Porous aluminum with a density of 0.27g∕cm3 was produced by the spacer method. The sound absorbency of the material is significantly improved by inserting an air gap between the sample and the rigid back surface; in this manner, a sound absorption coefficient near unity can be achieved over a significant portion of the audible range. The present data agree with the theoretical analysis in the previous study by Lu et al. [J. Acoust. Soc. Am. 108, 1697 (2000)]; this in turn shows that the present results can largely be attributed to the size (= approximately 50μm) of apertures connecting pores in the material.

Porous Al specimens with porosities of 85%–95% and pore sizes of 212–300to610–700μm were fabricated by the spacer method, and their sound absorption coefficients were investigated by the transfer function method. The sound absorption coefficient increased with porosity. However, there was no apparent correlation between pore size and sound absorption coefficient. Also, the sound absorption coefficient depended on the thickness. The sound absorption coefficient of a specimen was significantly improved by the modification of aperture size, even if the porosity and pore size were unchanged. Therefore, it is suggested that sound absorption behavior is strongly affected by not only the porosity and pore size but also the aperture size for porous Al fabricated by the spacer method.

Porous Al specimens with a pore size range from 212-300 to 850-1000 mm and a porosity range from 77 to 90% were produced by the powder-metallurgical spacer method, and their electrical properties were experimentally investigated. The electrical resistivity increased with an increase in porosity; on the other hand, it was negligibly affected by the pore size when the pore size was sufficiently small. The experimental results agreed with the theoretical results obtained using the unit-cell model in which size of apertures at cell walls are taken into consideration. However, at the maximum pore size in the range investigated, the measured value was much higher than the calculated one. This is likely to be related to the large variation in the local density of the cross section.

The room-temperature damping properties of porous aluminum fabricated by the spacer method were investigated using the method of lateral resonant vibration in cantilever holding. In particular, the effects of the porosity and pore size, which are the representative parameters of porous metals and can be controlled well by spacer method, on the damping properties were focused on. The damping capacity increased with increasing porosity and pore size. Local stress concentration arising from the heterogeneity of porous structures seems responsible for the enhanced damping capacity under the condition in which the main damping mechanism is amplitude-dependent dislocation damping. The present results point out the importance of the porous structure control in damping properties.