WO2006021605A1 - Insertion units which are microperforated for use as sound absorbers - Google Patents

Abstract

The invention relates to the design and production of devices and materials which can absorb an acoustic wave, which provide an alternative to fibrous or porous materials and which have similar or better sound absorption properties than microperforated materials. The inventive devices include meshes or similar acoustic elements with submillimetric holes, which can be easily inserted into surfaces that are exposed to the acoustic wave. The characteristics of the resulting absorption spectrum depend on the design of said elements, namely: hole in the mesh, number of elements to be inserted for a determined surface area, size of the rear cavity (with optional individual adjustment), etc. In addition, the devices have excellent mechanical properties which enable easy insertion and cleaning without the loss of acoustic characteristics. In this way, the devices are suitable for acoustical treatments in which hygiene, cleanliness and/or extraordinary usage situations are essential factors.

Description

Title

MICROPERFORATED INSERTION UNITS FOR USE AS

ACOUSTIC ABSORBENTS.

Technical sector

Acoustics, buildings, vehicles for road and air transport, industrial noise, environment, ...

State of the art For the acoustic conditioning of the most varied types of enclosures, porous or fibrous materials are normally used capable of absorbing the unwanted incident acoustic energy in the range of medium and high frequencies (approximately from 200 Hz).

In certain cases, these types of devices made with these materials, which have rough surfaces and open pores are not the most appropriate for health requirements (they are considered carcinogens and also absorbers of all types of dust and bacteria) and / or cleaning (white cameras), as well as in other cases in which their presence should not be relevant and should pass as unnoticed as possible even being transparent optically. On other occasions it is acoustically conditioning spaces with extreme technical requirements such as the nozzles of aviation engines, etc., where classic absorbent acoustic materials are not viable.

Moreover, the traditional absorbents mentioned above need relatively large thicknesses to effectively absorb in the regions of the acoustic spectrum of low and medium frequencies.

On the other hand, the problem of absorption at low frequencies has undergone a significant increase in recent years not only in the fields of architectural acoustics and building, but also in noise control. Just as the medium and high frequencies of the noise spectrum can be controlling by means of the classic porous silencers, at very low frequencies, these silencers require large thicknesses of the acoustic covers, which in addition implies loss of flow in the roughness of the treatment. Therefore, it would be ideal to obtain an absorbent material free of porous or fibrous materials, which would produce high insertion losses in the range of very low frequencies, either passively or actively by means of the insertion of an Active Noise Control system CAR. At the same time, high stability and cleaning thereof are desirable without prejudicing its acoustic properties.

Most of the previous absorbent systems are based on the use of microperforated panels as theoretically proposed by D.Y. Maa since the 80s of the last century (D.Y. Maa, Potential of microperforated panel absorbers, J. Acoust. Soc. Am. 104 (5), Nov 1998, 2861-2866).

In this type of panels, the acoustic absorption is a consequence of the acoustic resistance to the air flow of the system (mainly of the reactive type) and a low mass reactance. In the microperforated materials (Microperforated Panels MPP), the energy losses are due to the effects of thermal and viscous gradients that occur in the perforations to the passage of the acoustic wave, in this case essentially the resistive part of the characteristic acoustic impedance of the material. The sound absorption frequency band in a specific microperforated panel depends essentially on the diameter of the perforations, the thickness of the panel, the percentage of perforation thereof (total area of the perforations / area of the panel surface), as well as the space of air enclosed between the panel and the rigid rear structure on which it rests.

Since the 1990s, these types of absorbents have been used successfully in certain constructions and restricted situations in which the use of Classic absorbent materials are unfeasible for technical or health reasons

The main restrictions of these acoustic absorbers are their costs due to the difficulty of drilling the most varied surfaces and materials

(metal, glass, methacrylates, ceramics, etc.) with holes whose diameters are sub-millimeter, which can vary between 0.06 mm to 1 mm, with varying drilling percentages between 1 and 30%, which would often lead to hundreds of perforations per square decimeter, or in the case of flexible sheets, the need to use a protective cover in turn also perforated.

At present, the use of microperforated panels has been limited to panels for making false ceilings with absorbent acoustic properties, which among its advantages include: reduction of risks in enclosures with high health conditions; cleaning possibilities; mechanical sensitivity during assembly and installation; physiological effects due to the abrasion and dispersion of the fibers, etc.

