Foam material sound absorption

The present invention relates to a sound absorbing member comprising a fiber grid insert disposed between and bonded to two open cell foam panels.

The construction and automobile industries are known to use insulation 
board from foam material for noise abatement. The sound-absorbing effect 
results from the thickness of the panel, its mass, increased preferably 
through embedded heavyweight layers. This way of sound absorption using 
foam material is thereby very expensive in terms of material , requires 
much volume, involves increased weight, and remains nevertheless 
relatively ineffective. 
The problem underlying the invention is to show a way toward optimizing the 
ratio between material expense and sound absorption utilizing plastic foam 
materials. The invention does not choose the avenue of sound insulation, 
but that of sound absorption utilizing the not self-suggestive insight 
that such is possible with plastic foam materials. 
This is accomplished through the teaching set forth in the main claim. The 
subclaims represent favorable advancements of the panel so used. 
Using certain foam material panels for the first time according to the 
principle of the flow-optimized absorber, instead of sound insulation, the 
solution achieves a greater and even optimal sound absorption in the area 
of audible frequencies at minimum material expense and thus also minimum 
weight loading of the object being insulated. The sound absorption panel 
may have a uniform thickness between 1 and 10 mm. But areas of varying 
thickness, within this range, may occur also on one and the same foam 
material sound absorption panel. The same applies with regard to the 
homogeneity regarding a flow resistance value on one and the same panel 
and/or various panel areas. The corresponding bending strength value is 
optimal for the self-supporting property and the absorption behavior. It 
can be accomplished favorably in terms of manufacture, through an embedded 
grid which codetermines the flow resistance of the entire panel: This 
applies especially when using foam materials with a low inherent hardness, 
for manufacturing reasons. As the case may be, such a grid insert, may 
also be integrated in the hot-compacting of a heavier foam material panel, 
where the hot-compacting makes for the adjustment of the flow resistance 
value from 20 to 120 rayls. The adjustment of the flow resistance value 
can also be accomplished by skin formation on one broadside of the panel. 
Such skinning may be effected by customary calendering of one foam 
material broadside under pore reduction to the extent giving the 
respective flow resistance. But the resistance may also be obtained by 
applying on one panel broadside a cover foil which through its own 
porosity produces the respective flow resistance (together with the foam 
material and/or a grid insert). In the spacing arrangement of a so 
fabricated foam material sound insulation panel relative to the reflecting 
wall, the broadside with the skin will then be on the far side of the foam 
material sound insulation panel, from the sound-reflecting wall. An 
appropriate bearing edge of the entire panel guarantees a fool-proof, 
correct spacing relative to the sound-reflecting wall.

The sound absorption panel S designed as a shaped component is composed of 
three layers. These are a bottom layer from compacted plastic 
foam-material, that is, the layer 1 facing the source of noise, and a 
corresponding layer 2 on the wall or hood side, i.e., facing the sound 
reflecting surface. Embedded between these two layers is a 
sound-transparent nonelastically bendable grid 3 as a third layer. This 
layer may be wire mesh or a grid from other materials. In the embodiment 
it is a fiberglass fabric with a thermally responding resin coating. The 
bending strength of the entire panel ranges at approximately 20 
Newton/mm.sup.2. Self-supporting, the shaped component is thus mounted 
before a wall with an intervening space, for instance on an engine 
compartment hood, utilizing the 4 through-holes for fasteners. 
The sound absorption panel (5) consists of open-cell, soft elastic plastic 
foam material. The flow resistance of the foam material panel S including 
grid, which overall is about 3 to 5 mm thick, ranges suitably between 20 
and 120 rayls. Allowing for the applicable frequency range (in the engine 
compartment of a motor vehicle), the flow resistance is optimally 80 
rayls. But the indicated limit ranges up to 20 rayls, for one, and 120 
rayls, for another, still yield considerable sound absorption effects as 
compared to what otherwise would have to be expended with customary 
insulating foam materials, in thickness, in order to obtain the same sound 
insulation value. 
Raising parts of the panel from the general plane E-E causes a variation of 
the space from the sound-reflecting wall. According to FIG. 5, this 
section is designed as a cup-shaped recess 5. Variably selected in 
contingence on frequency, one of these spaces is marked x. It equals about 
lambda/4 (and/or an integer multiple thereof) of the frequency range to be 
absorbed forwardly. The rises of the reflected wave impinge on the forward 
plane E.sub.1 --E.sub.1 which is closer to the sound emitter. As follows 
from FIG. 1 and the relief drawing relative to FIG. 3, the forward panel 
sections 6 may be step wise advanced or retracted in the interest of a 
nonvibrating structure, especially with very low sections 6. The plastic 
foam material tolerates the respective lengthwise stretching superbly. In 
the case of especially jagged panel structures, thermally deformable 
material is used for the grid 3. 
The mounted holes 4 are so located that the most favorable connecting 
points between these areas will be covered. 
As follows from FIG. 5, the layers 1 and 2 are drawn around the rounded 
edges 7 and/or forced into the inside corners essentially under retention 
of a uniform layer thickness and structure, due to the only slight 
thickness of , at any rate, less than 10 mm. The very slight thickness 
variations do not subtract from the absorption effect because the mass, 
and especially the flow resistance to air passage as such does not change. 
The layers 1 and 2 may be bonded already previously, instead of during the 
shaping process, for instance by hot-sealing and/or adhesive bonding. 
Thermally responding adhesive favors the irreversible shaping. The thermal 
bonding joins the foam material layers through the meshes 3'. 
Concomitant with this compression shaping to the specified air flow 
resistance, a protective skin 8 can be formed through melting of the outer 
areas of the foam structure, which skin prevents moisture, oil etc. from 
penetrating into the final product. The skin 8 may as well be produced 
already before on the porous layer material. Such a skin, especially when 
wholly or for the msot determining the flow resistance value by its 
adapted porosity, is located on the far panel broadside from the 
sound-reflecting wall. 
All of the new characteristics mentioned in the description and illustrated 
in the drawing are inventionally essential, also if not expressly made a 
part of the claims.