US2502016A - Diffraction type sound absorber - Google Patents

Description

Y Mt/oE/vr Sol/N0 1 m'c/oz-wr sowvo 5 8 5 W041 1 13 L v March 28, 1950 OLSON 2,502,016

DIFFRACTION TYPE SOUND ABSORBER Filed NOV. 30, 1943 6 Sheets-Sheet 1 INK/DENT sod/V0 WALL Mei a 4 -I I a 8 Zinnentor Harry 1 01/6070 $1 1424.

Gttomeg March 28, 1950 v OLSON 2,502,016

DIFFRACTION TYPE SOUND ABSORBER Filed Nov. 30, 1943 6 Sheets-Sheet 2 0 000000 0 o o e 0 0 o o o 9 e 0 00 0 0 0 0 00000 a 0 0 0 o o o 9 0 0 0 0 ooo e g m o0 g Q 0 0 0a o o 0 0 a Q 0 0 0 0 0 a o o a d 16 OOOO 0 16 O0 000 0 0 09 0 ac 0o 14 oco oo 00 0 0 90000000 6 00-000 ooo oooo 0 0 ooooooooo o o oz' l5 ooooooo-oe o oooooooo g o o oo 000006 20 0 2 oooooooq 0 20 9 uoobog go 000000000 OOOOQQQQQ 0 3nontor 000060000 attorney March 28, 1950 OLSON 2,502,016

DIFFRACTION TYPE souma' ABSORBER Filed Nov. 50, 194:5

6 Sheets-Sheet 3 2*;3 J 19% T fi 2 1 E2 lhwcmor Harry E Olson March 28, 1950 H. F. OLSON DIFFRACTION TYPE-scum) ABSORBER 6 Sheets-11991. 4

Filed Nov. :50, 194:5

ISnvcntor Har 1%] 0 L80 n n March 1950 H. F. OLSON DIFFRACTION TYPE SOUND ABSORBER 6 Sheets- Sheet 5 Filed Nov. 30, 1943 u (a nventoz $550M March 28, 1950 H. OLSON mmuc'rxon TYPE sounn ABSORBER 6 Sheets-Sheet 6 Filed NOV. 30, 1943 j m E E N m. 1 ,m R mm A Ma Q A 6 m I t H W A z m fl/ 0W 5 XML. m 5 a H m f w m w m W m 1 mm 1' Kw m m u MM w w 4 Z I 0 iS memc's FIPMdE/Mff mam-442$ PEI? 555mm @mmmeg Patented Mar. 28, 1950 OFFICE DIFFRACTION TYPE SOUND ABSORBER Harry F. Olson, Princeton, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application November 30, 1943, Serial N0. 512,320 13 Claims. (cl. 181-33) This invention relates to acoustic absorbers, and more particularly to an acoustic absorber of the diffraction type.

The practice of providing sound absorbing materials in rooms, auditoriurns, ofllces, apartments, factories, etc. is well known. Conventional sound absorbing materials used in this connection are usually designed for dual p ses, namely, as a building or wall material and as a sound absorber. To be satisfactory for wall structures, these materials must be quite rigid, and in order to provide suflicient rigidity, the inherent properties of these materials are such as to impart to them acoustical impedances which are high compared to the characteristic acoustic impedance of air in the free atmosphere, being usually from ten to twenty times as great. As a consequence, a

large amount of incident sound energy is reflected in spots, so to speak, whereby to take advantage I of the phenomenon known as diffraction for improving the efliciency of the absorbers. However, existing materials are not particularly suitable for use in this "spo fashion, being usually relatively heavy and cumbersome, difllcult to install. and ineflicient.

The primary object of my present invention is to provide an improved difiraction or spot type acoustical absorbing unit which is free from the aforementioned and other similar limitations which characterize prior art absorbers.

More particularly, it is an object of my present invention to provide an improved difl'raction type acoustical absorber which readily lends itself to use as a spot" type absorber and which can be installed easily even by an unskilled person.

Another object of my present invention is to provide an improved diffraction type acoustical absorber which, for a given amount of absorbing material, provides an extremely low reverberation time compared to absorbers of the prior art.

