NL1028909C2 - Broadband sound reduction with acoustic resonators. - Google Patents

Description

BROADBAND SOUND REDUCTION WITH ACOUSTIC RESONATORS

The present invention relates to a method for broadband reduction of the sound emitted from a vibrating structural element. The invention also relates to a sound-reducing panel construction for broadband reduction of the radiated sound.

Structural noise arises because vibrating structures make the air in the environment of that construction vibrate, which vibrations are propagated by the air and are experienced as noise. Many techniques are known for effecting a reduction of the vibrations of the air in the environment of the vibrating structure and therefore for reducing the radiated sound.

Two important areas of application can be distinguished. The first field of application is the broadband reduction of the radiated sound of a vibrating structure that vibrates as a result of a mechanical excitation, for example a motor that is mechanically connected to a casing. The second field of application 25 is the reduction of radiated noise from a vibrating structure, which vibrates as a result of an acoustic excitation. Consideration should be given here to constructions that are placed between the acoustic source and the listener and thus have a sound-proofing effect, for example a wall between two rooms, an aircraft cabin that reduces the flow noise, a partition wall in a car between engine and passenger cabin.

1 028 90 9! 2

A number of types of constructions are known with which noise with which the noise occurring in an enclosed space, such as a cabin, and caused by air or contact noise can be reduced. Most of such constructions work according to the principle of sound absorption or at least reducing the reflection of sound against the walls of the enclosed space. The structural elements of a cabin can, for example, be designed to be sound-absorbing, for example by providing them with Helmholtz resonators or quarter-wave resonators. A drawback of both types of resonators is that only sound absorption within a limited frequency band can be realized.

The US patent US 5 959 265, for example, describes a sound absorption-based system. The system can absorb sound in a construction element such as a panel. In the patent, the panel is provided with a number of resonators with a length of one-fourth wavelength of the sound to be reduced. Standing waves that are phase-shifted by half a wavelength with respect to the wave front reflected in the mouth of the resonators interfere destructively with this wave front, resulting in a noise reduction. However, these known quarter-wavelength resonators also have a sound absorption which is limited to a very narrow sound frequency band, which is determined by the length of the resonator. Hardly any reduction occurs outside this narrow frequency band.

30 Kitts, ZT, 2000, "An analytical study of the weak radiating cells as a passive low frequency noise control device", graduation report, State University, Blacksburg, Virginia, USA, describes another principle for passive 1 0 2 8 9 0 9 3 noise reduction. Instead of absorbing sound, the source strength of the sound source is minimized by distortion of the transmitted sound field by means of a system of weak-radiating cells in a panel. The source strength of the surface of the panel is minimized by means of two mechanically coupled surfaces which, when placed on a vibrating surface, are almost out of phase and have equal strength over a wide frequency range. The untreated substrate is divided into characteristic areas and provided with weakly radiating cells. The outer fixed element is directly connected to the vibrating structure and is called the frame of the cell. The frame of the cell is stiff, so that the speed of the frame of the cell is approximately the same as the speed of the substrate. The cell is connected to the frame of the cell via a flexible medium with a certain rigidity, thereby creating an enclosed cavity. This enclosed air volume and the flexible medium provide a certain compliance to the cell thereby creating a mass spring system. This allows the source strength to be reduced, so that a certain amount of noise reduction can be achieved. A drawback of the system of weakly radiating cells is that the system increases the source strength by a 100 factor at a single frequency, because the cell at that frequency resonates in phase with the frame of the cell. This means that at that frequency an enormous amplification of the sound is provided instead of a reduction of the sound. Further drawbacks are that relatively many mechanical parts are required and that the system is relatively heavy.

It is therefore an object of the present invention to provide a method for broadband reduction of the sound radiated by a vibrating construction element 1 0 2 8 9 0 9 4. It is also an object of the present invention to provide a sound-reducing panel construction in which the sound emitted by the panel can be broadband reduced.

