WO1984002940A1

WO1984002940A1 - An arrangement in acoustic systems

Info

Publication number: WO1984002940A1
Application number: PCT/SE1984/000027
Authority: WO
Inventors: Krister Amneus
Original assignee: Krister Amneus
Priority date: 1983-01-28
Filing date: 1984-01-27
Publication date: 1984-08-02
Also published as: DK451584D0; DK451584A; EP0162049A1; NO843741L

Abstract

In an acoustic system which includes a substantially panel-like member (10; 13; 18; 26; 30; 34) which can be set into oscillatory motion transversally of its main extension for the purpose of generating, or damping and absorbing air-borne sound, the aforesaid member (10; 13; 18; 26; 30; 34) exhibits in a centrally located region (12; 23; 29) of considerably smaller area than the total area of the member, an air-permeability which is so much greater than that of the remainder of the member that said region dynamically dampens oscillations in the member proportionally to change in velocity of the oscillatory motion in said region.

Description

AN ARRANGEMENT IN ACOUSTIC SYSTEMS

The present invention relates to an arrangement in acoustic systems of the kind which include a substantially panel-like member which can be set in oscillatory motion transversally to its geometrical extensions, in order to generate, or dampen and absorb air-borne sound.

Such acoustic systems can be divided into two main groups, namely sound-generating and sound-absorbing and/or sound-screening systems. The sound-generating group includes loud speakers or microphones, which operate with an- electric signal and a moving diaphragm. Such acoustic arrangements are normally connected to an air-filled chamber and, in this way, become suspended against an air-spring formed by the air in the chamber. Thus, the actual acoustic transducer has been given a mechanical resonance frequency, the value of which depends upon^' the relationship between the spring force in the diaphragm suspension and the mass in motion. The natural resonance frequency is called f , and may, for example, lie at about 20 Hz in the case of a conventional low-frequency loud speaker, which is driven by a voltage applied to an osci^atory coil in a magnetic field. At this resonance free .c , the resistive oscillation damping in the acoustic circ-.t is minimal, and the motional amplitude of the dia¬ phragm is at a maximum - in other words damping in the oscil¬ lation circuit is at a minimum - resulting in non-linear distortion of the oscillatory motion of the diaphragm. When the loudspeaker is mounted in an air-filled, closed chamber, the resonance frequency f is caused to change to f_D ⁼f_s ys+1 , where f. is the new resonance frequency obtained in a pressure chamber system (airtight, sealed chamber volume), and

) -1, where is the ratio between the compliance of the loudspeaker and the compliance of the air enclosed in the chamber. The new resonance frequency f. is always higher in frequency than f , and damping in this motional system is also minimal at the resonance frequency f_fa, and the distortion thus maximal. In order to apply some frictional damping to the

OMP! moving system, it is normal to introduce mineral -fi bre material into the loudspeaker cabinet used, and therewith increase the degree of damping at the resonance frequency f_b. Only a modest increase in the damping of the system is achieved, however, and non-linear distortion and a high acoustic Q-value constitute problems for pressure-chamber loudspeakers, which become particularly pronounced at high sound-pressure levels.

With regard to the other main group of acoustic cabinets for loudspeakers, bass-reflex cabinets, other resonance frequency values are obtained, the resonance frequency in this case being designated f_Q. In this case, this frequency appears simultaneously with two antiresonance frequencies f. and f₂, which lie respectively beneath and above the resonance frequency f_Q, and approximately in accordance with ff fp-'f \ • Bass-reflex cabinets are provided with an acoustic port which connects the cabinet interior with the surroundings. The port gives rise to resonance with the loudspeaker at a given frequency, which - as before- is determined by the compliance ratio s_^ and the size A of the port. At the resonance frequency f_Q, the mouth area A of the port is added to the area of the loudspeaker and therewith loads the loudspeaker diaphragm, and the motional amplitude becomes minimal, and therewith also damping of the oscillating circuit, when the oscillating coil moves in the magnetic field. Similar to the former kind of construction - the pressure-chamber loudspeaker resonance conditions are obtained in which the oscilla¬ tion amplitudes are at a maximum, namely at the two anti¬ resonance frequencies f.. and f , at which damping in the system is also at a minimum. This problem of deficient damp- ing at discrete resonance frequencies generally occurs in acoustic constructions with oscillating diaphragms, and one object of the invention is to contribute substantially to improving the linearity of the oscillatory motion of the dia¬ phragm, by applying mechanically active motional damping to the system.