Still another object of my present invention is to provide an improved acoustical absorber as above set forth which is highly effective at the low and high ends of the audio frequency spectrum, as well as in the intermediate range, in contrast to conventional absorbing materials 2 which are effective over a rather limited intermediate range only. This makes my improved absorbers particularly useful for installations where either low or high frequency noises, or both, are in abundance, as in factories, industrial oiilces, plants, restaurants, bowling alleys, and the like, while being also especially suitable in auditoriums and other places where people congregate in relatively large numbers, ofllces, apartments and the like.

A further object of my present invention is to provide an improved acoustical absorbing unit as above set forth which, while designed primarily as an acoustical absorber. may also be made decorative or artistic in appearance for use in homes, theatres, museums, and other large auditoria.

Still a further object of my present invention is to provide an improved acoustic absorbing unit as aforesaid which can be rendered decorative by the application of paint to the outer surface thereof without impairing the absorbing qualities thereof, in contrast to conventional absorbers, the efliciency of which is usually reduced by the application of paint or the like thereto due to the hard surface which the dry paint provides.

Another object of my present invention is to provide an improved acoustical absorbing unit of the type set forth which can be easily removed for cleaning and the like and equally easily remounted for use.

A further object of my present invention is to provide an improved acoustic absorber as aforesaid which is extremely light in weight.

It is also an object of my present invention to provide an improved acoustical absorber which has an extremely high absorption coefficient, which has a much lower cost per unit of absorption than do acoustic absorbers of the prior art, and which is highly efficient in use.

In accordance with my present invention, the absorber comprises a casing which encloses a relatively large fluid filled space and the wall structure of which is constituted at least in part by a material which is pervious to acoustical waves. This material is of a character such that it offers a relatively large dissipative impedance to the passage of sound waves therethrough from the exterior of the casing to the interior, while the space enclosed by the casing is of sufliciently large volume to offer a relatively small impedance to the passage of the sound waves through the casing. The absorber units may be made in the form of box-like structures Or in any other suitable form, and any desired number of such units may be mounted at suitable points or "spots 3 throughout a room which is to be corrected acoustically.

The material of which the casing is made may take various forms. In one construction, this material is constituted by an outer layer of cardboard having a plurality of openings therein of such dimensions as to permit sound waves to pass therethrough substantially freely, the absorbing material backingup and lining the cardboard layer. In thisparticular modification, the absorbing material is constituted by a plurality of layers of thin paper having fine slits therein disposed at random and in sufficient communication with each other to provide a plurality of continuous passages which establish communication between the exterior of the casing and the space therein. The inner, sound dissipating layer is of a limp material and may be stapled or otherwise suitably secured to the outer, rigid, car board layer to provide a rigid, self-contained, selfsupporting structure.

In another form of the. invention, the material is constituted by a felted, fibrous material the fibers of which are intertwined in suiliciently compact relation to provide a plurality of passages of small cross-sectional dimensions affording communication between the exterior of the casing and the space therein. In any case, the small passages in the absorbing material provide a very high acoustical resistance which dissipates the sound energy passing therethrough, the large volume of fluid in the space within the casing having sufficient capacitance to provide a low impedance to the incoming sound waves whereby the waves will enter the passages and the energy thereof will be readily dissipated.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with additional objects and advantages thereof, will best be understood from the following description of several embodiments thereof, when read in connection with the accompanying drawings in which Figuresl and 2 are bar graphs showing, in a general way,-the absorptive and reflective properties of certain types of sound absorbing wall structures,

Figures 3, 4 and 5 are similar graphs showing the absorptive properties of absorbers formed in accordance with my present invention,

Figure 6 is a central, sectional view through one form of absorbing unit constructed in accordance with my present invention,

Figure '7 is an enlarged fragmentary view, partly in section, of a portion of the wall structure of the unit shown in Fig. 6,

Figure 8 is a perspective view of a blank from which an absorber such as shown in Fig. 6 may be constructed,

Figure 9 is a view thereof in partially assembled form,

Figures 10 and 11 are perspective views of two completed absorbing units each of somewhat different shape and each formed from a blank similar to that shown in Fig. 8,

Figure 12 is an electric wiring diagram showing, by way of illustration, the electrical system analogous to the acoustical system of Fig. 6,

