Metal foams, especially close-celled foams, are generally regarded as poor sound absorbers. This paper studies the sound absorption behaviour of the open-celled Al foams manufactured by the infiltration process, and the mechanisms involved. The foams show a significant improvement in sound absorption compared with close-celled Al foams, because of their high flow resistance. The absorption performance can be further enhanced, especially at low frequencies, if the foam panel is backed by an appropriate air gap. Increasing the air-gap depth usually increases both the height and the width of the absorption peak and shifts the peak towards lower frequencies. The foam samples with the smallest pore size exhibit the best absorption capacities when there is no air gap, whereas those with medium pore sizes have the best overall performance when there is an air gap. The typical maximum absorption coefficient, noise reduction coefficient and half-width of the absorption peak are 0.96-0.99, 0.44-0.62 and 1500-3500 Hz, respectively. The sound dissipation mechanisms in the open-celled foams are principally viscous and thermal losses when there is no air-gap backing and predominantly Helmholtz resonant absorption when there is an air-gap backing.

The effects of the compaction and liquid-state sintering conditions on the structure of the resultant Al foams manufactured by the sintering and dissolution process have been studied. The cell morphology and size of the final Al foam closely match those of the NaCl particles used. The foam porosity is 2-4% higher than the initial volume percentage of NaCl in the Al/NaCl powder mixture due to the contributions from green porosity and shrinkage. The microstructure of the Al matrix is characterised by an interconnected metallic framework with the presence of discontinuous interstices and fine voids as well as primitive particle boundaries in the cell walls. The optimum ranges of compaction pressure and sintering temperature are 200-250 MPa and 670-680• C, respectively. Adding elemental Mg powder in the compacts has little effect on sintering. Adding elemental Sn together with Mg powders creates a densified Al matrix.

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