The arrangement according to the invention can also be used to advantage, for applying damping to an oscillating circuit active in absorbing or screening air-borne sound. At present, sheets of fibrous materials are almost exclusively used with various mounting means - so-called false ceilings - for the purpose of controlling the acoustics and damping noise in offices and industrial locales. The adsorbents often have the form of relatively thin panels, adjusted to module sizes of 1200 x 600 mm. The panels are normally mounted in a plane-parallel array in relation to the ceiling by a special profile construction which is arranged about 200 mm below the ceiling. The panels are given a density which imparts self- supporting properties thereto at a material thickness which can vary between about 70 mm and 15 mm. One disadvantage with constructions of this nature is that such a panel is unable to remain passive, when exposed to acoustic energy at low frequencies, or when exposed to acoustic pressure pulses. As a result hereof, the panel will react to sound waves and will generate a self-induced sound of a so-called random nature, unless the oscillatory motion cannot be developed as a sym¬ metric oscillatory motion in the panel, and therewith produce an oscillation maximum in the region of the geometric centre point of the panel..

Thus, the present invention is intended to provide an arrangement with which the aforediscussed problems can be at least substantially overcome.

To this end, it is proposed in accordance with the in- vention that in an arrangement of the kind described in the introduction said member, in a centrally located region of considerably smaller area than the total area of said member, exhibits such an increased air-permeability that said region dynamically dampens oscillations in said member proportional - ly to change in velocity of the oscillatory motion in said region. This greatly improves the transient properties of the acoustic system, since the arrangement according to the in¬ vention is able to affect the oscillating mass favourably, so that it is accelerated and decelerated more rapidly when momentarily excited by a force. The arrangement according to the invention also provides improved low-frequency properties and reduced distortion, above all at high oscillation ampli¬ tudes. The arrangement according to the invention is in¬ expensive to produce, in relation to the results achieved, and can be reproduced with great accuracy.

OMPI WlPO According to the invention, the region of increased air permeability can, to advantage, be encircled by a tubular element, whose longitudinal axis extends in the direction of the oscillations, thereby to further improve the reproduca- bility and function of the arrangement. For the purpose of tuning its function, the tubular element may be provided with an additive flow resistance, so as to increase the re¬ sistance to the flow of air through the element. This enables the proportions between static and dynamic flow resistance to be set in a ready fashion.

According to another preferred embodiment of the in¬ vention the aforesaid member comprises a porous, air-permeable panel, which externally of said region is provided on at least one of its sides with an air-impermeable blocking layer, e.g. a layer of plastics film or aluminium foil.

In order to obtain a high system accuracy and a high acoustic efficiency, especially with acoustic systems intended for absorbing and damping air-borne sound, an advantage is gained when the aforesaid member is stiffened along its de- fining edges.

With an arrangement such as that which includes a planar absorbent in the form of a panel suitable for mounting horizontally, the panel should be able to support itself, with¬ out any appreciable sag at the centre part thereof from the plane of the geometri c- extension of the panel, when the panel is only supported along an edge region thereof, e.g. to a width of 20 mm. Consequently, the panel must be relatively rigid. According to an advantageous embodiment of the inven¬ tion, in a planar absorbent provided with a panel which is re- inforced along its defining edges, there is created in said panel a moment of force which, when the planar absorbent is mounted in position, counteracts the tendency of the panel to sag under the influence of gravitational forces. Through this arrangement there is obtained a diaphragm absorbent in which the panel can be slender and smooth, with subsequently good oscillating tendencies. Thus, it is poosible to obtain great¬ ly improved absorption at low frequencies, while enabling the absorption of the panel to be increased in the mid frequency region, since the tendency of the panel to osci l late nas now been expanded upwardly in frequency, due to a reduced weight. When the panel comprises a sheet of mineral fibre, its weight can be reduced by about 20 - 40 % in comparison with known ab sorbents, while in principle retaining its sagging factor, or the panel can be made much thinner while substantially retain ing its weight, at an unchanged sagging factor, which favour¬ ably affects the acoustic function.