Figures 1341 and 13b are similar electrical wiring diagrams corresponding to the acoustical system' of P18. 1,

Figures 14a and 14!: are similar wiring dia- 4 grams corresponding to the acoustical system of each of the units shown in Fig. 5,

Figure 15 is a view showing a plurality of absorbing units in accordance with my present invention mounted in a room requiring acoustical treatment,

Figure 16 is a graph showing the reverberation time of the room of Fig. 15 both before treatment and after treatment thereof, .1

Figure 17 is a graph showing the comparative absorption coefficients of several prior art acoustic absorbing materials and also of my improved absorber, and

Figures 18 to 23, inclusive, are fragmentary sectional views showing other forms of absorber casing wall structures constructed in accordance with my present invention.

Referring more particularly to the drawings, wherein similar reference characters designate corresponding parts throughout, there is shown, in Fig. 1, a wall faced or lined with conventional absorbing material I.

The width of the several bars 8, 5 and I is shown in approximately proportionate relation to indicate that there is less sound reflected by the surface of the material I and still less from within the material I than is received by this material. A reduction in the amount of reflected sound by the relatively high impedance absorbing material I of the prior art may be obtained by spacing the material I from the wall, as shown in Fig. 2. In practice, however, it is not feasible to provide a large space between the absorbing material I and the wall, so that the condition represented by Fig. 1 is the one usually encountered.

Fig. 3 illustrates diagrammatically the sound absorptive qualities of an absorber constructed in accordance with my present invention and comprising a pair of wall members I, 9 spaced from each other a distance to provide a space ll of relatively large volume. As shown in Fig. 3, the incident sound represented by the bars 3 strikes the front surface of the wall I and only arelatively small amount thereof, as shown by the bars H, is transmitted to the space Ill. S uch sound as is transmitted by the wall It then'strikescharacteristic impedance of the air. 3 It will be noted that the sound transmitted by;

a single layer of the material of which my improved absorbers are made, as shown by the bars H of Fig. 3, is somewhat higher than that transmitted by a single layer of the prior art absorbing materials. However, if two layers are used, as shown in Fig. 3, the net amount of sound transmitted is negligible, as shown by the thin bars II. By placing a single sheet 8 of my improved absorbing material at a substantial distance from the wall to provide a space in of relatively large volume, as in Fig. 4, the same effect is produced as in Fig. 3 because the sound which is transmitted by the layer of material I strikes the wall, is reflected thereby back to the material 8, and must pass through the latter once more. Thus,

the sound passes through the absorbing material l twice before entering the room again and therefore almost entirely dissipated.

In this figure, the bars I- indlcate the incident sound and the bars Band 1; indicate the sound reflected by the material I.

When used as a wall material as shown in Fig. 4, the absorption per unit area of the absorbing material 8 is approximately unity. n the other hand, if the material is applied in spots, or as discrete units l4 spaced from each other as shown in Fig. 5, the efliciency is considerably improved due to diffraction of thesound. The absorption per unit area under these conditions is considerably greater than unity. Obviously, therefore, from an economic standpoint, it is desirable to take advantage of the phenomenon of diffraction to improve the efliciency.

One of the ways in which use may be made of diffraction is to scatter the sound absorbing material throughout the room in the form of small, box-like units. One such unit, which I have termed an acoustic sink because of its ability to absorb the sound energy received by it, is shown in Figs. 6 to 11, inclusive. This comprises a closed, hollow casing having a composite wall structure which includes an outer layer of cardboard or other suitable sheet material l5 and an inner layer II. Th layer I5 is approximately 3 thick and is provided with a plurality of openings l6 which are large enough to permit the passage of acoustical waves therethrough substantially freely, the openings it being suitably distributed over the entire area of the layer IS. The inner layer H, which constitutes the absorbing material proper, is suitably secured to the inner surface of the outer wall material [5 and may be constituted by any one or more of a variety of sound absorbing materials which I have found suitable. Among these is one constituted by approximately 60 layers of very thin paper having a plurality of natural holes or perforations therein which are arranged at random, so that they do not line up, but which have suflicient communication with each other to provide a plurality of continuous, irregular passageways through which sound waves may pass from the exterior of the unit It to the interior thereof, as clearly shown in Fig. 7. Thus, the material I! forms a system of long, narrow slits through which the sound waves must pass to reach the space within the unit I4.

