Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a device for
absorbing the acoustic energy emitted within a ship~s
hull and, more particularly, to a device devised for
equipping a submarine. The present invention also relates
5to a modular acoustic screen forming part of such a
device.
During their military missions, submarines have
to be as unobtrusive as possible so as to prevent enemy
navies locating them. This involves especially their
10electromagnetic and acoustic ~'silence". Acoustic
emissions, in particular, arise from the many apparatuses
which are necessary to the operation of the submarine,
even when the latter is stopped. These emissions are
unavoidable and have to be absorbed, at least partially,
15within the submarine itself in such a manner as to reduce
the acoustic energy, which is propagated into the water
after traversing the hull of the submarine, to a level
which does not enable the latter to be detected.
~Solid-borne~ transmission of noise generated by
20the various apparatuses can be attenuated, or indeed
eliminated, by incorporating suspensions and/or dampers
into the support of each apparatus. Likewise "liquid-
bornell transmission of noise can be considerably at-
tenuated by interposing adapted dampers in the piping in
25question. On the contrary, the propagation of air-borne
noise can certainly be attenuated at the source, for
example by means of suitable cowlings, but in no case can
it be totally eliminated.
Attempts have already been made to absorb the
3Qair-borne noise by covering the inner wall of the hull
of the submarine with a layer of porous material (glass
or mineral wool, plastic foam etc.) constituting an
absorbent screen. This solution is relatively effective
from the acoustic point of view but represents a serious
35drawback in terms of heat transfer. The atmosphere of a
submarine is heated in fact by thermal radiation released
especially by the engines. This heating can be combated
with the aid of air-conditioning units, especially in the
inhabited parts, and by heat exchange with the water
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which bathes the outer surface of the submarine, through
the latter's hull, especially in the non-inhabited zones
under the influence of high-power engines (for example
nuclear engines) necessary ~or the propulsion of the
submarine.
As a result of the material constituting them and
of their construction, good acoustic screens exhibit bad
heat-transfer characteristics. When such a screen lines
the internal wall of the submarine, the heat exchange
with the sea water is considerably reduced and there is
effectively no longer a benefit from the cold source
formed by the sea water in order to cool the air heated
by the operation of the engines.
The object of the present invention is therefore
to produce a device for absorbing the acoustic energy
emitted within a ship~s hull, which does not act as a
screen for heat transfer towards the outside of the ship,
through the latter's hull.
The object of the present invention is also to
produce a modular acoustic screen for such a device,
which exhibits the required thermal "transparency" and
acoustic 'lopacity" whilst at the same tLme being light-
weight and compact, easy to mount and to maintain and
having good impact properties and low manufacturing cost.
These aims of the invention, together with others
which will appear following the present description, are
achieved with a device for absorbing the acoustic energy
emitted within an at least partially submerged ship's
hull, which comprises a bed of an acoustically absorbent
material drilled through by passages allowing air to flow
through the bed and means for fixing the bed at a
predetermined distance from the inner wall of the hull in
such a manner that this bed constitutes an acoustic
screen for the air-borne transmitted noise in the hull of
the ship, this screen allowing free convection of the air
into the passages traversing the bed and into the space
provided between the hull and the bed in order not to
restrict the cooling of the air by heat exchange with the
water wetting the outside of the hull.
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Thus by moving the bed away from the hull and by
providing air passages through the bed, an e~fective heat
exchange is maintained between the air and the water
through the hull, necessary for cooling this air, despite
the indispensable presence of the bed for ensuring the
acoustic unobtrusiveness of the ship.
According to one characteristic of the device
according to the invention, devised for being employed in
an underwater ship whose hull is supported by means of
circular ribs proliferating in parallel planes, the bed
is formed from a plurality of adjacent elementary modular
acoustic screens, each supported between two adjacent
ribs.
According to a first embodiment of the invention,
each elementary modular screen is fixed between two ribs
by means of at least one strap passed around the screen
and solidly connected at its ends to means for fastening
to two adjacent ribs. A bar may be associated with each
strap in order to grip an elementary modular screen
between them and to fix it to the two ribs.
According to a second embodiment of the inven-
tion, each elementary screen is fixed with the aid of
small columns extending beyond some of the drilled
passages in the screen in order to bear on the internal
wall of the hull in such a manner as to define the
predetermined distance separating this wall from the
screen by jamming the screen between the base of these
small columns bearing against the hull and two adjacent
ribs.
