Building Acoustics

Sound absorbers 157

Plastic fibres products, e.g. polyester fibres materials, are also becoming popular
for sound absorption. The diameter of the fibres in these products is normally larger than
for the mineral wool products, being of the order 20–50μ. The aforementioned
anisotropy also applies to these products. Other types of porous materials are also
commercially available. Products comprise glass and metals in a sintered form. One may
also find aluminium expanded as foam but to be effective as an absorber the pores must
be interconnected. There also exist products using fibres of aluminium compressed into
sheets of thickness 2–5 mm. These are intended for suspended ceilings.

Porous

material

Porous

material

Cavity

Cavity

Membrane

a)

b)

c)

Panel

d) Cavity

Figure 5.1 Basic types of acoustic absorber.

5.2.2 Membrane absorbers

By definition, a membrane shall have no stiffness but thin metal sheets are usually
included when talking of membrane absorbers. The prerequisites for obtaining a
reasonably high absorption factor are low surface weight and high internal losses in the
membrane (plate) material. Aluminium or steel panels having a thickness of 0.5–1.0 mm
mounted at a certain distance from a hard wall or ceiling normally absorbs in a limited
frequency range only, having a statistical absorption factor α usually less than 0.5–0.6.
By using thin sheets of plastic materials instead of metal much better performance may
be achieved.

5.2.3 Helmholtz resonators using perforated plates

Helmholtz resonator absorbers are based on the principle that the air in the holes (or
slots) of the plate represents a mass and that the air volume of the cavity behind the plate
represents the spring stiffness in an equivalent oscillator, i.e. a simple mass-spring
system. To absorb or dissipate acoustic energy one certainly must include a resistive