Sound absorption analysis of thermally bonded high loft nonwovens
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.