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Banana fiber sound insulation

In a recent study published in the journal Applied acousticsBrazilian researchers have analyzed pseudostem fibers extracted from banana trees as a potential sound-absorbing material.

To study: Acoustic performance of banana pseudostem fiber. Image Credit: PICJON/Shutterstock.com

The fibers were glued together using a natural adhesive without the use of binders. Sound absorption coefficients were measured using an impedance tube and transfer function technique for a frequency range of 100 to 6300 Hz. The fibers demonstrated a coefficient of 0.89 at 6300 Hz, which was better than coconut fiber and similar to yucca fiber.

Fund

Noise pollution is a growing concern in urban and metropolitan cities. It affects directly or indirectly, physically or psychologically, human beings and their activities. This requires well-planned building designs and effective acoustic materials.

Acoustical materials can be categorized into soundproofing materials and sound absorbing materials. The basic difference between the two types of materials is that sound absorbing materials reflect incoming sound waves while sound absorbing materials absorb sound waves and convert them into another form of energy, such as heat.

The most commonly used sound absorbing materials are fiberglass and rock wool. However, these materials are known to deteriorate their surrounding environment upon extraction. Thus, readily available natural fibrous materials have attracted much attention as sound absorbing materials.

There are three types of sound absorbers viz. porous absorbers, membrane absorbers and resonance absorbers. In the case of porous absorbers, the sound waves circulating in the cellular or fibrous structure are gradually damped and transformed into heat. Membrane absorbers are non-porous and effective below the frequency range of brass. They are placed in front of a rigid wall with a certain air gap. The waves cross the membrane towards the air gap but remain trapped inside. Resonance absorbers are similar to membrane absorbers except that they are rigid and perforated.

Banana trees only flower once in their lifetime. After that, they are cut to make room for new plants. For tropical regions like Brazil, banana trees are abundant and the fibers extracted from their pseudostem are used in the paper and textile industries. Additionally, their highly watery bodies are the result of highly porous fibers that can effectively absorb sound.

About the study

In this study, the researchers made three types of samples made of banana pseudostem fibers to analyze their sound absorption potential. The first type of sample, sample A, was composed only of fibers of length 10 mm and 70 mm. Their sample thicknesses were 7 and 10 mm, and sample densities were 80 and 120 kg/m3, respectively. In type B samples, the fibers were glued together using natural cassava starch and water. In the C type samples, the bonded samples obtained in the B samples were sandwiched between two sheets composed of gypsum and water. The top layer was perforated while the bottom layer was not.

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Subsequently, the three types of samples were tested in SW466 impedance tubes. The transfer function method was used to calculate sound absorption coefficients and transmission loss coefficients. Several microphones were placed on both sides of the sample inside the tube with a sound source on one side. The analysis was performed in the frequency range from 100 to 6300 Hz.

Comments

All samples showed similar results with a maximum absorption coefficient of 0.20 up to the frequency of 1600 Hz. Above 2500 Hz, samples with higher density and higher thickness showed a better sound absorption. However, with similar standards, the glued samples had a lower sound absorption coefficient than the unglued samples.

This could be due to the reduction of the porous structure. The C samples demonstrated varying coefficients at different frequency ranges without any direct correlation to the density or thickness of the samples. However, C samples with equal gypsum layer thickness on each side showed better performance than samples with unequal gypsum layer thicknesses.

The unglued and glued samples with the highest densities and thicknesses, i.e. A7 and B7, respectively, demonstrated the highest sound absorption coefficient. Both samples demonstrated lower sound absorption than sisal fiber and fiberglass. However, they exhibited higher sound absorption than coir at frequencies above 4000 Hz. C samples had the highest noise reduction coefficient.

conclusion

In summary, the researchers in this study tested the sound absorption potential of banana pseudostem fiber. They prepared three types of samples with varying density and thickness, viz. unglued, glued with natural cassava starch, and unglued, glued samples sandwiched between two sheets of gypsum.

On two plasterboards, one was perforated and the other unperforated. The impedance tube and transfer function method revealed that the sandwiched sample with the highest sample density and thickness had the highest sound absorption coefficient. Thus, banana pseudostem fibers with high density and resonance absorber configuration, i.e. covered with a rigid perforated layer, can be an effective natural and environmentally friendly noise reduction material.

Source

Mendes, C., Nunes, M., Acoustic performance of banana pseudostem fiber, Applied acoustics 191, 2022, 108657, https://www.sciencedirect.com/science/article/abs/pii/S0003682X22000317

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