By Surbhi JainReviewed by Susha Cheriyedath, M.Sc.Sep 13 2022In an article recently published in the journal Applied Acoustics, researchers discussed the results of the preliminary research on mixed porous/nano-fibrous materials for noise attenuation within aircraft cabins.
Study: Noise attenuation inside airplane cabin: Preliminary results on combined porous/nano-fibrous materials. Image Credit: Ryan Fletcher/Shutterstock.com
Background
The aeronautics sector is constantly looking for ways to increase cabin comfort as a means of competing on the world stage. However, weight restrictions and production costs place limits on this quest for a noiseless cabin.
The location and kind of engine have an impact on cabin noise. A careful approach using the frequency range between 800 and 1800 Hz must be taken while looking for a soundproof material for an airplane cabin. Sandwich panels and polyurethane foams are two materials that are frequently employed in the automotive sector. The low weight and low cost of fabrics and textile-based sound absorption materials are two further appealing qualities. These materials have a restricted range of uses while being an affordable option for passive noise suppression.
The so-called elastic waves metamaterials are a new kind of active noise-canceling device. The modification of the current materials to address particular issues is one potential answer for the aerospace industry. In the past, the electrospinning method has been used to create nanomembranes for a variety of purposes. Potential materials for reducing cabin noise include electrospun nanomembranes. Furthermore, the literature has proven the potential advantages of adding nanostructures to composites. One additional layer of potential enhancement might be viewed as the addition of nanostructures to nanofibers.
About the Study
In this study, the authors used cellular materials (aerogel and melamine foams) and poly (vinylidene fluoride-cohexafluoropropylene) (PVdF-HFP) nanomembranes doped with carbon nanotubes to create a number of lightweight and reasonably priced combination systems which were further evaluated. Each of the components of the composite system had an impact on the overall acoustic behavior of the system. Experimental evidence and mathematical models demonstrated that the interaction of nanomembranes and biological components resulted in a behavior akin to a Helmholtz resonator.
The team captured the resonant effect physical phenomena using the suggested analytical correction of the sound absorption coefficient. Aerogel, by itself, was not a good sound-insulating material, however the combination of nanomembrane and aerogel resulted in sound absorption coefficients of about 0.7 at a thickness of 4.0 mm at 6000 Hz. The sound absorption coefficient of the melamine foam combined with the nanomembrane peaked at 0.97 at 1600 Hz and a thickness of 12 mm. The inclusion of a 60.00 lm thick nanomembrane changed the frequency of melamine foam with a density of 6 kg/m3 from the high range to the mid-range and opened up a new market for a novel class of cellular composites for the acoustic insulation of commercial airline cabins.
The researchers examined how cellular materials' acoustic properties could be impacted by nanomembranes doped with carbon nanotubes. The intended frequency range, which corresponded to the typical range of cabin interior noise for mid-size jet aircraft, was between 800 Hz and 1800 Hz. The ultimate objective was to lower some cabin noise frequencies between 800-Hz and 1800-Hz.
Observations
For the pure PVdF-HFP nanofibers, the average fiber diameter had a normal distribution and was around 243.33 ± 0.75 nm; however, the addition of CNT seemed to encourage a reduction in fiber diameter. Despite the aerogel's high porosity of 95%, it was stiff because of the modest ratio between the average size of the pores of 77.06 ± 12.71 μm and the thickness of the walls of 30.34 ± 11.37 μm.
Up to 2000 Hz, all groups behaved in a consistent manner at low/medium frequencies. A combination of the aerogel dominant behavior and the large porous size of 2.38 μm was responsible for group #5's subpar performance. The second group behaved in a totally different way; the sound absorption coefficient rate development was almost exponential beyond 2000 Hz.
The sound absorption coefficient of the melamine foams, which were 12 mm and 14 mm, peaked at high frequencies in the range of 5500–6000 Hz. These frequencies were converted to mid-range frequencies in the range 1200–1600 Hz when the nanomembranes were included. The wall's oscillations generated enough energy that its thin thickness of 4.0 mm was unable to disperse it.
The proposed model had a 98% accuracy rate in capturing not only the resonance frequency but also in forecasting the sound absorption coefficient. The transition of peak frequencies from high-range to mid-range was accurately predicted by the model.
Aerogel by itself was not a good sound-insulating material, however, the combination of nanomembrane and aerogel resulted in sound absorption coefficients of about 0.7 at a thickness of about 4.0 mm at 6000 Hz. At 5500 Hz, the melamine foam's sound absorption coefficient peaked at 0.95.
The sound absorption coefficient of the composite system of aerogel, nanomembranes, and melamine foam was 0.97 at 1600 Hz. The resonant vibrational nanomembrane behavior and the melamine foam's function as a Helmholtz resonator were two simultaneous effects that resulted in this behavior.
Conclusions
In conclusion, this study developed an experimental inquiry on a novel class of composite systems based on the aeronautical necessity to reduce cabin noise at frequencies between 800 Hz and 1800 Hz. In a serial association, PVdF-HFP nanomembranes doped with carbon nanotubes, melamine foams, and polyimide aerogel were used to build lightweight and affordable sound absorption composite systems. Each of the components of the composite system had an impact on the overall acoustic behavior of the system. The presence of nanomembranes created the potential for resonant vibrations and raised the viscosity of friction.
The authors mentioned that the physical phenomenon and its resonant effect might both be explained by the suggested model. They believe that the proposed technology could be very beneficial for commercial airplane cabin acoustic insulation.
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References
Leão, S. G., Monteiro, E. C., dos Reis, M. O., et al. Noise attenuation inside airplane cabin: Preliminary results on combined porous/nano-fibrous materials. Applied Acoustics, 109009 (2022). https://www.sciencedirect.com/science/article/abs/pii/S0003682X22003838
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