WO2012175280A2

WO2012175280A2 - Method and apparatus for determining the passage of audible sound energy through a double-wall structure

Info

Publication number: WO2012175280A2
Application number: PCT/EP2012/059704
Authority: WO
Inventors: Alexander Peiffer
Original assignee: Eads Deutschland Gmbh
Priority date: 2011-06-10
Filing date: 2012-05-24
Publication date: 2012-12-27
Also published as: DE102011103958A1; WO2012175280A3

Abstract

The invention relates to a method for determining the passage of audible sound energy through a double-wall structure which comprises a first wall, a second wall and a double-wall space in between. In order to take the double-wall effect into consideration with lower computational complexity and more precisely, the following steps are proposed: a) abstraction of the double-wall structure as a double-wall model by splitting into a plurality of subsystems which are formed from a volume of fluid representing the double-wall space and at least one mass layer which represents the walls, which adjoins the volume of fluid and which corresponds to the mass of the wall, b) calculation of the complex eigen modes for the subsystems, c) calculation of the modal impedance matrix for the introduction of sound into the connected subsystems, and d) calculation of the coupling loss factor for the double-wall structure.

Description

Method and device for determining the passage of acoustic sound energy through a double wall structure

The invention relates to a method and a device for determining the passage of acoustic sound energy through a double-wall structure. The invention finds particular application in the field of aviation, in order to determine the noise level in cabins of aircraft with a certain fuselage structure, so as to be able to influence the noise development within the cabin even in the design phase. Aircraft cabins are often designed with an outer wall and an inner wall and a space in between. For the design of aircraft, it is advantageous to have a tool in hand, which determines the noise passage or the sound reduction measure of such wall structures already in the design phase.

Generally speaking, the invention is about acoustic calculation. In particular, there are methods for calculating the coupling loss factor between rooms. The coupling loss factor indicates how an acoustic energy density in one room is coupled to an acoustic energy density in another room if the construction to be calculated is arranged therebetween.

With the invention, an improved method and an improved device for the acoustic calculation of double walls are provided. Double walls are widespread, especially in aviation, but also in sound insulation. With the method and the device, a tool is to be created to construct the double walls and to determine the acoustic throughput. It should be possible to predict how loud it will be in one of the two rooms separated by the double walls. A special factor that is at stake here is the soundproofing dimension.

The literature has already described many approaches to the calculation of sound reduction and coupling loss factors by structures. Reference is made to the following references:

[1] A. Peiffer, S. Tewes, S. Brühl, SEA Modeling of Double Wall Structures, DAGA 2007

[2] Shorter, Langley, "On the reciprocity relationship between direct field irradiation and diffuse reverberant loading," JASA, 2004

[3] Shorter, Langley, Vibro-Acoustic Analysis of Complex Systems, JSV

[4] W.R.Graham. Boundary Layer Induced Noise in Aircraft, Part I: The Flat Plate

Model, J. Sound and Vibration, 192 (1), 101-120, 1996

[5] US 6 090 147 A. The following description is based on the knowledge of the present references [1] to [5]; Reference is made to each of the above references [1] to [5] for more details on the acoustics calculation.

The previous method for calculating the sound energy flow through double wall structures is described in more detail in reference [1]. For this purpose, sound incidence at a certain angle is used to calculate what sound intensity will pass through the wall. For this purpose, incoming waves are averaged at all possible angles. For all typical sidewall superstructures in helicopters and aircraft, the double wall effect dominates the transmission of sound in the frequency range between 100 Hz and 600 Hz. A double wall has a higher sound reduction index compared to a single wall with the same surface weight. This can be justified, for example, by the fact that an airborne sound wave must again overcome the resistance during the transition from the medium air into the heavy and hard medium wall and again into the medium air for each wall. The soundproofing measure is not easily available by adding the soundproofing dimensions of two single walls, as the walls interact with each other. These reactions are dependent on the respective wavelength.

According to the literature [1], this double-wall effect is taken into account in the statistical energy analysis (SEA) by so-called double-wall connectors, the calculation of which is carried out by means of the theory of transfer matrices. In this case, infinite areas are assumed. Methods and apparatus enabling such a technique for acoustic computation of double wall structures are integrated in commercial products, such as e.g. in the commercially available program VAOne (ESI Group) or the Open Source Project Open SEA. The entire state of the art and the previous methods and devices for calculating double-wall structures are based on the transfer matrix methods as explained in [1]. In this case, infinite areas are assumed. As a result, transverse resonances are taken into account very inaccurately and the predictions of the simulation are inaccurate.

