Electrospun membrane as Sound Barrier

Fig 1. Schematic of electrospun membrane as sound absorbance.

The effectiveness of a passive fiber-based sound absorbance material involves several parameters such as porosity, tortuosity, fiber diameter, density, airflow resistance, thickness etc. and the absorbance coefficient of a material is dependent on the amount of energy absorbed by it. Mathematically, it is the ratio of the energy absorbed by the material to the energy incident upon its surface. Nonwoven fabric has been used extensively as a sound absorbance barrier and has been known to perform well for absorbing high frequency sound. A nonwoven nanofibrous mesh has the potential to be a good sound absorbance material as its high surface area and tortuous path through the thickness of the mesh exposes more material for sound absorbance while retarding airflow.

Tests on electrospun nanofiber mesh have supported its use as sound absorbance material. Comparing the sound absorption coefficient of electrospun silica fibers of different diameter to glass wool, Akasaka et al (2014) found significant improvement in sound absorption of electrospun fibers over glass wool at frequency of 1600 to 6400 Hz. There is also a general improvement in sound absorption with smaller fiber diameter from 8.24 µm of glass wool down to electrospun fibers of various diameters with the best absorption at diameter of 670 nm. A slight reduction in sound absorption was recorded for electrospun fibers with diameter of 520 nm although it may suggest that optimum fiber diameter is around that range. Alba et al tested the effect of adding a nanofiber veil (1% of composite) to polyester wools and it was found that the absorption coefficient increases from a range of 300 Hz to 2000 Hz and up to 30% for some frequencies. Rabbi et al (2013) sandwiched nanofibrous membrane of polyacrylonitrile and polyurethane between two nonwoven layers of polyethylene terephthalate and wool. All materials containing electrospun membrane(s) were found to significantly increase its absorbance from about 500 Hz to more than 6000 Hz. Between electrospun polyacrylonitrile and polyurethane, the former was found to exhibit better sound absorbance. Comparison was also made with various thickness (1, 3 and 5 g/m²) and layers (1, 2 or 3 within four nonwoven layers) and it was found that multiple layers of thinner membrane give better result than single thick layer although thicker layer. Chang et al (2016) tested the sound absorption properties of a thick block of electrospun PVP nanofibers with diameter of about 200 nm and compared against commercial cotton sound absorption mat. Although the thickness of the block used in the experiment is not stated, the PVP nanofibrous block demonstrated significantly better absorption coefficient than cotton at frequency range of 200 Hz to 1600 Hz. At frequency of 800 Hz, the absorption coefficient of the nanofibrous block was at a maximum of 0.9 while commercial sound-absorbing cotton was less than 0.5. Hurrel et al (2021) attempted to use Biot- and Darcy-type mathematical models to explain the acoustical behaviour of a thin layer of nanofibers membrane on a foam substrate. The acoustic behavior of electrospun poly(methyl methacrylate) (PMMA) fiber membrane on a melamine foam substrate was tested. With a fiber diameter of 440 nm and membrane thickness of 22 µm on top of the melamine foam, the increase in the absorption coefficient is close to 75% at some frequencies such as 2000 Hz. This is explained by over 100% increase in the real part of the surface impedance and a small drop in the imaginary part of the surface impedance which contributes to extra absorption. Their study also suggests that the classical model for flow resistivity of fibrous media does not work when the diameter of nanofibers becomes comparable to the mean free path.

Noise reduction material used in aircraft interior needs to be light and occupy a small space while having good sound absorbance property. Asmatulu et al (2009) tested the sound absorbance property of electrospun polyvinyl chloride mat of different thickness and with fiber diameters ranging from a few hundred nanometers to a few microns. Fiber diameters of about 200 to 500 nm with thickness of 0.5 mm were better able to absorb sound at the higher frequency (>5000 Hz). As the thickness of electrospun fiber increases, the sound absorbance shift towards the lower frequency. However, the absorption coefficient drops too. When the fiber diameter goes beyond 500 nm, the sound absorbance shift towards the lower frequency with thicker mesh but absorption coefficients remains the same. Petrone et al (2016) tested the performance of multiple stacked layers of electrospun PVP fibers membrane (average diameter of 2.8 µm) with minimum of 6 layers as sound absorbers in aircraft. As the layers of PVP membranes increases, the peak absorption shifted towards lower frequency (about 300 Hz) when the weight was 18 g. With 6 layers at a weight of 7g, the peak absorption is at about 700 Hz. The sound absorption coefficient was consistently above 0.8 at its peak across all samples. In contrast, glass wool fibres at 12 g showed better sound absorbance at higher frequency. Sound absorption coefficient of 12 g electrospun fiber multi-layer membranes performs better than aerogel (12 g) and polyester (10 g) across most frequency range from 300 Hz to 1600 Hz. For glass wool fibers (12 g), sound absorption starts to surpass electrospun PVP layers at frequency above 650 Hz.

The ability of nanofibrous coating to absorb sound and its frequency range is also dependent on the underlying material used. Experiments by Trematerra et al (2014) showed that with a thin layer (10 µm thickness) of nylon nanofibers (diameter 150 - 200 nm), sound absorption on kenaf (thickness 40 mm) is improved at frequency between 500 to 1500 Hz while on felt (thickness 3 mm), it is at frequency of more than 4000 Hz. For 40 mm thick foam, the significant sounds absorption was recorded at frequency of 1000 to 1500 Hz. Application of nanofibers does not result in a reduction in the sound absorption ability of the underlying substrate. It either improves the absorption coefficient or widens the frequency range at which the maximum absorption coefficient is reached.

Addition of nanoparticles into the electrospun fiber matrix was also found to have a significant effect on its sound absorption properties. Gao et al (2016) conducted a series of tests using polyvinyl alcohol (PVA) with different loading of TiO₂ and ZrC nanoparticles. Using ZrC nanoparticles, sound absorption properties of the composite shifted to the higher frequency above 2500 Hz. However for TiO₂ nanoparticles, the improvement were found at the lower frequency range from 500 Hz to 1500 Hz. The size of nanoparticles was also found to influence the sound absorption properties with TiO₂ nanoparticles size of 200 nm showing better sound absorption in the lower frequency range compared to 10 nm nanoparticles.

Physical characteristic of electrospun membrane may also be varied to alter its sound absorbance property. Mohrova and Kalinova (2012) used electrospun polyvinyl alcohol (PVA) membrane to demonstrate the degree of fiber fusion on its acoustic properties. Since PVA is water soluble, exposure of PVA nanofiber to water vapor will cause it to gradually dissolve over time. With this, they were able to change the physical characteristic of the fibrous membrane which subsequently affected its sound absorption coefficient.

Image of surface of PVA nanofiber structures by scanning electron microscope SEM for layers after water vapour action during time of 60 seconds (b) and 120 seconds (c), without liquid water or water vapour action (a) and structure after action of water in the liquid condition to nanofiber layer during the time of 60 seconds (d). Magnification = 5000X and scale bar = 20µm [Mohrova and Kalinova. Journal of Nanomaterials, vol. 2012, Article ID 643043, 4 pages, 2012. doi:10.1155/2012/643043. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]