EP0024044A2 - Sound absorber, in particular for anechoic chambers - Google Patents

Abstract

1. Sound absorbing cladding, especially for anechoic chambers, having a plurality of adjacent, substantially wedge-shaped sound absorbers of sound absorbing material, characterized in that a) the base surface (3) of the sound absorber is spaced from the acoustically hard chamber wall, b) the sound absorber is provided with at least one recess (4) in the region of the base surface (3), the cross-section of which tapers towards the interior of the sound absorber, and c) the base surface (3) of the sound absorber is not acoustically tightly sealed.

Description

The invention relates to a sound absorber, in particular for anechoic rooms, which is wedge-shaped and can be attached at a distance from a room wall.

For example, to record directional characteristics of sound sources or to check and calibrate sound transducers or sound measuring devices, examinations are carried out in extremely low-reflection sound measurement rooms, which are referred to as anechoic rooms and allow sound propagation as in the free field without being influenced by reflections. To prevent reflection, such rooms are lined with sound-absorbing materials, which usually have a strongly structured surface structure. Particularly suitable for this purpose are wedge-shaped absorber elements, ie elements whose cross section increases in the direction of the room wall. These known wedge-shaped absorbers are either symmetrical, with their two converging surfaces having the same angle of inclination with the base surface (DE-PS 809 599) or asymmetrical, with only one wedge surface of the absorber element being inclined, while the other is at right angles with the Base area forms (DE-PS 25 02 846). The zigzag shape of the surface, which is created when several of these wedge-shaped absorbers are arranged, results in a continuous transition from the air into the sound-absorbing material, since a preferred plane for reflections is no longer available.

An improvement in the absorption capacity of wedge-shaped absorbers of this type can be achieved by arranging them at a certain distance from the wall of the room, wherein slots can also remain free between the absorber wedges (DE-PS 878 731). The acoustic mechanism of action of these air gaps between the base surface of the absorber wedges and the reverberant room wall is not known in detail. It is conceivable that effects such as are known in so-called Helmholtz resonators occur. In any case, the additional interface for sound propagation, which is formed by the base surface, should absorb additional sound energy if the sound waves at the base surface exit into the gap to the wall of the room, are reflected there and then re-enter the base surface.

It seems certain that, given the length of the absorber wedges, increasing the distance to the wall of the room improves the degree of sound absorption at low frequencies and thus dampens them better. The lower limit frequency f _o is the frequency at which the sound reflection factor r increases beyond a value of 0.1, that is to say the sound pressure amplitude of the reflected wave corresponds to more than 1/10 of the amplitude of the incident wave. The value of the sound reflection factor r of 0.1 corresponds to a sound absorption level of 99% of the incident sound energy. Below the lower limit frequency f, the sound reflection factor r rises above 0.1, so that sufficient sound absorption can no longer be achieved for lower frequencies, while for all higher frequencies the sound reflection factor r is usually well below 0.1 if a suitable porous material for the sound absorber is used.

The reason for the appearance of a quite clearly defi Nated lower limit frequency for sufficient sound absorption, below which there is a steep drop in sound absorption, should be that at even lower frequencies the antinode of the maximum speed of the incident wave and reflected on the reverberant wall of the room comes to lie at such a distance from the reverberant wall of the room that exceeds the length of the sound absorbers installed there. It has been shown that the lower cut-off frequency can be easily shifted to lower frequencies by giving the wedge-shaped sound absorbers a greater length that protrudes from the wall of the room, the lower cut-off frequency being approximately the one whose wavelength is four times the value of the distance Top of the absorber elements from the room wall corresponds. It follows from this that sound absorption sufficient for anechoic rooms can only be achieved for those frequencies whose wavelength il is less than four times the distance of the tip of the absorber elements from the wall of the room, so that the dimension λ / 4 is still in the absorber material at a distance of the wall of the room comes to rest. The absorption material in the immediate vicinity of the room wall is relatively less effective, since the sound velocity in the fully reflecting plane of the reverberant room wall is by definition zero and only takes on finite and ever increasing values at a distance from the reverberant wall of the room until the maximum at a distance reached by Z / 4.

A distance between the base surface of the dimensionally predetermined absorber elements and the reverberant wall of the room thus leads to a shift in the material distribution of the absorber elements away from the wall of the room The lack of material above an installation of the base surface on the wall of the room does not lead to any noticeable disadvantages, while the material added compared to an installation of the base surface on the wall of the room due to the gap at the front of the absorber elements has a considerable damping effect in the region of the lower limit frequency f and thus acoustically is extremely effective. In addition to any other effects in the sense of an additional destruction of sound energy, a distance between the base surface of the absorber elements and the room wall with otherwise the same dimensions, in particular the same wedge length, gives the absorber elements the advantage of lowering the lower limit frequency without additional material.

