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Title:
A SOUND DAMPING DEVICE
Document Type and Number:
WIPO Patent Application WO/2017/191293
Kind Code:
A1
Abstract:
A sound damping device comprises a first flow channel device (4) and a second flow channel device (6) separated from one another in the elongation of said first and second flow channel devices (4, 6) by at least one intermediate wall (8; 8a, 8b, 8c, 8d, 8e) as seen in the elongation of said first and second flow channel devices (4, 6), said intermediate wall being at least partly provided with a sound attenuating means (9), wherein said first flow channel device (4) has a first inlet (5a) and a first outlet (5b) for a first flow and said second flow channel device (6) has a second inlet (7a) and a second outlet (7b) for a second flow. According to the invention, at least said second flow channel device (6) is provided with an adjustable pressure influencing means (35) for creating a predetermined dynamic pressure difference over said sound attenuating means (9) with respect to the first flow channel device (4).

Inventors:
CORIN, Ralf (Vårgärdsvägen 79, Saltsjöbaden, S-133 36, SE)
Application Number:
EP2017/060725
Publication Date:
November 09, 2017
Filing Date:
May 04, 2017
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
SONTECH INTERNATIONAL AB (Energivägen 6, Kungsängen, S-196 37, SE)
International Classes:
G10K11/16; F01N1/06; F01N1/08; F01N1/10; F01N1/16; F01N1/24; F16L55/027; F16L55/033; F24F13/24; G10K11/175; G10K11/178
Domestic Patent References:
WO2006098694A12006-09-21
WO2006098694A12006-09-21
WO1997027370A11997-07-31
Foreign References:
US3113635A1963-12-10
US4665549A1987-05-12
US5692054A1997-11-25
US20150014091A12015-01-15
US3113635A1963-12-10
DE19730355C11999-03-18
US2043416A1936-06-09
US4665549A1987-05-12
US5692059A1997-11-25
Attorney, Agent or Firm:
IPRACRAFT AB (P.O. Box 24166, Drottning Kristinas väg 67, Stockholm, S-104 51, SE)
Download PDF:
Claims:
CLAIMS

1. A sound damping device comprising a first flow channel device (4) and a second flow

channel device (6) separated from one another in the elongation of said first and second flow channel devices (4, 6) by at least one intermediate wall (8; 8a, 8b, 8c, 8d, 8e) as seen in the elongation of said first and second flow channel devices (4, 6), said intermediate wall being at least partly provided with a sound attenuating means (9), wherein said first flow channel device (4) has a first inlet (5a) and a first outlet (5b) for a first flow and said second flow channel device (6) has a second inlet (7a) and a second outlet (7b) for a second flow, characterised in that at least said second flow channel device (6) is provided with an adjustable pressure influencing means (35) for creating a predetermined dynamic pressure difference over said sound attenuating means (9) with respect to the first flow channel device (4). 2. A sound damping device according to claim 1, wherein said adjustable pressure influencing means (35) is provided with means (26, 28, 42, 48) for reducing alternatively increasing the pressure of the flow path of said second flow channel device (6) in relation to the pressure of the flow path of said first flow channel device (4). 3. A sound damping device according to any one of claims 1 or 2, wherein said adjustable pressure influencing means (35) is provided with means (26, 28, 42) for adjusting the flow velocity of the flow path of said second flow channel device (6) in relation to the pressure of the flow path of said first channel device (4). 4. A sound damping device according to any one of claims 1 to 3, wherein said adjustable pressure influencing means (35) is provided with means (29, 31) for adjusting the length of the flow path of said second flow channel device (6) in relation to the length of the flow path of said first channel device (4). 5. A sound damping device according to any one of the preceding claims, wherein said

pressure influencing means is provided with a control means.

6. A sound damping device according to claim 5, wherein said control means comprises means (29, 31) for manual adjustment of said pressure influencing means.

7. A sound damping device according to claim 5, wherein said control means is provided with 5 a control device (36) associated with a sensor means (38, 52), for controlling the pressure of said first flow path or second path in response to said sensor means.

8. A sound damping device according to claim 7, wherein said control device comprises a computer (36).

10

9. A sound damping device according to claim 7 or 8, wherein at least one sensor member (32, 52) of said sensor means (38, 52) is arranged at or inside said first channel device (4).

10. A sound damping device according to any one of claims 7 to 9, wherein at least a sensor 15 member (32, 52) of said sensor means (38, 52) is arranged at or inside said second channel device (6).

11. A sound damping device according to any one of claims 7 to 10, wherein said pressure influencing means (35) comprises a fan device (39), said sensor means being a pressure or

20 flow sensor (52), the speed of the fan being controlled in response to said pressure or flow sensor (52).

