delany-Bazley-Miki (DBM) model. The characteristic absorption spectra for these highly porous materials are reported to depend mainly upon the pore morphology and porosity [7,11]. The DBM empirical model was specifically developed to model the acoustic behaviour for these relatively simple structures and the good fit, as observed in [11], is expected.…”

“…Technological limitations prevent attaining the same high levels of porosity and small cell sizes and offer limited potential to alter the pore morphology in-process. These structural differences tend to lead to lower flow resistivities for the metallic counterparts [7] and inferior absorbers.…”

“…Fibre felts, or sintered metal fibres [7,8] offer the closest matching performance to those of glass wool and polymer foams, as shown in Fig 1, but offer little or no structural function. Fig 1 shows the absorption behaviour for a so-called "bottleneck-type" porous metal structure (from [9]) made by casting liquid metal into the spaces between a packed bed of dissolvable salt beads [9,10].…”

Sound Transmission Loss Enhancement in an Inorganic‐Organic Laminated Wall Panel Using Multifunctional Low‐Density Nanoporous Polyurea Aerogels: Experiment and Modeling

“…The STL values were reached up to 40 dB, in contrast to traditional acoustics materials, such as polyurethane foam, which can only reach up to 5 dB . Furthermore, despite most passive materials (e.g., open‐celled materials), which have weak acoustic performance in sonic absorption at low frequency range (i.e., less than 5 kHz), the aerogels showed remarkably high acoustic absorption and attenuation over the entire low frequency range. Such a great attribute originates from the random multiscale heterogeneous structural elements and hierarchical porosities of the aerogels which provide highly tortuous flow streams for air molecules.…”