According to one advantageous embodiment of the inventio the aforesaid moment of force is created by means of a frame which encircles and holds the panel and which is provided wit a groove for receiving an edge portion of said panel, the sides of said frame being arranged to exert thrust forces whic act towards the centre of the panel, substantially in the plan thereof. Alternatively, or in addition thereto, moment of force can be produced by means of at least one asymmetrically located panel -engaging zone of a groove which accommodates the edge portion of the panel and which is formed in a -frame encircling and holding said panel. In this respect, particular advantage is afforded when the groove has a bottom, which slopes towards one side, so that when the panel is inserted into the groove, said groove-bottom will tend to bend the panel to an extent corresponding to the angle at which the groove-bottom slopes and to the extent to which the panel is inserted in the groove, as a result of the excentric engage¬ ment of said groove-bottom with the inserted panel-edge.

According to an alternative method, or to a supplementary method, particularly in the case of large panels, moment of force may be created by means of a spring force which acts on the central region of the panel and which counteracts the force of gravity acting on the panel.

As an alternative to producing said moment of force with the aid of said frame or said spring force, or as a supple¬ ment thereto, moment of force can be created in the panel , in accordance with the invention, by covering one side of the panel with a layer of material, such as a layer of .glue or paint having shrinkage properties, capable of exerting con- tractional forces in said side of the panel.

The invention will now be described in more detail with reference to a number of embodiments thereof illustrated schematically in the accompanying drawings.

Figure 1 is a plan view of a substantially panel-like absorbent according to a first embodiment of the invention. Figure 2 is a sectional view of an edge part of an ab¬ sorbent whose edges are stiffened by means of a frame.

Figures 3 and 4 are respectively a plan view and a sec¬ tional view of a centre part of an absorbent having a second embodiment of the arrangement according to the invention. Figure 5 illustrates a variant of the embodiment il¬ lustrated in Figures 3 and 4.

Figures 6 and 7 are respectively a plan view and a sec¬ tional view of an absorbent having a third embodiment of the invention. Figure 8 illustrates the centre part of a loudspeaker cone, in which a damping arrangement according to the in¬ vention is incorporated in a dust cap.

Figure 9 is a plan view of a corner portion of a planar absorbent according to a further embodiment of the invention. Figure 10 is a sectional view taken on the line X-X in

Figure 9.

Figure 11 illustrates schematically and in section an alternative method of producing an arrangement for counter¬ acting the gravitational forces acting on the planar ab- sorbent.

In Figure 1 the reference 10 identifies a panel-like ab¬ sorbent mounted in a frame 11, which may optionally be omit¬ ted, and provided with a valve 12, which may extend through the absorbent. The absorbent 10, which may be impervious to air or exhibit a certain degree of air-permeability, may be arranged to be set in oscillatory motion transversally to the plane of the drawing by the action of air-borne sound.

In Figure 2 there is illustrated a fibre-material absorbent 13 which is covered on both sides with surface layers 14. The reference 15 identifies a frame, shown in cross- section, having a groove 16 in which an edge portion of the absorbent 13 is received and fixed with the aid of glue beads 17.

JOMPI Figures 3 and 4 illustrate a central part of an, for example, absorbent panel-like element 18 provided with, for example, surface stabilizing layers 19, 20. Arranged in the element 18 is a through-passing hole, which accommodates a tube 21, which may be made of metal, such as aluminium, and which is secured in said hole with the aid of an adhesive. As indicated in chain lines 22, the length of the tube 21 may exceed the combined thickness of the element 18 and associated surface layers. One end of the tube 21 lies in the same plane as the layer 19, and may be fitted with a perforated end plate 23, in the manner shown, which in turn may be covered with a layer 24 of staple fibre. Alternatively, if the outer layer 19 is made pervious to air, e.g. comprises a woven glass- fibre fabric, said layer can be left intact, and in this way replaces the staple fibre layer 24 and optionally also the plate 23. The element 18 can be caused to oscillate in the direction of the longitudinal axis of the tube, by air-borne sound.

As illustrated in Figure 5, the tube 21 may optionally have inserted therein a substantially flow-resistive insert 25. In other respects, Figure 5 corresponds to the previously described Figure 4.

In the embodiment illustrated in Figures 6 and 7 a panel¬ like element 26, for example an absorbing element, is pro- vided with an air-permeable surface layer 27 and an air- impermeable surface layer 28, which may comprise a sheet of plastics film. In the region of the centre of element 26 there is arranged in layer 28 a hole 29, to allow air to flow through the element in said region. The element 26 can be caused to oscillate in a direction transversal to its geo¬ metric extension, by means of air-borne sound.