12. A sound damping device according to claim 11, wherein said pressure influencing means (35) comprises a loudspeaker (48), said sensor means comprising a microphone (32), the

25 sound volume of said loudspeaker (48) being adjustable in response to said microphone.

13. A sound damping device according to any one of claims 7 - 12, wherein said pressure

influencing means comprises any one of an adjustable iris (28) and at least one adjustable baffle (26).

30 14. A sound damping device according to any one of the preceding claims, wherein said sound attenuating means (9) comprises a sound absorbing element (16).

15. A sound damping device according to claim 14, wherein said sound absorbing element (16) comprises a porous material.

16. A sound damping device according to claim 15 wherein said porous sound absorbing 5 element (16) comprises a micro-perforated sheet or plate (17)

17. A sound damping device according to claim 16, wherein said micro-perforated element is provided with micro-slits (18).

10 18. A sound damping device according to claim 16 or 17, wherein the thickness of the sheet or plate (17) is in the range 10"9 m - 2 mm, more particular 10"8 m - 1 mm, even more particular 10"7 m - 0,9 mm.

19. A sound damping device according to claim 15, wherein said porous sound absorbing 15 element comprises a piece of sintered metal or sintered cemented carbide.

20. A sound damping device according to any one of the claims 15 - 19, wherein the air flow resistance of the sound attenuating means (9) is in the range 100-10 000 RaylsMKs, more particular in the range 200 - 1000 RaylsMKs, even more particular 300 - 500 RaylsMKs-

20

21. A sound damping device according to claim 14, wherein said sound absorbing element (16) comprises a membrane damping element in the form of a non-perforated sheet (17).

22. A sound damping device according to claim 21, wherein the thickness of the membrane 25 damping element is in the range 10"9 m - 1 mm, more preferably 10~8 m - 0,7 mm, even more preferably 10"7 m - 0,5 mm.

Description:
A Sound Damping Device

BACKGROUND OF THE INVENTION

The present invention relates to a sound damping device comprising a first flow channel device and a second flow channel device separated from one another in the elongation of said first and second flow channel devices by at least one intermediate wall as seen in the elongation of said first and second flow devices, said intermediate wall being at least partly provided with a sound attenuating means, wherein said first flow channel device has a first inlet and a first outlet for a first flow and said second flow channel device has a second inlet and a second outlet for a second flow, Such a sound damping device is known from WO 2006/098694. Another such sound damping device is known from US 3 113 635.

Different kinds of sound attenuating means in the form of micro-perforations are disclosed in, DE 197 30 3555 and WO 97/27370. More particularly, DE 197 30 3555 discloses sound attenuating means provided with micro-perforations in the form of circular holes, while WO 97/27370 discloses sound attenuating means provided with micro-perforations in the form of micro-slits.

US 2 043 416 and US 4 665 549 disclose a sound damping device for flow channels, including microphones and cancelling loudspeakers creating a cancelling noise in the duct.

In US 5 692 059 is described a two fan system provide with two fans with separate motors and a double ventilation duct, provided with a fan noise sensor arranged at each fan and a microphone placed at a distance from the air outlet of the double ventilation duct. The speed of motors of the fans is individually adjusted based on input from the microphone until the noise is cancelled.

OBJECT OF THE INVENTION

The object of the invention is to provide an improved silencing of ducts. SUMMARY OF THE INVENTION

This object has been achieved by the sound damping device as initially defined, wherein at least said second flow channel device is provided with an adjustable pressure influencing means for creating a predetermined dynamic pressure difference over said sound attenuating means with respect to the first flow channel device.

Hereby is achieved an improved sound attenuation in ducts with hot or cold flowing gaseous fluids, i.e. gases or liquids or a mixture thereof, such as ventilation ducts or ducts for exhaust gases, or ducts for transportation of solid materials, such as vacuum or pressure ducts for transportation of grains or waste disposal bags.

Suitably, said adjustable pressure influencing means is provided with means for reducing alternatively increasing the pressure of the flow path of said second flow channel device in relation to the pressure of the flow path of said first flow channel device.

Alternatively, or in addition, said adjustable pressure influencing means is provided with means for adjusting the flow velocity of the flow path of said second flow channel device in relation to the pressure of the flow path of said first flow channel device. Alternatively, or in addition, said adjustable pressure influencing means is provided with means for adjusting the length of the flow path of said second flow channel device in relation to the length of the flow path of said first channel device.

By the above alternatives or combinations, a possibility is achieved for adjusting the pressure difference in the first and second channel devices, such that a predetermined sound attenuation is achieved over the sound attenuation means. Furthermore, the damping device can be adjusted to dampen desired frequencies. Suitably, said pressure influencing means is provided with a control means.

Hereby is achieved a possibility of adjusting the pressure influencing means, such that a predetermined pressure difference is achieved over the sound attenuating means.