In references [2] and [3], a technique for simulating acoustic structures by hybrid methods is generally explained. This will simulate the The system to be examined is deterministic with regard to a predictable behavior and statistically considered with respect to an unsafe behavior. For example, certain interfaces of the acoustic system are well known and well predictable in acoustic behavior, while other limits of the system are not well known or can be calculated in an unpredictable manner, therefore average behaviors are considered.

Specifically, many models can be relatively well calculated and predicted for large wavelength excitation, while excitation by wavelengths that are small in relation to system areas or interconnections are often difficult to predict. In the methods and the devices for calculation by such hybrid methods, therefore, both deterministic thermals and statistical thermals are used. For more details on these hybrid methods, see documents [2], [3], [5].

An approach of using a hybrid method to calculate the acoustic behavior of an aircraft structure excited by turbulence in the air boundary layer on the outer skin is described in [4]. In this case, only the outer wall is considered as a single plate with air boundary layer. The aim of the reference [4] was to enable a prediction of the noise by the excitation of the outer skin by air turbulence.

For double-wall structures, only the transfer matrix methods have hitherto been possible.

Disadvantages of the known methods for calculating the acoustic behavior of double walls are the computation effort in the integration over the different angles, the abstraction of the infinitely extended wall and the uncertainties regarding the impact angles to be considered. The object of the invention is therefore to provide a method, which is significantly improved, in particular with regard to increased speed, improved accuracy and lower computational complexity, and a corresponding device for acoustic simulation of double walls.

This object is achieved by a method according to claim 1 and a device according to the independent claim.

Advantageous embodiments are the subject of the dependent claims.

The invention provides a method for determining the passage of acoustic acoustic energy through a double wall structure comprising a first wall, a second wall and a double wall space therebetween, comprising:

a) abstraction of the double wall structure as a double wall model by division into a plurality of subsystems, which are formed from a volume of fluid constituting the double wall space and at least one mass layer representing one of the walls, which adjoins the volume of fluid and which corresponds to the mass of the wall,

b) calculating the complex eigenmodes for the subsystems,

c) calculating the modal impedance matrix for sound introduction into the connected subsystems and

d) calculation of the coupling loss factor for the double wall structure.

Further, the invention provides an apparatus for determining the passage of acoustic sound energy through a double wall structure comprising a first wall, a second wall, and a double wall space therebetween, the apparatus comprising:

a) an abstraction unit for the abstraction of the double wall structure as a double wall model by division into a plurality of subsystems, which consists of a volume of fluid representing the double wall space and at least one of the walls forming mass layer adjacent to the fluid volume and which corresponds to the mass of the wall,

b) a eigenmode calculation unit for calculating the complex eigenmodes for the subsystems,

c) an impedance matrix calculation unit for calculating the modal impedance matrix for sound introduction into the connected subsystems, and d) a coupling loss factor calculation unit for calculating the coupling loss factor for the double wall structure. Furthermore, the invention provides a computer program product with program codes which, when loaded into a computer, make it possible to carry out the method according to the invention and thus make the computer an exemplary embodiment of the device according to the invention. In a particularly preferred embodiment of the invention, the hybrid methods are used for the calculation of the double-wall connector. In this case, an analytical model of the double-wall cavity, whose front and rear sides are covered with the respective basis weight of the double-walled shells, is preferably created. For this model, the sound transmission through this system is calculated by means of hybrid methods.

The inventive method, the sound transmission through the double wall is described much more precise. Transverse resonances are considered correctly.

According to a preferred embodiment, the double wall connector is calculated by means of hybrid methods and then the energy flow through the double wall is carried out by methods of statistical energy analysis by means of this double wall connector thus determined. It is preferably provided that at least one of the calculation steps is carried out by means of hybrid formulation of the double wall structure or of the subsystems, wherein a deterministic determination method is carried out for longer wavelengths and a statistical determination method for smaller wavelengths.

In a further preferred embodiment, it is provided that step a) comprises: creating a first subsystem consisting of the first wall, the double wall space and the second wall,

Creating a second subsystem consisting of the double wall space and the second wall,

Create a third subsystem consisting of the first wall and the double wall space. In a preferred embodiment, it is provided that the energy passage through each of the first to third subsystems is determined by means of the hybrid formulation.

Preferably, it is further provided that each of the eigenmodes of the subsystems is used as a complex eigenmode in a modal analysis of the entire double-wall model in order to numerically calculate the coupling loss factor.

