EP1875461A1

EP1875461A1 - Broadband sound reduction with acoustic resonator

Info

Publication number: EP1875461A1
Application number: EP06733033A
Authority: EP
Inventors: Hans Wijnant Ysbrand; Marieke Henriëtte Cathrien HANNINK; Jacob Piet P/a Universiteit Twente VLASMA
Original assignee: Twente Universiteit
Current assignee: Twente Universiteit
Priority date: 2005-04-29
Filing date: 2006-04-28
Publication date: 2008-01-09
Also published as: NL1028909C2; WO2006118443A1

Abstract

The present invention relates to a method for broadband reduction of the sound radiated from a vibrating structural element, comprising of determining a centre frequency, around which frequency the sound level can undergo broadband reduction, and arranging a number of resonators in a structural element provided with an acoustically hard outer surface, wherein the resonators debouch in said acoustically hard outer surface, wherein the length (L) of the resonators is roughly equal to co/2fc, with co being the sound velocity, and wherein the resonators are embodied to provide a porosity (Ω) of the structural element of between about 0.1 and 0.9.

Description

BROADBAND SOUND REDUCTION WITH ACOUSTIC RESONATOR

The present invention relates to a method for broadband reduction of the sound radiated from a vibrating structural element. The invention also relates to a sound-reducing panel construction for broadband reduction of the radiated sound. Structure-borne sound results because vibrating constructions set into vibration the air in the vicinity of this construction, these vibrations being propagated through the air and perceived as noise. Many techniques are known for bringing about a reduction in the vibrations of the air in the vicinity of the vibrating construction, and therefore for reducing the radiated sound.

Two important fields of application can be distinguished. The first field of application is the broadband reduction of the sound radiated by a vibrating construction which vibrates as a result of a mechanical excitation, for instance a motor which is connected mechanically to a housing. The second field of application is the reduction of sound radiated by a vibrating construction which vibrates as a result of an acoustic excitation. Constructions must be envisaged here which are placed between the acoustic source and the listener, and thus have a sound- screening effect, for instance a wall between two rooms, an aircraft cabin which reduces the aerodynamic sound, a partition wall in a car between the engine and passenger space. A number of types of construction are known with which the sound occurring in an enclosed space such as a cabin and caused by airborne sound or impact sound can be reduced. Most such constructions operate in accordance with the principle of sound absorption or at least of reducing the reflection of sound against the walls of the enclosed space. The structural elements of a cabin can for instance be given a sound- absorbing form, for instance by being provided with Helmholtz resonators or quarter-wavelength resonators. A drawback of both types of resonator is that sound absorption can only be realized within a limited frequency band.

The American patent US 5 959 265 for instance describes a sound absorption-based system. The system can absorb sound in a structural element such as a panel. In the patent the panel is provided with a number of resonators with a length of a quarter wavelength of the sound to be reduced. Standing waves which are phase-shifted by a half wavelength relative to the wave front reflected in the mouth region of the resonators interfere destructively with this wave front, and this results in a sound reduction. These known quarter- wavelength resonators however 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.

Kitts, Z. T., 2000, "An analytical study of the weak radiating cells as a passive low frequency noise control device", graduate thesis, State University, Blacksburg, Virginia, U.S.A., describes another principle for passive sound reduction. Instead of absorbing sound, the strength of the sound source is minimized by deforming the transmitted sound field by means of a system of weak radiating cells provided in a panel. The source strength of the surface of the panel is minimized by means of two mechanically coupled surfaces which, when they are placed on a vibrating substrate, are almost out of phase and have an equal strength over a wide frequency range. The untreated substrate is divided into characteristic regions and provided with weak radiating cells. The outer solid element is directly connected to the vibrating structure and is referred to as the frame of the cell. The frame of the cell is rigid so that the speed of the frame of the cell is roughly 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 the cell with a determined compliance, whereby a mass spring system is created. The source strength can hereby be reduced so that a certain degree of sound reduction can be effected. A drawback of the system of weak radiating cells is that the system increases the source strength by a factor 100 at a single frequency, since the cell resonates in phase with the frame of the cell at this frequency. This means that at this frequency an enormous amplification of the sound is produced instead of a reduction in the sound. Further drawbacks are that a relatively large number of mechanical components are necessary 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 structural element. It is also an object of the present invention to provide a sound-reducing panel construction in which the sound generated by the panel can undergo broadband reduction.

