Relationship between Sound Absorption Property and Microscopic Structure Determined by X-ray Computed Tomography in Urethane Foam Used as Sound Absorption Material for Automobiles

Abstract

Recently, the application of porous soundproof materials to automobiles is changing from the use of sound insulation materials to the use of sound absorption materials. A method for attaining high absorption performance in the low-frequency region without increasing the material weight is thus needed. We investigated the use of X-ray computed tomography (CT) scanning for investigating the microscopic structure of soft urethane foam, a low specific gravity resin material, in a nondestructive manner. Our testing reveals that it is an effective tool for observing the microscopic organizational structure of a low specific gravity resin material and that the cell size of urethane foam, as measured by X-ray CT, affects the sound absorption characteristics. It also shows that reducing cell size shifts the peak frequency of the sound absorption coefficient downward.

Effect of Microscopic Internal Structure on Sound Absorption Properties of Polyurethane Foam by X-ray Computed Tomography Observations

“…As a result, the sound velocity inside the material decreases, and the peak frequency of the absorption coefficient decreases, as discussed in the previous report. 8) Furthermore, as shown Figs. 10 and 11, for instance, even when the micro cell diameter remained almost the same (0.3 mm), the frequency of the specimens with 10% thicker cell frame walls shifted downward.…”

“…This supports the findings of our previous study. 8) We performed multiple regression analysis among the mean cell diameter, mean thickness of the cell frame walls, and tortuosity. The equation we obtained for tortuosity takes into account both the cell diameter and the thickness of the walls.…”

“…7) We previously demonstrated that X-ray computed tomography (CT) scanning is an effective non-destructive technique for observing the microscopic structure of soft polyurethane foam used as a sound absorbing material. 8) And we identified a strong relationship between the cell diameter and sound absorption performance of polyurethane foam.…”

“…This is consistent with the results of our previous study. 8) The relationship between the peak frequency and the mean cell frame thickness is shown in Fig. 11.…”

“…8) Observing a material's internal structure using Xrays requires the use of X-rays with a wavelength suitable for the density and specific gravity of the material. Likewise, equipment must be used that can supply the proper voltage for generating the X-rays because their penetration capability depends on their wavelength and the density of the material.…”