Note: Descriptions are shown in the official language in which they were submitted.
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ACOUSTIC SHIELDING DEVICE FOR DAMPING OF DISTURBING
TRAFFIC NOISE
TECHNICAL FIELD
The present invention relates to an acoustic screening device for damping
disturbing noise, especially such noise which arises from road traffic, rail
traffic and the like. The invention especially relates to an acoustic
screening
device configured as a module consisting of a framework, preferably of
concrete, and a noise-absorbing cassette arranged thereon. A plurality of
modules can be placed in a row one after another with a view to creating an
integrated acoustic screening device or barrier.
BACKGROUND ART
Traffic noise from roads and railways has become an ever increasing problem
in present-day society, and the environmental effect, especially for local
residents, is often obvious and very disturbing. Demands for effective
measures are therefore made by members of the public and the authorities.
Disturbing, high noise levels need to be effectively damped and various
techniques are already currently being used. For example, excavated materials
are used to build noise-damping embankments, and wooden or concrete
constructions are used to a great extent to screen disturbing noise along
roads
and railways. However, these measures provide a limited noise protection.
Noise measurements and calculations have shown that an acoustic-screening
device is more effective if this is provided with noise-absorbing material
against that part of the screen which faces towards the noise source.
Effective
SUBSTITUTE SHEET (RULE 26)
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noise-absorbing materials of this kind are constituted by mineral wool or the
like, alternatively by foamed plastic with communicating pores, but these
materials are less suitable for use outdoors and in dirty environments such as
along roads and railways. The constructions, apart from damping disturbing
noise, must also be able, of course, to withstand weathering, be cleaned and
also function in different climates.
Technology for damping disturbing noise from roads and railways is thus
previously known and, in some cases, also patent-protected.
SE513102 describes, for example, a method for producing noise-absorbing
and/or vibration-damping units made of rubber waste from tyres, for example,
which have been cut up and mixed with bonding agent. The units can be used
as noise screens alongside roads or railways, holder elements/lifting elements
being embedded in the material before it solidifies. These units do not solve
all
the problems associated with arranging stable and effective acoustic screening
devices along roads and railways at reasonable cost.
W02005033412 describes an acoustic screening device consisting of a
concrete framework on one side of which a mesh is cast into the concrete. The
mesh is integrated with a noise-damping material. The drawback with this
construction is, inter alia, that the noise absorber cannot be exchanged when
damaged. Moreover, the damping capacity of this acoustic screening device is
limited by the structural configuration of the absorber. The width of the
concrete profile also makes it difficult for the acoustic screening device to
be
able to be placed sufficiently close to the noise source.
FR2683368 describes an acoustic screening device for damping disturbing
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noise, for example noise arising from traffic on roads, railways and the like,
comprising a framework, preferably of concrete, and a noise absorber arranged
thereon. The noise absorber is stated to be arranged exchangeably on the
framework. Spacer elements are arranged behind the noise absorber in such a
way that a certain air gap is formed between the noise absorber and the
framework. The air gap can be between 10 and 50 mm, but is typically 20 mm.
The noise absorber is fitted against the concrete element on a substantially
flat
surface. The front/outer surface of the noise absorber is arranged
substantially
flat against the front limit surface of the concrete element. The noise
absorber
is stated essentially to be made of wood particles agglomerated with a bonding
agent such as cement or a mixture of mineral bonding agent including cement,
silicate or similar known substances and consists of a front corrugated part
and
a rear flat part. The noise absorber, by virtue of its configuration with
corrugated front side, is intended to absorb noise, but is not expressly
intended
to let noise or air through to the air gap behind the plate. As a result of
the
properties of the incorporated material, it is not possible to control or
regulate
the flow resistance through the plate. Any flow of air through the plate is
also,
moreover, barely possible, which hence will not act as a noise absorber. The
fact that the plate contains wood fibres means that it is scarcely weather-
resistant and hence less suitable for outdoor use. Furthermore, the wood fibre
material is relatively heavy to handle, since it is so solid and, inter alia,
contains cement. The noise absorber is also stated to be mechanically fragile
and is fitted with the aid of screws through the noise-absorbing material. The
device does not integrate any air gap former in a cassette, nor does it form
in
the air gap a number of air cells which are substantially closed.
The prior art within this field does not therefore solve the problem of
damping
disturbing traffic noise in a flexible and effective manner and at reasonable
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cost.