According to a first aspect of the invention a method is hereby provided for broadband reduction of the sound radiated by a vibrating structural element, comprising of:

- determining a centre frequency (f_c) , around which frequency the sound level can undergo broadband reduction;

- arranging a number of resonators in a structural element provided with an acoustically hard outer surface, wherein the resonators debouch in said acoustically hard outer surface, wherein the length (L) of the resonators is roughly equal to the sound velocity (C₀) divided by twice the determined centre frequency (f_c) , and wherein the resonators are embodied to provide a porosity (Ω) of the structural element, which is defined as the quotient of the cross- section (A_r) of the mouth of the resonators and the characteristic area (A_c) of the structural element, of between about 0.1 and 0.9.

The lengths of the resonators are roughly equal to c_o/2f_c, which signifies here that the lengths (L) of the resonators have values which can vary between c_o/4f_c and 3c_o/4f_c) with c_o the sound velocity. At a length of exactly c_o/2f_c an optimal reduction is however obtained in most cases. Such resonators will also be referred to hereinbelow as λ/2- wavelength resonators.

When the porosity (Ω) now amounts to between about 0.3 and 0.6, a particularly good result can be obtained. Calculations on the basis of a one-dimensional model have shown that, if the length of the resonators is tuned to a centre frequency of about 1000 Hz (i.e. L=17 cm), minimal and average sound reductions equal 30 dB and 38 dB respectively over a bandwidth of 250 Hz, equal 16 dB and 30 dB respectively for a bandwidth of 500 Hz and equal 6dB and 23 dB respectively for a bandwidth of 750 Hz. These values are reached at varying porosities.

In a determined preferred embodiment the resonators take a tubular form, which produces a simple construction. In a more preferred embodiment the resonators are prismatic tubes, i.e. the tubes have a practically constant cross-sectional surface along the respective lengths of the tubes. These latter tubes are relatively simple to manufacture.

It is otherwise noted that the operation of the present invention is practically independent of the cross-sectional shape (round, square, rectangular, polygonal or any other random shape) of the resonators. A slight curving of the resonators is also allowed. The curving does not reduce the effect of the resonators, or hardly so. In a further preferred embodiment the resonators have an acoustically hard inner surface. Acoustically hard in the sense of the present invention is understood to mean a surface having an absorption coefficient (GL) of less than 0.2. In other embodiments said inner surface can take an acoustically absorbing form.

In a further preferred embodiment acoustically absorbing material, such as for instance mineral wool or rock wool, is arranged in the resonator. The resonator opening can also be sealed with a thin foil. In both embodiments undesired particles such as dust and the like are hereby prevented from being able to enter the resonators.

Although the method according to the invention enables a sound reduction in a wide frequency band, the method can also comprise of providing resonators of different length for the purpose of reducing the radiated sound level in respective wide frequency bands around respective centre frequencies

(f_c) •

The resonators preferably have a constant cross-section so that they can be easily realized. The cross-sections of the resonators can however also differ from each other.

In a particularly advantageous embodiment the method comprises of providing a structural element comprising a honeycomb structure on which is provided an acoustically hard skin plate provided with perforations. In this manner a structurally strong element can be provided easily and quickly. If the honeycomb structure and/or the skin plate are then also manufactured from aluminium, this has the advantage that the panel is resistant to corrosive environments, high temperatures and moisture. The core of the honeycomb structure can also be manufactured from fibre-reinforced plastic, such as glass fibre or carbon fibre composites. In both cases the panels can hereby be made resistant to a moist and/or corrosive environment.

The method described herein not only comprises of reducing the radiated acoustic power when the structural element is acoustically excited, for instance when the structural element is used as partition wall and is irradiated with an airborne sound source. The method can also comprise of reducing the radiated acoustic power when the structural element is mechanically excited, for instance when the structural element is set into vibration by a mechanical vibration source. The fields of application are therefore very extensive. The structural elements can for instance be applied in the motor industry (panel between engine and cab, doors and roofs) , the aircraft industry (trim panels, cabin ceiling, cargo space ceiling) , domestic appliances (housing) , construction (lightweight silent walls, cleanrooms, dead rooms) , medical applications (sound reduction of MRI scanners), traffic (sound screens), as well as in industry and machine construction (housing or sound screens, turbines) .

