GB2263281A

GB2263281A - A sound-damping material for underwater use

Info

Publication number: GB2263281A
Application number: GB9200602A
Authority: GB
Inventors: Ingvar Andersson
Original assignee: Barracuda Technologies AB
Current assignee: Barracuda Technologies AB
Priority date: 1990-03-06
Filing date: 1992-01-13
Publication date: 1993-07-21
Also published as: SE9000794L; SE9000794D0; DE4200773A1; GB9200602D0; FR2686725A1; CA2057733A1; SE500795C2

Abstract

A sound-damping material for underwater use includes a layer of solid material having a thickness of at least 1 cm and an open structure. The material has open alveole structures of 2-20 alveoles per centimeter which, in use, are filled with water. The front surface of the material is preferably structured with crests and troughs. The material is a plastic material having a glass transition temperature for frequencies of 50-500 Khz which is near to a temperature of use. The layer preferably includes an open-cell cellular plastic which is preferably a reticulated polyurethane foam. <IMAGE>

Description

2263281 A SQ3AndDampingi-MIlterial- for Underwater Use Underwater

sound-damping material can be put to many uses. For example. such material can be used to eliminate disturbances in depth sounding processes. or as protection against acts of terrorism that are directed against pipelines and offshore rigs, for example.

Those frequencies whose damping is of interest in the present context lie between about 50 and 500 Khz, corresponding 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 antireflex processing, where the front and rear surfaces of a coating reflect with mutually extinguishing phase angles. The reflection ability with this type of damping is highly dependent on frequency, however.

An object of the present invention is to provide a soundabsorbent material which will primarily provide a good effect within the frequency range of 75-300 Khz, preferably up to 500 Hz. This object is achieved with a material 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, particularly at lower frequencies, preferably frequencies down to 50 Khz, it is also preferred to provide a profiled surface structure, e.g. of the "egg-carton type", in accordance with Claim 5. Such surface structures are also advantageous at high frequencies.

2 Surprisingly good effects have been achieved with experiments in which there was used a so-called reticulated foam, which is a material known as a packaging materialf filter material, and also as a material used in the construction of loudspeakers. Such commercially available, known material, is normally manufactured by detonating 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-f illed structures cannot be achieved if the material is 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 transparent. 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. The latter effect is the most important. Therefore, the material must not only have structure of mutually connected cavities, but 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 transition temperature, and it is therefore for best effect necessary 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 have a glass transition temperature for frequencies of 50-500 Khz 3 which is near the intended temperature of use.

is Furthermore, the surface of the material is preferably structured. so as to obtain reduced reflection at primarily progressively lower frequencies. one preferred method of achieving this involves passing 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 which a knife is nounted. 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 is preferably between 10 and 30 mm. optionally, the residual flat surfaces of slabs cut in this manner may be fastened to flat slabs of corresponding 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 used in boat paints. Examples of such substances are organic tin compounds or copper powder.

Reticulated plastic foam materials may be produced from different plastics. The most common plastics at present, however, are the polyurethane plastics, e.g. polyester or polyether-based plastics, which react with isocyanate in a hydroxyl terminated state. Because the polyether-based plastic is the more water-resistant of these plastics, it is the plastic that is preferred according to the present invention, even though experiments have shown that the polyester-based plastic is equivalent to the polyetherbased plastic from the aspect of sound absorption.

4 The inventive material can be bent and cut in an appropriate manner to cover objects of different shapeso e.g. cylindrical, conical and spherical shapes, and can be glued or mechanically fastened to the surfaces of the objects concerned. The invention can therewith be applied advantageously to objects having metal surfaces and objects made,, e.g., of construction cellular plastic, or more generally expressed materials which have a higher or a lower density than water.

It may also he 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 alveoles 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 surface of the material is structured, whereas the internal structure is of greater significance when damping higher frequencies.

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 alveolar structures.

is The invention will now be described in more detail with reference to an exemplifying eiment thereof and also with reference to the accompanying drawings. Figure 1 is a sectioned view of an absorber having a surface structure of the "egg-carton type"; and Figures 25 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 cells (thickness 20 mm, density 200 kg/m 3). 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 Yaxes of said Figures, approximately normalized in Db.

Example 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.

6 ExamRle 2 (FigurgL_,U 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 dimensions respectively (the tested slab was square). It will be seen from the graph that damping is the lowest frequency range is how greatly improved in comparison with damping achieved in Example 1.

Example 3 (Figure 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 satisfactory.

Example-4 (Figure 5) 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, good absorption was obtained over the whole of the range.

These preferred reticular foams are polyurethane foam. summary of different test results shows that open, nontransparent alveolar structures function well at p 7 7 is frequencies which exceed 100 Khz when the average size of the alveoles is greater than 0.5 mm, and preferably 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.

The preferred reticulated foams commercially available at present have 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 kg/m 3 in the case of Examples 3 and 4. The compressibility (compressibility with 40%) in the former case is 2.63.6 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 (11Astro TurfO) with usable results above 150 Khz, and 20 mm felt (needled and fullered), which gave poor absorption.

8 claims 1. A sound-damping material for underwater use, characterized byalayerof anopenstructure of solid material which presents open alveole structures having 2-20 alveoles per cm which are intended, in use, to be filled with water, said layer having a thickness of at least 1 cm, and being a plastic material having a glass transition temperature for frequencies of 50-500 Khz which is near to a temperature of use.

Claims

2. A sound-damping material according to Claim 1, c h a r a c t e r i z e

d in that the layer includes an open-cell cellular plastic.

3. A sound-damping material according to Claim 2, c h a r a c t e r i z e d in that the cellular plastic is a reticulated-type plastic.

4. A sound-damping material according to Claim 3, c h a r a c t e r i z e d in that the material in the cellular plastic is a polyurethane-type material.

5. A sound-damping material according to Claim 4, c h a r a c t e r i z e d in that the polyurethane material is a polyether-type material.

6. A sound-damping material according to Claim 4, c h a r a c t e r i z e d in that the polyurethane material is a polyester-type material.

9 7. A sound-damping material according to Claim 1, c h a r a c t e r i z e d in that the layer is substantially non-transparent.

8. A sound-damping material according to Claim 1, c h a r a c t e r i z e d in that the material has an undulating structure on one side thereof.

9. A sound-damping material according to Claim 5. c h a r a c t e r i z e d in that the structure has a thickness of from between 10 and 40 mm calculated between the crests and troughs of said undulations.

10. A sound-damping material according to Claim 5, characterized inthatitiscomposedof a flat material and a surface structured material affixed to said flat material.

11. A sound-damping material according to Claim 1, character i zed in that when dry andnot filled with water, said solid material has a modulus of elasticity such that compression of the material to 40% in a dry state requires a pressure of 2-6 Kpa.