The elementary modular acoustic screen used for
constituting the bed takes the form of a parallelepipedal
unit made from a porous material suitable for absorbing
acoustic radiation, this unit being traversed by passages
~hose outlets are distributed over two opposing faces of
the unit.
Other characteristics and advantages of the
device according to the invention will appear on reading
the description which will follow and by examining the
attached drawing in which:
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- Figure 1 is a partial view, in perspective, of
the hull of a ship equipped with the device for
absorbing acoustic e~ergy according to the
invention,
- Figure 2 is a partial view, in perspective and
in cross section, of an elementary modular
acoustic screen for~ing part of the device
according to the invention and of its means for
fixing to the hull of the ship and
- Figure 3 is a similar view to that of Figure 2
and represents another embodiment of the means
for fixing an elementary modular acoustic
screen forming part of the device according to
the invention.
lS Refexence is made to Figure 1 of the attached
drawing where a part of the hull 1 of an underwater ship
has been shown. In this case this hull 1 is generally
cylindrical and it is supported by`means of "ribs" 2
taking the form of circular beams having a T-shaped cross
section which are welded to the internal wall of the hull
(appro~imately one quarter of these beams have been shown
.in Figure 1).
According to the present invention, the hull 1 is
lined, at a distance E from its internal wall, by means
of a bed made from an acoustically absorbent material of
thicXness D, this bed being formed by a mosaic of elemen-
tary modular screens 3 placed next to each other. Also
according to the invention, use is made of beams 2 in
or er to fix the screens 3 at the distance E from the
hull 1. The means for fixing these screens will be
described hereinbelow when considering Figures 2 and 3.
Only one of the elementary screens utilized for
constituting the bed of the device according to the
invention has been shown~in Figure 1 in order to preserve
the clarity of the latter. An essential characteristic of
the invention thus appears in Figure 1, namely the
presence of a space of radial thickness E between the bed
consti~uted by screens 3 and the internal wall of the
hull 1, this space communicating with the atmosphere of
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the submarine by virtue of passages 4 traversing the
thickness of each screen. In the enlarged sectional view
of a screen according to the invention shown in Figure 2,
it apFears that these passages 4 are of cylindrical shape
and completely traverse the thickness D of the screen.
The outlets of these passages on two opposite faces of
the screen are preferably distributed uniformly over
these surfaces, for example in a staggered fashion.
It is known that, in this case, the air can flow
through the screen via the passages 4, either in one
direction or in the other, and into the annular space of
thickness E located between the screens constituting the
bed and the internal wall of the submarine (this flow of
air being effected according to the directions of the
arrows shown in Figure 1). Air heated within the sub-
marine by the engines and other apparatuses operating in
the latter can thus flow, by free convection, against the
hull of the ship in order for heat exchange thereat with
the sea water located beyond the hull, which exchange has
the effect of freshening this air by contributing thereby
to its conditioning.
Moreover, and in accordance with the main object
of the device according to the invention, any acoustic
emission internal to the submarine is largely absorbed by
the bed of acoustic screens 3 fixed to the rib 2, without
the presence of this bed impeding the heat exchange
necessary between the atmosphere of the submarine and the
sea water, which exchange would deteriorate unacceptably
if, conventionally, use is limited to an acoustic screen
in direct contact with the internal wall of the hull 1.
Various means for fixing the screens 3 to the
ribs 2 can be utilized. Figure 2 shows a first embodiment
of these fixing means, which essentially comprise a strap
5 provided at each of its extremities with means for
fastening to facing flanges 6 of two adjacent ribs 2.
These means may comprise an right-angle bracket 7 which
is fastened to the lower face of one flange of a rib 2
(as per the figure), an angle bracket solidly connected
to, on the one hand, one end of the strap 5 and, on the
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other hand, to a welded nut 8 provided for interacting
with a bolt 9. The fixing means may furthermore comprise
a bar 10, constituted for example by a metal pipe having
crushed ends and drilled by a hole. The bolt 9 passes
into this hole and this bolt is tightened in the nut 8 in
order to grip the screen 3 between the bar and the strap.
The screen is thus solidly fixed to two adjacent ribs, as
shown in Figure 2.