According to another aspect of the present invention, there is provided a sound-reducing panel construction, wherein the panel construction comprises a panel provided with an acoustically hard outer surface in addition to sound radiation-reducing means for broadband reduction around a centre frequency (f_c) to be selected of the sound radiated from the panel in vibrating state, wherein the sound radiation-reducing means comprise a number of resonators arranged in the panel and debouching in said hard outer surface of the panel, wherein the resonators have a length (L) of about the sound velocity (c₀) divided by twice the determined centre frequency (f_c) , and wherein the number of resonators and the dimensions of the resonators are embodied so as to provide a porosity (Ω) of the structural element, which is defined as the quotient of the cross-section (A_r) of the mouth of the resonators and the characteristic area (A_c) of the structural element, of between about 0.1 and 0.9.

In a particularly advantageous embodiment a panel construction is provided, wherein a resonator is formed by a tube which debouches into one or more further tube parts arranged adjacently of the tube. This embodiment is preferably formed by a tube which is placed in a larger enclosed space. An advantage of this embodiment is that the total length of the resonators is smaller, so that a panel construction of reduced thickness (d) can be realized.

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 on the basis of the following description of several preferred embodiments thereof. Reference is made in the description to the accompanying figures, in which:

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

Figure 2 shows a top view of the cross-section (area A_r) of the resonator and the characteristic region A_c thereof; Figure 3 is 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=IO.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 sound-reducing panel according to the invention;

Figure 6 shows a graph of the transmission loss as a function of the 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 sound- reducing panel 1, consisting of a top plate 2, for instance manufactured from aluminium or other suitable material, in which a large number of openings 4 are arranged. Cylindrical resonators 3, provided with an acoustically hard base and preferably with an acoustically hard wall, connect to openings 4. The resonators are in open connection with the vicinity via said openings 4. In the embodiment shown in figure 1 all resonators have roughly 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 length (s) of the resonators is determined mainly by the centre frequency (f_c) around which the sound reduction must be realized. The distribution of resonators 3 over plate 2 is shown in more detail in the enlargement of figure 2. The area of resonators 3 in cross-section is indicated in the figure with the designation A_r. Around each of the resonators can be defined a region which is designated with the term characteristic region A_c. In the embodiment shown in figures 1 and 2 the upper plate 2 of 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 uniformly over the surface of plate 2.

In other embodiments (not shown) the resonators are arranged in irregular manner over the surface of plate 2. 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 construction (plate) can vibrate in different ways at different frequencies. At those frequencies where a part of the construction vibrates violently, resonators can be applied at that position which are designed for such frequencies. Other parts of the construction (plate) can then be provided with resonators which are tuned to other frequencies at which these other parts vibrate violently. The embodiment of the resonators and the positioning thereof is thus adapted subject to the properties of the different structural parts, i.e. depending on how the different structural parts vibrate per frequency range, in order to produce an optimal sound reduction of the sound radiated by the entire construction.

The quotient of the cross-section of a resonator and the characteristic area is defined as the porosity Ω (=A_r/A_c) .

With a correct adjustment of the length L of the resonators and the cross-section of each of the resonators A_r in relation to the quantity of resonators distributed over the surface of the panel, an unexpectedly great sound reduction of the radiated acoustic power can be obtained over a relatively wide frequency range. It has been found that, if the length L of resonator 3 is equal to the c/2f, with f as the selected frequency around which the sound reduction is desired, a considerable reduction can be brought about in the frequency ranges from about (k+1/4) (c_o/L) to (k+3/4) (c_o/L) , with c_o being the sound velocity in the relevant medium, for instance air or water, and k a whole number greater than or equal to zero. The sound reduction takes place around the centre frequency F_c which is defined as c_o/2L.

Figure 3 shows the incident sound wave (B₁) , the sound field (A₁) reflected from panel 2, and the transmitted sound wave (B₄). The transmission loss is defined as 10*log ( JB₁ZB₄I²) , wherein B₄ is the transmitted sound wave and B₁ the incident sound wave.

Figure 4 shows the transmission loss calculated using a one-dimensional model as a function of the frequency of the panel according to the invention. Curve 1 shows the transmission loss as a function of the frequency when the porosity equals 0, i.e. when not a single resonator is arranged in the vibrating surface 2 and the structural element behaves as an isotropic panel. In this case the transmission loss is the transmission loss of a standard single plate, this transmission loss being determined in the shown frequency- range by the so-called mass law. This means that the sound reduction, and thereby the transmission loss in the panel increases by about 6 dB when the frequency is doubled.