Preferably, said control means comprises means for manual adjustment of said pressure influencing means.

Preferably, said control means is provided with a control device associated with a sensor means, for controlling the pressure of said first flow path or second flow path in response to said sensor means.

Suitably, said control device comprises a computer.

Hereby, a computer controlled adjustment of the pressure influencing means is achieved.

Suitably, at least a sensor member of said sensor means is arranged at or inside said first channel device.

Alternatively, or in addition at least one sensor member of said sensor means is arranged at or inside said second channel device.

Preferably, said pressure influencing means comprises a fan device, said sensor means being a pressure or flow sensor, the speed of the fan being controlled in response to said pressure or flow sensor.

Suitably, said pressure influencing means comprises a loudspeaker, said sensor means comprising a microphone, the sound volume of said loudspeaker being adjustable in response to said microphone.

Preferably, said pressure influencing means comprises any one of an adjustable iris and at least one adjustable baffle. Suitably, said sound attenuating means comprises a sound absorbing element.

Preferably, said sound absorbing element comprises a porous material. Suitably said porous sound absorbing element comprises a micro-perforated sheet or plate.

Preferably said micro-perforated element is provided with micro-slits.

Suitably, the thickness of the sheet or plate is in the range 10 "9 m - 2 mm, more particular 10 "8 m - 1 mm, even more particular 10 "7 m - 0,9 mm.

Alternatively, said porous sound absorbing element comprises a piece of sintered metal or sintered cemented carbide. Preferably, the air flow resistance of the sound attenuating means is in the range 100-

10 000 Rayls MK s, more particular in the range 200 - 1000 Rayls M Ks, even more particular 300 - 500 RaylsMKs-

Hereby, an optimal noise reduction is achieved

Yet alternatively, said sound absorbing element comprises a membrane damping element in the form of a non-perforated sheet.

Suitably, the thickness of the membrane damping element is in the range 10 "9 m - 1 mm, more preferably 10 ~8 m - 0,7 mm, even more preferably 10 "7 m - 0,5 mm.

According to other alternatives, the micro-perforated sound absorbing element may comprise materials such as wovens, nonwovens , textiles, sintered material, such as sintered PTFE-films (e.g. Gore-Tex ® ), or nano-porous materials with defined air-flow resistance. DRAWING SUMMARY

In the following, the invention will be described in further detail by reference to the accompanying drawings, in which Fig. 1A is a schematic cross-section of a passive sound damping device including a sound attenuating means;

Fig. IB is an enlargement of the portion IB in Fig. 1A; Fig. 2A illustrates a sound attenuating means;

Fig 2B is an enlargement of the sound attenuating means shown in Fig. 2b; Fig 2C is a cross-section along line IIC-IIC in Fig 2B;

Fig 3A is a schematic cross-section of another kind of passive sound damping device;

Fig 3B is a schematic cross-section of a manually adjustable sound damping device;

Fig 3C is a schematic cross-section of an active sound damping device;

Fig 4A is a schematic cross-section of an alternative manually adjustable sound damping device;

Fig 4B is a schematic cross-section of another active sound damping device;

Fig. 5A is a schematic cross-section of an alternative active sound damping device utilising a controllable fan;

Fig. 5B is a schematic cross-section of an alternative active sound damping device utilising a pair of controllable fans;

Figs. 6A-6B is a schematic depiction of the general principle of sound damping device utilising one or more active loudspeakers; Fig. 7A is a schematic cross-section of an alternative active sound damping device utilising a controllable fan;

Fig. 7B is a schematic cross-section of an alternative active sound damping device utilising a pair of controllable fans; Fig. 8A is a schematic cross-section of an alternative active sound damping device utilising a controllable fan as well as an active loudspeaker; and

Fig. 8B is a schematic cross-section of an alternative active sound damping device utilising a pair of controllable fans and a pair of active loudspeakers.

DETAILED DESCRIPTION

Figs. 1A and IB shows a passive sound damping device 2, comprising a first flow channel device 4 having a first inlet 5a and a first outlet 5b, and a second flow channel device 6 having a second inlet 7a and a second outlet 7b. The first flow channel device 4 is straight, while the second flow channel device 6 is curved in such a way that the second flow channel device 6 and the first flow channel device 4 have portions of a contact region 3 including an intermediate wall 8a, 8b, 8c, 8d, 8e, respectively, while the second flow channel device 6 does not contact the first flow channel device 4 between the

intermediate walls 8a, 8b, 8c, 8d, 8e, i.e. at non-contact regions 10a, 10b, 10c, lOd, lOe etc. The portions of a contact region 3 including an intermediate wall 8a, 8b, 8c, 8d, 8e is according to this embodiment at least partly constituted by a sound attenuating means 9. At non-contact regions 10a, 10b, 10c, lOd, lOe etc., there is a distance between the first and second channels 4, 6, the fluid in said non-contact regions being part of the

surrounding fluid, i.e. each non-contact region is an open space. In Fig IB is shown the centre axis Al for the first flow channel device 4 along its

longitudinal extension and the centre axis A2 for second flow channel device 6 along its longitudinal extension, thus also indicating the first and second flow paths 12, 14 of the first and second flow channel devices 4, 6, respectively. Thus, as a result of the

configuration of the first and second flow channel devices 4, 6, the second flow path 14 is longer than the first flow path 12.

