EP3080522B1

EP3080522B1 - A sound absorbing panel

Info

Publication number: EP3080522B1
Application number: EP13821521.5A
Authority: EP
Inventors: Jesper Nielsen
Original assignee: Kvadrat Soft Cells AS
Current assignee: Kvadrat Soft Cells AS
Priority date: 2013-11-28
Filing date: 2013-11-28
Publication date: 2018-05-30
Anticipated expiration: 2033-11-28
Also published as: WO2015079289A1; KR20160079023A; EP3080522A1; KR102044727B1

Description

TECHNICAL FIELD

The present invention pertains to panels that can be used to cover interior surfaces in buildings, for instance in auditoriums, open-plan offices, etc. and more specifically to such panels for use in buildings with thermally activated building systems (TABS) in which balancing acoustics and thermal comfort is a well-recognized challenge

BACKGROUND OF THE INVENTION

In such fields as architecture and interior design there is often a need for panels for covering of boundaries of a room, such as the ceiling, the walls, or partitions placed within the room. Such panels can serve purely aesthetic purposes, but can also be used to actively alter a room's characteristics, for instance relating to acoustic and thermal properties of the room.
Panels used to determine the acoustic properties of a room often comprise a frame structure supporting a plate of a sound-absorbing material, such as mineral wool, gypsum or a thin wood membrane. Although such panels can offer quite excellent solutions relating to acoustic regulation of rooms, the thermal properties of such panels, such as their thermal transparency, are seldom optimal and may in fact be very far from optimal it is a problem with known panels simultaneously to optimize the acoustic properties and the thermal properties and hence to use the panels in an attempt to optimize the overall comfort of a room.
In US 4,330,046 a sound barrier panel is described. Said sound barrier is for isolating a noise source of the type emanating from manufacturing areas for food The sound barrier comprises a panel made of sound attenuating material having a plurality of longitudinally extending side by side co-planar sections separated by V-shaped ribs so as to define a plurality of longitudinally extending side by side cavities. The cavities are provided with acoustical absorptive material therein so that the sound waves emanating from the noise source are first absorbed by the acoustical absorptive material and thereafter partly attenuated by the panel and partly deflected thereby so as to be absorbed again by the acoustical absorptive material. The panel is preferably transparent, and portions of at least some cavities may be left free of acoustical absorptive material so as to provide undistorted vision from one side of the panel to the opposite side thereof. The sound barrier is intended to be disposed so that the open faces of the cavities confront the noise source. Alternatively, the cavities are separated by flat panel sections instead of V-shaped ribs, which sections are free of acoustical absorptive material to provide undistorted vision there through.
In US 2009/0178882 is described another sound panel- a so called acoustic panel- and a method is disclosed for an improved acoustic panel comprising a sound absorbing member defined by a first and second face surface and a plurality of peripheral edges A sound blocking member is defined by a first and second face surface and a plurality of peripheral edges. The first face surface of the sound blocking member is secured relative to the second face surface of the sound absorbing member for blocking the transmission of sound through it. In another embodiment, the first face surface of the sound blocking member is spaced relative to the second face surface of the sound absorbing member for decoupling the sound blocking member from the sound absorbing member. An air handling ceiling apparatus comprising an acoustic panel is known from EP 1 959 207 A1 . A problem with the above mentioned panels is that they are thermally isolating as well as sound absorbing, which render them practically useless in building thermally activated building systems, wherein usually the ceiling is water cooled.
In order to balance the acoustic and thermal comfort of a building, especially one that is equipped with thermally activated systems (TABS), the applicant of the present application has in EP 2 444 561 suggested a sound dampening, but thermally transparent panel comprising a substantially rigid frame, a front face and a rear face, one or more sound-absorbing elements, and sub-regions that acoustically connect the front face of the panel with the rear face of the panel, and in which sub-regions sound-absorbing elements are not present, whereby said sub-regions ensure thermal transmission through the panel, the panel comprising brackets that together with frame portions provide receptacles for said sound-absorbing elements, wherein said brackets comprise a central portion provided with apertures for providing access of a sound field to side faces of the sound-sbsorbing elements.
However, in many practical situations the above mentioned panel has proven to impair the operation of the TABS due to the thermally insulating properties of the sound absorbing regions. Hence, the applicant has realized a need to improve said panel in order to provide an optimized balance between sound absorption and interference with the operation of a TABS.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to provide
According to the present invention, the above-mentioned and other objects are fulfilled by a panel according to claim 1. During use, the surface onto which the panel is mounted will cool (or heat) the air in the first air gap down to substantially the same temperature as the surface itself, and by providing means for providing an airstream through the first air passage, this cool air will be transported to the second air gap and therefore cool the front cover (and the one or more sound absorbing elements). Hereby the front cover of the panel will achieve the same temperature as the surface (e.g. ceiling) onto which the panel is mounted. Therefore, the comfort experienced in a room wherein the inventive panel is mounted will not be impaired, because the temperature conditions that exist on the surface onto wich the panel is mounted are "moved" to the front cover of the panel. Hereby is thus achieved a sound absorbing panel, which is thermally transparent. This is a tremendous advantage in buildings which are employed with thermally activated building systems (TABS), such as cooling of a ceiling. Hitherto, it has been virtually impossible to provide satisfactory sound dampening is such rooms due to the thermal insulating properties of conventional sound absorbing panels, which thereby has lead to less satisfactory solutions, such as using carpets on the floors in order to at least suppress back-scattering of sound the floor.
In a preferred embodiment of the panel according to the invention, the one or more sound absorbing elements are box shaped. Hereby is achieved a rather practical solution, especially suitable for building several panels together to form one larger unit.