The sound absorbing layer I1 is itself a limp material and therefore not self-sustaining. It is for this reason that I provide the outer, rigid layer l5 of cardboard or the like and secure the absorbing layer ll thereto by means of staples ill or in any other suitable manner, as by stitching, or the like. After securing the sound absorbing layer II to the outer, supporting layer 15 to form the wall structure of the absorbing unit I, the composite wall structure may be blanked out in the form shown in Fig. 8 and thereafter assembled by bending up the sides, top and ends, as shown in Fig. 9, to provide a closed casing, as shown in Figs. and 11, the latter form being cubical, and the former being in the shape of a rectangular parallelepiped.

The casing it thus provided by the composite wall structure I5, I! encloses a cavity or space IQ of relatively large volume which is filled with air or any other suitable, compressible fluid. If the inertance of the air in the openings I6 is M1, the acoustic resistance ofiered by the openings I6 is n, the inertance of the passageways formed by the slits or the like in the absorbing layer I1 is Mz. the acoustical resistance offered thereby is re, and the acoustical capacitance of the air in the space I9 is C, then the corresponding elec' trical system will be as shown in Fig. 12. The acoustic elements which control the performance inertance M1 and the resistance r; are small compared to the inertance M: and the resistance n. The impedance of the absorber or sink I4 is the sum of the input acoustic resistance and the acoustic reactance designated, respectively, as r. and Is- The eiTect of difl'raction in improving the emciency of acoustic absorption by the use of an absorbing unit or sink as above described will be readily understood by means of the following theoretical consideration of the acoustic impedance concept when considered in connection with Figs. 1, 3, 13a, 13b, 14a and 1411: In the case of Fig. 1, it is assumed that the surface of the material l is relatively large, (that is, that it covers the walls or the ceiling or both in a room). Under this condition, the acoustic resistance TA of the air is its characteristic impedance. The acoustic impedance of air is 42 acoustical ohms per square centimeter. The acoustic resistance and acoustical reactance of the absorbing material I are designated as m and :m, respectively, and the corresponding electrical system is shown in Fig. 13a. The dissipation or power P which is lost in a sound wave of pressure 1: due to TM is given by the equation:

u? A+ M)+ M' (1) Referring to Equation 1, it will be seen that maximum absorption is obtained when am=0 (corresponding to Fig. 13b) and TA=TM. For these conditions, Equation 1 reduces to Reference will now be made to the improved acoustic sink ll of my present invention and particularly to Fig. 5. The air impedance contains both a resistive and reactive component. Therefore, in this case, the sound waves are convergent. The resistive component of the air is designated as TA and the reactive component of the air due to its inertance is designated MA. The

acoustic circuit is shown in Fig. 5 and the corresponding electrical analogue in Fig. 14a. The impedance of the acoustic sink I4 is composed of the acoustic resistance r, and acoustic reactance as which may be obtained from Fig. 6. In order to obtain a direct comparison with conventional acoustic materials, it may be assumed that the reactive component $5 is negative .and equal in magnitude to the acoustic reactance due to MA. The acoustic circuit then is reduced to one corresponding to Fig. 14b.

As a specific example, let it be assumed that the sink is approximately a cubic foot in volume and that the frequency under consideration is cycles per second. Under these conditions, the acoustic resistance TA is approximately 4 acoustic ohms. To obtain maximum absorption, Ts should also be equal to 4 acoustic ohms. For this condition, the dissipation in Ts is The preceding analysis shows that the characteristic impedance of the air is reduced by using unit sound absorbers and separating them in space. The characteristic impedance of the air may be considered to be the internal impedance of a generator which has an internal driving pressure 12. As the above analysis shows, this means that it is possible to absorb more sound per unit area under these conditions because the effective internal impedance of the generator is smaller. The desirability of building the absorbers in units and appropriately distributing them throughout the room is therefore apparent. In this way, maximum use can be made of the absorbing material.