In this sense, we have only found the following patents in the US patent office:

1. Pat. N ⁰ 6,617,002 Microperforated polymeric film for sound absorption and sound absorb using same.

2. Pat. N ⁰ 6,598,701 Shaped microperforated polymeric film sound absorbers and methods of manufacturing the same

3. Pat. N ⁰ False ceiling 5,740,649 (1998), Fuchs et At the.

In all of them it is the absorbent characteristics of false ceilings made from very thin microperforated plastic films, showing their suitability as acoustic materials broadband absorbents lacking fibrous materials (mineral wool, etc.) that are harmful to health.

4. International patent WO 02/03375 A1 (10.01.2002) "Shaped microperforated polymeric film sound absorbers and methods of manufacturing the same" by K.B. Word and P.A. Martinson, in which the absorbent acoustic properties of a thin plastic film and the process for its manufacture are described. The patent includes the graphs of the sound absorption spectra obtainable with this procedure.

In all of them, the films are of the order of 100 μm, which really constitutes a handicap against their resistance to external agents (easy rupture, delicate handling, difficult cleaning since these pores quickly fill up with the environmental dust, etc. ), in addition to its mechanical fragility.

It should be noted that the acoustic absorption curves depending on the frequency obtained with the procedure claimed in this patent are perfectly equivalent to those advocated by the aforementioned authors.

Description of the invention.

- Brief description of the invention. The principle of operation of a microperforated acoustic panel consists in obtaining a resistance to adequate air flow through a certain number of perforations with diameters less than 1 mm. and drilling percentages in the order of 1 -10%. Behind the panel must have an air cavity that acts in these conditions as a vibrating mass - spring system capable of absorbing a relatively wide band of frequencies of the incident acoustic energy. Said frequency band can be conveniently extended by simply having several panels in parallel with intermediate air spaces calculated ad hoc. They can also an extension of the frequency band to be absorbed is obtained by simply curving the microperforated panel or the wall that constitutes the air cavity.

In particular, the selection of the configuration of the holes (diameter and number) not only determines the range of frequencies to be absorbed but also the efficiency of the absorbers in that range.

The panels can be rigid, in which case the theories of Maa must be applied; but they can also be flexible, taking this situation into account in the calculation of membrane-induced vibrations, which give rise to supplementary absorption.

In all these situations it is required to have sophisticated technologies to perforate the panels of the most varied constitutions, operations that are expensive given that they are very specific situations and that in the case of these sub-millimeter holes, special drilling machines, lasers, jets must be used of pressurized water etc., which will depend on the type of material to be drilled.

The previous situation is perhaps responsible for the restricted use of this type of materials.

It would seem very convenient to replace this complicated technology with a system of simple application that facilitates the a priori realization of the starting material, or to condition the treatment of already implemented surfaces a posteriori, for the sake of a controlled increase in its acoustic absorption.

The present invention consists in the design and construction of small mechanical devices (MIUs) that can be inserted very easily and comfortably in any panel that covers a more or less reflective surface (separated from it a certain space) in such a way that allow its transformation into an absorbent device to the incident sound on it. This type of panels, thus treated, are applicable both in public or private use rooms (building acoustics, conference and concert halls, ...); They can also be of great application in vehicles (car roofs, airplane cabins, ...), in enclosures where cleaning plays a fundamental role (hospital rooms, operating rooms, food preparation rooms, white microelectronic research chambers ,. ..), etc., since they can be cleaned with detergents or other antibacterial media

It has the advantage of its low weight and volume compared to fibrous materials, which allows the development of CAR devices of reduced thicknesses, of vital importance in which the use of volume and space plays a fundamental role in the design of an enclosure, passenger compartment, etc.

In the classic microperforated panels, normally the distribution of the perforations is carried out on a regular basis over the entire surface considered until reaching the percentage of perforation appropriate to the diameter of the holes, rear air space and amplitude and frequency of the central band of the spectrum to absorb.

On the contrary, and according to the studies carried out by the authors, the regular distribution of sub-millimeter perforations can be replaced by small distributions of concentrated and separated perforations at greater intervals, so that the result of the absorption spectra are exactly equivalent in both cases.

Films made with perforated polymers, with perforations of the order of 4 to 20 mils (100 μm - 0.5 mm) can be filled with dirt, dust, etc. especially for the smaller diameters, and given the extreme thinness of the film (50 μm typical) its cleaning is highly difficult, in the case that it can be done.

Another aspect to note is that in MPP the maximum efficiency of the same is obtained when the thickness of the panel is identical to the diameter of the perforations, while in the proposed MIU the total absorption does not vary fundamentally although the thickness of the panel is increased by a factor of ten, which allows the insertion of the devices in almost any type of panel or cover.