The first and second flow channels devices 4, 6, have substantially the same cross-section across their respective longitudinal extension.

In Figs. 2A - 2C, a sound attenuating means 9 is shown in the form of a micro porous material, in the form of a micro-perforated sound absorbing element 16. According to this variant, the micro-perforated sound absorbing element 16 comprises a sheet or plate 17 of plastic or metal, such as stainless steel or aluminium provided with micro-slits 18.

The micro-slits 18 of the sound absorbing element are preferably made by cutting the sheet or plate 17 by means of a knife roll having a wavy shape against another edge, hereby resulting in a first slit edge 20a and a second slit edge 20b partly pressed out of the material plane. Subsequently, the first and second slit edges 20a, 20b are pressed back by a subsequent rolling operation. Hereby, micro-slits 18 of a predetermined length 22 and predetermined width 24 are created. The width 24 is preferably in the range 10 "9 -10 3 m.

The slits may be positioned across or parallel to or at any angle in relation to the elongation of the axes Al, A2.

The length 22 of the micro-slits 18 may be as small as 10 "9 m, but may depend on the size of the sheet 17.

Cutting may instead be performed by use of laser or a water jet cutter.

The micro-perforations may not only be made in the form of micro-slits, but also in the form of micro-cracks or circular holes. In the case of circular holes, have a diameter of 10 "9 m -1 mm, more particular 10 "8 m - 0,9 mm, even more particular 10 "7 m - 0,7 mm. The absorption degree of such a sound attenuation element 16 depends i.a. on the perforation degree. It is not difficult to calculate mathematically the perforation degree of a sheet or plate provided with circular holes. However, the perforation degree of a sheet or plate provided with micro-slits or micro-cracks is much more difficult to calculate. It is therefore preferable to measure the airflow resistance in accordance with the accepted method described in ASTM C 522 - 73 for achieving a comparable size of the perforation degree, utilising the unit Rayls M Ks It should in this context be noted that 1 Rayls M Ks =

1 N-s/m 3 = 1 Pa-s/m = 1 kg/s-m 2 .

The optimal air flow resistance of the micro-porous sound absorbing element 16 is is preferably in the range 100 - 10 000 Rayls M Ks, more preferably in the range 200 - 1000 RaylsMKs, even more preferably 300 - 500 Rayls M Ks-

The thickness of the sheet or plate 17 is in the range 10 "9 m - 2 mm, more preferably 10 "8 m - 1 mm, even more preferably 10 "7 m - 0,9 mm.

Alternatively, the micro-porous sound absorbing element may comprise a sintered metal or sintered cemented carbide, such as WC or TiC.

Instead of utilizing a micro porous material, such as a micro perforated plate of a sintered material, the sound absorbing element 16 may comprise a membrane damper in the form of a thin sheet without perforations. The thickness of such a membrane damper in the range 10 "9 m - 1 mm, more particularly 10 "8 m - 0,7 mm, even more particularly 10 "7 m - 0,5 mm.

A fluid flow, e.g. by a liquid, such as water, or a gas, such as air in the first and second flow channel devices 4, 6 will create noise. The noise may in addition be created by use of a pump or a fan connected to one or both of the channels 4, 6, e.g. in a ventilation system or a water in a water cooling system of a ventilation system. The noise may alternatively be created by use of a pump or a fan or a compressor or a combustion engine

The different flow lengths 12, 14 will create a phase shift between the flow of the first flow channel device 4 and flow of the second flow channel device 6. The phase shift of the flows on either sides of the sound absorbing element 16 will cause a loss of energy, in turn damping the noise transported by the flows. It should be noted that it is immaterial whether the flow of the first flow channel device 4 and flow of the second flow channel device 6 are substantially counter directed or directed in substantially the same direction at the sound absorbing element.

Counter flow may be the case in a ventilation duct, where the first flow channel device of the sound damping device acts as inlet for outdoor air and the second flow channel device acts as outlet for indoor air, or vice versa.

Flow in the same direction may be the case where the sound damping device is adapted to be inserted into a single duct, e.g. a ventilation duct, a chimney, an exhaust for a combustion engine or a water tubing. It should also be noted that the number of intermediate walls 8a, 8b, 8c, 8d, 8e including a sound absorbing element 16 could be as few as a single one, or as many as one hundred or more.