DISCLOSURE OF INVENTION
One object of the invention is to solve the abovementioned problem and to
provide a device, an acoustic screening device, of the type stated in the
introduction, having a small number of component parts and which can be
configured as a simple, compact and relatively cheap unit which effectively
damps disturbing traffic noise from roads and railways.
Another object of the invention is to provide an acoustic screening device
which can be placed in an outdoor environment, which withstands different
climates and which can be easily cleaned of dirt.
A further object of the invention is to provide an acoustic screening device
which can be placed close to the source of the disturbing noise, but can
nevertheless allow normal maintenance of the road or railway.
Another object of the invention is that the noise absorber shall be easily
exchangeable if it is damaged or if it is wished to alter the noise-damping
characteristic on a certain section, or if it needs to be replaced for other
reasons.
Yet another object of the invention is that the construction shall be cheap to
produce, install and maintain.
These and further objects and advantages are achieved according to the
invention by a device according to the distinguishing features stated in the
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characterizing part of the first patent claim.
The invention thus relates to an acoustic screening device for reducing
disturbing noise, especially noise arising from road traffic, rail traffic and
the
like, comprising a framework, preferably of concrete, and a noise absorber
arranged thereon. The invention is characterized in that an air gap former is
arranged integrated in the cassette behind the noise-absorbing mat in such a
way that an air gap is formed between the noise-absorbing mat and the
framework, in that the air gap former is arranged to form a number of cells or
compartments between the noise-absorbing mat and the framework, the cells
or compartments being arranged to form substantially closed air volumes, and
in that the noise-absorbing mat is arranged to be, at least to some degree,
noise
and air permeable.
Further distinguishing features and advantages of the invention emerge from
the following, more detailed description of the invention, as well as from the
appended drawings and other patent claims.
BRIEF DESCRIPTION OF DRAWINGS
The invention is described in greater detail below in some preferred
illustrative
embodiments on the basis of the appended drawings.
Figure 1 shows a section through an inventive module and a noise absorber
fitted on the front side of the module.
Figure 2a shows in greater detail an inventive noise-absorbing cassette,
viewed from the front.
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Figure 2b shows in greater detail the gap-forming mesh of the cassette.
Figure 3 shows a section through the cassette according to Figure 2a.
Figure 4 shows in greater detail a part of the cassette shown in Figures 2 and
3, from the rear.
Figure 5 shows a detail of the air gap former and its wall construction with
curved, compartment-forming surfaces, which makes the noise absorber an
absorber of the "locally reacting type".
Figure 6 shows the inventive module according to Figure 1 placed on a bed of
gravel/macadam alongside a railway track.
Figure 7 shows a chart of the noise absorption factor as a function of
frequency, the so-called noise absorption characteristic.
Figure 8 shows how a number of inventive modules have been fitted together
to form noise barriers.
Figure 9 shows the noise barriers in greater detail set out by the side of a
railway track.
DESCRIPTION OF PREFERRED EMBODIMENTS
Figure 1 shows in profile an inventive acoustic screening device 1, in the
form
of a module, for screening and absorbing disturbing noise, for example from
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road traffic, rail traffic or the like. The invention especially relates to an
acoustic screening device 1 consisting of a framework 2 made, preferably of
fibre-reinforced concrete, and a noise absorber 3, mounted thereon, in the
form
of a cassette. The module is designed such that a plurality of units can be
placed in a row one after another to create an integrated acoustic screening
device. The fibre reinforcement preferably consists of synthetic plastic
fibres
in order to avoid the presence of electrically conductive material in the
framework 2, whereby measures for electric earthing of the modules are
rendered redundant so that they can be omitted. A future maintenance factor is
thus eliminated, since earth connections would otherwise need to be checked
and maintained at regular intervals.
The cassette 3, too, consists of purely non-electrically conductive material.
The
cassette 3 is fitted, with the aid of, for example, insulating gun nails, to
the
front side of the framework 2, i.e. that side of the module which is intended
to
be directed towards the disturbing noise source.