In the shown case the centre frequency f_c is chosen to equal about 1573 Hz, which amounts to a resonator length L of 0.1 m. when resonators 3 of such a length are arranged in vibrating plate 2, there occurs an increase in the transmission loss, and therefore an increase in the sound reduction. For values of the porosity Ω up to about 0.1 the reduction is obtained close to both frequencies C₀/4L, 3C₀/4L, i.e. at about 786 Hz and 2360 Hz respectively. For values of the porosity above 0.1 a reduction is obtained over the whole frequency range, i.e. from about C₀/4L to 3C₀/4L around the centre frequency of f_c=1573 Hz, this corresponding to a resonator length of L=O.1 m. Curves 2-6 in figure 4 show the respective transmission losses at a porosity of 0.30, 0.40, 0.45, 0.50 and 0.65. It can be inferred from the figure that relatively great transmission losses are obtained at porosity- values between 0.3 and 0.6, while maximum reduction is obtained at a porosity of about Ω = 0.45. The method loses some of its efficiency at porosity values above 0.9. Maximum reduction occurs when the porosity is such that the area below the curve is minimal.

Curve 6 for instance shows the transmission loss at a porosity of Ω = 0.45. The figure shows clearly that the transmission loss is greater than would be expected as a result of the mass law from a minimum frequency of about 800 Hz up to a maximum frequency of about 2100 Hz. Particularly in the range between 1700 and 1900 Hz an extra transmission loss of about 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 for instance 4720 Hz.

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

The material of vibrating plate 2 and of the interior of resonators 3 preferably takes an acoustically hard form, i.e. with an absorption coefficient of less than 0.2. As long as the absorption coefficient is sufficiently low, any random material can in principle be used so that the panel can also function properly in extreme conditions, such as at high temperatures, high humidities and/or in 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 on two sides with skin plates 7 ' . A large number of perforations is arranged in one of the skin plates 7, 7' in the above stated manner. The honeycomb structure behind each of the holes now functions as a resonator. This is a panel which is very easy to manufacture and which can be used in a large number of applications. In figure 6 the transmission losses in such a honeycomb panel are shown as a function of the frequency. The figure shows that relatively high transmission losses (compared to a panel without resonators, as represented by curve 1) and therefore a relatively large sound reduction can also be realized in such a honeycomb structure at for instance a porosity of 0.45. Figure 7 shows an alternative embodiment of the present invention. Instead of being prismatic tubes, resonators are as it were folded back in this embodiment. The folded resonator 10 comprises a first tubular part 11 which is arranged in plates 2,2', and to either side of which connect tubular parts 12 and 13. The tube parts 12 and 13 provided on either side are closed off at about two-thirds of the thickness (d) of the whole panel using a partition wall 14 and 15 respectively. Tube parts 12 and 13 can optionally be filled with absorption material. In the shown embodiment the resonator mouth 17 is also sealed using a thin foil 16. This is embodied such that opening 17 is covered but the operation of the resonator in the relevant frequency range is not affected, or hardly so. The advantage of the shown embodiment is that the total thickness of the panel defined between front plate 2 and rear plate 2' can be limited. When a tube length of about 10 cm is for instance necessary, it is possible to suffice with a thickness D of the panel of about 8 cm. In a particularly advantageous embodiment the configuration shown in figure 7 is realized by placing a tube in a space enclosed by walls, wherein some intermediate space remains between the outer end of the tube directed toward plate 2 ' and the inner side of plate 2'. Tube parts 12,13 are then formed by the space around tube 11.

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

Claims

1. Method for broadband reduction of the sound radiated from a vibrating structural element, comprising of: - determining a centre frequency (f_c) , around which frequency the sound level can undergo broadband reduction between a minimal frequency (f_min) and a maximal frequency

* ^-max ' '

- arranging a number of resonators in a structural element provided with an acoustically hard outer surface, wherein the resonators debouch in said acoustically hard outer surface, wherein the length (L) of the resonators is roughly equal to the sound velocity (c₀) divided by twice the determined centre frequency (f_c) , and wherein the resonators are embodied to provide a porosity (Ω) of the structural element, which is defined as the quotient of the cross-section (A_r) of the mouth of the resonators and the characteristic area (A_c) of the structural element, of between about 0.1 and 0.9.

2. Method as claimed in claim 1, wherein the porosity (Ω) amounts to between about 0.3 and 0.6.

3. Method as claimed in claim 1 or 2, wherein the porosity (Ω) amounts to about 0.44.

4. Method as claimed in any of the foregoing claims, comprising of providing tubular resonators in the structural element.