In an embodiment of the panel according to the invention, the sound absorbing elements are manufactured from mineral fibers such as glass or stone wool, but could be made from melamine foam, glass textile, cellulose (paper) based materials, ceramic substances or other porous materials, The sound absorbing elements could also be membranes or perforated materials, working as membrane or resonance absorbers.
In a further preferred embodiment of the panel according to the invention, the dimensions of the one or more sound absorbing elements are chosen according to the lowest frequency at which substantial sound absorption shall take place.
In a further preferred embodiment of the panel according to the invention the thickness of the one or more sound absorbing elements is in the range of 20 mm to 100 mm, which has proven to be especially good in absorbing undesired noise and its reverberations in the audible frequency range.
In a preferred embodiment of the panel according to the invention, the front cover is a sheet of flexible material, e.g. woven textile material. Preferably, the front cover is tensioned regardless of for instance changes in the temperature and humidity of the surrounding environment and aging effect of the front cover material itself.
In a further preferred embodiment of the panel according to the invention, the front cover is of a material or structure which allows air to diffuse through it, whereby heat exchange between the surface onto which the panel is mounted and the surrounding environment is further enhanced due to exchange of heat through air transport.
The frame of the panel may in yet another embodiment further comprise an apperture in each of the ends of the frame that are furthest away from the first air passage between the first and second air gap, said apertures being placed on said frame for allowing air to flow from the surroundings of the panel directly into the first air gap for compensation of the air which diffuses out of the second air gap through the front cover.
Advantageously, the first air passage is placed in the center of the panel.
Preferably, the panel has an elongated shape with a longitudinal axis, which is longer than its transversal axis. The panel may have any of the following shapes: box shaped, oval shaped or elliptical shaped
In a further preferred embodiment, the panel further comprises a second and third additional air passage between the first and second air gap each of said second and third air passages being placed at the respective end of the panel that is furthest away from the first air passage Hereby a forced air circulation may be achieved, wherein air is forced through the first air passage from the first or second air gap, and further through the two second and third air passages towards the first air passage again. Hereby the flow of air would be forced to substantially travel along all the surface of the sound absorbing elements as well as the back side of the front cover, whereby a very efficient cooling of the front cover (and one or more sound absorbing elements) is achieved.
in a practical embodiment of the panel according to the invention, each of the second and third air passages are provided as a gap between the frame and one or more sound absorbing elements. Hereby is ensured that the air will flow over substantially all of the inner side of the front cover, whereby a more effective temperature exchange with said front cover is achieved.
In an alternative embodiment of the panel according to the invention, the first air passage is placed adjacent to the frame of the panel, the panel further comprising a second air passage between the first and second air gaps, said second air passage being provided adjacent to the part of the frame, which is furthest away from the first air passage.
In a preferred embodiment of the panel, the frame completely surrounds the edges of the panel. Hereby it is possible to tension the front cover by its attachment to the frame.
In a further preferred embodiment of the panel according to the invention, the first air gap, which is defined by the distance between the one or more sound absorbing elements and the surface when the panel is mounted on said surface, is 10 mm to 25 mm wide.
In a further preferred embodiment of the panel according to the invention, the second air gap is, which is defined by the distance between the one or more sound absorbing elements and the front cover, is 8 mm to 15 mm wide.
Preferably, the speed of the airstream through the first air passage is adjusted, so that a minimum of energy is used, still achieving a temperature in the second air gap close to that of the surface that the panel is mounted on.
In a further preferred embodiment of the panel according to the invention, the means for providing the airstream through said first air passage is a fan placed in or adjacent to the first air passage. Such a fan may be connected to the power grid in a normal way, e.g via a transformer built into the panel A fan is low in energy consumption and may be virtually silent, whereby the sound environment of the room in which the inventive panel is used is not impaired by the application of a fan as means for providing said airstream through the first air passage. A specially built fan may be used or an off-the-shelf fan, such as those used for cooling various parts of a computer, may be used. The fan may be configured to aperate at a fixed, predefined, effect.
In a further preferred embodiment of the panel according to the invention, the panel comprises means (e.g. two temperature sensors) for monitoring the temperature difference between the air in the two air gaps.
In a further preferred embodiment of the panel according to the invention, the panel further comprises two temperature sensors placed adjacent to or on the same side of the one or more sound absorbing elements, at two different distances from the first air passage in the direction of flow of air, said temperature sensors being connected to a controller, which controller is operatively connected to the fan. Hereby the temperature difference between the two air gaps is monitored in a very practical way, because if the temperature of the front cover (and sound absorbing elements) and the surface has been evened out, then there will be substantially no difference in the two measurements. The difference in the temperature measurements can therefore be used to control the fan.
Hence, in a further embodiment of the panel according to the invention, the controller is configured to compare the temperature measurements from the two temperature sensors and regulate the operation of the fan in dependence of the numerical value of said comparison. Said comparison could e.g. be the calculation of a ratio.
However, in a further embodiment of the panel according to the invention, said comparison is a calculation of the difference (e.g. the numerical difference) of the temperature measurements from the two temperature sensors, and wherein the fan is regulated in such a way that

