FR2878643A1 - Anechoic cell for constituting anechoic surface covering of large immersed/submersible surface, has front diaphragm deformed by pressure of acoustic wave traversing water to drive viscous fluid via perforations of resistor plate - Google Patents

Abstract

The cell has a front diaphragm (28) that is initially deformed by pressure of an acoustic wave (46) traversing the water (48), in a direction of a surface (2) to drive back a part of viscous fluids through circular perforations (36) of a resistor plate (34). The driving of the part of the fluids permits to absorb a large part of the kinetic energy by reducing produced high shearing constraints and by pushing a rear diaphragm (30). The front diaphragm is flat or offers a convex surface to the acoustic waves. The plate has a density of 1045 equidistant holes of 1 millimeter diameter in a surface of 13 square centimeter and uses viscous fluid such as silica oil of 2000 centistokes, so that characteristic acoustic impedance of the cell ranges between one time and 0.1 times that of the water.

Description

Ex. No. 1

  The present invention relates to acoustic absorption devices for use in liquid environments, for example immersed in water. These devices will be called thereafter "anechoic devices". The invention also extends to anechoic surface coatings, such an anechoic coating comprising one or more individual devices according to the invention.

  It has been proposed to construct an anechoic cell comprising a shallow recess with a cylindrical wall arranged in a metal plate and covered with a flexible diaphragm enclosing a volume of viscous liquid between the diaphragm and the base of the recess. viscous liquid flowing in the radial direction of the recess when acoustic energy impinges on the diaphragm.

  These anechoic cells are highly dependent on how the diaphragm is separated from the base of the recess. Under certain conditions, for example in case of high static pressure, this separation is reduced by the deformation of the diaphragm and therefore exceeds the critical damping conditions, making the cell unstable.

  The anechoic device according to the present invention comprises at least one flexible diaphragm, a substantially inflexible resistance plate, and a volume of fluid normally trapped between the flexible diaphragm and the resistance plate, the bending of the diaphragm under the impact of a acoustic wave passing a portion of the fluid through the resistance plate.

  In a preferred embodiment of the invention, the anechoic device comprises a pair of expandable diaphragms spaced from each other, one being a front diaphragm and the other a rear diaphragm, a fluid volume trapped between the diaphragms, and a resistance plate dividing the volume into two chambers communicating with each other through the resistance plate, bending the front diaphragm under the shock of an acoustic wave passing the fluid from a chamber into the another through the resistance plate.

  In another aspect of the invention, the anechoic device comprises a cell body having a substantially cylindrical opening closed at both ends by extensible diaphragms trapping between them a volume con-holding a fluid said volume being divided into two chambers by a plate of resistance arranged between the diaphragms.

  Preferably, the device comprises a recess coaxial with the cylindrical opening and communicating with it, the rear diaphragm being mounted in said recess.

  Preferably, in a device as described in the preceding paragraph and mounted on a surface so that the front diaphragm will be directed in the opposite direction to the surface and that the rear diaphragm will face it, the environment ambient that can freely access the diaphragm facing the surface.

  Preferably, an anechoic device according to the invention, comprising front and rear diaphragms, will comprise a circumferential flange coming to apply to the contiguous end of a cylinder mounted against a surface, said cylinder having openings allowing the environment ambient to freely access the rear diaphragm.

  Preferably, a device as described in any one of the preceding paragraphs comprises means for compensating the local static pressure.

  In a device as described in the previous paragraph and comprising two diaphragms, the local static pressure compensation members may comprise, for example, a deformable sponge-like multicell material mounted against one of the diaphragms. But, preferably, the local static pressure compensation members comprise a sealed bellows containing a gas such as air, mounted against one of the diaphragms.

  According to another aspect of the invention, a resistance plate suitable for an anechoic device as described above comprises a series of holes, slots, or other perforations in a rigid plate. Preferably, an anechoic device according to the invention has an acoustic impedance of between one and 0.1 times the acoustic impedance characteristic of the liquid environment in which it is intended to be used.

  The acoustic impedance of a device according to the invention may advantageously be around one-third of the characteristic acoustic impedance of said liquid environment.

  The separation of the devices in a coating as mentioned above will preferably be between 0 and 0.5 of the maximum wavelength of the incident sound at the highest frequency to be received by the coating.

  According to another aspect of the invention, a surface anechoide coating comprises a series of devices, all in accordance with the invention, closely juxtaposed.

  It should be noted that the dimensions and the physical properties of the resistance plate and its supporting structure are such that the movement of the plate under the impact of the incident acoustic wave will be small in comparison with the movement of the fluid.

  The invention is hereinafter described in more detail with reference to a preferred, non-limiting embodiment, illustrating the above features and showing others, shown in the accompanying drawings, in the following: which: - Figure 1 is a cross section of an anechoic cell according to the invention; and FIG. 2 is a plan view of an insulated portion of a resistance plate suitable for the present invention. For illustrative purposes, the anechoic cell is shown in FIG. 1 mounted against a surface 2 by means of rivets 4 and immersed in water 48, but it is obvious that neither surface 2 nor rivets 4, nor water 48 are part of the cell subject of the present invention.