As pointed out heretofore, most absorbing materials known heretofore are made to serve dual purposes. They are designed to serve both as wall materials and as acoustic absorbing materials. For this reason, acoustic materials of the prior art are not as efficient acoustically as it is possible to obtain by designing for maximum sound absorption. Furthermore, it is rather difflcult to incorporate a large space between the wall and the sound absorbing material, and therefore absorbing materials are usually placed directly against or very close to the wall, as in Fig. 1. Consequently, the acoustic reactive component :m of most materials is usually quite large, and this results in a further loss in efliciency.

Referring again to Fig. 6, it will be seen that, in the acoustic sink of my present invention, the reactive components are due to the inertance of the holes IS, the inertance of the material I! and the acoustic capacitance of the volume of air in the space l9. In general, the reactance due to the inertance is quite small. Therefore, the controlling reactance is due to the acoustic capacitance. The acoustic capacitance of a volume of air is given by the equation where p=density of air c=velocity of sound V=volume of the enclosure (the space is in the present case).

Equation 4 shows that the acoustic impedance can be made small by making the volume of the space H] large. This volume can be made practically any appropriate value in the acoustic sink. The most logical value would be such that the reactance due to the acoustic capacitance would be substantially equal to the reactance due to the combined inertance of the holes IS, the material I'! and the air load at the lowest frequency to be absorbed.

Referring to the acoustic circuit of Fig. 6, the acoustic resistance Tl due to the holes I6 is negligible. Therefore, the important acoustic resistance is re. The acoustic resistance due to the material I! should be equal to the average characteristic acoustic resistance of the air in order to obtain maximum absorption. The acoustic resistance of the material depends upon the type of material used. A slit can be made to exhibit practically a pure acoustic resistance as shown by the following equation:

p=density, in grams per cubic centimeter,

d=thickness of the slit normal to the direction of flow, in centimeters,

l=-width of the slit normal to the direction of flow,

in centi eters,

w=length of the slit in the direction of flow, in

centimeters,

(0:21), and

=frequency, in cycles per second.

This is the type of resistance shown in Figs. 6 to 11, inclusive. The air enters the holes IS in the outer, cardboard wall portion l5 and follows a tortuous path through the layers of paper or the like .41, as clearly shown in Fig. 7. The holes in the paper allow the air to pass from one layer of paper to another layer, these layers being shown by single broken lines in Fig. 7 for the sake of clearness. The slits are formed by the layers of paper.

An acoustic resistance can also be obtained from an aperture or tube of small diameter. The

acoustic impedance of a tube of small diameter is given by where R=radius of the tube, in centimeters, =viscosity coefficient (1.86x10- for air), w=21rf,

f=frequency, in cycles per second, l=length of the tube, in centimeters, and =density, in grams per cubic centimeter.

In the case of cloth or other finely screened material, the material has holes of small diameter between the crossed threads. Equation 6 shows that such a material constitutes an acoustic resistance due to these small holes. The magnitude of the acoustic resistance may be adjusted by an appropriate size of holes in the apertures between threads and by the number of layers of cloth or the like. It has been found that cloth is a very good acoustic resistance, and a sink employing a number of layers of cloth or similar material l'la as the sound absorbing layer is shown in Fig. 18.

Hair felt or a material known commercially as Ozite" is a combination of slit and hole type of acoustic resistance. A wall structure employing hair felt or the like Nb and suitable for use as the casing of my improved absorbing unit II is shown in Fig. 19. This is also a good acoustic resistance, the effect being enhanced by vibration of the hairs or fibers themselves.

Porous material, that is, material with fine holes, may also be made from a porous paper such as blotting or filter paper, as shown in Figs. 20, 21, 22 and 23. The material may or may not be sufficiently rigid to be self-sustaining, and if it is not, it may be secured to an outer, cardboard layer as above described with reference to Figs. 6 to 11, inclusive. One material which I have found especially suitable is a felted material made from wood pulp fibers and sulphite, the fibers being accreted by suction from a pulp solution in known manner. This material, like blotting paper and the like, is constituted by a plurality of intertwined fibres of various lengths and arranged indiscriminately in random directions to provide a plurality of irregular, random passages of very small cross-sectional dimensions. These passages afford communication between the exterior of the casing comprising each of the units I4 and the space is enclosed thereby and provide the requisite dissipative impedance to the acoustical energy received thereby.