As long as the gap between the base surface of the absorber elements and the wall of the room is not too large, the lower cut-off frequency can be reduced in this way without additional material, but the installation depth of the sound absorber must be increased in any case, i.e. the thickness of the other Inside the room wall adjoining absorption area. This reduces the usable room size. For a further reduction of the lower limit frequency, increased use of material is also inevitable if a maximum value of the gap between the base surface of the absorber elements and the wall of the room is not to be exceeded.

In contrast, the invention has for its object to provide a sound absorber of the type outlined in the preamble of claim 1, which allows a further lowering of the lower limit frequency without additional use of material for a given installation depth.

This problem is solved by the character nenden features of claim 1.

This measure of making a recess in the base area of the absorber surprisingly results in a significant reduction in the lower cut-off frequency without any other additional changes to the absorber elements or their spatial arrangement. If one determines the frequency response or the reflection factor r in accordance with DIN 52 215 in Kundt's pipe, a lower cut-off frequency is obtained for the sound absorber according to the invention, which is significantly lower than the lower cut-off frequency of the same absorber in the same installation position, but which has no recess in its base area . A comparison measurement showed that the cut-off frequency was reduced from 94 Hz to 84 Hz, ie by more than 10%. It is obvious that the recess also leads to a corresponding saving in material and in particular weight. Furthermore, fluctuations in the density of the absorption material of the sound absorbers in the order of magnitude of + 10% can be absorbed with a constant installation depth, so that the cut-off frequency determined for a nominal density is retained even in the case of fluctuations in density, which would have a greater impact without such a recess in the base area have the cutoff frequency.

A scientific clarification of these phenomena has not yet been achieved. The effects which can be achieved according to the invention were also not to be expected, since the removal of material in the area of the base area - apart from the material and weight savings which would naturally be expected - only expected disadvantages in the acoustic area. It is true that the base area lies in an area of the sound absorber that is not preferably effective with regard to sound absorption in the manner explained in the introduction, in particular not at low frequencies, the large frequencies of which Wavelength in this range gives only relatively low speed amplitudes and thus damping possibilities. In any case, this measure would at most have expected a more or less slight, but in any case existing reduction in the total sound absorption and thus also the sound absorption in the range of the lower cut-off frequency, but in no case an improvement in sound absorption especially in the range of the lower cut-off frequency . At higher frequencies, measurements actually result in a reduction in sound absorption due to the material recess at certain frequencies in the manner to be expected theoretically, but this reduction in sound absorption is not critical in the case of sound absorbers according to the invention, since at higher frequencies the limit value of the sound reflection factor r of 0.1 in one as in the other case, it is safely undercut. Through empirically determined special shaping of the recess, a reduction in sound absorption compared to a smooth base surface can also be specifically excluded if necessary, even at certain higher frequencies.

To explain the phenomenon found, an analogy with the mode of action of the distance or gap between the base surface of the sound absorber and the wall of the room is also ruled out; because in the case of such a gap, the improved sound absorption at low frequencies results simply from the fact that absorber elements of a predetermined wedge length receive a greater distance from the reverberant wall of the room through the gap with their tips and thus in the theoretically explainable and initially explained manner at lower frequencies become increasingly effective. On the other hand, would be the case with absorber elements lying against the wall of the room without any other change in position to form such a gap If, in the area of the base areas, slices were cut out, there would be no better sound absorption properties at lower frequencies, but in the expected manner there would only be a, albeit very slight, reduction in the total absorption due to the fact that material, even if at a non-critical point, has disappeared. In contrast to this, however, according to the invention, the mere measure of removing material through the recess in the area of the base area results in a significant lowering of the lower limit frequency and a greater independence of sound absorption from fluctuations in the spatial density of the sound absorption material, which must therefore be based on completely different mechanisms of action. It can be assumed that such recesses in the area of the base surface also result in a slight reduction in the total sound absorption due to the absence of sound absorption material, but this effect is more than compensated for by a still indefinable phenomenon, especially in the range of lower frequencies, which ultimately leads to leads to a significant improvement in the sound absorption properties, especially in the range of low frequencies.