the airstream through the first air passage between the first and second air gap is increased if said difference is above a predefined threshold value, or
the airstream through the first air passage between the first and second air gap is decreased if said difference is below said predefined threshold value.

In an alternative embodiment of the panel according to the invention, said comparison is a calculation of the numerical difference of the temperature measurements from the two temperature sensors, and wherein the fan is regulated in such a way that

the fan is turned on or kept, on if said difference is above a predefined threshold value, or
the fan is turned off or kept off if said difference is below said predefined threshold value.

Hereby a power saving mode of operation is achieved, wherein the fan is only operating when needed.
In a further preferred embodiment of the panel according to the invention, the surface onto which the panel is to be mounted forms part of a thermally activated building system. Due to the forced airstream between the first and second air gaps, the sound absorbing elements will not function as thermally insulating thereby making the panel according to the invention especially suitable for being used in connection with TABS, e.g. a water cooled roof or ceiling.
In a further embodiment of the panel according to the invention, the surface onto which the panel is to be mounted is a ceiling or a wall.
In a further embodiment of the panel according to the invention, said controller is furthermore operatively to the thermally activated building system.

BREIF DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings. In the following, preferred embodiments of the invention are explained in more detail with reference to the drawings, wherein

Fig. 1: shows an embodiment of a panel according to the invention,
Fig. 2: shows a cross section of the embodiment of the panel illustrated in Fig. 1, Fig. 3 shows a further embodiment of a pane! according to the invention,
Fig. 4: shows a further embodiment of a panel according to the invention.
Fig. 5: shows a further embodiment of a panel according to the invention, and
Fig. 6: shows a cross section of a further embodiment of a panel according to the invention.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings. in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout. Like elements will, thus, not be described in detail with respect to the description of each figure.
Referring to Fig. 1 and 2, which show a panel 2 according to the invention as seen from above (Fig. 1), and as seen from the side (Fig. 2). In Fig. 2, the panel is mounted on a ceilling 6 inside a building. The illustrated panel 2 comprises a substantially rigid frame 4, a front cover 8 placed on said frame 4 and being configured to face away from said surface (e.g. ceiling 6) onto which the panel 2 is to be mounted. The panel 2 also comprises a sound absorbing element 10 placed within said frame 4 in such a way that a first air gap 12 is formed between the sound absorbing element 10 and the ceiling 6 when the panel 2 is mounted on said ceiling 6, and a second air gap 14 is formed between the sound absorbing element 10 and the front cover 8. The illustrated panel further comprises a fan 16 placed in a first air passage 18 between the first air gap 12 and the second air gap 14. The fan 16 is configured for providing an airstream through said first air passage 18. The fan 18 may be connected to the power grid in a normal way e.g. via a transformer built into the panel 2. In the illustrated embodiment the first air passage 18 is placed in the center of the panel 2.
The panel 2 shown in Fig. 1 and Fig. 2 further comprises a second and third additional air passage 20, 22 between the first and second air gap 12, 14, each of said second and third air passages 20, 22 being provided as a gap between the frame 4 and the absorbing elements 10. Hereby a forced air circulation may be achieved, wherein air is forced through the first air passage 18 from the first air gap 12, and further through the two second and third air passages 20, 22, towards the first air passage 18 again as indicated by the arrows. Hereby, the flow of air would be forced to substantially travel along ail the surface of the sound absorbing element 10 as well as the back side of the front cover 8, whereby a very efficient cooling of the front cover 8 (and sound absorbing element 10) is achieved if the ceiling 6 is cooked by for example a TABS. By placing the second and third air passages 20, 22 as a gap between the frame 4 and sound absorbing element 10 at the ends of the panel 2 that are furthest away from the first air passage 18, it is ensured that the air will flow over substantially all of the inner side of the front cover 8, whereby a more effective temperature exchange with said front cover 8 is achieved.