  The anechoic cell 14 has a body 24 having a substantially cylindrical central opening 26 closed at each end by a rubber front diaphragm 28 and a rubber rear diaphragm 30 enclosing a volume 32 containing viscous silica oil. The volume 32 is divided into two chambers 38 and 40 by a rigid resistance plate 34. The cell is provided with a recess 42 coaxial with the opening 26 and communicating with it, in which is mounted the rear rubber diaphragm 30 and A flange 20, peripheral to the body 24 of the cell, comprises a plurality of threaded bores 13 through which screws 16 pass to screw into the threaded bores 22 pierced in the front edge of a wood. 4-- 2878643 bucket 8 with cylindrical walls. The base of the plug is provided with bores 6 through which rivets 4 to penetrate into the surface 2. The cylindrical side walls of the plug 8 are provided with openings 10 through which the surrounding water 48 can freely penetrate, and they leave a cylindrical space 12 between the air bellows 44 and the surface a. The purpose of the bellows 44 is to eliminate any temporary pressure effects occurring in the water contained in the cylindrical space 12 but to transmit to the rear diaphragm 30 the static pressure due to the immersion depth.

  The body 24, the diaphragms 28 and 30, the resistance plate 14 and the air bellows 44 are held in place by rivets and bolts, not shown for clarity. Any suitable method of assembly may be used for this purpose, the only condition being the watertightness of the system to the external water and the leakage from the interior to the pressures supported during operation.

  For maximum efficiency, the front diaphragm 28 should be flat or provide a convex surface for acoustic waves moving in the water. It will therefore be desirable that at the time of mounting the apparatus the volume 26 is slightly overloaded with viscous fluid.

  Resistance plate 34 itself has a plurality of circular perforations 36 made in a plate, which is particularly shown in FIG. 2. Experience has shown that in practice it is necessary to manufacture a resistance plate having a 1.045 equidistant holes 1 mm in diameter in a surface of 13 cm and using as a viscous fluid, silica oil of 2000 centistokes. This produces a device whose characteristic acoustic impedance is about one third of that of water, the device being particularly effective with a characteristic acoustic impedance of between one time and 0.1 times that of water.

  The usual machining processes are somewhat complicated when applied to the manufacture of plates having such a hole density.

  From the foregoing it can be seen that in operation the static pressure height due to the depth applies to both the front diaphragm 28 and the rear diaphragm 30. The transient pressure variations produced by acoustic waves only reach the diaphragm before 28, the other diaphragm 30 being immunized against these variations by the tortuous path leading to the cylindrical space 12 and, even more so, by the repulsion of the air bellows 42. Therefore, the pressure of an acoustic wave 46 passing through the water 48 and reaching the front diaphragm 28 deforms the latter first towards the surface 2 and thus forces a portion of the viscous fluid through the perforations 36 of the resistance plate 34, a large part of the kinetic energy is thus absorbed by overcoming the high shear stresses thus produced and by pushing the rear diaphragm 30. During the negative alternation of the acoustic wave, the diaphragm arr Thirty 30 relaxes, returns the viscous fluid through the resistor plate 34 and restores the front diaphragm 28 substantially to its original position. Thus, a harmonic simple harmonic movement is established, driven by the impact of the acoustic waves on the front diaphragm 28. The experiment revealed that the reflection loss for acoustic waves striking a cell of this kind and possessing physical properties as described above was of the order of 20 decibels.

  It should be emphasized again here that any movement of the resistance plate or the body of the cell during the impact of an acoustic wave is negligible in comparison with the movement of the diaphragm and the viscous fluid.

  An anechoic coating for large immersed or submersible surface may be made by fixing a plurality of cells such as the one just described to said surface. Experience has shown that in order to obtain the best results in this case, the separation of the cells must not exceed half the maximum wavelength of the incident sound at the highest frequency to be received by the coating.

  If one analytically considers a single cell, and in particular the flow of fluid through a single hole of the resistor plate 34, the equation of fluid movement in the hole is 2u + 1 J _ 1 11+ J wherein: J = velocity of the particles in the viscous fluid, r = radial coordinate, = dynamic viscosity coefficient, = density of the viscous fluid, = time, = longitudinal coordinate.

  For sinusoidally varying pressures, it can be shown that the solution of equation (1) is 8) i Z = + - iu-) (2) 3 in which: Z = specific surface acoustic impedance, = thickness of the resistance plate, a / = radius of the hole.

  For a series of holes per unit area, the solution becomes: 8) 4 ut.) PX Z = ana, + 3 'Fr n e2 (3) We see that the critical dimension is now the radius of the holes, which is perfectly stable and independent of the static pressure. The bandwidth for a 20 dB decibel loss is given by the formulas: 6 p. 2 = 5 (4). where 20 is the bandwidth for a reflection loss of 20 decibels.