The invention is susceptible of various modifications and alternative forms, so the drawings and explanations included in this patent are by way of example. It should be understood that their intention is not limited to the examples described, but quite the opposite; The intention is to cover all modifications, equivalences and alternatives that match the spirit and purpose of the invention as defined in the claims.

Detailed description of the invention. The object of the present invention is to simplify and reduce alternative absorbent acoustic systems based on the use of surfaces formed with submillimeter pores.

It also allows replacing the required perforation mesh (set of perforations generally forming a grid), in order to have the necessary perforation coefficient (usually less than 10%), especially

The absorbent surface, by another meshing of a unit cell of notably larger dimensions, in which a set of sub-millimeter pores would be concentrated in its nodes, calculated in such a way that its resulting perforation percentage is equivalent to the previous one.

This is achieved by replacing the complicated sub-millimeter drilling process of multiple perforations, (which generally requires sophisticated high technology), with a much smaller number of perforations with diameters within the range of millimeters or centimeters; perforations that are subsequently covered with sub-millimeter meshes of easy acquisition (and reduced cost) in the industrial market. There are bad sides ranging from 25 μm to mm of light, and with them devices such as those in Figures 1 and 2 can be formed with which the perforated holes will be covered on the starting material, or on existing coatings such as it happens with the false plaster ceilings, side or perimeter panels in enclosures and hoods for noise isolation in machinery, panels in airplanes and motor vehicles (truck cars, etc.), transparent separations (in the manner of glass walls) in enclosures, in which you want to maintain a landscape distribution. In the industrial field, these devices can be arranged inside silencers and noise level reducers.

Figure 3 schematizes the principle of operation of the invention: it is mainly about replacing the perforated panel on the left (fig. 3a) of numerous sub-millimeter perforations, of expensive and sophisticated technology when it must be metallic, ceramic, glass etc., on the right (fig. 3b) in which a small number of large perforations covered with a mesh (metal, plastic, etc.) whose light is equivalent to the diameter of the perforations of fig. 3a and so that the percentage of perforation provided by all of these "microperforated insertion units" (MIU) is equivalent to that of the first situation (figure 3a).

Figures 1 and 2 show examples of fixing the meshes to the plates or surfaces to be treated. The arrangement of Figure 1b is ideal when the surface to be acoustically conditioned is of reduced thickness, as is the case of a metal plate of thickness less than a millimeter, or pre-molded covers of fabrics or plastics impervious to the flow of air used in cabins of vehicles, etc. The case of Figure 1c is specially designed when the surface to be treated is flaccid, such as for example interior plastic cover fabrics. In these cases, the submillimeter mesh is attached to an adhesive circular crown that can be glued over the holes made in the cover. The arrangement of Figure 2 is applicable to the case of panels of appreciable thicknesses, such as false plaster ceilings, vertical panels of partial separation of enclosures, screen screens etc., in which the neck of the "MIU" is embedded in the housings. of the perforations previously made in the panels. In this case, the length of the cavity can be regulated by means of the lower cylinder that has one of its bases closed, which makes it possible to tune the resonance frequency of the system. Different lengths will allow to form spectra of controlled bandwidth, through an adequate selection of the respective resonance frequencies.

Eventually, the "MIUs" of Figure 1 can be installed in thin or very thin panels, if a riveting device is available, in the manner and shape of those used in automatic closures of clothing, where the lower end of the cylinder opens and folds over the panel or fabric.

It is worthwhile to insist that Figure 2 presents the case in which MIUs are installed in spaces whose cavities are of great magnitude (suspended ceilings, vertical partial separation panels, etc.) and hinder the tuning of the desired frequency band . The closed mobile cylinder can be moved so that the air cavity after the desired value can be adjusted. The total absorption bandwidth can be significantly increased and adapted to the different ambient noise conditions to be mitigated, without more than tuning in groups the different MIUs, adequately varying the length of the cylinders that make up the cavities. Detailed Description of the Figures

Figure 1a-1 Plant a MIU on a bearing plate in which a perforation has been performed that is coated with a submillimeter mesh Figure 1a-2. Cut of a MIU on a bearing plate in which a perforation has been performed that is coated with a sub-millimeter mesh.

Figure 1b Fixing parts of the sub-millimeter mesh (to be inserted between them) in the form of an "automatic" in the perforation carried out in the supporting plate.

Figure 1c When the supporting plate is very thin, it may be appropriate to fix the mesh by means of an adhesive circular crown.