From the DD-PS 81 712 panels used for wall cladding are known which, in addition to having a visually appealing appearance, should also have sound absorption properties and, for this purpose, can also be produced from open-cell foamed plastics, that is to say from a sound absorption material. These panels are essentially U-shaped and therefore have a large cavity on their side adjacent to the wall, which serves to save weight and should also be acoustically effective in order to achieve sound absorption, especially at low frequencies. Obviously, an effect like a Helmholtz resonator, as he suspects, is being considered Lich also occurs in the known sound absorber wedges for anechoic rooms explained above; that a gap is provided between the base surface of the absorber wedges and the wall of the room. However, these known panels for general sound absorption in use-related sound-stressed rooms are decorative wall claddings with a certain acoustic effectiveness that is neither precisely defined in terms of absorption frequencies nor in terms of sound absorption. The inside of the central web of the U-shape corresponds in terms of sound technology essentially to the base surface of generic absorber wedges, while the side legs of the U-shape only bridge the distance to the wall in order to allow the panels to be glued there. In order to create generic sound absorber wedges for anechoic rooms, a long, slender wedge would have to be attached to the outside of the center web of the U-shape, with a length that corresponds to λ / 4 of the lower cut-off frequency. In order to be able to lower this even further without extending these wedges, the recess according to the invention would then have to be provided on the inner surface of the central web of the U-shape corresponding to the wedge of the generic type, a measure for which both the constructive and the functional aspect from the DD -PS 81 712 no information can be found. The flat wedge shape of the center web of the U-shape, which can also be found in DD-PS 81 712, obviously only serves to increase the rigidity of the panels and is therefore not comparable to generic, slim wedges with a length of 1 m, for example, because the extremely flat wedge shape of these known panels results in no significant increase in installation depth.

For the recess in the base area of the Schallab sorbers different cross-sectional shapes can be selected, provided that the condition specified in claim 1 remains fulfilled that the clear cross-section of the recess decreases towards the interior of the absorber. For example, the outline of the recess on the base surface can have the shape of a polygon. In a longitudinal section of the absorber element, the recess preferably has the outline shape of a triangle, rectangle or trapezoid perpendicular to the room wall.

Such a rectangular sectional shape can completely penetrate the base surface, which is, for example, a square or a rectangle of larger area, so that the recess is open on two opposite sides. This is the case, for example, if a wedge with a rectangular base is cut from the base surface in such a way that a continuous slot results. If a symmetrical wedge is cut into the absorber, there is at the same time a linear increase in the two side surfaces of the recess.

A non-linear increase occurs in the case of forms of the recess which taper in a pointed manner, i.e. H. with pyramid or conical design. In addition, mixed forms are also conceivable in which the tapering takes place in steps or a combination of wedge and rounded edge is present.

The recesses can either be subsequently cut or punched into an already manufactured sound absorber, or can be provided in the sound absorber from the outset by using suitably designed shapes. It does not matter whether the sound absorbers are provided with a shaft or only have a wedge or pyramid shape. The area of the recess in the base area of the sound absorber can be in one vary wide range, for example, all edges of the recess can coincide with the side lines of the base surface. The base area of the recess in the plane of the base area is preferably about half to a quarter of the base area.

The depth of the recess, which extends from the base surface to the apex of the recess, can also vary within a wide range. For example, it can extend almost to the tip of the sound absorber. The depth of the recess is preferably one third to one eighth of the total lining depth, the total lining depth being understood to mean the total length of the sound absorber, including the distance of the sound absorber from the room wall.

The sound absorber itself can consist of the usually open-cell materials, for example plastic-bound mineral fibers or foam plastic. The finished sound absorbers can be impregnated on their surface. Furthermore, the plastic-bound mineral fiber layers can be covered with a fleece or a textile surface protection made of synthetic or natural fibers. A close-knit, but acoustically transparent, knitted sheath, which consists of synthetic fibers, which are at least flame-retardant, is particularly suitable for this. Such a coating also forms a kind of trickle protection for mineral fiber particles. With such a flame-retardant knitted fabric as a covering in combination with sound absorption material based on mineral fibers, all official fire protection regulations can be met. The textile covering cannot be used as a food source by microorganisms, so that no microorganisms can settle on the absorber, which is also sometimes important. Try mold Mushrooms, yeasts, gram-positive and gram-negative bacteria as well as spore-forming agents have shown that no infection occurs even in a humid atmosphere. Such an elastic knitted sheath sticks to the rough surface of the mineral fiber material due to static friction, with a certain clawing occurring. By resilient biasing of the enclosure _A bsorberelemente greater height can be constructed from individual mineral fiber sheets, which are stabilized by the envelope in their mutual position and overall spans. In the area of the base surface of the absorber wedges, the covering is pulled inward by its elastic property, so that its edge comes to lie in the area of the recess and no thickenings such as beads occur due to the material of the covering on the base surface, which would, for example, make attachment more difficult . This is a particular advantage of such a sheathing in the case of sound absorbers according to the invention, but the use of such an elastic tubular sheathing made of knitted material, which in particular in the direction of the stitch row can have considerably greater extensibility than in the direction of the wales, is not limited to the sound absorber elements according to the invention.