In Fig. 20, the wall structure of the casing is constituted by a porous material llc of the type just previously described, the material l'lc being preferably corrugated and glued to the outer cardboard layer l5. If desired, an inner cardboard layer may be also glued to the corrugated material llc, as shown in Fig. 21. The cardboard layer lie is similar to the outer cardboard layer l5, being provided with openings Ilia which correspond to the openings I6.

In the modification of Fig. 22, the sound absorbing material "11 is provided with ridges which may be glued to the outer cardboard wall member Is. In general, the felted type of sound absorbing material has suflicient rigidity to be self-sustaining, and does not require the supporting cardboard layer I 5. In such case, the cardboard layer may be entirely dispensed with and the acoustic sinks made solely of a single layer of porous material We, as shown in Fig. 23.

Fig. 17 shows the relative absorption coefilclents of three conventional sound absorbing materials and also my improved acoustic sink. The three curves B. C and D show the absorption coeflicients of three standard materials which may be identified, respectively, as AMA type V, AMA type VI, and AMA type IV. It will be seen that these materials are quite ineffective in the lower region of the audio spectrum and are only fairly effective in the region between 500 C. P. S. and 1000 C. P. S., falling off again above 1000 C. P. S. The curve A, on the other hand, clearly shows that my improved acoustic sinks not only possess a higher absorption coeflicient in the mid-frequency range, but that they are vastly more effective in both the lower and the higher frequency ends of the audio spectrum.

In Fig. 15, there is shown a room treated with a plurality of acoustic sinks M in accordance with my present invention. The sinks are all mounted adjacent to the ceiling, certain ones being mounted on the walls of the room in spaced relation thereto by means of brackets or the like 25, and certain other ones being mounted on the beams or panels 26 which depend from the ceiling. The sinks mounted on the walls are spaced therefrom to aiford the incident sound ample opportunity to reach all the surfaces of the sink so that advantage may be taken of diffraction. The curve E of Fig. 16 shows the reverberation time of the room depicted in Fig. 15 before treatment with the sinks of my present invention. and the curve F shows the reverberation time of the same room after twenty-four such sinks were mounted therein at distributed points or spots in the room. It will be noted from the curves of Fig. 16 that the reverberation time was reduced in the low frequency region from approximately 3 seconds to approximately 0.6 second, and that avery substantial reduction was also obtained in the reverberation time at the higher frequencies.

From the foregoing description, it will undoubtedly be apparent to those skilled in the art that I have provided an improved sound absorbing unit which is not only highly efficient in use, but which is very compact and is easy to install and to take down. Although I have shown and described a number of modifications of my present invention, it should be apparent to those skilled in the art that many other variations thereof, as well as changes in the ones described, are possible. For example, where the material 11 is of a rigid nature, it may be made up into a box-like structure separately from the outer,

10 cardboard layer It and placed within the box formed by the latter, after which the entire unit is sealed or otherwise suitably secured along the edges to provide a unitary, self-contained structure. Where the sound absorbing material is of the limp type, it may be desirable, in some cases, to provide an inner, as well as an outer supporting wall structure, as in Fig. 21. In such case, an inner boa; which is next adjacent to the space l9 may first be formed and placed within the outer box formed by the layer 15, the material l'l being interposed in the spacev therebetween. So far as the layers !5 and 15a are concerned. the

openings l6 and 18a therein may be circular, F as shown, or they may be of any other suitable shape. In all forms of the invention thus far described, a minimum amount of material 11, Ha, etc. may be used to provide the requisite acoustic resistance because it is terminated by the acoustic capacitance of the space l9. In other forms of my present invention, a single layer of relatively rigid, non-porous material may be used for the casing wall structure and a plurality of diaphragms or other similar vibratory members mounted over openings therein, the diarhragms or the like being exposed at the front to the atmosphere and at the rear to the space 19. When incident sound strikes such diaphragms, the latter are set into vibration and offer suflicient impedance to the sound to dissipate the energy thereof. Thus, in a sense, the vibratory diaphragms render the wall structure pervious to the sound waves in a manner similar to the porous structures.