Viewed in profile, the acoustic screening device 1 is configured with a base
4,
a middle section 5 and a top part 6. The base 4, or the ground support,
constitutes a compact and heavy part and is intended to hold its acoustic
screening device 1 in position despite, for example, powerful wind blasts from
passing trains or vehicles. The forces in the wind blasts can be approximately
as large out from the track, which is caused by the pressure created in front
of
the train, as in towards the track, by suction forces created once the train
has
passed. The module 1 is therefore heavy and very stable so as not to be
dislodged from its position when a train passes. Should the acoustic screening
device 1 move, this can create serious risk of accident. Trains must be able
to
pass at speeds of up to 250 km/h and the acoustic screening device 1 must also
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be able to endure various weather phenomena and be stable in all conceivable
weather situations, for example withstand hurricane winds. In order to achieve
maximum stability, the centre of gravity of the framework 2 should lie as
close
to its geometric centre as possible.
The middle section 5 is located on or close to the front edge 7 of the base 4
in
such a way that the cassette 3 can be arranged as close to the noise source as
possible. The cassette 3 is preferably fitted with the aid of insulating
bolts, but
can also, of course, be fitted differently. By virtue of the fact that the
front side
of the acoustic screening device 1 is substantially flat and does not have any
projecting parts, the module is more easily put in place than previously known
acoustic screening devices. The front side of the noise-absorbing cassette 3
is
thus arranged substantially in line with the front edge 7 of the base 4 and
with
the front edge 8 of the top part 6. This means that the module requires less
space on the ground, which is advantageous when placed on a railway
embankment.
The configuration of the framework 2 with a base 4 which projects forward (to
the left in the figure) to a lesser degree means that the foundation for the
acoustic screening device 1 in, for example, a railway embankment can be
placed slightly farther out from the railway track than was previously
possible.
This enables, inter alia, the maintenance work which often needs to be carried
out on the railway embankment, inter alia with the aid of a so-called macadam
plough, to be done without affecting the acoustic screening device 1 or its
foundation. It is thus possible to use the module more effectively for damping
of disturbing noise from rail traffic. The front surface of the acoustic
screening
device 1 is substantially flat and vertical by virtue of the fact that the
front
edge 7 of the framework 2 lies substantially level with the front surface of
the
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cassette 3 and the front edge 8 of the top part 6.
In the upper surface of the base 4, behind the middle section 5, an attachment
fitting 9 for a lifting yoke or the like can be placed in a recess provided
for
this, for example a sleeve rod anchor.
Acoustic screening devices 1 can be set out, for example, with the aid of two
slings hooked into eyelets screwed onto the modules, which eyelets are
removed from the module following assembly. No special lifting device is
therefore required for handling of the modules, which means that the full
capacity of the lifting gear is not reduced by the weight, for example, of a
lifting yoke.
Figure 2a shows from the front, and in greater detail, the noise-absorbing
cassette 3. The cassette 3 consists of an outer frame 11 of, for example,
plastic.
On the front side of the cassette 3, i.e. that side which, following fitting
against
the framework 2 of the acoustic screening device 1, is facing towards the
noise
source, a grille or mesh 12 (see Figure 2b), also preferably made of plastics
material, is arranged. In or against the mesh 12 there is suitably arranged a
noise-absorbing material, for example a rubber material. The noise-absorbing
material can, for example, be integrated with the mesh 12, or cast into the
mesh 12, or alternatively glued or otherwise applied to the mesh 12 with a
view to stiffening the mesh 12, which allows even a thin mat 13 to be used. If
the mat 13 is sufficiently rigid, it can be placed loose behind the mesh 12.
The
cassette 3 is fitted on the front side of the middle section 5 of the
framework
and is protected at the top against rain, for example, by a protruding portion
on
the top part 6 of the framework 2 (see Figure 1).
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Figure 3 shows a section through an inventive cassette 3 along the line A-A
shown in Figure 2. In the frame 11, the mesh 12, the noise-absorbing material
in the form of a mat 13 and an air gap former 14 are arranged.
The noise-absorbing material in the mat 13 consists at least partially of a
fine
granulate of, for example, reused rubber, which is held together by a bonding
agent intended for this purpose. Such a bonding agent can be a dual-
component polyurethane glue, which makes the mat 13 flexible and resilient
even after hardening. The mat 13 is at least partially air permeable by virtue
of
the fact that it contains small communicating pores or channels, so that a
flow
resistance arises in the mat 13. The material in the mat 13 is also durable
and
withstands, for example, cleaning with water without the noise-absorbing
effect being affected or impaired. The granulate size and degree of compaction
affects the flow resistance and can be adapted for different applications. In
the
case of damping of traffic noise from railways, granulate of about 3 mm size
is
used and the mat 13 has a density of about 650 kg/m3. With a mat thickness of
about 10 mm, its weight is about 6.5 kg/m2.