The sound absorbers provided with the recess according to the invention can be arranged in several rows or meandering for the construction of anechoic rooms. Provided that the number of the form is chosen, it is expedient to form the base surface of the sound absorber square to the to be arranged in another row sound absorber by 90 ⁰ rotated to mount.

The invention is explained in more detail with reference to the drawing and the following description.

• Fig. ₁ einen erfindungsgemäßen Schallabsorber in einer räumlichen Ansicht ohne Umhüllung,
• Fig. 2 eine Draufsicht auf den Schallabsorber gemäß Fig. ₁, jedoch mit Umhüllung und in seiner Zuordnung zu einer Raumwand, und
• Fig. 3 ein Diagramm mit zwei Reflexionskurven, in dem ein bekannter Absorber und ein erfindungsgemäßer Absorber verglichen werden.

It shows

• _{1 shows} a sound absorber according to the invention in a spatial view without an envelope,
• Fig. 2 is a plan view of the sound absorber according to FIG. ₁ , but with a cover and in its assignment to a room wall, and
• 3 shows a diagram with two reflection curves, in which a known absorber and an absorber according to the invention are compared.

The sound absorber shown schematically in Fig. 1 has a wedge-shaped upper part 1, which is symmetrical in the present embodiment. A rectangular shaft 2 adjoins the upper part 1 and has a square base surface 3. In this cuboid shaft 2 a wedge-shaped recess 4 is cut symmetrically to the longitudinal axis of the absorber from the base surface 3, so that a continuous slot is formed.

The sound absorber shown consists of mineral fiber material which is produced in layers 7 of limited thickness. Layers 7 of mineral fiber material of this type are stacked in the illustrated manner to form the sound absorber in the example in each case with the same area, and if necessary the individual layers 7 can be stitched together, for example by adhesive. A further securing of the positions of the layers 7 in the composite takes place by means of a sheathing 8 illustrated in FIG. 2 in the form of an elastic tube, which is essentially only stretchable in its circumferential direction of the stitch row, which is illustrated in FIG. 2 by arrow 9. In the direction of the stitches according to arrow 9, however, the hose-like sheath 8 has a very high degree of extensibility and is, for example, opposite unstressed condition to three times the circumference or even more stretchable. Such stretchability can be achieved well with knitwear, in the simplest case the stitch loops lying in the direction of the wales are pulled out to a considerable length in the knitting or knitting machine, so that the stretchability in the direction of the stitch row can be achieved even with simple, smooth goods. that the long stitch loops lying in the direction of the wales are deformed in the direction of the course. For the reasons described in the introduction, the sheathing 8 is of narrow mesh and is produced on a circular knitting machine which works with a large number of needles. The sheath 8 consists in the example of synthetic, flame-retardant fibers, which belong to the group of acrylic fibers. In the present case, acrylonitrile-based synthetic fibers were used.

• - Reißfestigkeit 1,8 - 2,2 g/dtex
• - Bruchdehnung -45 - 55 %
• - relative Knotenfestigkeit - 95
• - Kochwasserschrumpf 1 - 2 %
• - spezifisches Gewicht ₁,₃₅ g/cm
• - Feuchtigkeitsaufnahme 1 - 2 %
• - Glasumwandlungspunkt 83 - 85°C
• - Lichtechtheit (Blauskala) Note 6 (in jeder Farbtiefe, d. h, kein Vergilben)

The synthetic fibers have the following physical properties:

• - Tear resistance 1.8 - 2.2 g / dtex
• - elongation at break -45 - 55%
• - relative knot strength - 95
• - Cooking water shrink 1 - 2%
• - density _{1, 35} g / cm
• - moisture absorption 1 - 2%
• - Glass transition point 83 - 85 ° C
• - Light fastness (blue scale) grade 6 (in any color depth, i.e. no yellowing)

