Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
A Sound-Dampina Material ~or Underwater Use
Underwater sound-damping material can be put to many
uses. For example, such ma~erial can be used to elimi-
nate disturbances in depth sounding processes, or as
protection against acts of terrorism that are directed
against pipelines and offshore rig~, for example~
Those frequencies whose damping is of interest in the
present context lie between about 50 and 500 Khz, corre-
sponding to wavelengths in water of about 3 mm-0.3 mm.
Sound is reflected back in the arrival direction of both
objects which are lighter than water and objects which
are heavier than water. A principle for damping of the
reflexes is conceivable in an analogous method as by
ant$reflex processing, where the front and rear surfaces
of a coating reflect with mutually extingui hing phase
angles. The reflection ability with this type of damping
is highly dependent on frequency, however.
An ob~ect of the present invention is to provide a sound-
absorbent material which will primarily provide a good
effect within the frequency range of 75-300 Khz, prefera-
bly up to S00 Hz. Thi~ object i8 achieved with a mate-
rial constructed in accordance with the invention and
having the characteristic features set forth in Claim 1.
In order to achieve a good sound-damping effect, par-
ticularly at lower frequencies, preferably frequencies
down to 50 Khz, it is also preferred to provide a pro-
filed surface structure, e.g. of the "egg-carton typen,
in accordance with Claim 5. Such surface structures are
al60 advantageous at high frequencies.
Surprisingly good effects have been achieved with experi~
ments in which there was used a so-called reticulated
foam, which is a material known as a packaging material,
filter material, and also as a material used in the
construction of loudspeakers. Such commercially avail-
able, known material, is normally manufactured by deto-
nating gas present in the cells of the material, or by
removing the partition walls between adjacent cells.
Experiments have shown that a good absorption effect for
water-filled structures cannot be achieved if the materi-
al i8 too pliable. Thus, natural sponges provide a poor
absorption effect. Carpets or mats which have a long
pile in the sound direction also give a relatively poor
result, as do also different skeleton-like structures
which have penetrating openings or are otherwise trans-
parent. The mechanism of damping can be seen to be a
combination of internal friction in water which in the
undulation is forced to take different paths, wherein the
different paths taken confuse the phase pattern, and the
internal friction of the dampening material itself. ~he
latter effect is the most important. Therefore, the
material must not only have structure of mutually con-
nected cavities, ~ut also a proper propensity to absorb
sonar energy. In order to achieve this, it is necessary
for the plastic material to be near to its glass transi-
tion temperature, and it is therefore for best effect
neceasary to select the material differently if it is to
be used in northern winter waters (temperatures near 0 )
and if it is to be used in tropical waters. As the glass
transition temperature is also different for different
frequencies, the choice of material must ha~e a glass
transition temperature for frequencies of 50-500 Khz
which is near the intended temperature of use.
Furthermore, the surface of the material is preferably
structured, so as to obtain reduced reflection at primar-
~ly progressively lower frequencie~. one preferred
method of achieving this involves pas~ing a slab of
compressible material through the roll nip of rolls which
are provided with patterns of obtuse studs which leave in
the nip a free centre plane in whicn a knife is ~ounted.
In this way, when the cut sheet leaves the roll nip and
returns to its original form, the sheet is divided along
two complementary surfaces of egg-carton configuration.
The distance between the top and the bottom of each
surface i5 preferably between 10 and 30 mm. Optionally,
the residual flat surfaces of slabs cut in this manner
may be fa~tened to flat slabs of corre6ponding material.
When the material is to be used under water for long
periods of time, the material is preferably coated with
an antifouling substance of the kind u~ed in boat paints.
Example~ of such suDstances are organic tin compounds or
copper powder.
Reticulated plastic foam materials may be produced from
different plastics. The most common plastic~ at present,
however, are the polyurethane plastics, e.g. polyester or
polyether-based plastics, which react with i60cyanate in
a hydroxyl terminated state. Because the polyether-based
pla~tic is the more water-resistant of these plastics, it
is the plastic that is preferred according to the present
lnvention, even though experiments have shown that the
polyester-based plastic is equivalent to the polyether-
based plastic from the aspect of sound absorption.