Preferably, the double wall model is obtained by simple abstraction of the concrete double wall structure, assuming a length or width corresponding to the length and width of the double walls and a corresponding depth as in the concrete double wall structure. As mass for the walls of the model, the masses of the structural walls are assumed.

In the following the invention will be explained in more detail with reference to the attached drawings. Showing: 1 shows a typical section of an aircraft with examples of double-wall structures;

FIG. 2 shows a schematic representation to illustrate an abstraction of a double-wall construction to be calculated; FIG.

Fig. 3 is a schematic representation of the abstracted double wall construction;

4 shows a schematic illustration to illustrate the abstraction of the double-wall construction with regard to the acoustic paths;

5 a schematic representation of the individual subsystems for the acoustic simulation of the double-wall construction; and

6 is a schematic representation of a flowchart for calculating the coupling loss factor of the double-walled connector.

In Fig. 1, the typical structure of a fuselage segment 10 is shown as an example of a structural element of an aircraft. At the aircraft fuselage are a plurality of double wall structures 12 which separate the interior 14 of the passenger compartment 16 from the outside environment 18 and / or a cargo hold 20.

In order to be able to estimate which noise level prevails in a particular design of an aircraft fuselage 10 in the interior 14, it is advantageous to determine the soundproofing dimension for the cabin walls. Since the cabin walls have the double wall structures 12, it is necessary for this purpose to determine the soundproofing dimension of the double wall structures 12. This sound reduction index is significantly influenced by the double wall effect at certain frequencies.

In the following, a method is described in order to determine the coupling loss factor of the double-wall structures 12 and thus the sound reduction index.

For this purpose, a concrete double wall structure 12 from the aircraft body segment 10 will be considered in more detail below, which is shown in outline in FIG. 2.

The double wall structure 12 has an inner wall 22 with an inner lining 24 and an outer structure 26 with an outer wall 28. Between the inner wall 22 and the outer wall 28 an inner volume with insulating material is present.

Investigations, which are explained in the literature [4], have shown that the curvature of the real double wall structure 12 has only little influence on the acoustic energy flow. Therefore, in order to simplify the real double wall structure 12, a double wall model 30 is assumed, as is shown in more detail in FIG. 3. The double wall model 30 has a first wall 32 that represents the inner wall 22 and the inner panel 24, a second wall 34 that represents the outer structure 26 with the outer wall 28, and a double wall cavity 36 between the two walls 32, 34 that carries the inner volume represents the insulating material.

The model is constructed so that it still images the real double wall structure 12. Reference is again made to FIG. 2. The walls 32, 34 have a length lx which corresponds to the mean length lx of the curved walls 22, 28 along the line of curvature (in the middle between the wall structures). speaks, and a width ly, which corresponds to the width of the ly to be examined double wall structure.

Further, in the real double wall structure 12, the volume V _{0 of} the space between the inner wall 22 and the outer wall 28 is determined. In the double wall model

30 is then expressed as depth t, i. as the distance between the walls 32, 34, the value t is assumed according to the following formula:

Vo

t =.

lx - ly

Alternatively, specifically the real depth t of the double wall structure 12 used and the volume V ₀ = Ix ly ^■ * t calculated therefrom.

Further, in the double-walled model 30, the masses of the first wall 32 and the second wall 34 are assumed to be area-related mass values by dividing the mass of the inner structure with inner wall 22 and inner wall 24 by the area of the inner structure:

In this case: A ₀ = I x.l y. Accordingly, the area-related mass of the second wall 34 is represented by dividing the mass of the outer structure by its area:

Thus, the double wall model 30 shown in FIG. 3 is obtained, wherein the first wall 32 has the areal mass nY and the dimension ly x Ix, the second wall 34 has the areal mass m _s ' and the dimensions ly x Ix and wherein as a distance between the walls 32, 34, the depth t has been adopted according to the above formula. The sound propagation speed and the density in the walls 32, 34 and the double-wall cavity 36 are assumed according to the respective material selection for the real double wall structure 12. In this case, the double-wall cavity 36 is considered as fluid-filled volume (fluid volume).

This results in a very simple double-wall model 30, which is nevertheless well suited for determining the soundproofing dimension through the real double-wall structure 12, whose acoustic response can be calculated quite well with the following methods.

The method is intended to determine the energy flow of vibrational energy from a cavity with an acoustic oscillation field (diffuse field) through the double wall structure 12 into a second cavity separated by the double wall structure 12. Specifically, the passage of acoustic energy from the outside environment 18 through the double wall structure 12 in the interior 14 is to be determined. As indicated in FIG. 3, the outside environment 18 accordingly forms a transmitting cavity 38 and the interior 14 forms a receiving cavity 40, the acoustic energy passing through the double-wall structure 12.