According to a first aspect of the invention, a method is thereby provided for broadband reducing the sound radiated from a vibrating structural element, comprising: - determining a center frequency (fc) around which frequency the sound level can be broadband reduced; - arranging a number of resonators in a construction element provided with an acoustically hard outer surface, wherein the resonators open into said acoustically hard outer surface, wherein the length (L) of the resonators is approximately equal to the sound speed (c0) divided by twice the determined center frequency (fc), and wherein the resonators are arranged to have a porosity (Ω) of the structural element, which is defined as the quotient of the cross section (Ar) of the mouth of the resonators and the characteristic surface (Ac) of the structural element, to be provided between approximately 0.1 and 0.9.

The lengths of the resonators are approximately equal to c0 / 2fc, which here means that the lengths (L) of the resonators have values that can vary between c0 / 4fc and 3c0 / 4fc) with c0 the sound speed. However, with a length of exactly c0 / 2fc, an optimum reduction is achieved in most cases. In the following, such resonators will also be referred to as λ / 2-wavelength resonators.

If the porosity (Ω) is now between approximately 0.3 and 0.6, a particularly good result can be achieved. Calculations based on a one-dimensional model 1028909 5 have shown that if the length of the resonators is tuned to a center frequency of approximately 1000 Hz (ie L = 17 cm), minimum and average noise reductions over a bandwidth of 250 Hz are equal to 30 dB 5 and 38 dB respectively, for a bandwidth of 500 Hz equal to 16 dB or 30 dB and for a bandwidth of 750 Hz equal to 6 dB and 23 dB respectively. These values are achieved with varying porosities.

In a particular preferred embodiment the resonators are of tubular design, which results in a simple construction. In an even more preferred embodiment, the resonators are prismatic tubes, that is to say, the tubes have a substantially constant cross-sectional area over the respective lengths of the tubes. The latter tubes are relatively easy to manufacture.

Incidentally, it is noted that the operation of the present invention is substantially independent of the cross-sectional shape (round, square, rectangular, polygonal or any other shape) of the resonators. A slight curvature of the resonators is also permitted. The curvature does not, or hardly, reduce the operation of the resonators.

In a further preferred embodiment, the resonators have an acoustically hard inner surface. By acoustically hard in the sense of the present invention is meant a surface with an absorption coefficient (a) of less than 0.2. In other embodiments, the said inner surface can be acoustically absorbed.

In a further preferred embodiment, acoustic-absorbing material is provided in the resonator, such as, for example, mineral wool or rock wool. The resonator opening can also be closed with a thin foil. In both embodiments it is hereby prevented that unwanted particles, such as dust and the like, can end up in the resonators.

Although the method according to the invention allows a noise reduction in a broad frequency band, the method may also include providing resonators of different length for reducing the radiated sound level in respective broad frequency bands around respective center frequencies 10 (fc).

The resonators preferably have a constant cross-section so that they are easy to realize. However, the cross sections of the resonators can also be different. j

In a particularly advantageous embodiment, the I

method providing a structural element comprising a honeycomb structure on which an acoustically hard skin covering provided with perforations is provided. In this way, a structurally strong element 20 can be provided simply and quickly. If the honeycomb structure and / or the skin plating are also made of aluminum, this has the advantage that the panel is resistant to corrosive environments, high temperatures and moisture. The core of honeycomb structure can also be made of fiber-reinforced plastic, such as glass fiber or carbon fiber composites. In both cases the panels can therefore be made resistant to a moist and / or corrosive environment.

The method described herein comprises not only reducing the radiated sound power when the structural element is acoustically excited, for example when the structural element is used as a partition wall and is irradiated by an air sound source. The method may also include reducing the radiated sound power when the structural element is mechanically excited, for example when the structural element is vibrated by a mechanical vibration source. The application areas are therefore very extensive. The structural elements can for example be used in the automotive industry (panel between engine and cabin, doors and roofs), the aircraft industry (trim panels, ceiling] cabin, ceiling loading space), white goods (casing), construction (lightweight quiet walls, cleanrooms , dead rooms), medical applications (noise reduction from MRI scanners), traffic (noise barriers), as well as in industry and mechanical engineering (housing or noise barriers, turbines).