Figure 8 illustrates a central part of a loudspeaker cone 30, which has arranged centrally thereof a dust cap 31 provided with an attachment flange 32. Mounted in the dust cap is a tube 33, which may, for example, be of similar de¬ sign to the tube of Figure 4 or Figure 5, and which is able to move transversally to the plane of the drawing, together with the cone 30.

"BU EA

_O PI The respective areas of the central regions 12, 24, 29, 33 of the panel-like members 10, 18, 26 and 30, described with reference to Figures 1 - 8, are small in relation to the remainder of said members, while the resistance to the flow of air through said regions is so increased and adapted , optionally with the aid of additional resistances 23, 24, 25, that when the panel-like members oscillate perpendicularly to their main geometric extension, said regions 12, 24, 29, 33 dampen said oscillations dynamically, in proportion to the change in the velocity of the oscillatory motion in said regions. Thus, the aforesaid regions 12, 24, 29, 33 with as¬ sociated, optional additional resistance constitute means for damping the oscillations of the oscillating members 10, 18, 26, 30 in which they are arranged. In the embodiment illustrated in Figure 9, a planar ab¬ sorbent includes a panel-like member 34 mounted in a frame 35. The member 34, which may be air impermeable or have a degree of permeability to air, is provided centrally thereof with an arrangement according to any of Figures 3 - 7. The panel-like member is intended to be held fixed along the edges thereof while the remainder of the panel is intended to be able to oscillate in a direction transversally to the plane of the drawing under the effect of air-borne sound, with said oscillations increasing towards the centre of the panel-like member. In the Figure 9 and Figure 10 embodiment it is assumed that the panel-like member 34 comprises a fibre material 36, which is covered on one side thereof with a surface layer 37 of, for example, a woven fibre-glass fabric. The sides or parts 38 of the frame 35 are shown mitred at the corners, and are held together by means of angle pieces 39, which are suitably made of plastics. The connection between the parts 38 themselves, and between the frame 35 and the panel-like member 34 may also, to advantage, be supple¬ mented with an adhesive bond. The frame parts 38 comprise profiled sections having arranged therein a groove which accommodates the edge portions 40 of the panel 34, said groove being defined by portions 41, 42, 43 of the profiled sections forming the walls and bottom of the groove. The

OMPI section-portions forming the groove walls 41,42 extend in¬ wardly from the edges of the panel-like member 34. Extending in opposite directions are L-shaped section-portions 44,45 which, together with the section-portion 43 forming the bottom of the groove, define a channel which accommodates the angle pieces 39. The angle pieces fill the entire cross-section of the associated channel, and have a part 46 which projects out¬ wardly through an outwardly open slot, -and which serves to accomplish an acoustically dampened abutment against a support- ing or bearing structure for the planar absorbent. In order to avoid generation of alient noise, and in order to provide an air seal around the planar absorbent, the absorbent is resting on its supporting structures via strips of plastics or rubber. As illustrated, the section-portion 43 is inclined to the hori- zontal, and the inner surfaces of the section —portions 41 , 42 converge towards the portion 43. Thus, when the profiled sections 38 are inserted evenly on the panel-like member 34, the respective edge portion of said member will be slightly compressed, and the peripheral surface of the member will ultimately come into contact with the portion 43 forming the bottom of the groove, whereupon said peripheral surface is deformed and induces a stiffening and bending moment of force in the panel 34, which flexes the panel 34 elastically, down¬ wardly in Figure 10, this downward flexing of the panel reach- ing a maximum at the centre of the panel 34. The resonance frequency of the panel is influenced and raised by this moment of force. The magnitude of said force can be adjusted, by adjusting the extent to which the profiled sections are pushed onto the panel 34. In the Figure 10 embodiment, the layer 37 is arranged on what will become the lower surface of the planar absorbent. Consequently, when the planar absorbent is brought to its intended mounting position, the induced moment of force will counteract the gravitational forces acting on the panel 34, so as substantially to avoid sagging of the panel and to obtain in substantially the plane of the panel a neutral plane favourable to the symmetry of the oscillations.

