In the article on metal foams by John Banhart and Denis Weaire (Physics Today, July 2002, page 37), I saw no discussion of possible acoustical absorption applications. Controlled interconnection of pores is, of course, necessary for broadband sound absorption. Possibly that property, combined with certain of its structural abilities, would make metal foam a unique high-performance structural material.
When doing architectural acoustics consulting in New York and Chicago, Tony Paolello of the New York City Transit Authority and George Krambles of the Chicago Transit Authority both expressed the hope that an economical, easily cleaned, sound-absorbing, and extremely durable material would be found for lining subway tunnels. Could metal foams be such a material? Combined with damped sheet steel backup for sound isolation, these materials would seem ideal for the subway system's sound-absorbing surfaces. Since metal foams can be rigid enough to bear the weight of workmen, transit systems might also be able to use them to make noise barriers that can pivot from the vertical to the horizontal to become maintenance walkways.
Banhart replies: Aluminum foams are, almost by definition, materials with closed cells that have no interconnections; therefore, the foams are poor sound absorbers. However, as liquid metal foams solidify, thermal stresses occur: The solidified foams usually have cracked cell walls, which significantly increase sound absorption. In addition, by slightly rolling thin sheets of foam from, say, a thickness of 10 mm to 9 mm, further mechanical cracking occurs, and the interconnections between adjacent cells widen. Thus, sound absorption increases even more. The result is an absorber that has its maximum between 1 and 5 kHz with a peak absorption coefficient of up to 95%. By placing an air gap between foam and a rigid wall, one can shift the frequency curve to lower frequencies.
Altogether, metal foam is not a very good sound absorber; other materials—glass wool, for example—show an almost constant 99.9% absorption over a wide frequency range. However, other metal foam properties—namely, high weight-specific stiffness, good crash-energy absorption ability, and nonflammability—might make them marketable for sound absorption panels. Reportedly, the Japanese railways are using aluminium foam panels to damp the shock waves caused by trains as they enter tunnels, so the application suggested by David Klepper is worth evaluating.
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