According to another aspect of the present invention, a sound-reducing panel construction is provided, wherein the panel construction comprises a panel provided with an acoustically hard outer surface and with sound-radiation reducing means for broadband around a selectable center frequency (fc) for reducing the vibrating state from the panel radiated sound, wherein the sound radiation reducing means comprise a number of resonators arranged in the panel and ending in the said hard outer surface of the panel, the resonators having a length (L) of approximately the sound speed (c0) divided by twice the determined center frequency ( fc), and wherein the number of resonators and the dimensions of the resonators are formed around a porosity (Ω) of the structural element, which is defined as the quotient of the cross section (Ar) of the mouth of the resonators and the characteristic surface area (Ac) of h The construction element, to be provided between approximately 0.1 and 0.9.

1028909 8

In a particularly advantageous embodiment, a panel construction is provided, wherein a resonator is formed by a tube which opens into one or more further tube parts arranged next to the tube. This embodiment 5 is preferably formed by a tube which is placed in an enclosed (larger space. An advantage of these embodiments is that the total length of the resonators is smaller so that a panel construction with reduced thickness (d) can be produced.

The invention also relates to a vehicle, in particular an aircraft, provided with the panel construction according to the invention defined herein.

Further advantages, features and details of the present invention will be elucidated with reference to the following description of some preferred embodiments thereof. Reference is made in the description to the accompanying figures, in which:

Figure 1 is a perspective view of a first preferred embodiment of a noise-reducing panel according to the invention;

Figure 2 is a top cross-sectional view (surface Ar) of the resonator and its characteristic region Ac;

Figure 3 shows a schematic representation of the incident, reflected and transmitted sound;

Figure 4 shows a graph of the transmission loss in the preferred embodiment of Figure 1 for a number of different values of the porosity of the panel, with L = 10.9 cm and a mass per unit area of 0.015 kg / m2;

Figure 5 shows a perspective view of a second preferred embodiment of a noise-reducing panel according to the invention; 1 0 2 8 9 0 9 9! Figure 6 is a graph of transmission loss as a function of frequency for a second preferred embodiment of the invention; and Figure 7 shows a schematic cross-section of a further preferred embodiment of the invention.

Figure 1 shows a first preferred embodiment of the noise-reducing panel 1, consisting of an upper plate 2, for example made of aluminum or another suitable material, in which a large number of openings 4 are arranged.

Connect cylindrical resonators 3 with an acoustically hard bottom and preferably with an acoustically hard wall to the openings 4. The resonators are in open communication with the environment via the said openings 4. In the embodiment shown in Figure 1, all resonators 15 have approximately the same length L. In the examples described herein, the resonators have a length of 0.09 m.

Other lengths of the resonators are of course also possible. The choice of the length (s) of the resonators is mainly determined by the center frequency (fc) around which the noise reduction must be realized.

The distribution of the resonators 3 over the plate 2 is shown in more detail in the enlargement of Figure 2. The surface of the resonators 3 in cross-section is indicated in the figure by the designation Ar. Around each of the resonators an area can be defined, which is designated by the term characteristic area Ac. In the embodiment shown in figures 1 and 2, the upper plate 2 of the panel 1 is provided with a large number of resonators, of which only sixteen resonators 3 are shown in figure 2. The resonators are arranged equally over the surface of the plate 2.

In other embodiments (not shown), the resonators are arranged irregularly over the surface of the plate 2 1 0 28 9 0 9 10. In a particularly advantageous embodiment, the positions and / or the lengths of the different resonators are adapted to the dynamic properties of the plate.

Some parts of the structure (plate) can vibrate in different ways at 5 different frequencies.