The inventive material can be bent and cut in an appro-
priate manner to cover objects of different shapes, e.g.
cylindrical, conical and spherical shapes, and can be
glued or mechanically fastened to th~ surfaces of the
objects concerned. The invention can therewith be ap-
plied advantageously to objects having metal surfaces and
objects made, e.g., of construction cellular
plastic, or ~ore generally expressed materials which have
a higher or a lower density than water.
It may also be suitable to saturate the material with a
wetting agent or the like, so that its poor structures
will be readily filled with water.
A large number of experiments have been made with various
materials, which can be summarized as fOllows.
open, water-filled alveolar structures having 2-20 alve-
oles per centimeter have been found to function best,
while so-called reticular foam has been found to function
best of all. Sonar experiments have shown that the
material should not be transparent, although it can be
slightly transparent. When damping frequencies in the
range beneath 100 Khz, it is essential that the outer
sur~ace of the material is structured, whereas the inter-
nal structure is of greater significance when damping
higher frequsncies.
Those materials which have been tested and found highly
suitable, with respect to availability, are materials
which are normally used in filters, and materials which
are used in the manufacture of loudspeaker constructions
and the like. Other available materials are fibreboard
in which the fibres are mutually connected in a space
system, or appropriately positioned stacks of expanded
metal with slots which together form the alveolax struc-
tures.
The invention will now be described in more detail with
reference to an exemplifying embodiment thereof and also
with reference to the accompanying drawings. Figure 1 is
a sectioned view of an absorber having a surface struc-
ture of the "egg-carton type"; and Figures 2-5 illustrate
absorption curves obtained with different materials.
The slab of material illustrated in Figure 1 comprises
reticulated foamed plastic which has been cut in the
manner described in the introduction.
In the following Examples, damping of sound reflection at
different frequencies has been measured for different
slabs which have been mounted on slabs of hard cellular
plastic with closed cellg (thickness 20 mm, density 200
kg/m3). The test equipment was immersed in water and the
absorption material well saturated. The frequency is
plotted in logarithmic scale along the X-axes of the
Figures, while a damping scale is plotted along the Y-
axes of said Figures, approximately normalized in Db.
Exam~le 1 (Figure 2)
A plain, non-patterned slab having a thickness of 20 mm
and produced from a material having 15-25 alveoles per
inch in reticulated plastic foam. A very good damping
effect was obtained at frequencies above 80 Khz, although
damping was poorer at lower frequencies.
Example 2 CFigure 3~
A slab made of the same material as in Example 1 but with
an "egg-carton structure", such that the slab had a
smallest thickness of 15 mm and a largest thickness of 25
mm. The pattern-repeat of the surface structure was
60/90 mm of its manufactured length and breadth dimen-
sions respectively (the tested slab was square). It will
be seen from the graph that damping is the lowest fre-
quency range is now greatly improved in comparison with
damping achieved in Example 1.
E~ (Figu~e 4)
A flat slab of reticulated plastic foam having an average
of 60 aveoles per inch (alveole size about 0.4 mm). It
will be seen from the graph that damping was not satis-
factory.
Exam~le 4 (Fiau~Q ~)
An egg-carton structured slab of reticulated foam plastic
having an alveole size of 7-15 alveoles per inch (1.7-3.6
mm). The structure or pattern had a pattern-repeat of
60/90 mm.
As shown by the graph, qood absorption was obtained over
the whole of the range.
These preferred reticular foams are polyurethane foam. A
summary of different test results shows that open, non-
transparent alveolar structures function well at
frequencies which exceed 100 Khz when the average size of
the alveoles is greater than 0.5 mm, and prefera~ly
greater than 1 mm, and smaller than 5 mm, and preferably
smaller than 2.5 mm. The material should also have a
surface structure or pattern having a pattern-repeat or
the like of less than 100 mm, particularly for the lower
frequencies.
Ths preferred reticulated foams commercially available at
present ha~e a bulk density in a dry state of 26-32 kg/m3
in the case of Examples 1 and 2, and a bulk density of
20-24 kgJm3 in the case of Examples 3 and 4. The com-
pre~sibility (compressibility with 40%~ in the former
case is 2.6-3.6 and in the latter case 3.0-5.0 Kpa.
Among other materials that were tested can be mentioned
non-woven PVC of corresponding thickness, which when flat
exhibits good results (>6 Db over 100 Khz for a thickness
of 15 mm), artificial turf ("Astro Turf") with usable
results above 150 Khz, and 20 mm felt (needled and
fullered), which gave poor absorption.