If the width of the frame 11 is dimensioned to about 40 mm, an air gap 15 of
about 30 mm width is formed behind the mat 13, i.e. the distance between the
mat 13 and the concrete framework 2 of the module is about 30 mm. The
width of the air gap 15 is here achieved by an air gap former 14, for example
made of a plastics material, which forms cells or compartments 16 and which
makes the absorber acquire the character of a "locally reacting absorber". The
width of the air gap 15 and the size of the cells 16 is determinant for the
range
of frequency at which the noise will most effectively be absorbed. The width
of the air gap 15 can be altered and adapted by the width of the frame 11 and
of the air gap former 17 being changed, or by the mat 13 being otherwise
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arranged at a certain distance from the middle section 5 of concrete framework
2. Also determinant for the total noise absorption effect is the air flow
resistance of the noise-absorbing mat 13, in combination with the width of the
air gap 15. The flow resistance in the noise-absorbing material in combination
with the air gap forms an acoustically equivalent electric, so-called RC
circuit.
The absorber or the cassette 3 thus has a better adjustable and adaptable
noise
absorption capacity than previously known systems, and, above all, it damps a
frequency range which, more easily than previous systems, can be adapted to
the character of the disturbing noise, so that maximum reduction, in terms of
dB(A) units, is achieved. Should the noise absorber/the cassette 3 be damaged
in any way, it is also easy to replace it with a new cassette 3. Previously
known
alternatives require, for example, whole concrete blocks to be exchanged.
Figure 4 shows the rear side of the mat 13 and the air gap former 14 placed
thereon, which is configured with a number of substantially circular cells 16
or
compartments. In the assembled state, the cells 16 thereby form a number of
closed volumes between the mat 13 and the concrete framework 2. The cells
16 can also, of course, have a shape other than that shown. Each cell 16
receives some of the noise and spreading of the noise inside the air gap 15 is
prevented with the aid of the wall construction of the air gap former 14,
having
curved surfaces which, by virtue of their thereby acquired rigidity, minimize
the noise transmission between the cells. The noise is thus absorbed primarily
in each cell 16 individually. This type of noise absorber is termed "absorber
of
the locally reacting type". This absorber type is characterized in that the
frequency for the absorption maximum can be more easily adapted to the
desired frequency range, at the same time as the absorption factor is higher
at
absorption maximum.
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Figure 5 shows in greater detail a part of the air gap former 14, here made of
plastic, and its walls 17, which have curved surfaces with high rigidity and
form closed cells 16, which air gap former, at the same time as it forms the
gap, also makes the noise absorber 3 a so-called "locally reacting absorber".
By this is meant that the noise which enters through the mat 13 at one place
is
prevented by the air gap former 14 from spreading sideways in the noise
absorber 3. Reflected noise must now exit along the same path as it came into
the noise absorber 3. In this way, the noise absorber 3 is "locally reacting".
For
a locally reacting absorber, it is easier to control the absorption maximum to
the frequency range which leads to a maximum reduction in a number of
dB(A) units. Even obliquely incoming noise will find its absorption peak at
the
frequency which is determined by the thickness of the mat 13 and the width of
the air gap to the solid wall, here, for example, the concrete framework.
Figure 6 shows the inventive acoustic screening device 1 according to Figure
I placed on a bed 18 of gravel/macadam alongside a railway track 19, placed
on sleepers 20. The railway embankment 21 usually consists of coarse
macadam. The coarseness in the material makes noise able to pass through
and, moreover, cannot be smoothed. The acoustic screening device 1 is
therefore placed preferably on a bed 18 of finer material, for example
shingle,
which can also be compacted and smoothed so that it does not let through
noise to an undesirable extent. This finer material is kept separate from the
macadam, for example, with the aid of a geotextile 22.