At those frequencies at which a part of the structure vibrates violently, resonators can be used on site that are designed for such frequencies. Other parts of the construction (plate) can then be provided with resonators that are tuned to other frequencies at which these other parts vibrate violently. Thus, depending on the properties of the different structural parts, that is, depending on how the different structural parts vibrate per frequency range, the design of the resonators and the positioning thereof are adjusted in order to provide an optimum noise reduction of the sound radiated from the entire construction.

The quotient of the cross section of a resonator and the characteristic surface is defined as the porosity Ω (= Ar / Ac).

By properly matching the length L of the resonators, and the cross section of each of the resonators Ar in relation to the amount of resonators distributed over the surface of the panel, an unexpectedly large noise reduction of the radiated sound power over a relatively wide frequency range can be achieved. be achieved.

!

It has been found that if the length L of the resonator 3 is equal to the c / 2f with f the selected frequency around which the! noise reduction is desired, a substantial reduction in the frequency ranges from approximately (k + 1/4) (c0 / L) to (k + 3/4) (c0 / L) by c0 the sound speed in the relevant medium, for example air or water, and k can be an integer greater than or equal to zero. The 1 0 28 90 9 11 noise reduction takes place around the central frequency Fc, which is defined as c0 / 2L.

Figure 3 shows the incident sound wave (Βχ), the sound field reflected from the panel 2 and the transmitted sound wave (B4) .The transmission loss is defined as 10 * log (| B1 / B4 | 2), where B4 is the transmitted sound wave and Bj is the incident sound wave.

Figure 4 shows the transmission loss calculated with the aid of a one-dimensional model as a function of the frequency of the panel according to the invention. Curve 1 gives the transmission loss as a function of the frequency when the porosity is 0, that is, when no resonator is arranged in the vibrating surface 2 and the structural element behaves as an isotropic panel. The transmission loss in this case is the transmission loss of a standard single plate, which transmission loss in the displayed frequency range is determined by the so-called mass law. This means that the noise reduction and hence the transmission loss in the panel 20 increases with a doubling of the frequency by approximately 6 dB.

In the case shown, the center frequency fc is chosen to be approximately 1573 Hz, which amounts to a resonator length L of 0.1 m. When resonators 3 of such a length are arranged in the vibrating plate 2, an increase in the transmission loss occurs and therefore an increase in noise reduction.

For values of the porosity Ω to approximately 0.1, the reduction is obtained near both frequencies C0 / 4L 3C0 / 4L, i.e. at approximately 786 Hz and 2360 30 Hz, respectively. For values of porosity above 0.1, a reduction is obtained over the entire frequency range, i.e. from about CO / 4 L to 3 CO / 4 L around the center frequency of fc = 1573 Hz, which corresponds to a 1 0 2 8 9 0 9 12 resonator length of L = 0.1 m. Curves 2-6 of Figure 4 represent the transmission losses at a porosity of 0.30, 0.40, 0.45, 0.50 and 0.65, respectively. It can be seen from the figure that relatively large transmission losses are obtained with porosity values between 0.3 and 0.6, while maximum reduction is obtained for a porosity of approximately Ω = 0.45. The method loses part of its efficiency for porosity values above 0.9. Maximum reduction occurs when the porosity is such that the area under the curve is minimal.

For example, curve 6 shows the transmission loss with a porosity of Ω = 0.45. It is clearly visible from the figure that the transmission loss is greater than would be expected as a result of the mass law from a minimum frequency of approximately 800 Hz to a maximum frequency of approximately 2100 Hz. In particular in the range between 1700 and 1900 Hz, additional transmission loss of approximately 50 dB (80-30) can be realized.

Figure 4 shows that an additional transmission loss can also be realized at the higher harmonics, i.e. at 4720 Hz, for example.

It has been found that the sound reduction to be achieved hardly depends on the cross section Ar of the resonator and the shape of the resonator in cross section as long as the porosity of the panel is selected in the above-mentioned correct range. Although a circular cross-section is shown in the drawings, the resonators can also have any other shapes, such as elliptical, triangular, rectangular or square, without appreciably changing the sound-reducing effect of the resonators.

