Depending on the manufacturing method used, metal foams can
be either mostly open celled or mostly closed celled. Open-celled
metal foams can be manufactured using polyurethane foams as
a matrix base.38 The resulting metal foam has values of porosity
between 75-95%. Evidently the vibration of the skeleton can be
neglected when modeling the sound absorption properties of metal
foams. Open-celled metal foams with cell sizes from 0.2-0.5 mm are
commercially available for high-temperature applications.