  Similarly, a solution for viscous fluid flow through a resistance plate having a slot length of per unit area would be: 12./JQ 6 Z = 3 + 5 d (6) where d, is the width of slit, and 2o. 2 i (7) The terms obtained to denote the band length in equations 5 to 7 are somewhat disturbing because / - as well as O or d, are not independent variables but are related by the matching conditions which are : 8 & tr n o4 - = () W for holes (8) where (c) W is the specific acoustic impedance of water and 12 m-- (C 3 ==) vV for slots (9) If we consider the case of circular holes distributed symmetrically, as in FIG. 2, we can dismantle that: ((-ev) J (11) / TG, by eliminating between equations (5) and (11) ) and positing OL 'h on ap 2 fi = 3 (f) 80 and similarly, for a slit distribution of d K where b is half of the separation 2b of the center-to-center slits: ("c) & K = 12 x W where x is the transversal coordinate of a slot.

  The theoretical maximum value of the bandwidth of either type of plane plate is given by setting 2 U 3 b 2 and that equation (8) becomes 16 Ve b3 (10) (12) (13). ) K = 1 which gives, for circular holes: V '20 max = 80 (14) and for slots: (C)% i max = 12 1.q (15) It would be clear from these equations that the best result is with a resistance plate pierced with slots, but this advantage is more than exceeded in practice by the greater rigidity of the plate pierced with circular holes and this is the reason why it is preferred to use this end in the elements. and anechoic cells of the present invention.

  There is a maximum limit to the lateral dimensions of the resistance plate beyond which its impedance decays rapidly. It has been found that it is convenient to use a series of holes of a density of about 500 holes for 6, 5 cm 2, each hole having about 1 mm in diameter, and to use a resistance plate of dimensions 2 mm thick, but it is obvious that these dimensions are interdependent and function of the fluid used, and that the individual dimensions will have to be set using the criteria presented as suggestions for solving a particular problem.

  The particular embodiment of the invention which has just been described and shown should not be considered as limiting the scope of the invention. In particular, the shape of the perforations of the resistance plate, although circular, preferably could be that of slits or any other shape, although these are more difficult to achieve. Similarly, the particular structure of the anechoic cell shown is only one example and not a limitation of the invention.

  RENENDICA TIONS =========================== 1.-Anechoide device characterized in that it comprises at least one flexible diaphragm, a plate essentially inflexible resistance, and a volume of fluid normally trapped between the flexible diaphragm and the resistance plate, the bending of the diaphragm under the impact of an acoustic wave passing a portion of the fluid through the resistance plate.

  2. An anechoic device according to claim 1, characterized in that it comprises a pair of expandable diaphragms spaced from one another, one being a front diaphragm and the other a rear diaphragm, a volume of fluid trapped between the diaphragms, and a resistance plate dividing the volume into two chambers communicating with each other through the resistance plate, flexing the forward diaphragm passing the fluid from one chamber to the other through the plate. resistance.

  3. Anchoide device according to claim 2, characterized in that it comprises a cell body having a substantially cylindrical opening closed at both ends by the extensible diaphragms, the volume of fluid and the resistance plate being contained in the opening cylindrical.

  Anchoic device according to claim 3, characterized in that the cell body has a recess coaxial with and communicating with the cylindrical opening, the rear diaphragm being mounted in said recess.

  5. An anechoic device according to any one of claims 2 to 4, characterized in that it is mounted on a surface so that the front diaphragm is directed in the opposite direction to the surface and the rear diaphragm is directed towards it and that it comprises organs allowing the ambient environment to freely access the diaphragm directed towards the surface.

  6. Anchoide device according to claim 5, characterized in that it comprises a circumferential flange contacting the contiguous face of a cylinder mounted on the surface, said cylinder having openings allowing the ambient environment of freely access the rear diaphragm.

  7. Anchoide device according to any one of claims 2 to 6, characterized in that it comprises means for compensating the local static pressure.

  8. Anchoic device according to any one of claims 2 to 7, characterized in that it comprises a deformable multicellular material, similar to the sponge, mounted against the rear diaphragm to compensate for the local static pressure.

  9. An anechoic device according to any one of claims 2 to 7, characterized in that it comprises a sealed bellows containing a gas, mounted against the rear diaphragm to compensate for the local static pressure.

  10. Anchoide device according to any one of claims 1 to 9, characterized in that the resistance plate comprises a series of perforations in a rigid plate.

  11. An anechoic device according to any one of claims 1 to 10, characterized in that it has an acoustic impedance between one time and 0.1 times the acoustic impedance characteristic of a liquid environment in which it is intended to be used.

  12. Anchoic device according to claim 11, characterized in that its acoustic impedance is equal to one third of the acoustic impedance characteristic of a liquid environment in which it is to be used.

  13. Anchoic device according to any one of claims 1 to 12, characterized in that the front diaphragm has a convex surface outwardly.

  14. Anechoic devices according to any one of claims 1 to 13, characterized in that they are arranged on a surface and constitute an anechoic coating for this surface.

  15. Anchoide devices according to claim 13, characterized in that they are separated by a distance at most equal to half of the maximum wavelength of the incident sound at the highest frequency to be received by the coating. .