Maximum sound damping for a particular frequency is achieved if the phase shift of a particular frequency in the first and the second flow channel devices 4, 6 on either sides of the sound absorbing element 16 is half of a wave-length (λ/2) for parallel portions of the flow channels 4, 6, where a parallel grazing sound direction will extend at the sound absorbing element 16.

While using the formula λ = speed of sound / frequency:

For f = 500 Hz

λ = 340 m/s / 500 Hz = 0,68 m = 68 cm

Thus, for 500 Hz, maximum sound damping is achieved for

λ/2 = 34 cm

For

f = 5000 Hz

λ = 340 m/s / 5000 Hz = 0,068 m Consequently, for 5000 Hz, maximum sound damping is achieved for

λ/2 = 34 mm.

Thus, in order to achieve maximum sound damping for the embodiment of Figs 1A-1B, the flow paths 12, 14 of the first and second flow channel devices 4, 6 should be such that the second flow path 14 along the central axis A2 of the second flow channel device 6, and from the first intermediate wall 8a to the second intermediate wall 8b (or the second intermediate wall 8b to the third intermediate wall 8c etc.) is in the range 34 mm - 34 cm longer than the first flow path 12 along the axis Al of the first flow channel device 4.

In this connection, it should be noted that a spiral configuration (such as two rolled sheets) would not be suitable, since it would not only cause damping at half a wave-length (λ/2) but also amplification at one wave length (λ).

Fig. 3A shows an alternative passive sound damping device 2, comprising a first flow channel device 4 and a second flow channel device 6 both having a square cross-section. The exterior of the first and second flow channel devices 4, 6 are straight and parallel. Also the axes Al and A2 are straight. However, the interior of the second flow channel device 6 is provided with fixed baffles 26a, 26b, 26c, 26d, 26e, 26f, causing the flow of the second flow channel device 6 to take a second flow path 14, said second flow path 14 being longer than the first flow path 12, along the axis Al.

The intermediate wall 8 of first and second flow channel devices 4, 6 is provided with the baffles 26a, 26c, 26e. Between the baffles 26a, 26c, 26e a sound absorbing element 16 is provided. By placing the baffles 26a - 26f at predetermined distances from one another it is possible to reduce noise over the sound absorbing element 16 in accordance with what is explained in connection with figures 1A - 2C.

According to an alternative, it would be possible to manually adjust the noise reduction by allowing one or more or even all the baffles 26a - 26f to be moved - separately or in groups - in relation to one another during operation of the source of noise.

In Fig. 3B is shown an alternative adjustable sound damping device 2, according to which one or a plurality of the baffles 26a - 26f are pivotable about a hinge 27 by means of an electric motor or step motor substantially across the axis A2 of the second flow channel device 6b. A potentiometer 29 is connected to each motor or step motor 34 such that they motors can be operated individually by manually turning the potentiometers 29.

Alternatively, a plurality of electric motors or step motors 34 may be connected to a common potentiometer 29, hereby allowing them to be operated together.

Alternatively, the baffles 26a-26f may instead of an electric motor or step motor 34 be provided with a handle for manual operation.

It is contemplated that only some of the baffles 26a - 26f may be provided with electric motor or step motor 34, moved individually, group-wise or all together. In particular, the baffles 26b, 26d and 26f may instead be fixed or manually adjustable individually, group- wise or all together.

In Fig. 3C is shown an alternative active sound damping device 2. A sensor device 37, including three microphones 32a, 32b, 32c connected to the first flow channel device 4 and a sensor unit 38 connected to a computer 36. The electric motors or step motors 34 of each baffle 26a - 26f are controlled individually, group-wise or all in common by said computer 36. Hereby, the baffles are controlled by the computer 36 in response to the microphones 32a - 32c to take a position where the sound damping is optimal over the sound absorbing element 16, i.e. where the noise sensed by the microphones 32a - 32c is as low as possible. The number of microphones 32 may be less than three, i.e. one or two, or more than three, i.e. four, five or any number depending on the length of the first and second flow channel devices 4, 6. As explained above, the noise reduction is achieved by created a pressure difference over the sound absorbing element 16.

Of course, in case of moving baffles group-wise by, a single electric motor or step motor 34 may be used, connected via a suitable transmission to the group of baffles to be adjusted.

Of course, adjustment may instead be performed by moving of baffles by means of an electric motor or step motor in the elongation of the channel. It should be noted that microphones 32a - 32c together with an amplifier and headphones, may be used by the operator in the embodiment of Fig 3B for listening to the amplified noise while adjusting the baffles manually.

It is contemplated that in the variants of Figs. 3A - 3B, the baffles 26a - 26f etc. may instead be arranged in such a way that they are opposite to one another, however on the two walls of the square being perpendicular to the intermediate wall 8.

