Researchers Create Effective and Economical Sound-Absorbing Nanofoam

This is the initial nanopowder of nanomagnetite (Fe3O4) with a specific surface area of 70 m2/g. (Credit - FEFU press office)

A new breakthrough material capable of reducing the noise level by 100% has been developed by a young scientist from the Far Eastern Federal University (FEFU) along with others. The new foam material is more efficient compared to standard analogs, minimizing the level of noise transmission by 20-22 dB.

The foam reacts to sound waves of both high as well as low frequencies, which can harm human health.

Partnership Development

Alexey Zavjalov, postdoc, researcher at the Academic Department of Nuclear Technologies School of Natural Science, FEFU, worked as a part of the international team of Russian and South Korean scientists under Professor S.P. Bardakhanov. Alexey's research performance led to the development of the nanofoam—the new noise-absorbing composite material. The outcome of the work was reported in 'Applied Acoustics'.

The problem of noise is the problem of modern technogenic civilization. In South Korea, cities are equipped with round-the-clock working stationary and mobile networks for noise levels monitoring. The urbanization level of such territorially small countries as South Korea is much higher than in Russia. However, in our country this problem is still crucial for big cities. The development of new noise-absorbing materials is especially interesting for the automotive industry. Modern people spend a lot of time driving cars and the noise level inside the vehicles' directly determines the quality of life. For East Asian countries, the issue of noise control is relevant for high-speed rail lines. Porous materials are exceptional sound absorbers but their noise-absorbing properties can be considerably improved by injecting nanoporous grit into the foam structure and forming internal channels in it.

Alexey Zavjalov, Researcher, Academic Department of Nuclear Technologies School of Natural Science, FEFU

Harmfulness of the Low Frequencies Noises

Together with the fast development of nanotechnology, there have been numerous attempts to mix nano- and microsized materials to design a modified material with improved strength, dynamical, elastic, and vibrational properties. The acoustic factors of such materials could not be fundamentally improved thus far.

Foam materials are most frequently used for soundproofing applications. They offer the right quality at a reasonable cost, but thus far have been effective against high-frequency noise only. Simultaneously, low frequencies can be a lot more damaging to human health.

Infra- and low-frequency vibrations and noise (below 0.4 kHz) are most detrimental and dangerous for human health and life. Particularly unfavorable is their lifelong impact, which leads to critical pathologies and diseases. Complaints on such oppressions surpass 35% of the sum total of complaints on risky environmental conditions.

The foam material, created by Russian and Korean researchers, exhibited promising results at medium frequencies and, thus, more specialized low-frequency noise tests are required.

Cheaper and Easier for Application than Aerogel

The enhanced acoustic features of the latest hybrid nanofoam were acquired by extra impregnation of the basic off-the-shelf sound-absorbing foam with porous granules of magnetite and silica nanoparticles. The porous foam was immersed in nanopowder suspensions in the liquid, exposed to ultrasonic treatment and dried.

The nanoparticles granules formed in the result can be compared structurally to a commonly known group of materials—aerogel. It has not only superior thermal insulation properties but also has a good noise-proof property. But aerogels are relatively expensive and complex when used in structures. The new material, developed according to the scheme put forth by the FEFU scientist, is structurally similar to aerogel but is free of such limitations as high price and engineering issues.

Composite Technology

The sound absorption mechanism the new foam is based on the fact that its sound-absorbing surface is considerably scaled because of the presence of numerous nanopores in the particles injected, as well as the location of these particles in the foam matrix in the form of separate channels. Nanoparticles disperse the energy of a sound wave converting it into heat. The soundproof properties of the material grow.

The researchers discovered that the composite structure is most effective for noise reduction. Thin layers of foam infused with nanoparticles are linked to each other in a "sandwich"-construction. This design considerably enhances the soundproof properties of the resulting material. The study’s outcome also indicates that the more foamy material is infused with nanoparticles, the better its sound absorption is.

In some approximation, any material can be represented as a network of weights connected by springs. Such a mechanical system always has its own frequency bands, in which the oscillations propagate in the system relatively freely. There are also forbidden frequency bands in which the oscillations rapidly fade out in the system. To effectively extinguish the transmission of oscillations, including sound waves, the materials should be alternated in such a way that the fluctuations that propagate freely in the first material would be in the forbidden band for the second layer,. Of course, for our foam material, this idealization is too crude. However, it allows us to clearly illustrate the fundamentally conditioned necessity of creating a "sandwich" structure.

Alexey Zavjalov, Researcher, Academic Department of Nuclear Technologies School of Natural Science, FEFU

Research Outcome

The research revealed the effectiveness of the technique of foams impregnation with nanomagnetite or nanosilica, which form granules up to several hundred micrometers (consistent with the pore sizes of the modified foam material) and pores measuring approximately 15 nm. This small incorporation provided a more complex and branched 3D network of nanochannels which resulted in extra absorption of noise energy.

Owing to the technique used, noise absorption efficiency was attained in the range of 2.0-6.3 kHz and at lower frequencies of 0.5-1.6 kHz. The degree of absorption was raised by 60-100% and the sound transmission was lowered by 20-22 dB, irrespective of the type of nanofiller.

There is room to further improve the sound absorbing properties of the new material for medium and low frequencies using the "active control" strategy. First of all, this refers to the materials obtained by using a magnetite nanopowder. Active noise protection systems have long been used in the world. The main idea is to detect the noise acoustic fields "online" and to generate sound waves in antiphase by means of loudspeakers. That allows achieving a significant reduction of noise in a given area. Concerning the nanofoam, it's proposed to adapt this approach and to actively exert on a material saturated with granules of magnetite nanoparticles by magnetic fields. This will achieve even better noise reduction.

Alexey Zavjalov, Researcher, Academic Department of Nuclear Technologies School of Natural Science, FEFU

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