As will be understood, a bending moment can be induced in the panel in ways other than that illustrated. For example, the section-portions 41, 42 can be caused to slope relative

.OMPI to the plane of the frame, such that the edge regions of the panel are flexed elastically, therewith to generate a bend¬ ing and stiffening force in said panel. It is also possible to create the whole or part of the desired force in the actual page! 34 itself, by covering one side of the panel with a coating of material, which will exert contraction forces on the panel, such as a layer of an adhesive or paint having shrinking tendencies. The frame 35 is suitably made of aluminium, a plastics material or sheet metal. The panel 34 may to advantage comprise a sheet of fibre-glass, and thickness of the panel may be from a few millimetres and up¬ wards, depending upon the density of the material from which the panel is made, and the size and shape of said panel. For example, a panel covered with a painted fibre-glass fabric and measuring 1200 x 600 x 18 mm and having a density of _3 55 kgm can replace a conventional panel having a thickness of 25 mm while being similar in all other respects, there be¬ ing obtained approximately twice the dynamic compliance of the panel for induced oscillatory energy, in addition to a saving of about 30 % with respect to the amount of fibre material used.

In the embodiment illustrated in Figure 11 , the desired moment of force can be generated, either totally or in part_» by means of a spring force. The Figure 11 embodiment includes a panel 34 whose edge regions rest on supporting members 48 carried by a ceiling structure through attachments 49.^' The panel 34 is provided at its centre 50 with an arrangement according to any one of Figures 3 - 7. At 51 there is shown schematically a tension spring which acts between the central region of the panel and a transverse member 52; alternatively, the spring may act between said central region and the ceiling structure 53. As will be understood, the spring counteracts the sagging tendencies of the panel 34 due to the gravitational forces acting thereon. When applying the invention in conjunction with loud¬ speakers, the dynamically active oscillation-damping means may have the form of an opening which penetrates the central part of the diaphragm (the cone 30) and which permits air to pass from the front side of the diaphragm to the rear side thereof, to a certain extent. The opening is then provided ^" with additive flow resistance and optionally also with a viscou friction component (the tube 33). When using a tube, such as the tube 33, of given area and given length, the tube may be placed in the normally used airtight dust cap covering the moving coil. In this case, the longitudinal axis of the tube is presumed to extend in the oscillating direction of the diaphragm. This fundamental dynamic damping means can be tuned, for example, as follows: Assume that the oscillations to be dampened originate from a 10' ' bass-loudspeaker of the high-compliance kind having, for example, a natural resonance frequency in free air of f = 22 Hz, and that the loudspeake is to be equipped with a matched dynamic damping means in the centre of a mechanically stable dust cap, that the dust cap has a diameter of 50 mm, and that the diameter of the oscilla ting coil is 39 mm. The unit shall be matched to operate in a pressure-chamber casing having a volume of 30 dm . If there is used a flow channel which penetrates the magnetic circuit, then this channel may be tuned by arranging therein a flow resistance and may suitably be arranged co-axially with the opening in the dust cap, and also suitably be given sub¬ stantially the same diameter as said opening. The damping means should suitably be tuned to approximately f and have a tube extension which is not too short, said length pre- ferably being equal to, or somewhat greater than the diameter of the opening. This fundamental dimensioning of the damping means is not critical, but may serve as a guidance for reach¬ ing a practical construction.

According to the general formula for a resonance f of p the opening, with a box volume V. , there is obtained:

2TT If ( - V ) (0,96 VA - t_) HZL

where c can be approximated to 3448 dm .• sec^"1, and the volume of the tube, V = A

P P outlined parameters of V may generally be ignored. When the the aforedescribed construction are entered into the equation, it is found that the area A of p the opening should lie on a diameter of about 16 mm at a tube length t of about 20 mm, when a perforated cover plate is used in which the total area of the perforations is about 50 %, in order for the resonance frequency of V. to reach about 22 Hz. The tube is filled with a suitable flow resistive material (e.g. matted acryl fibres, 20 mm) and may be terminated against the surroundings at the mouth of the tube, by means of the perforated plate, which is preferably made of rigid aluminium and light - suitably, such a plate is placed at both ends of the tube, to avoid displacing the resistive material located therein. Because the motional amplitude becomes maximal at this geometric location at transient sound passages and at the resonance frequency f of the acoustic system, there is obtained with the described arrangement an effective, dynamic motional damping which favourably affects distortion and diaphragm amplitude. It is important to ensure that the flow resistance in the tube is sufficiently high to avoid air-penetration noise. If the flow resistance is too low, the air which flows rapidly through the arrangement may generate disturbing sound effects. Other conceivable embodiments may include a perforated plate without the tube, said plate being adhesively bonded to a layer of, for example, staple fibres, etc. The damping func¬ tions of the acoustic system is favourably affected when a channel is located in the magnetic circuit, said channel form¬ ing a stationary, viscous or purely resistive additional para¬ meter.