Figure 7 shows a chart of the noise absorption factor as a function of
frequency, the so-called noise absorption characteristic for an inventive
noise
absorber/cassette 3 with a noise-absorbing mat 13 having a thickness of about
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mm and having an air gap 15 between the mat 13 and the concrete
framework 2 of about 30 mm. A noise absorption factor > 0.9 is here achieved
for noise within a frequency range of 700-1050 Hz, which is especially well
suited for optimal damping of noise from rail traffic. If the width of the air
gap
5 15 is increased or reduced, a more effective noise damping is obtained
within
other frequency ranges which can be relevant to other types of disturbing
noise, such as, for example, road traffic noise, which requires a somewhat
lower frequency of noise absorption maximum compared with, for example,
railway noise. The dashed curve in the chart shows what absorption factor is
10 obtained if the absorber is of the so-called "non-locally reacting type"
without
air gap former, i.e. with only air in the gap and without local cells 16. From
the
continuous black curve, it can be seen that with an air gap former and
absorption of a "locally reacting character", a more well formed noise
absorption maximum is obtained, which, through the altered width of the air
gap, can be easily adapted to the particular noise source type, for example
road
traffic or rail traffic.
Figure 8 shows how a number of inventive modules have been fitted together
to form noise barriers placed along either side of a railway track. The top
part
6 of the modules is provided with a substantially flat upper surface 10, which
allows it to be trodden, for example by persons who need to be
evacuated/cleared from a train which has come to a halt out on the line, for
example, due to a minor accident, mechanical faults, fire or the like. The
upper
surface 10 of the top part 6 can also be provided with an anti-slip pattern,
such
as, for example, a chequered plate pattern, to facilitate treading and reduce
the
risk of slipping. An appropriate anti-slip protection of a suitable non-
electrically conductive material can be fitted onto the upper surface 10 of
the
top part 6. The patterning can also have a certain effect on the propagation
of
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the noise which escapes between the train and the acoustic screening device 1.
The module can also be provided with steps 23 on the rear side of the middle
section 5, which makes it easier for evacuating persons to come down to the
railway embankment.
The modules can also be supplied complete with evacuation doors 24. The
module is in this case provided with recesses/openings 25 through the middle
section 5 and top part 6 of the framework 2. The openings 25 are closed off by
doors 24, which are self closing by, for example, spring action (not shown),
and which are locked in the closed state to prevent them from being
accidentally opened by, for example, the air pressure from a train and from
thus reducing the noise-damping effect of the acoustic screening device 1. The
locking can be released and the door 24 opened, for example, by a vertical
pressure against the top edge of the door 24.
Figure 9 shows the noise barriers in greater detail and how doors 24 are
arranged in the openings 25 of the modules to permit evacuation of staff from
the track region.
Modules which are placed on the end sections of the acoustic screening device
1 endure significantly greater forces than other modules in the barrier and
are
preferably configured with a sloping upper surface (not shown). These
modules, too, can be provided with cassettes/noise absorbers 3.
With the present invention, it is thus possible, with a small number of
component parts, to configure an acoustic screening device 1 as a simple,
compact and cheap unit which effectively damps disturbing noise from roads
and railways. Moreover, an acoustic screening device 1 according to the
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invention, by virtue of its construction, withstands different climates and
can
easily be cleaned of dirt with, for example, water. The acoustic screening
device 1 according to the invention, by virtue of the fact that the base 4,
the
ground support, is substantially located on the rear side of the framework 2,
can also be placed close to the source of the disturbing noise, and can thus
to a
greater degree prevent noise from escaping between the bogie space formed by
the bottom side of a rail coach and the side of the coach and the acoustic
screening device, yet nevertheless allow normal maintenance of the railway
tracks.
The fact that the acoustic screening device 1 is placed so close to the track
means that, at "platform-close" distance, the screen forms a seal for the
noise
which escapes from the bottom edge of the body edge of the railway coach. By
virtue of the fact that the bottom side of the coach, together with the body
sides
drawn down into the bottom edge, forms the so-called bogie space, a new type
of damping is obtained, which is termed chamber damping by virtue of the fact
that the ballast, too, provides certain noise absorption. Moreover, the
acoustic
screening device 1 provides, together with the body side, a so-called channel
damping.
By virtue of the construction with a noise absorber in the form of a cassette
3,
this can be exchanged, if damaged or worn out, in order to adapt the noise
damping characteristic. The construction is, in its entirety, relatively cheap
to
produce, install and maintain.
The description above is primarily intended to facilitate understanding of the
invention. The invention is not, of course, limited to the specified
embodiments, but rather other variants of the invention are also possible and
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conceivable within the scope of the inventive concept and the scope of
protection of the following patent claims.