Looking at the double wall model 30 according to FIG. 3, it is clear that in this case the two walls 32, 34 are excited, but that the walls 32, 34 influence one another. There are thus quite different eigenmodes or eigenmodes which are determined by the different interfaces.

In order to determine the eigenmodes of the double-walled model 30 more precisely than heretofore with less computational effort, the system of the double-walled model 30 becomes further abstracted by being divided into a plurality of subsystems, which are explained in more detail below with reference to the illustration in FIGS. 4 and 5.

Fig. 4 schematically illustrates subsystems for an SEA method and related resonant and non-resonant paths.

4, there is a first path 1 of direct passage through the double wall model 30, with the eigenmodes determined by the two walls 32, 34 and the double wall cavity 36 being predominant. There is also a second path marked 2, where vibrations of the first wall 32 are transmitted to the receiving cavity. There is also the third path marked 3, where the second wall 34 is excited by the transmitting cavity 38. Accordingly, the subsystems 1, 2 and 3 shown in more detail in Fig. 4 are set up, wherein the first subsystem 1 of the first wall 32, the second wall 34 and the double wall cavity 36 is formed therebetween, the second subsystem 2 of the first wall 32 and the double wall cavity 36 is formed and the third subsystem 3 of the second wall 34 and the double-wall hollow space 36 is formed.

The double wall cavity 36 is assumed to be a complex fluid layer, and the walls 32, 34 are assumed to be mass layers corresponding to the panel mass of the real double wall structure 12.

The method for calculating the coupling loss factor of the real double wall structure 12 can be explained as follows, as illustrated in the flowchart of FIG. 6, as follows: First, the three subsystems shown in Fig. 5 consisting of complex fluid and bulk layer are formed (step A).

Subsequently, the complex eigenmodes for each subsystem are calculated (step B).

For the calculation of these complex eigen methods hybrid methods are used, as described exactly in [3] and [5]. Thus, for example, for larger wavelengths, deterministic methods, such as the finite element method, and for smaller wavelengths, statistical methods, e.g. the statistical energy analysis used. The concrete calculation methods are described exactly in [3] and [5]. For the specific case of sound transmission through wall structures, the method was also described earlier in [4] in the literature. Software operating according to these methods is available in the market, e.g. VAOne of the company ESI Group (formerly of the company Vibro-Acoustics). With the complex eigenmodes, the modal impedance matrix for radiation in the interconnected subsystems can be calculated (step C). Thus, the coupling loss factor of the double wall connector can be calculated.

The double wall connector can thus be considered in a simple manner accordingly.

Concretely, an SEA can be performed with the double-wall connector thus determined, similar to [1], but with a more accurate double-wall connector. Specifically, an analytical modal analysis is performed with the double-wall model 30 illustrated in FIG. 3, which in concrete terms represents an abstraction of the real double-wall structure 12, but nevertheless can be calculated quite easily. In the method in question, the double wall is abstracted. Soft masses are assumed and an air volume between them to calculate the coupling loss factor.

As shown in Fig. 2, for abstraction, a curved aircraft element having two frames 42, 44, an intermediate volume V ₀ and a lining (inner lining 24) is abstracted to the double wall model 30 shown in Fig. 3. For this purpose, a first frame is used taken 42, the average Spantlänge Ix, the width Ly, and the depth t. this yields the abstracted volume _V0. the masses are similar easily abstracted. You simply take the mass of the outer structure and the mass of the inner structure and normalized or averages them to From these values, one then forms the rectangular system shown in Fig. 3 with the respective thicknesses of the walls 32, 34 and the double wall cavity 36. The method uses hybrid methods for calculating the double wall connector in Fig. 3 illustrated analytical model of Doppelwandkavität - double wall cavity 36 -, the front and back with the respective The basis weight of the double wall shells is occupied. For this double wall model 30, the sound transmission is calculated by this system by means of the known hybrid method.

In the acoustic simulation of double walls, the double wall effect is dominant for a wide frequency range. This frequency range starts at about the double wall resonance of about 100 Hz. This effect could be calculated by the deterministic finite element method, in which the whole system of the double wall is divided into small elements. Due to the large computational effort, this method is limited to smaller subsystems. An alternative method would be the so-called SEA method. Here, the double wall effect is considered using an analytical formulation. So far, this formulation is based solely on transfer matrix methods that use infinite layers of masses and fluid. The ultimate transmission of energy is obtained by averaging over certain angles of incidence. The disadvantage of this method is that it does not work for double walls of realistic size and that there is no valid rule for selecting the angle range for averaging.