The material of the vibrating plate 2 and of the inside of the resonators 3 is preferably acoustically hard 1028909 13, i.e. with an absorption coefficient of less than 0.2. As long as the absorption coefficient is sufficiently small, any material can in principle be used, so that the panel can also function properly under extreme conditions, such as under high temperatures, high humidity and / or corrosive environments.

Figure 5 shows a second preferred embodiment of the invention. In this embodiment the panel is constructed from a honeycomb structure 6 which is provided with skin plates 7 'on two sides. A large number of perforations is provided in one of the skin plates 7, 7 'in the aforementioned manner. The honeycomb structure behind each of the holes now functions as a resonator. This is a very easy to manufacture panel that can be used in a large number of applications. Figure 6 shows the transmission losses in such a honeycomb panel as a function of the frequency. The figure shows that even in such a honeycomb structure with a porosity of 0.45 relatively high transmission losses (with respect to a panel without resonators, as represented by curve 1) and therefore a relatively high noise reduction can be achieved.

Figure 7 shows an alternative embodiment of the present invention. Instead of prismatic tubes, resonators are folded over in this embodiment.

The folded-over resonator 10 comprises a first tubular part 11 arranged in the plates 2,2 ', to which tubular parts 12 and 13 connect on either side. The tube parts 12 and 13 provided on either side are sealed at approximately two-thirds of the thickness (d) of the total panel with the aid of an intermediate wall, respectively 14 and 15. The tube parts 12 and 13 may or may not be filled with absorption material. In the embodiment shown, the resonator mouth 17 is also closed with the aid of a thin foil 16. This is designed in such a way that the opening 17 is covered, but the operation of the resonator in relevant frequency range is not or hardly affected. The advantage of the embodiment shown is that the total thickness of the panel defined between the front plate 2 and the rear plate 2 'can be limited. If, for example, a tube length of approximately 10 cm is required, a thickness D of the panel of approximately 8 cm will suffice. In a particularly advantageous embodiment 10, the configuration shown in Figure 7 is achieved by placing a tube in a space enclosed by walls, with some space between the end of the tube directed towards plate 2 'and the inside of the plate 2'. remains. The tube parts 12, 13 are then formed by the space around the tube 11. i

The present invention is not limited to the preferred embodiments thereof described herein. The rights sought are defined by the following claims, within the scope of which many modifications are conceivable.

1028909

Claims (37)