Figure 2. MIU whose maximum frequency absorption of the spectrum can be tuned by varying the length of the cavity by means of the adequate displacement of the closed tube. Figure 3a. Microperforated panel for hundreds of holes or thousands of submillimeter perforations.

Figure 3b MIU with absorption spectrum equivalent to that achieved according to model 3a.

Figure 4.- In a continuous line, the variation of the acoustic absorption coefficient is shown as a function of the frequency, obtained experimentally in a standing wave tube, of a MIU formed by a 0.5mm thick and 30mm diameter plate with a 5mm diameter hole covered with a 100μm mesh of light, resulting in a drilling coefficient of 0.81%. 5 cm air cavity between the MIU and the rigid bottom of the standing wave tube. The absorption spectrum theoretically calculated for the MIU parameters described above is presented in dashed lines.

The theoretical result corresponding to a traditional microperforated with an equivalent absorption spectrum that would be obtained from a microperforated with 6 holes per cm ² with a diameter of 0.25mm and a panel thickness of 0.2mm on Ia is presented in a dotted line same cavity of 5cm.

Figure 5.- In a continuous line, the variation of the acoustic absorption coefficient is presented as a function of the frequency, obtained experimentally in a standing wave tube, of a MIU formed by a 0.5mm thick and 100mm diameter plate with 28 6mm diameter holes covered with a 35μm mesh of light, resulting in a drilling coefficient of 1.31%. 20cm air cavity between the MIU and the rigid bottom of the standing wave tube.

The absorption spectrum theoretically calculated for the MIU parameters described above is presented in dotted line. The theoretical result corresponding to a traditional microperforated with an equivalent absorption spectrum that would be achieved from a microperforated with 13712 holes per cm ² with a diameter of 35μm and a panel thickness of 35μm on the same cavity is presented in dashed lines. 20cm

Example of embodiment of the invention. In the following examples (figures 4, 5) the absorption spectra obtained with different devices measured experimentally in an impedance tube (Kundt tube) are presented, according to the procedure standardized in ISO Standard 10534 Part. 1/2 "Determination of sound absorption coefficient and impedance in impedance tubes". The absorption thus measured, corresponds to normal acoustic incidence on the surface of the sample, whose results will show an absorption spectrum with a maximum (which depends on the characteristics of the microperforated material, and the air cavity between it and the rigid wall of the tube) followed by a minimum repeating this sequence for higher frequencies of the excitation signal.

Typically, it is desired that the bell of the first maximum absorption is as wide as possible, that its maximum value reaches values of the absorption coefficient α∞1 and that at frequencies lower and higher than that corresponding to α «1 in which the coefficient Absorption takes values of 0.4, are separated in the largest possible range, as in the case of Figure 5. We have developed a series of mathematical algorithms for predicting the results of the absorption to be achieved (magnitude and frequency spectrum) that must be used for the design of MIUs in a given noise situation.

In the figures, the experimental results measured in impedance tube are shown in a continuous line, in line of points the theoretical values calculated with the theory developed for this purpose, and in line of lines the acoustic absorption corresponding to the MPP of traditional realization

Claims

Claims 1. A microperforated insertion unit, MIU, for use as an acoustic absorber, composed of a film-bearing adhesive device, which holds a mesh with a sub-millimeter light with a certain number of threads per unit area and whose open surfaces (lights) vary between 25 μm and 0.5mm. These units specially designed for light or very thin panels, normally have a circular perimeter, but can have square or any other surface. Normally the percentage of perforation of all the MIUs arranged on the surface to be treated must reach values between 0.5 and 10% and in which the resulting total sound absorption spectrum can be calculated through the known procedures of the technique ( see article by DY Maa, for example), depending on the thickness of the air cavity, light of the mesh, thickness of the same and percentage of perforation. 2. A MIU with the characteristics and functions of claim 1, made from tubular devices such as those of Figure 2, capable of being inserted in thick panels (plasters, wood, metal sheets, ...). These devices can have any other section (square, hexagonal, ...) that adapts to the aesthetics of the site. 3. Idem of claim 2, but can be inserted into a thin surface (tissues, thin sheets, thin films, etc.) by riveting the free tubular surface (opposite end to that contained in the mesh). 4. Idem of 1, 2 and 3, with the meshes that can be: metallic, nylon, or any other material that fulfills the necessary functions and purposes including colors for the case of taking into account decorative aspects. 5. The use of these devices in the CAR active control systems, with which a remarkable acoustic absorption is allowed in the range of very low frequencies with very low total thicknesses compared to those required when using traditional absorbent materials in which They use porous materials.