The new method proposed here overcomes these problems.

Generally, the physical system that is behind the double-walled connector is a system of concentrated mass-cavity-concentrated mass. The method now calculates the complex eigenmodes of this subsystem using the hybrid methods known from [3], [4] and [5] to calculate the energy flow through the respective compounds of that system. The energy flow paths taken into account are shown in FIG. 4 and designated 1, 2 and 3. Each path 1, 2, 3 is described by complex modes of the cavity subsystem - concentrated top walls. The concentrated masses or cover walls are considered as membranes without tension or bending stiffness.

Using such a simple model with simple subsystems, corresponding complex eigenmodes of these subsystems are obtained with which the coupling loss factor can be determined numerically by means of a modal analysis. can be expected. The accuracy is much greater than that with the transfer matrix method.

The method takes into account all local resonances of the double wall system. No averaging over angles or the like is needed. This makes it much easier to correctly account for the double wall effect in the calculation of the acoustic behavior of the real double wall structure 12.

Overall, a method for the hybrid formulation of the double-walled connector is obtained in the statistical energy analysis.

The corresponding steps may be implemented in software and stored on a computer program product, possibly together with known program codes for performing the hybrid procedures. For example, The subsystems can be entered in the software for hybrid processes. However, with currently existing software, a model, for example an FE model, with the corresponding properties would have to be created artificially.

By loading such a program or appropriate programming, a computer can be made into a device for carrying out the method.

LIST OF REFERENCE NUMBERS

10 aircraft fuselage segment

12 double wall structure

14 interior

16 passenger compartment

18 outdoor environment

20 cargo hold

22 inside wall

24 interior paneling

26 external structure

28 outer wall

30 double wall model

32 first wall

34 second wall

36 double wall cavity (DW cavity)

38 transmitting cavity (Sending Cavity)

40 receiving cavity (Receiving Cavity)

42 first frame

44 second frame

Vo intermediate volume

A site double-walled systems consisting of complex fluid layer and mass layer (s) corresponding to the panel mass

B Compute complex modes of the subsystems

C Calculate modal impedance matrix for irradiation into the connected subsystems

D Calculate the coupling loss factor for the double wall connector

Claims

claims

1 . A method of determining the passage of acoustic sound energy through a double wall structure comprising a first wall, a second wall, and a double wall space therebetween, comprising:

a) abstraction of the double wall structure as a double wall model by division into a plurality of subsystems, which are formed from a volume of fluid representing the double wall space and at least one mass layer representing one of the walls, which adjoins the fluid volume and which corresponds to the mass of the wall,

b) calculating the complex eigenmodes for the subsystems,

d) calculation of the coupling loss factor for the double wall structure.

2. The method according to claim 1,

characterized,

in that at least one of the calculation steps is carried out by means of hybrid formulation of the double-wall structure or of the subsystems, wherein a deterministic determination method is carried out for longer wavelengths and a statistical determination method for smaller wavelengths.

3. Method according to one of the preceding claims,

characterized,

that step a) contains:

Creating a first subsystem consisting of the first wall, the double wall space and the second wall,

Creating a second subsystem consisting of the double wall space and the second wall, Create a third subsystem consisting of the first wall and the double wall space.

4. The method according to claim 2 and claim 3,

characterized,

that by means of the hybrid formulation the energy passage through each of the first to third subsystems is determined.

5. The method according to any one of the preceding claims,

characterized,

that each of the eigenmodes of the subsystems is used as a complex eigenmode in a modal analysis of the entire double-wall model to numerically calculate the coupling loss factor.

A computer program product that can be loaded into a computer and that contains program codes for performing the method of any one of the preceding claims on the computer.

An apparatus for determining the passage of acoustic sound energy through a double wall structure comprising a first wall, a second wall, and a double wall space therebetween, the apparatus comprising:

a) an abstraction unit for the abstraction of the double wall structure as a double wall model by division into a plurality of subsystems, which are formed from a volume of fluid representing the double wall space and at least one mass layer representing one of the walls, which adjoins the fluid volume and which corresponds to the mass of the wall,

c) an impedance matrix calculation unit for calculating the modal impedance matrix for sound introduction into the connected subsystems and d) a coupling loss factor calculation unit for calculating the coupling loss factor for the double wall structure.