Other changes and modifications, including pyramid-like, conical, spherical and other similarly shaped hollow casing structures, will, no doubt, readily Suggest themselves to those skilled in the art. I therefore desire that my invention shall not be limited except insofar as is made necessary by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. A diffraction type acoustic absorber comprising a self-contained, closed casing enclosing an air-filled space therein, said casing being constituted by an acoustically porous material having a plurality of fine passages therethrough of such fine dimensions that they offer a relatively large dissipative impedance to the passage of said waves therethrough between the exterior of said casing and said space, and said space having a volume of such magnitude that the air therein offers a relatively small impedance to the passage of said waves through said casing.

2. A diffraction type absorber according to claim 1 characterized in that said casing comprises a laminated wall structure of which said material forms a part.

3. A diffraction type absorber according to claim 1 characterized in that said casing has a wall structure comprising a layer having a plurality of openings therein of sufficient size to permit the passage of sound waves therethrough substantially freely and a layer of said material lining said first named layer.

4. A diffraction type absorber according to claim 1 characterized in that said casing has a wall structure comprising a layer having a plurality of openings therein of sufficient size to permit the passage of sound waves therethrough substantially freely and a layer of said material lining said first named layer, and characterized further in that said material is disposed between said first named layer and said space.

' 531d space.

6. A diffraction type absorber according to claim 1 characterized in that said passages are of slit-like formation and are arranged at random in said material.

7. A ditlraction type absorber according to claim 1 characterized in that said material is constituted by a plurality of layers of thin sheets each provided with a plurality of slit-like openings therein, said openings being arranged at random throughout said sheets but having sufficient communication with each other to provide a plurality of continuous passageways between the exterior of said casing and said space therein constituting said passages.

8. A. diffraction type absorber according to claim 1 characterized in that said material is constituted by a plurality of intertwined fibers arranged indiscriminately in random directions and in relatively compact relationship whereby to provide said plurality oi passages each of small cross-sectional dimension.

9. An acoustic absorber according to claim 1 characterized in that said passageways are disposed at random throughout said wall structure.

10. An acoustic absorber according to claim 1 characterized in that said passageways are irregular in shape and extend in random directions through said wall structure.

11. An acoustic absorber according to claim 1 characterized in that said body has an overall acoustic impedance approximately equal to the characteristic acoustic impedance of air in free atmosphere whereby a minimum oi. the sound wave energy received thereby is reflected therefrom.

12. An acoustic absorber according to claim 1 characterized in that the reactance ofiered by the asoaow air in said space due to the capacitance thereof is substantially equal to the reactance provided by said wall structure and the load thereon at substantially the lowest frequency to be absorbed by said body.

13. A diflraction type acoustic absorber comprising a self-contained, closed casing enclosing a fiuid filled cavity therein, said casing comprising a unitary structure capable of being suspended in the ambient for reception of acoustical wave energy incident thereon from any direction and being constituted by an acoustically porous material having a plurality oi fine passages therethrough of such fine dimensions that they ofler a relatively large dissipative impedance to the passage of said waves therethrough between the ambient and said cavity, and said cavity having a volume or such magnitude that the fluid therein offers a relatively small impedance to the passage of said waves through said casing.

HARRY F. OLSON.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 1,385,741 Dillon July 26, 1921 1,483,365 Mazer Feb. 12, 1924 1,7 26,500 Norris Aug. 27, 1929 1,816,769 Fisk July 28, 1931 1,832,741 Sersen et al Nov. 1'7, 1931 1,851,208 Nicolson Mar. 29, 1932 1,903,201 Steur et al Mar. 28, 1933 1,926,679 Kellogg et al Sept. 12, 1933 2,101,568 Woodbury Dec. 7, 1937 2,160,638 Bedell et al May 30, 1939 2,221,001 Lucius Nov. 12, 1940 2,308,869 Eckardt Jan. 19, 1943 FOREIGN PATENTS Number Country Date 8529/27 Australia Aug. 7, 1928 102,914 Australia Jan. 30, 1938 406,384 Great Britain Mar. 1, 1934 409,502 Great Britain May 3, 1934 Italy u July 17, 19 0 Certificate of Correction Patent No. 2,502,016 March 28, 1950 HARRY F. OLSON It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 7, lines 41 and 42, for the equation O= read 0= pc 7 p6 and that the said Letters Patents should he read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed andsealed this 12th day of September, A. D. 1950.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

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