US 3177970 A
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Description (OCR text may contain errors)
A. BOSCHI A ril 13, 1965 SOUND-ABSORBING PANELS WITH TAPERED HOLES THERETHROUGH Fig.1
I -s w United States Patent 3,177,970 SOUND-ABSORBING PANELS WITH TAPERED HOLES THERETHROUGI-I Antonio Boschi, Milan, Italy, assignor to Societa Applicazioni Gomma Antivibranti S.A.G.A. Societa per Azioni, Milan, Italy Filed Apr. '18, 1961, Ser. No. 103,879 Claims priority, application Italy, Jan. 21, 1961, 977/61 3 Claims. (Cl. 181-33) The present invention refers to sound-absorbing panels, the main eifective component of which comprises a porus layer, of mineral fibers for example, the thickness of which is proportioned so as to obtain a possibly high degree of acoustic absorption.
For mechanical support and also aesthetic reasons, the porous layer is often glued to the rear face of a foraminated relatively thin plate, consisting of a sheet of aluminium or other metal, asbestos-cement or plastics, for example.
It is known, however, that the absorbing power of porous or fibrous layers satisfactorily covers acoustic frequencies which are superior to about 2000 c./s., while it is practically insignificant for frequencies lower than 1000 to 1500 c./s.
The main object of this invention is to increase in as simple a way as possible, the sound absorbing power of the above panels in the range of low acoustic frequencies.
The above object is attained by the present invention,
without applying any supplemental component elements,
by providing the porous layer with flared holes therethrough, the small ends of which open into one at least of the holes in the foraminated plate.
It has been found that each of such apertures, in combination with the respective hole in the plate, is able to form a good low-frequency resonator provided that the thickness of the porous layer and the holes therein and the volume of said holes are suitably proportioned. To
. obtain remarkable results, the porous layer thickness should amount at least to one centimetre; however, in practice, it is not convenient to go below about 1.5-2 em, all the more so that layers of small thickness are not suflicient for a satisfactory absorption of high frequencies.
Of course, owing to the variety of the porous materials available in this field, and considering the known differently compacted mats of vegetable or mineral fibres, it is practically impossible to give, in this place, a general rule for the proportioning of the said apertures.
Generally, it is advisable to start, in every specific case, from the general rules valid for Helmholtz classic resonators, and subsequently improve the results through experimental test.
In the appended drawing:
FIGURE 1 shows schematically in cross-sectional partview a sound absorbing covering according to an embodiment of this invention.
FIGURE 2 is a similar sectional view of a further embodiment and FIGURE 3 is a diagram showing the sound absorbing properties of a panel of the type shown on FIGURE 1.
In the drawing, denotes a facing platen made of aluminum stamped sheet in the form of a rectangular shallow tray having a flat'bottom wall 11 and four side walls 12 (only one of which is visible). In the wall 11 numerous through holes 14 are bored, in a preferably uniform distribution. In the tray 10 a porous layer 15 is placed, consisting of a mat of mineral wool equalling in thickness the height of the side walls 12.
The layer 15 is glued to the bottom wall 11 so as to form a unitary group with the tray.
In the layer '15 (FIGURE 1) numerous apertures 16,
17 are provided, flaring towards the free (upper) surface of the layer.
The plan cross-sectional profile of such apertures has no particular importance: so, for example, the aperture 16 is shown having a frusto-conical shape (that is having a circular plan cross-sectional profile), the small end of which is centered on one of the holes 14. On the contrary, the aperture 17 is substantially elliptical in plan and its small end includes two holes 14.
To form a sound absorbing covering, on a ceiling 18 for example, the panel 10, 15 is hung on the ceiling by means of suitable known suspension members, not shown, anchored to the tray 10. In most cases, the panel is not applied in direct contact of the ceiling 18, but it is hung at a distance D which can be determined experimentally.
It has been found that such a distance affects, in a certain degree, the tune of the resonators formed by the apertures 16, 17 with the respective holes 14. More particularly, there is an upper limit for such a distance, which amounts to about the double of the thickness of the porous layer 15, and which, together with the lower limit of zero value (distance D nil), defines the adjustment range of the panel, that is the range in which the response to the low frequencies varies according to the distance D.
Beyond the said upper limit, the response remains constant and depends upon the factors already mentioned in the introductory paragraphs of this description. In FIG- URE 3 the continuous curve 20 indicates the sound absorbing characteristics of a panel identical to that shown in FIGURE 1, but lacking the apertures 16, 17; it will be seen from the curve that the efficiency of such a panel becomes appreciable for the frequencies higher than 18002000 c./s., While it is insignificant for lower frequencies. On the contrary, the dotted curve 21 refers to a panel according to FIGURE 1, in which 20% of the total number of holes 14 were associated with flared apertures such as 16, 17 in the layer 15; the curve shows a clear improvement in the range 500-3000 c./s., due to the action of the resonators 14, 16 and 14, 17. According to FIGURE 1, the porous layer 15 can be obtained by depositing a porous material or fibres on a flat horizontal support having on its upper face a number of pyramidally shaped projections of a form complemental to that of the apertures 16, 17.
The embodiment shown in FIGURE 2 shows the manner in which such apertures can be obtained by punching. In this embodiment, the porous layer 15 is formed by a stack of elemental layers 15a, 15b each of which consists of a mat of fibres. In the elemental layers straight apertures 16a, 16b respectively, are punched having areas which increase from the lowermost layer towards the uppermost one, and the elemental layers are stacked in registry in such a Way that the apertures 16a, 16b result coaxial therebetween to jointly form a flared aperture equivalent to that denoted by 16 on FIGURE 1.
It is to be understood that the invention is not limited to the embodiments shown on the drawing. Actually, it is presumed that a skilled person will be able to carry out the present invention in any circumstance and by using materials which will not necessarily be the same as those indicated hereinbefore.
What I claim is:
l. A sound absorbing panel comprising a body of sound absorbing material, said body being generally planar and having major opposite sides, a plurality of holes each of which opens through said opposite sides, said holes being defined by radially continuous peripheral walls extending continuously from one to the other of said opposite sides, said holes being progressively larger .35 from said one tosaid other opposite side, and said panel including a facing platen covering said one side of said body, said platen having apertures therethrough smaller than the holes in said body at said one side thereof, said apertures in the platen connecting with the holes in said v body, said panel being arranged for attachment to a sur- 7 face with said other opposite side adjacent to said surface.
2. A sound absorbing panel as claimed in claim 1, wherein said holes in said body are evenly tapered from said one to said other opposite side of said body.
3. A sound absorbing panel as claimed in claim 1, wherein said body comprises a pluralityof layers of material, each layer having holes therethrough larger in size than the holes in the next layer towards said platen, the holes in the respective layers being axially aligned.
4- References Qited by the Examiner UNITED STATES PATENTS Pannati 181-33 LEO SMILOW, Primary Examiner.
C. F. KRAFFT, M. SUSSMAN, Exal ainers.