If the flow resistance is not sufficiently high, there will be obtained in-the closed box a pronounced flow through the dynamic damping means, due to the high internal pressure in said box. The flow through the damping means is smaller in the case of an alternative construction incorporating the bass-reflex principle (low internal pressure), and the in¬ tended oscillation damping can suitably be adjusted, by simply reducing the flow resistance. A suitable consctruc- tional variant of the dynamic damping means may then be ef¬ fected as before, although possibly without the flow re¬ sistance in the tube and then solely with a layer of staple fibre (e.g. 50 gm grade) glued onto a perforated aluminium plate in the mouth of the tube facing the surroundings. In all applications, there should be arranged in the centre of the magnet, if possible in the region of the extension of the oscillating coil, a ventilation channel, the through-flow properties of which may be adjusted with the aid of a flow resistance, so that the internal chamber volume V. is able to communicate with the damping means. When the so-called spider (the resilient holder of the moving coil) is permeable to air, ventilation can be effected solely through the spider. In the case of diaghram type arrangements intended for the absorption or screening of air-borne sound, the damping means can be constructed in a similar fashion. A sheet of mineral fibre, or an air-impermeable panel, is then suitably mounted along its defining edges, so that the oscillation amplitude is there practically zero, and the absorbent or the like can be made in the size ratios of 1:1 or 2:1 , and may be flexible to oscillatory motion perpendicular to its extension in the plane.

The absorbents incorporated in a false-ceiling structure may be 1200 x 600 x 20 mm in size, and - as before mentioned - may be mounted in a suitable frame structure. The natural resonance frequency of the absorbent can be measured by applying an accelerometer, e.g. on a support plate of small area (about 30 mm diameter) at the centre of the absorbent. A slight stroke on the accelerometer can be registered by, e.g., an FFT-analyzer, where the time function and spectrum of the motion is recorded. Thus, the natural frequency of the absorbent is obtained, and analogously with the afore- given formula for calculating loudspeaker design, it is also possible to determine the area and axial length of the damping means. When mounting the arrangement at a given distance below a ceiling, each absorbent or panel constructed in accordance with the invention can be considered to operate on a rearwardly applied spring (oscillating volume) V , which corresponds substantially to the area of the absorbent or panel multiplied by the distance to the ceiling. In this way, each absorbent or panel is acoustically impedance matched to the air, as a result of the oscillation damping effect obtain¬ ed with the present invention, and absorption at low frequency UJ E

> OMPI ^ι W is greatly improved as well as the acoustic efficiency. When the absorbent or panel constitutes a partition wall in an otherwise substantially closed chamber, there is obtained a defined chamber volume V^. In order to increase absorption and to dampen any resonance phenomena, an absorbent material, e.g. 50 - 100 mm thick mineral wool web having a density of,

_3 for example 24 kgm , can - in both cases - be arranged on a rearwardly lying wall or ceiling, with an air-space between said material and the absorbent or panel. In this manner the frequency range of the low-frequency absorption may be favour¬ ably broadened, with a defined chamber volume V^. If, in the mentioned case, the absorbent has a natural frequency f of 22 Hz and the volume V or V_u is 144 dm (distance of the absorbent from the ceiling or rearwardly lying wall 200 mm), the length of the tube can, for practical reasons, be made equal to the thickness of the absorbent, i.e. a length of 20 mm, wherewith - according to the formula for port resonance - the tube should have a diameter of about 56 mm, when a per¬ forated plate with a 50 % total hole area is used. In order to ensure that the damping means provides full efficiency, said means should be provided with a layer of staple fibres over the holes in the cover plate, and when the absorbent comprises an air-permeable fibre material, it should be pro¬ vided on one side thereof with a plastics film which short- circuits the flow resistance of the absorbent, except in the immediate location of the damping means, for example in accordance with Figures 6 and 7. Such a fibrous membrane-li e absorbent should also be provided with a surface stabilizing cover layer of, for example, fibre-glass fabric, on the other side thereof, for example on the side facing the sound source. A surface stabilizing cover layer has a damping effect on the fibre core, so as to prevent its surface from breaking-up into discrete portions, which execute troublesome random oscilla¬ tions, and also assists in concentrating the oscillation energy in the absorbent to the central region thereof. With an absorbent or panel provided with an arrangement according to the invention, there is obtained, even without a rearward¬ ly lying volume V or V^ (air spring), a dynamic moment which opposes the oscillatory motion, as a result of the in¬ crease in flow through the central region of higher air- permeability, so that the absorbent or panel functions with a dynamically variable additional mass. Consequently, the arrangement according to the invention has a pronounced favour¬ able effect, even when mounted freely in a room, in order to screen-off an acoustic source for example.

