Civil Engineering Reference
In-Depth Information
A number of other methods, most of them non-standard, are also in use both in
laboratories and in the field (in situ methods). Development of in situ methods usable for
low frequencies is particularly challenging, specifically from a few hundred Hertz and
downwards.
For development purposes it should certainly be an advantage to make a direct
calculation of the absorption factor based on material data and geometry for the absorber.
Reasonable accurate analytical methods exist for homogeneous materials with simple
geometry (plane absorbers), but certainly dependent on the accuracy of the material
parameters going into the models.
5.2
MAIN CATEGORIES OF ABSORBER
Commonly used acoustic absorbers (absorbing surfaces) may be divided into two main
groups:
a)
Porous absorbers, e.g. mineral wool, plastic foams, fabric etc.
b)
Resonator absorbers, either membrane or absorbers based on the Helmholtz
resonator principle.
Basic forms of absorbers used in practice are depicted in
Figure 5.1.
Porous
materials are often placed directly on to a hard surface or with a cavity behind to increase
the absorption at low frequencies (see a) and b)). Membrane absorbers, depicted in c),
may be a thin panel (or foil) of metal or hardboard, again placed at a certain distance
from a hard surface. For a resonance absorber of the Helmholtz type, shown in d), the
panel is perforated is various ways, normally by holes or slits. Combinations of the
abovementioned types are also generally found.
When designing for proper acoustic conditions in a given room, one should be
aware of other mechanisms present and capable of absorbing sound energy. In a room
having lightweight wall constructions one may unintentionally induce plate vibrations by
the sound field. This vibration energy may partly be dissipated in the plates themselves
and partly radiated to a neighbouring room. With the latter mechanism the energy is also
absorbed (non-reflected) as seen from the primary room. Thermal and viscous effects
also add to the loss of acoustic energy in the room. These effects are contained in a
“classical” part of the expression for the attenuation coefficient. More important,
however, is the relaxation or hysteretic phenomena. Depending on the moisture content
of the air in the room one may observe that these phenomena dominate the acoustic
energy losses at the higher frequencies (in the kHz range). This type of air absorption has
already been treated in the preceding Chapter 4 (see section 4.5.1.3).
5.2.1
Porous materials
Well-known porous materials are products of mineral fibres and plastic foams.
Commonly used are blankets of mineral “wool”, either glass or stone wool. These have
fibres with a diameter in the range 2-20μ, commonly 4-10μ. Due to the manufacturing
process the fibres will be distributed anisotropically. They will be randomly distributed
in a plane parallel to the outer surface of the blanket but there will be few fibres oriented
normally to this plane. One will find the mineral fibre products of the type “elastic”
blankets as well as compressed into stiff boards, the latter normally used in suspended
ceilings.
