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Sound absorption characteristics of specially designed high-loft nonwovens with minimum thickness were reported in this study. Three different polypropylene and polyester fiber-based high-loft, air-laid, and thermally bonded nonwovens varying in basis weight were produced. Heavier high-loft nonwoven samples at various thicknesses were formed using a specially designed mold. The sound absorption coefficients of samples with mass per unit areas ranging from 350 to 1575 g/m2 and with thicknesses ranging from 5 to 45 mm were measured. Acoustical absorptive behavior of the high-loft nonwovens was explained by analyzing the displacements of small air control volumes in a high-loft nonwoven and the air velocities in the impedance tube. Results indicate that the velocity and the total displacement of the small air volumes inside the fiber network have a major effect on sound absorption. High-loft nonwovens can be much more effective in terms of sound absorption if they are produced at the thickness at which average maximum velocity of the air is calculated highest. If there is a desire to absorb more acoustic energy, heavier nonwovens can be produced. It is suggested that relatively heavy nonwovens (from 700 to 1575 g/m2) can be produced thinner (5–10 mm) than the calculated thickness value based on the average maximum air velocity to get maximum sound absorption at lower thickness.

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This work presents the results of efforts focused on the development of relatively lightweight and fibrous acoustic webs. For this objective, nonwoven webs that contain bicomponent filaments with islands-in-the-sea cross sections were produced by spunbonding, which involves the extrusion of sea and island polymer melts through dies, cooling and attenuating the bicomponent filaments by high-velocity air streams. Nylon 6 and polyethylene were used as the island and sea polymers, respectively. Webs were hydroentangled with high-pressure water jets prior to the dissolving process to obtain fiber entanglement. Sea polymer was removed from the spunbonded nonwovens by using a reflux dissolution setup. Weight, thickness, air permeability, pore size and sound absorption coefficients of the nonwoven samples were measured before and after the sea polymer removal. Results demonstrated that sea polymer removal led to further bicomponent filament fibrillation, which affected sound absorption positively. The structure with the higher number of island fibers had better acoustical properties. Lightweight and fibrous acoustic nonwovens can be obtained with the method given in this study.

This work presents the results of efforts focused on the development of sound absorptive woven fabrics by the raising process. Four woven fabrics with rib and basket weave patterns were produced for the raising process. Micro-fiber-based polyester weft yarns were used in one set of rib and basket weave fabrics, while weft yarns comprising regular polyester fibers were used in the other set. Fabrics were subjected to dyeing and heat setting prior to the raising process. Fabrics were then passed one to three times through the raising unit in order to obtain fabrics with different voluminous characteristics and different quantities of fiber ends on the fabric surface. The mass per unit area, thickness, air permeability, and sound absorption coefficient of the fabrics were measured and surface images of the fabrics were taken. The solid volume fraction and airflow resistivity of the fabrics decreased significantly after the first and second raising passes. Increasing the number of raising passes up to two passes resulted in higher sound absorption (average increment of 20% at 5 kHz) in the higher frequencies at the expense of that in the lower frequencies. Sound absorption change beyond two passes was insignificant, though. The results demonstrated that raised fabrics having a lower solid volume fraction and airflow resistivity had better acoustical properties in the higher frequency region.