Fig. 4A shows an alternative passive sound damping device 2, comprising a first flow channel device 4 and a second flow channel device 6, both having a square cross-section. The exterior of the first and second flow channel devices 4, 6 are straight and parallel, and so are the axes Al, A2 and the flow paths 12, 14.

However, the second flow channel device 6 is provided with a fixed or manually adjustable flow restriction 28 coaxially with the axis 12 before or after the sound attenuating means 9 depending on the direction of flow.

After the fixed or manually adjustable flow restriction 28, as seen in the flow direction, the velocity of the flow will be lower than before the flow restriction 28. Furthermore, the flow restriction 28 will cause a pressure drop due to the entry loss at the abrupt contraction.

By using a set of parts of fixed flow restrictions of different sizes of the flow opening, it would be possible to manually adjust the pressure drop by exchanging to a part having another fixed flow restriction. The result will be different degrees of sound damping depending on the flow restriction used. The sound damping as such depends on the created pressure difference between the second flow path 14 in relation to the first flow path 12 over the sound absorbing element 16.

Alternatively, as shown in Fig. 2, by using a manually adjustable flow restriction 28, such as an iris connected to a shaft 30 provided with a grip or handle 31, it will be possible to adjust the flow opening of the flow restriction 28 by turning the grip or handle 31 until a desired damping is achieved over the sound absorbing element 16. Fig. 4B shows the sound damping device 2 of Fig. 4A, however including an active sound damping device 2 provided with said adjustable flow restriction 28 in the form of an iris provided with a shaft 30 connected to an electric motor or step motor 34. A sensor device 37 is provided with a microphone 32 connected to or in the near vicinity of the outside of the second flow channel device 4 adapted to sense the sound of the channels 4, 6. A sensor unit 38 is connected to a computer 36, controlling the movement of the shaft 30 in response to the sensor device 37.

The size of the flow opening of the flow restriction 28 is adjusted by means of the step motor 34, in response to the microphone 32 until the sensed noise is as low as possible, e.g. regarding a particular frequency or frequency range. Again, the noise reduction is achieved by creating a pressure difference over the sound absorbing element 16.

Of course, the microphone together with an amplifier and head-phones, may be used by the operator in the embodiment of Fig 4B while adjusting the iris manually.

Fig. 5A shows another alternative active sound damping device 2, comprising a first flow channel device 4 and a second flow channel device 6, both having a square cross-section. The exterior of the first and second flow channel devices 4, 6 are straight and parallel, and so are interior of the first and second flow paths 12, 14.

A pressure influencing means 35 is provided in the form of a fan device 39 comprising an electric fan motor 40 driving a fan 42 is arranged inside the second flow channel device 6. In the intermediate wall 8 of the first and second channel devices 4, 6 a plurality of sound absorbing elements 16 are provided at equal distances from one another.

By means of the fan 42, the air speed inside the second channel 6 will be faster than the air speed inside the first channel 4. The differences in air speed in the channels 4, 6 create in turn pressure differences over the sound absorbing element 16, such that noise in the channels 4, 6 is attenuated.

The microphone 32 connected to the outside of the second flow channel device 6 is adapted to sense the noise of the channels 4, 6. The sensed noise is analysed in a sensor unit 38 integrated in the computer 36 and controls the speed of the fan motor 40 by a suitable computer program.

By adjustment of the fan speed in response to the microphone 32, the noise in the flow channel devices 4, 6 is damped due to the created pressure difference over the sound absorbing element 16.

As shown in Fig. 5B, it is also possible by means of a computer 36 to individually control the speed of the motor 40a of a first fan 42a in the first channel device 4 and a the speed of a motor 40a of a second fan 42b in the second channel device 6 in response to a sound sensed by the microphone 32. According to this embodiment, substantially the whole longitudinal extension of the contact region 3, i.e. the intermediate wall 8 of the first and second channel devices 4, 6, is provided with a sound absorbing element 16.

It should be noted that in the embodiments of Figs. 5A and 5B, the number of microphones 32 may be more than one, cf. Fig 3C above. Furthermore, the microphone or microphones 32 may instead be arranged at the first flow channel device 4.

Of course, the fan or fans in the embodiments of Figs. 5A and 5B may be adjusted manually by means of a potentiometer, optionally while using the microphone or microphones 32 connected to an amplifier and headphones.

Fig. 6A shows the general principle of sound reduction by means of an active loudspeaker. Each flow channel device 4, 6 is provided with a fan 42 driven by a motor 40 at constant speed. A microphone 32a is connected to the outside of the second flow channel device 6 and is adapted to sense noise of the channels 4, 6.