During use, the ceiling 6 will cool the air in the first air gap 12 down to substantially the same temperature as the ceiling 6 itself, and by providing a fan 16 for providing an airstream through the first air passage 18. this cool air will be transported to the second air gap 14 and therefore cool the front cover 8 (and the sound absorbing element 10). Hereby the front cover 8 of the panel 2 will achieve the same temperature as the ceiling 6, onto which the panel 2 is mounted. Therefore, the comfort experienced in a room wherein the inventive panel 2 is mounted will not be impaired, because the temperature conditions, which exist on the surface (ceiling 6) onto which the panel 2 is mounted, are "moved" to the front cover 8 of the panel 2. Hereby is therefore achieved a sound absorbing panel 2, which is thermally transparent. This is a tremendous advantage in buildings that are employed with thermally activated building systems (TABS), such as cooling of a ceiling 6. Hitherto, it has been virtually impossible to provide satisfactory sound dampening in such rooms due to the thermal insulating properties of conventional sound absorbing panels, which thereby has lead to less satisfactory solutions, such as using carpets on the floors in order to at least suppress back-scattering of sound the floor.
The front cover 8 is a sheet of flexible material, e.g. woven textile material. Preferably, the front cover 8 is tensioned regardless of for instance changes in the temperature and humidity of the surrounding environment and aging effect of the front cover material itself. The frame 4 completely surrounds the edges of the panel 2. Hereby it is possible to tension the front cover 8 by its attachment to the frame 4.
The illustrated sound absorbing element 10 is substantial box shaped. Hereby is achieved a rather pratical solution, especially suitable for building several panels 2 together to form one larger unit. The sound absorbing element 10 is preferably manufactured from mineral fibers such as glass or stone wool, but could be made from meiamine foam, glass textile, cellulose (paper) based materials, ceramic substances or other porous materials. The sound absorbing elements could also be membranes or perforated materials, working as membrane or resonance absorbers. The thickness of the sound absorbing element 10 is preferably in the range of 20 mm to 100 mm, which has proven to be especially good in absorbing noise and reverberations in the frequency range of 300 Hz to 3500 Hz.
In the illustrated embodiment, the first air gap 12, which is defined by the distance between the sound absorbing element 10 and the ceiling 6. is approximately 10 mm to 25 mm wide, the second air gap 14, which is defined by the distance between the sound absorbing element 10 and the front 8 cover, is 8 mm to 15 mm wide.
Preferably, the speed of the airstream through the first air passage 18 is adjusted, so that a minimum of energy is used, still achieving a temperature in the second air gap close to that of the surface that the panel is mounted on.
Fig. 3 shows another embodiment of a pane! 2 according to the invention. The illustrated panel 2 comprises two temperature sensors 24, 26 for monitoring the temperature difference between the air in the two air gaps 12 and 14. The two temperature sensors 24, 26 are placed on the same side of the sound absorbing element 10 at two different distances from the first air passage 18 in the direction of flow of air (as illustrated by the arrows). The temperature sensors 24, 26 are connected to a controller 28, which controller 28 is operatively connected to the fan 16. Hereby the temperature difference between the two air gaps 12, 14 is monitored in a very practical way. because, if the temperature of the front cover 8 and the ceiling 6 has been evened out, then there will be substantially no difference in the two measurements. The difference in the temperature measurements can therefore be used to control the fan 16.
In one embodiment of the panel 2 acceding to the invention, the controller 28 calculates the difference (e.g. the numerical deference) of the temperature measurements from the two temperature sensors 24, 26, and regulates the fan 16 in such a way that

the airstream through the first air passage 18 between the first and second air gap 12, 14 is increased if said difference is above a predefined threshold value, or
the airstream through the first air passage 18 between the first and second air gap 12, 14 is decreased if said difference is below said predefined threshold value.