The tube forming the sheath 8 can first be prefabricated endlessly on the circular knitting machine, then cut to the desired length and at one end, for example, with an overcast chain stitch seam indicated at 10, such as a safety seam or an imitated safety seam, or also be closed in any other suitable way. The assembly of the tube thus formed on the sound absorber can be carried out in the simplest case by rolling up the tube after the seam has been applied and rolling it over the body of the sound absorber after it has been attached to the tip 9. In the case of unfavorable forms of the sound absorber, however, this can lead to technical difficulties, since there is strong static friction due to claws between the meshes ^: the casing 8 and the fibrous surface of the sound absorber. In such a case, a pipe with a smooth surface can be used in a manner not shown in detail, the diameter of which completely describes the outside diameter of the sound absorber and the outer circumference of which the tube forming the casing 8 can be fitted without problems. By pulling off this auxiliary assembly pipe between the sound absorber and the drawn-on hose, the hose contracts and thus forms the elastically relatively tight covering 8 on the outer circumference of the sound absorber, where no relative movements have to occur anymore. The casing 8 is kept slightly longer than the length L of the sound absorber shown in FIG. 2, so that the open end 8a of the casing 8 contracts over the base surface 3 and its edge can be placed in the recess 4. So that the rear end of the sound absorber forming and for storage part of the base surface 3 is covered by a flat layer of the casing 8 and material beads at the end of the casing are loosely in the recess 4, so that they are a clean storage of the sound absorber on the base surface 3 do not hinder.

The sound absorber according to the invention, which according to FIG. 2 has the length L, is arranged at a distance 5 from a room wall 6 and thus forms a cavity or gap between the room wall 6 and the absorber.

The length L together with the distance 5 forms the total lining depth of the sound absorber arrangement according to the invention.

3 shows the measurement results of a comparison measurement between a known wedge-shaped sound absorber without a recess and the wedge-shaped sound absorber according to the invention. The sound reflection curve A corresponds to the known absorber, while curve B corresponds to the absorber with the recess according to the invention. In both cases, an absorber wedge was used, the total length L of which was 850 mm and which was arranged at a distance of 50 mm from the room wall 6. A material made of plastic-bound mineral fibers with a density of 33.7 kg / m ^{3 was} used for both types of absorbers.

The comparison measurement shows that the cut-off frequency of the known sound absorber is 94 Hz, while the lower cut-off frequency of the sound absorber according to the invention is 84 Hz. The frequency range that can be used for sound measurements in the anechoic room has been increased by almost 11%. Sound absorbers for anechoic rooms have lower limit frequencies in the range of at most 100 Hz, and should be as low as possible with regard to the lower limit frequency.

Claims (12)

1. Sound absorber, in particular for anechoic rooms, which is essentially wedge-shaped and can be attached at a distance from a room wall, characterized in that in the region of the base surface (3) of the absorber adjacent to the room wall (6) at least one recess (4 ) is provided, the clear cross section of which decreases towards the interior of the absorber. 2. Sound absorber according to claim ₁ , characterized in that the clear cross section of the recess (4) decreases linearly. 3. Sound absorber according to claim ₁ or 2, characterized in that the clear cross section of the recess (4) has a rectangular shape. 4. Sound absorber according to one of claims 1 to 3, characterized in that the recess (4) is designed as a continuous slot. 5. Sound absorber according to claim 1, characterized in that the clear cross section of the recess (4) has a triangular, trapezoidal or circular shape. 6. Sound absorber according to one of claims 1 to 5, characterized in that the entry surface of the recess (4) is half to a quarter of the base surface (3) of the absorber. 7. Sound absorber according to one of claims 1 to 6, characterized in that the depth of the recess (4) is one third to one eighth of the total lining depth of an absorber mounted at a distance (5) to a room wall (6). 8. Sound absorber according to one of claims to 7, characterized in that it consists of plastic-bound mineral fibers. 9. Sound absorber, in particular according to claim 8, characterized in that a close-knit, but acoustically transparent, knitted covering (8) is provided as surface protection, which consists of synthetic fibers which are at least flame-retardant. ₁ 0. Sound absorber according to claim 9, characterized in that a tube (8) serving as a hose in the circumferential direction of the row of stitches (arrow 9) is extremely stretchable, preferably to more than three times the circumferential length compared to the unloaded state. ₁ 1. Sound absorber according to claim 9 or 10, characterized in that the absorber has a rough surface and the envelope (8) is held on it by clawing. 12. Sound absorber according to one of claims 1 to 7, characterized in that it consists of foam plastic.

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