The microphone 32 is connected to a sensor unit of a computer 36, the computer in turn controlling a sound generator 44. The noise generated by the sound generator 44 is amplified by an amplifier 46 connected to a pressure influencing means 35 in the form of a loudspeaker 48 arranged in an opening 50 of the second flow channel device 6. It should be noted that in this case, the fans 42 are considered a source of noise only, and are not to be to be included in the expression "pressure influencing means", since they are driven at constant speed and cannot be controlled to change the pressure inside the flow channel devices. As explained above, the noise source may instead be a e.g. combustion engine not providing an air flow inside the first and second channel devices. In response to the noise sensed by the microphone 32, the sound generator 44 generates a noise with inverted phase to the sensed noise. The computer 36 also controls the volume level of the loudspeaker 48 in response to the noise sensed by the microphone 32.

This effect is generally called active noise control (ANC) or active noise reduction (ANR). According to the invention, this effect is enhanced by the sound absorbing element 16 arranged in the intermediate wall 8 of the first and second flow channel devices 4, 6. According to this embodiment, the sound absorbing element 16 is arranged in the area of the loudspeaker 48.

It should be noted that the microphone 32 may instead be arranged at the first flow channel device 4.

Fig. 6B shows the general principle of sound reduction by means of two active

loudspeakers.

Compared to what is shown in Fig. 6A, the sound damping device 2 shown in Fig. 6B includes in addition to a first microphone 32a connected to the second flow channel device 6 a further microphone 32b connected to the first flow channel device 4. The microphones 32a, 32b are connected to the computer 36, controlling the sound generator 44, the sound of which being amplified by an amplifier 46 connected to the loudspeakers 48a, 48b at an opening 50 arranged in the second flow channel device 6 and in the first flow channel device 4, respectively. The sound absorbing element 16 arranged in the intermediate wall 8 of the first and second flow channel devices 4, 6 is according to this embodiment arranged along substantially the whole longitudinal extension of the first and second flow channel devices 4, 6. In the embodiment of Fig. 7 A, a pressure sensor 52 is connected via a first tubing 54a to the first flow channel device 4 and via a second tubing 54b to the second flow channel device 6. The cross-section of the first and second channels 4, 6 is square, respectively.

The pressure difference between the first flow 12 and the second flow 14 sensed by the pressure sensor 52 is analysed by computer 36. The computer controls the motor 40 of the fan 42 such that a predetermined pressure difference is achieved to reduce noise in the first and second flow channel devices 4, 6 by means of the sound absorbing element 16.

In the embodiment of Fig. 7B, each flow channel device 4, 6 is provided with a fan 46a, 46b, the motor 40a, 40b of which being individually controllable by the computer 36 in response to a pressure transducer 56a, 56b to the interior first and second flow device 4, 6, respectively.

The motors 40a, 40b including the fans 42a, 42b are individually controlled by the computer 36 to create a predetermined pressure difference such that noise in the first and second flow channel devices 4, 6 is reduced by means of the sound absorbing element 16. In the variants of Figs. 3C, 4B, 5A, 5B, 6A and 6B it has been described that the microphone 32 is to be connected to the outside of the flow channel device 6. However the microphone 32 may instead be arranged inside the flow channel device 6. Alternatively, the

microphone 32 may be mounted inside or connected to the outside of the first flow channel device 4. The microphone 32 may instead be arranged at a distance from the first and second flow channel devices 4, 6.

Furthermore, it has been shown that the microphones 32 are positioned opposite to the sound absorbing element 16 where such an element has a limited extension. The microphones may however be placed somewhere else along the first and/or second flow channel devices 4, 6. Of course, the loudspeaker 48 could be mounted to the flow channels directly towards the wall rather than at an opening. As shown in Fig. 8A pressure transducers 56a, 56b sense the dynamic pressure difference in the first and second flow channel devices 4, 6 and a microphone 32 senses the noise level thereof. The cross-section of the first and second channels 4, 6 is square, respectively.

Furthermore, the sensor unit 38 is integrated in the computer 36. Likewise, the sound generator 44 is integrated in the computer.

As shown in Fig 8B, each flow channel device 4, 6 is provided with a fan 42a, 42b, the motor 40a, 40b of which being individually controllable by the computer 36 in response to a pressure transducer 56a, 56b to the interior first and second flow device 4, 6,

respectively.

Furthermore, two microphones 32a, 32b are connected to a computer 36, the latter controlling the sound generator 44, the signal thereof being amplified by an amplifier 46 connected to a loudspeaker 48 at an opening 50 arranged in the first flow channel device 4.

The sound absorbing element 16 arranged in the intermediate wall of the first and second flow channel devices 4, 6 is according to the embodiment of Figs 8A and 8B arranged along substantially the whole longitudinal extension of the first and second flow channel devices 4, 6.