In another embodiment of the panel 2 according to the invention, the controller 28 calculates the difference (e.g. the numerical difference) of the temperature measurements from the two temperature sensors 24, 26, and controls the fan 16 in such a way that

the fan 16 is turned on or kept on if said difference is above a predefined threshold value, or
the fan 16 turned off or kept off if said difference is below said predefined threshold value.

Hereby a power saving mode of operation is achieved, wherein the fan 16 is only operating when needed.
In a further embodiment of the panel 2 according to the invention, said controller may be operatively connected to a thermally activated building system.
Fig. 4 shows an alternative embodiment of a panel 2 according to the invention having an elliptical shape.
Fig. 5 shows yet another alternative embodiment of the invention wherein the first air passage 18 is placed to the frame 4 of the panel, and wherein the panel 2 only further comprises a second air passage 30 between the first and second air gaps, 12, 14, said second air passage 30 being prodded adjacent the part of the frame 4 that is furthest away from the first air passage 18.
Fig. 6 shows a cross section of another embodiment of a pane! 2 mounted on a ceiling 6. The illustrated panel 2 is substantially similar to the panel 2 illustrated in any of the Figures 1-3. Hence, only the fifferences will be described. In the illustrated pane! 2, the front cover 8 is of a material or structure that allows air to diffuse through it, as indicated by the arrows going through the front cover 8, whereby heat exchange between the ceiling 6 onto which the panel 2 is mounted and the surrounding environment is further enhanced clue to exchange of heat through air transport. The frame 4 of the illustrated panel 2 further comprises an aperture 32 in each of the ends of the frame 4 that are furthest away from the first air passage 18 between the first air gap 12 and second air gap 14. These apertures 32 are placed in such a way on the frame 4 that air is allowed to How from the surroundings of the panel 2 directly into the first air gap 12 for compensation of the air that diffuses out of the second air gap 14 through the front cover 8.

Test results

A panel according to the embodiment illustrated in Fig. 1 has been tested and compared with a pane! without a fan in order to assess the thermal conductivity properties in connection with a TAB ceiling. The tests were performed by the independent German company HLK Stuttgart GmbH. The tests showed that the acoustical elements reduced the heat transport hrough panel by 41% if no fan was present. If however a fan, type CMP-Fan 25, from the producer König Electronic. Diameter 12 mm, was used as shown in e.g. Fig. 1 and 2, the heat transport was only reduced by 10%, thereby making the panel according to the invention virtually thermally transparent

LIST OF REFERENCE NUMBERS

In the following is given a list of reference numbers that are used in the detailed description of the invention.

2: panel,
4: frame,
6: celling,
8: front cover,
10: acoustic element,
12: first air gap,
14: second air gap,
16: fan,
18: first air passage,
20, 30: second air passage,
22: third air passage,
24, 26: temperature sensors,
28: controller, and
32: apertures in the frame.

Claims

A sound absorbing panel (2) to be mounted on a surface inside a building, said surface forming part of a thermally activated building system (TABS), said panel (2) comprising:
a substantially rigid frame (4),

a front cover (8) placed on said frame (4) and being configured to face away from said surface onto which the panel (2) is to be mounted,