Method for broadband reducing the sound radiated from a vibrating structural element, comprising: - determining a center frequency (fc) around which frequency the sound level is broadband between a minimum frequency (fmin) and a maximum frequency (fmax) can be reduced; 10. arranging a number of resonators in a construction element provided with an acoustically hard outer surface, wherein the resonators open into said acoustically hard outer surface, wherein the length (L) of the resonators is approximately equal to the sound speed (c0) divided by twice the determined center frequency (fc) and where the resonators are arranged around a porosity (Ω) of the structural element, which is defined as the cross-section quotient (Ar) of the mouth of the resonators and the characteristic surface ( Ac) of the structural element, to be provided between approximately 0.1 and 0.9. The method according to claim 1, wherein the porosity (Ω) is between approximately 0.3 and 0.6. Method according to claim 1 or 2, wherein the porosity (Ω) is approximately 0.44. Method according to any of the preceding claims, comprising of providing tubular resonators in the structural element. 5. Method as claimed in any of the foregoing claims, comprising of providing resonators whose inner surface is acoustically hard. Method according to one of the preceding claims, wherein acoustically absorbent material is provided in at least one of the resonators. 1028909 Method according to one of the preceding claims, wherein the resonator openings are closed with a thin film. A method according to any one of the preceding claims, wherein the shredding of an acoustically hard surface comprises providing a surface with an absorption coefficient (a) of less than 0.2. 9. Method as claimed in any of the foregoing claims, comprising of providing resonators of different length for reducing the radiated sound level in respective wide frequency bands around the respective center frequencies (fc). 10. Method as claimed in any of the foregoing claims, comprising of providing resonators with a constant cross-section. A method according to any one of the preceding claims, comprising providing resonators with a sealed bottom. 12. Method as claimed in any of the foregoing claims, wherein the cross sections of different resonators have a different size. 13. Method as claimed in any of the foregoing claims, comprising of providing a construction element comprising a honeycomb structure on which an acoustically hard skin covering provided with perforations is provided. The method of claim 13, wherein the honeycomb structure and / or the skin plating are made of aluminum. Method according to one of the preceding claims, comprising reducing the radiated sound power when the structural element is acoustically excited. 1028909 A method according to any of claims 1-15, comprising reducing the radiated sound power when the structural element is mechanically excited. 17. Method according to one of the preceding claims, wherein the lengths (L) of the resonators vary between (k + 1/4) (c0 / fc) and (k + 3/4) (c0 / fc) with c0 the sound speed and k an integer greater than or equal to zero. 18. Method as claimed in any of the foregoing claims, comprising of providing a construction element with resonators wherein the positions and / or the lengths of the different resonators are adapted to the dynamic properties of the construction element. 19. Sound-reducing panel construction, comprising a panel provided with an acoustically hard outer surface and sound-radiation reduction means for broadband around a selectable center frequency (fc) reducing the sound radiated from the panel in vibrating condition, wherein the sound-radiation reduction means comprises a number arranged in the panel and comprising resonators terminating in said hard outer surface of the panel, wherein the resonators have a length (L) of approximately the sound speed (c0) divided by twice the determined center frequency (fc), and wherein the number of resonators and the dimensions of the resonators are arranged around a porosity (O) of the structural element, which is defined as the cross-section quotient (Ar) of the mouth of the resonators and the characteristic surface (Ac) of the structural element, between approximately 0, 1 and 0.9. The panel structure of claim 19, wherein the porosity (Ω) is between approximately 0.3 and 0.6. The panel structure according to claim 19 or 20, wherein the porosity (Ω) is approximately 0.44. 1028 90 9 The panel structure of any one of claims 19 to 21, wherein the resonators are substantially tubular. 23. Panel construction as claimed in any of the claims 19-22, wherein the resonators have an acoustically hard inner surface. The panel structure of any one of claims 19 to 23, wherein an acoustically hard surface has an absorption coefficient (a) of less than 0.2. The panel structure of any one of claims 19 to 24, wherein resonators have different lengths for reducing the radiated sound level in a wide band around different center frequencies (fc). 26. Panel construction as claimed in any of the claims 19-25, wherein the resonators have a constant cross-section. 27. Panel construction according to one of the claims! 19-26, wherein the resonators have a sealed bottom. 28. Panel construction as claimed in any of the claims 19-27, wherein the cross sections of different resonators have a different size. 29. Panel construction as claimed in any of the claims 19-28, wherein the panel comprises a honeycomb structure on which a perforated acoustically hard skin plating 25 is provided. The panel structure of claim 29, wherein the honeycomb structure and / or the skin plating are made of aluminum. 31. Panel construction as claimed in any of the claims 19-30, wherein acoustic-absorbing material is provided in the resonator. 1 02 6 9 0 9 The panel structure of any one of claims 19 to 31, wherein the resonator opening is sealed with a thin film. 33. Panel construction as claimed in any of the claims 19-32, wherein the lengths (L) of the resonators vary between (k + 1/4) (c0 / fc) and (k + 3/4) (co / f0) with cD the. sound speed and k an integer greater than or equal to zero. 34. Panel construction as claimed in any of the claims 19-33, wherein the positions and / or the lengths of the different resonators are adapted to the dynamic properties of the structural element. 35. Panel construction as claimed in any of the claims 19-34, wherein a resonator is formed by a tube which opens into one or more further tube parts arranged next to the tube. 36. Vehicle provided with a panel construction according to one of the preceding claims. A method according to any of claims 1-18, wherein the structural element comprises a panel construction according to any of claims 19-35. 102890 9