When an acoustic fal se-cei.l ing, extending substantially from wall-to-wall, is constructed with the use of correspond- ing fibrous, air-permeable panel-like absorbents, although not reinforced along the edge regions thereof, there is ob¬ tained a complex Helmholtz resonator system, instead of the aforediscussed membrane-absorbent system. In such a Helmho'ltz resonator system, which is less favourable than the aforesaid membrane-absorbent system, the mechanical natural resonance frequency of the absorbents, because said edge regions are not reinforced, will appear at a much lower frequency, e.g. 10 Hz, than the specific Helmholtz resonator frequency, e.g. 200 Hz. The damping means according to the invention, however, will act, as previously, in an oscillation damping mode at both said natural resonance frequency, thereby reducing genera¬ tion of undesired sub-frequencies, and at the Helmholts re¬ sonance frequency, and pressure differences between the spaces above and below the false-ceiling will be equalized in a favourable fashion.

Claims

1. An arrangement in acoustic systems of the kind which include a substantially panel-like member (10; 13; 18; 26; 30; 34) which can be set into oscillatory motion trans- versally to its geometrical extension for the purpose of generation or for damping and absorbing air-borne sound, characterized in that said member (10; 13; 18; 26; 30; 34) exhibits in a centrally located region (12; 23; 29) of con¬ siderably smaller area than the total area of said member an air-permeability which is so much greater than the air- permeability of the remainder of the panel that said region dynamically dampens oscillation of said member proportionally to change in the velocity of the osci 11atory motion in said region.

2. An arrangement according to Claim 1 , characterized in that said region is enclosed by a tubular element (21; 33) whose axis extends in the direction of the oscillatory motion.

3. An arrangement according to Claim 2, characterized in that the tubular element (21; 33) is provided with an ad¬ ditive flow resistance (24, 25), to increase the resistance to the flow of air through said element.

4. An arrangement according to any one of Claims 1 -

3, characterized in that said member (26) has the form of a porous, air-permeable panel which is provided on at least one side thereof externally of said region (29), with an air- impermeable blocking layer (28), e.g. plastics film or alumini um foi 1.

5. An arrangement according to any one of Claims 1 -

4, characterized in that said member ( 10; 13; 18;26;30; 34) is stiffened along its defining edges.

6. An arrangement according to any one of Claims 1-5, which includes a planar absorbent in the form of a panel (34) suitable for mounting horizontally, characterized in that there is created in the panel (34) a moment of force which when the panel is mounted in position, counteracts the tendency of the panel to sag under the influence of gravitational forces.

7. An arrangement according -to Claim 6, characterized in that said moment of force is created by means of a frame (35) which encircles and holds the panel (34) and which has a URE

.. OMPI groove for receiving an edge portion (40) of said panel, the sides (38) of said frame producing pressure forces which act towards the centre of the panel, substantially in the plane • thereof.

8. An arrangement according to Claim 6 or Claim 7, characterized in that the moment of force is created by means of at least one asymmetrically located panel-engaging zone (43) of a groove which accommodates the edge portion (40) of said panel (34) and which is provided in a frame (35) encircling and holding said panel.

9. An arrangement according to Claim 8, characterized in that the groove has "a bottom (43) which slopes towards one side.

10. An arrangement according to any one of Claims 6 - 9, characterized in that moment of force is created by means of a spring force (51) which acts in the central region of the panel, and which counteracts the effect of gravity on the panel.

11. An arrangement according to any one of Claims 6 - 10, characterized in that the panel (34) is covered on one side thereof with a layer of material which exerts contrac- tional forces in said one side.