one or more sound absorbing elements (10) placed within said frame (4) in such a way that a first air gap (12) is formed between the one or more sound absorbing elements (10) and the surface when the panel (2) is mounted on said surface, and a second air gap (14) is formed between the one or more sound absorbing elements (10) and the front cover (8), wherein the panel (2) further comprises a first air passage (18) between the first and second air gap (12, 14) and means for providing an airstream through said first air passage (18), and wherein the means for providing the airstream through said first air passage is a fan (16) placed in, or adjacent to, the first air passage (18).
The panel (2) according to claim 1, wherein said one or more sound absorbing elements (10) are box-shaped.
The panel (2) according to claim 1 or 2, wherein the sound absorbing elements (10) are manufactured from any of the following materials: mineral fibers such as glass or stone wool, melamine foam, glass textile, cellulose based materials, ceramic substances, membranes or perforated materials, working as membrane or resonance absorbers.
The panel (2) according to claim 1, 2 or 3, wherein the dimensions of the one or more sound absorbing elements (10) are chosen according to the lowest frequency at which substantial sound absorption shall take place.
The panel (2) according to claim 4, wherein the thickness of the one or more sound absorbing elements (10) is in the range of 20 mm to 100 mm, whereby noise in the audible frequency range are substantially absorbed.
The panel (2) according to any of the preceding claims, wherein the front cover (8) is a sheet of flexible material, e.g. woven textile material.
The panel (2) according to any of the preceding claims, wherein the front cover (8) is of a material or structure that allows air to diffuse through it.
The panel (2) according to claim 7, further comprising an aperture (32) in each of the ends of the frame (4) that are furthest away from the first air passage (18) between the first and second air gap (12, 14), said apertures (32) being placed on said frame (4) for allowing air to flow from the surroundings of the panel (2) directly into the first air gap (12) for compensation of the air that diffuses out of the second air gap (14) through the front cover (8).
The panel (2) according to any of the preceding claims, wherein the first air passage (18) is placed in the center of the panel (2).
The panel (2) according to any of the preceding claims, wherein the panel (2) has an elongated shape, with a longitudinal axis, which is longer than its transversal axis.
The panel (2) according to claim 10, having any of the following shapes: box shaped, oval shaped or elliptical shaped.
The panel (2) according to claim 10 or 11, further comprising a second and third additional air passage (20, 22) between the first and second air gap (12, 14), each of said second and third air passages (20, 22) being placed at the respective end of the panel (2), which is furthest away from the first air passage (18).
The panel (2) according to claim 12, wherein each of the second and third air passages (20, 22) is provided as a gap between the frame (4) and the one or more sound absorbing elements (10).
The panel (2) according to any of the claims 1-7, wherein the first air passage (18) is placed adjacent the frame (4) of the panel (2), the panel (2) further comprising a second air passage (30) between the first and second air gaps (12, 14), said second air passage (30) being provided adjacent the part of the frame (4) which is furthest away from the first air passage (18).
The panel (2) according to any of the preceding claims, wherein the frame (4) completely surrounds the edges of the panel (2).
The panel (2) according to any of the preceding claims, wherein the first air gap (12), which is defined by the distance between the one or more sound absorbing elements (10) and the surface when the panel (2) is mounted on said surface, is 10 mm to 25 mm wide.
The panel (2) according to any of the preceding claims, wherein the second air gap (14), which is defined by the distance between the one or more sound absorbing elements (10) and the front cover (8), is 8 mm to 15 mm wide.
The panel (2) according to any of the preceding claims, wherein the speed of the airstream through the first air passage (18) is adjusted, so that a minimum of energy is used, still achieving a temperature in the second air gap close to that of the surface that the panel is mounted on.
The panel (2) according to any one of the previous claims, wherein the fan (16) is configured to operate at a fixed, predefined, effect.
The panel (2) according to any one of the previous claims, wherein the panel (2) further comprises two temperature sensors (24, 26) placed adjacent to, or on, the same side of the one or more sound absorbing elements (10), at two different distances from the first air passage (18) in the direction of flow of air, said temperature sensors (24, 26) being connected to a controller (28), which controller (28) is operatively connected to the fan (16).
The panel (2) according to claim 20, wherein the controller (28) is configured to compare the temperature measurements from the two temperature sensors (20, 22) and regulate the operation of the fan (16) in dependence of the numerical value of said comparison.
The panel (2) according to claim 21, wherein said comparison is a calculation of the numerical difference of the temperature measurements from the two temperature sensors (24, 26), and wherein the fan (16) is regulated in such a way that
- the airstream through the first air passage (18) between the first and second air gap (12, 14) is increased if said difference is above a predefined threshold value, or

- the airstream through the first air passage (18) between the first and second air gap (12, 14) is decreased if said difference is below said predefined threshold value.
The panel (2) according to claim 21, wherein said comparison is a calculation of the numerical difference of the temperature measurements from the two temperature sensors (24, 26), and wherein the fan (16) is regulated in such a way that
- the fan (16) is turned on or kept on if said difference is above a predefined threshold value, or

- the fan (16) is turned off or kept off if said difference is below said predefined threshold value.
The panel (2) according to any of the preceding claims, wherein the surface onto which the panel is to be mounted is a ceiling (6) or a wall.
The panel (2) according to any of the claims 20-24, wherein said controller (28) is furthermore operatively connected to the thermally activated building system.