In this figure the relative positions of the bar
10 and of the passages 4 which it surmounts have only
heen chosen in order to make the latter visible in cross
section for the clarity of the figure. Also to be noted
is a junction plate 13 placed above the gap separating
each rib from the adjacent edge of an elementary screen
3. This junction plate 13, fixed by bolting or adhesively
bonded, prevents an acoustic short circuit of the screen
3 and improves the appearance of the assembly. A coat of
paint deposited on the visible face of the screens, of
the ribs and of the junction plates further improves this
appearance. Of course, a paint is chosen which does not
alter the acoustic performance of the assembly.
Figure 3 shows anot~er embodiment of the means
for fixing an acoustic screen 3. The screen is recessed,
as shown, along two sharp edges in order to fit com-
plementarily the profile of two flanges 6 of adjacentribs. These flanges are provided on their lower face with
a double-sided adhesive tape 12 which enables the two
facing surfaces of the screen and of these flanges to be
adhesively bonded. A junction plate 13' having a T-shaped
cross section plugs the rib/elementary screen gap. Small
columns 11 passed into some of the passages 4 drilled in
the screen 3 are pushed into these passages in such a
manner as to butt against the hull 1 of the ship. These
small columns may be constituted simply with the aid of
the cores removed from the passages 4 drilled in the
screen. By pushing a ~irst small column 11 with the aid
of a second small column 11~ inserted in turn in the same
passage, as shown in Figure 3, and then by adhesively
bonding the second small column 11' in this passage and
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by cutting off the part of the small column 11' which
extends beyond the surface of the screen, a second
bearing surface for the screen is constituted which
completely immobilizes the latter in relation to the hull
1. It is possible to provide such small columns, for
example at the four corners of a screen and in the
vicinity of its central part, in order to thereby support
the latter completely away from the hull in such a manner
as to provide a free circulation space for the air to be
freshened by heat exchange with the sea water located on
the other side of the hull.
In order to constitute an acoustic absorption
device according to the invention, which is capable of
reducing the 'air-borne noise" of the submarine by 10 to
15 decibels, elementary acoustic screens are produced in
a bed of inorganic, glass or mineral fibers, or in a bed
made from foam of synthe~ic materials such as polymers.
In this respect, a preferred material is the polyimide
foam called Illmid sold by the IhLBRUCK company (FRG~.
Given hereinbelow, by way of example, are the charac-
teristics of an elemen$ary modular screen constituted
from this material and enabling the abovementioned
acoustic performance to be obtained:
size: 500 x 500 x 200 (in mm), the screen being disposed
at 100 mm from the hull,
combined surface area of the passage cross sections/sur-
face area of the screen: 40%,
density of the material constituting the screens
10 kg/m3.
This material is satisfactory in other respects,
especially in terms of its temperature and impact resis-
tance, in terms of the toxicity of the decomposition
products and the fumes which are emitted in the event of
combustion, considerations which are particularly sig-
nificant in the case of a material intended to be in-
stalled in an enclosed vessel such as the hull of a
submarine.
The ratio d/D of the diameter _ of a cross
section of a passage 4 to the thickness D of the screen
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and the ratio s/S of the combined surface area s of the
cross sections of the passages 4 to the surface area S of
one of the faces of the screen where these passa~es
emerge are parameters which the person skilled in the art
can use in order to adapt the thermal and acoustic
performance of the device according to the invention to
whatever specification is laid down in a specification
document. Thus, it will be possible to vary significantly
the ratio s/S, for example from 30 to 50%. The ratio d/D
can lie between 3 and 10, for example.
Of course, the invention is not limited to the
embodiments described and shown which have been given
only by way of example. Thus/ it would be possible to
substitute with other screens the elementary screen
drilled by cylindrical passages which is described
hereinabove. Thus, it will be possible to constitute the
device according to the invention with screens con-
stituted by strips of absorbent materials separated by
spaces and held together in frames, for example. In
general, any disposition permitting the provision of
arbitrarily shaped passages havin~ changing cross section
and/or of straight axes or not, through the thickness of
a unit made from an acoustically absorbent material,
could be adopted in order to produce the elementary
screens con~tituting the device according to the inven-
tion.
Likewise, the invention is not limited to the
improvement of the acoustic unobtrusiveness of an under
water ship and may find application in surface ships,
only a part of whose hull is in contact with the sea
water.