By sensing the pressure difference over the sound absorbing element 16 by means of the pressure transducers 56a, 56b and at the same time sensing the noise by use of a microphone 32 (Fig 8A) or microphones 32a, 32b (Fig 8B), it will be possible to create a data base of experience values for different operating modes. Hereby, a quick adjustment of the sound damping for different operating modes is achieved.

In all above described embodiments, the sound damping device 2 may be mounted in an existing or a prospected single duct, the first and second inlets 5a, 7a and the first and second outlets 5b, 7b being connected to a cut-off part of said existing single duct.

Alternatively, the sound damping device 2 may be mounted in a corresponding manner in an existing or prospected double duct, the first and second inlets 5a, 7a and the first and second outlets 5b, 7b being connected to the channels of the double duct.

The direction of first flow 12 and the second flow 14 may be is the same or substantially the same. Alternatively, the direction of said first flow 12 and said second flow 14 may be reversed or substantially reversed.

In all the above described embodiments, the intermediate wall may be a common single wall or a common double wall.

Furthermore, in all the above described embodiments, the sound attenuating means 9 is in the form of a sound absorbing element 16 as discussed in connection with Figs 2A - 2C, including the proposed alternatives.

In the embodiment shown in Figs 7B, 8A, and 8B, the length 22 of the micro-slits 18 may extend along the whole length of the flow channel devices 4, 6. Thus, the length 22 of the micro-slit 18 may be several meters.

It should be noted that it would be possible to arrange the sound absorbing element 16 along substantially the whole longitudinal extension of the first and second flow channel devices 4, 6 of Figs. 3A - 5A and 6A as well. Likewise, arranging the sound absorbing element 16 in the manner shown in Figs. 3A - 5A and 6A would be possible also in the embodiments of Figs 7B, 8A, and 8B. In that case, the length 22 of the micro-slits 18 is limited by the size of the sound absorbing element 16. The latter also relates to the embodiment of Figs. 1A and IB.

It should be noted that the length 22 and width 24 of the micro-slits 18 is chosen in combination with the number of slits (or any other kind of the above described micro- perforations), in such a way that sound absorbing element 16 has perforation degree with the above described range of air flow resistance. It should also be noted that the above described fans and the above described loudspeakers may be controlled manually.

It should be noted that it would be possible to combine any one of the adjustable pressure influencing means shown in 3B, 3C, 4A, 4B, 5A, 5B, 7 A, 7B, 8A, 8B and their described alternatives with one another.

The cross-section of the first and second flow channel devices 4, 6 has in all variants been described as being square. However, they may alternatively have another cross-section, such as polygonal or annular. In particular, the polygonal cross-section may be e.g.

rectangular, triangular, pentagonal, hexagonal, a rhombus etc, whereas the annular cross- section may be e.g. circular or oval.

It should be noted that the cross-section of the first flow channel device 4 may be different from that of the second flow channel devices 6.

From this follows that if the cross-section of the first flow channel device 4 and a second flow channel device 6 is oval, respectively, e.g. with a long and a short symmetry axes perpendicular to one another, the intermediate wall 8 may be a portion of the wall at one of the short symmetry axes.

In the case of Figs 3A - 3C, the baffles 26a - 26f may thus either be arranged at a portion of the walls of the short symmetry axis, or at a portion of the walls of the long symmetry axis. On the other hand, if the cross-section of the first flow channel device 4 and a second flow channel device 6 is circular, respectively, the baffles 26a - 26f may be arranged at the portion of the intermediate wall 8 and diametrically thereto. Alternatively, they may be arranged diametrically to one another, but off-set in relation to the portion of the intermediate wall. The corresponding relates to all other polygonal or annular cross-section of the first flow channel device 4 and a second flow channel device 6. Reference signs used

2 Sound damping device

3 Contact region

4 First flow channel device 5a First inlet

5b First outlet

6 Second flow channel device

7a Second inlet

7b Second outlet

8, 8a, 8b, 8c, 8d, 8e etc. Intermediate wall

9 Sound attenuating means

10a, 10b, 10c, lOd, lOe etc. Non-contact region

12 First flow path

14 Second flow path

16 Sound absorbing element

17 Sheet or plate

18 Micro-slits

20a First slit edge

20b Second slit edge

22 Length

24 Width

26a - 26f Baffle

27 Hinge

28 Flow restriction

29 Potentiometer

30 Shaft

31 Grip or handle

32, 32a, 32b, 32c Microphone

34 Electric motor or step motor

35 Pressure influencing means

36 Computer

37 Sensor device

38 Sensor unit

39 Fan device

40, 40a, 40b Motor

42, 42a, 42b Fan Sound generator, 46a, 46b Amplifier

, 48a, 48b Loudspeaker

Opening

Pressure sensora, 54b Tubing

a, 56b Pressure transducer