Sound absorption of open celled aluminium foam fabricated by investment casting method

The sound absorption behaviour of an open celled aluminium foam fabricated by investment casting method was examined and correlated with the pore size, sample thickness and backing air gap depth in the present study. The foam shows low sound absorption coefficients at relatively low frequencies or with relatively thin sample thickness due to its 90% or higher open porosity. As frequency or sample thickness increases, the sound absorption is significantly enhanced. Furthermore, if the sample is backed with an air gap, the approximate periodical resonant absorption peaks emerge from the background and shift towards low frequencies with increasing the gap depth. It is also found that the present aluminium foam has even better absorption performance than polyurethane foam, with almost the same pore parameters as the former. The observation on the pore surface and further experiments demonstrate that the difference is attributed to rougher pore surface of the aluminium foam that makes the sound wave loss increase through enhanced friction between the air and the surface.