Sound absorption in metallic foams

Cellular materials constitute a new and attractive class of products with a wide variety of structural and functional applications. Metallic foams, a term that encompasses all porous metallic materials, are a specific type of such products.

In addition to their low specific weight, metallic foams feature a series of mechanical, thermal and acoustic properties that make them particularly well suited applications in the automotive, biomechanical and construction industries.

Metal foams having open or semi open cells are said to have sound-absorbing capacity.It is known that they have higher damping capacity and natural vibration frequencies than the solid of which they are made — sandwich panels with metal foam cores offer significant potential for vibration and acoustic management — but this is not the same as sound absorption. Sound absorption means an incident sound wave is neither reflected nor transmitted; its energy is absorbed in the material. There are many ways in which this can happen:

1. by viscous losses as the pressure wave pumps air in and out of cavities in the absorber,
2. by thermal-elastic damping,
3. by Helmholtz-type resonators,
4. by vortex shedding from sharp edges,
5. by direct mechanical damping in the material itself, and so on.

Due to additional properties such as mechanical strength, fire resistance and damping capacity metal foams (closed cell aluminum foam especially) are very effective and demanded materials for acoustic absorption.

For example, glass wool is made of interlaced stone filaments forming a pad in which immobilized air is trapped. This structural arrangement yields very light weight and highly effective thermal and acoustic insulation, whose properties are internationally acknowledged. Nonetheless, given the low mechanical strength of these products, they are routinely combined with a perforated panel or board to ensure rigid, long-lasting support and a fine finish that at the same time contributes to enhancing acoustic performance. Five — to ten-mm aluminum foam perforated panels could feasibly be used for this purpose. The high corrosion resistance characteristic of aluminum would, moreover, make such foams particularly suitable in outdoor environments.

Type	Metal Foam	Polyester	Glass Fiber	Urethane Form
Acoustic absorption	over 0,70	over 0,70	over 0,75	over 0,65
Acoustic absorption term	Permanent	Quality deterioration	Sudden quality deterioration	Quality deterioration
Eco-friendly	100% recyclable	No recyclable	No recyclable	No recyclable
Human maleficence	Harmlessness	Harmlessness	Harmlessness	Poisonous gas (CO, Cyan gas) on fire
Nonflammability	Inflammable (heat-resistant) — under 780°C	Flammable	Heat-resistant — under 350°C	Bad heat resistant (heat-resistant) — under 100°C

Table.1.Comparison of different acoustic materials

Are metal foams good sound absorbers? Data reported here suggest a sound absorption coefficient between 80% and 95% may be achieved in selected frequency ranges in selected aluminum alloy foams.Compared with glass wool and polymer foams, the advantage of using metal foam as a sound absorber is obvious: self-supporting due to its rigidness and strength, fire retardance, low moisture absorption, and superior impact energy absorption capabilities.It is expected that sound absorbing metal foams will find wide range applications in noise and vibration control of aircrafts, automobiles, machinery and buildings.

Source:

http://www.metalfoam.net/papers/lu99 a. pdf