5. Method as claimed in any of the foregoing claims, comprising of providing resonators, the inner surface of which is acoustically hard.

6. Method as claimed in any of the foregoing claims, wherein acoustically absorbing material is arranged in at least one of the resonators.

7. Method as claimed in any of the foregoing claims, wherein the resonator openings are sealed with a thin foil.

8. Method as claimed in any of the foregoing claims, wherein the provision of an acoustically hard surface comprises of providing a surface with an absorption coefficient (α) of less than 0.2.

9. Method as claimed in any of the foregoing claims, comprising of providing resonators of different length for the purpose of reducing the radiated sound level in respective wide frequency bands around the respective centre frequencies (f_c).

10. Method as claimed in any of the foregoing claims, comprising of providing resonators of a constant cross- section.

11. Method as claimed in any of the foregoing claims, comprising of providing resonators with a closed 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 structural element comprising a honeycomb structure on which is provided an acoustically hard skin plate provided with perforations.

14. Method as claimed in claimed 13, wherein the honeycomb structure and/or the skin plate are manufactured from aluminium.

15. Method as claimed in any of the foregoing claims, comprising of reducing the radiated acoustic power when the structural element is acoustically excited.

16. Method as claimed in any of the claims 1-15, comprising of reducing the radiated acoustic power when the structural element is mechanically excited.

17. Method as claimed in any of the foregoing claims, wherein the lengths (L) of the resonators vary between (k+1/4) (c_o/f_c) and (k+3/4) (c_o/f_c) with c_o being the sound velocity and k a whole number greater than or equal to zero.

18. Method as claimed in any of the foregoing claims, comprising of providing a structural element with resonators wherein the positions and/or the lengths of the different resonators are adapted to the dynamic properties of the structural element.

19. Sound-reducing panel construction, comprising a panel provided with an acoustically hard outer surface in addition to sound radiation-reducing means for broadband reduction around a centre frequency (f_c) to be selected of the sound radiated from the panel in vibrating state, wherein the sound radiation-reducing means comprise a number of resonators arranged in the panel and debouching in said hard outer surface of the panel, wherein the resonators have a length (L) of about the sound velocity (c₀) divided by twice the determined centre frequency (f_c) , and wherein the number of resonators and the dimensions of the resonators are embodied so as to provide a porosity (Ω) of the structural element, which is defined as the quotient of the cross-section (A_r) of the mouth of the resonators and the characteristic area (A_c) of the structural element, of between about 0.1 and 0.9.

20. Panel construction as claimed in claim 19, wherein the porosity (Ω) amounts to between about 0.3 and 0.6.

21. Panel construction as claimed in claim 19 or 20, wherein the porosity (Ω) amounts to about 0.44.

22. Panel construction as claimed in any of the claims 19-21, wherein the resonators have a substantially tubular form.

23. Panel construction as claimed in any of the claims 19-22, wherein the resonators have an acoustically hard inner surface .

24. Panel construction as claimed in any of the claims 19-23, wherein an acoustically hard surface has an absorption coefficient (α) of less than 0.2.

25. Panel construction as claimed in any of the claims 19-24, wherein resonators have differing lengths for the purpose of reducing the radiated sound level in a wide band around different centre frequencies (f_c) .

26. Panel construction as claimed in any of the claims 19-25, wherein the resonators have a constant cross-section.

27. Panel construction as claimed in any of the claims 19-26, wherein the resonators have a closed 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 is provided an acoustically hard skin plate provided with perforations.

30. Panel construction as claimed in .claim 29, wherein the honeycomb structure and/or the skin plate are manufactured from aluminium.

31. Panel construction as claimed in any of the claims 19-30, wherein acoustically absorbing material is provided in the resonator.

32. Panel construction as claimed in any of the claims 19-31, wherein the resonator opening is sealed with a thin foil.

33. Panel construction as claimed in any of the claims 19-32, wherein the lengths (L) of the resonators vary between (k+1/4) (c_o/f_c) and (k+3/4) (c_o/f_c) with c_o being the sound velocity and k a whole number greater than or equal to zero.

34. Panel construction as claimed in any of the claims 19-33, wherein the positions and/or 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 debouches into one or more further tube parts arranged adjacently of the tube .

36. Vehicle provided with a panel construction as claimed in any of the foregoing claims.

37. Method as claimed in any of the claims 1-18, wherein the structural element comprises a panel construction as according to any of the claims 19-35.