US2935151A

US2935151A - Acoustic absorber

Info

Publication number: US2935151A
Application number: US534958A
Authority: US
Inventors: Bill G Watters; Jordan J Baruch
Original assignee: Bolt Beranek and Newman Inc
Current assignee: Bolt Beranek and Newman Inc
Priority date: 1955-09-19
Filing date: 1955-09-19
Publication date: 1960-05-03
Anticipated expiration: 1977-05-03
Also published as: DK108542C

Description

y 1960 B. s. WATTERS ETAL 2,935,151

ACOUSTIC ABSORBER Filed Sept. 19, 1955 3 Sheets-Sheet 1 INVENTORS BILL G. WATTERS Y 8 JORDAN J. BARUCH 20 F/Gi5 United States v, 2,935,151 Patented May 3, 1960 2,935,151 7 ACOUSTIC ABSORBER Bill G. Watters, Nahant, and Jordan J. Baruch, Newton, Mass., assignors to Bolt Beranek and Newman Inc., Cambridge, Mass, a corporation of Massachusetts Application September 19, 1955, Serial No. 534,958

' 6 Claims. at. 181-33) The present invention relates to apparatus for absorb ing acoustic ene igy, and more particularly absorbing acoustic energy while permitting the transmission of light.

Many attempts have been made throughout the years to provide a surface, such as a ceiling, that will absorb acousticcnergy and yet will permit the transmission of light through theabsorbent ceiling. Unfortunately, those materials that are known to be both acoustically absorbent and light-transmitting arerelatively flexible, not structurally rigidenough or otherwise suited for construction purposes. It is very desirable, however, to illuminate a room or other area by means ofsuspended light fixtures and yet to provide some technique for absorbing acoustic energy produced in the room or area and directed upward toward the li-ght fixtures, It has been proposed, for example, to'interconnect the light fixtures with acoustically absorbing panels. A ceiling light fixture may comprise, for example, a fluorescent lamp disposed within a reflector and covered by a translucent glass or other lightdiffusingmaterial. Conventional acoustically absorbent materials have been disposed between the reflectors of adjacent light fixtures,- forrning a false ceiling. This arrangement is subject to the principal disadvantage that the regions between the light fixtures along which the acoustically absorbent material is disposed do not permit the transmission of light so that large areas of thefalse ceiling remain unilluminated. If a large degree of acoustic absorption is to be produced, therefore, the amount of illumination available from the ceiling light fixtures is decidedly limited. In addition, the sizable translucent glass or other covers for. the fixtures reflect appreciable sound energy; directed. thereupon. Another prior-art technique for attempting to solve this problem has been to suspend lights below a fixed ceiling, to cover the lights with atranslueent or other screen, as of glass or plastic, and to suspend sound-absorbing. bafiies of rigid a s rptive ma i a sfroni the screen. The baffles may also be replacedby acoustically absorbent fibrous material housed in perforatedcontainers. If an appreciable degree of acousticabsorption is required, however, the baffles suspended from the translucent false ceiling covering the lights must be of considerable depth and the spacing therebetwe'eni'rnust be'small--a condition which materially interferes with the amount of available illumination.- In addition, such f depending batfies present an estheticall'y' unpleasant appearance and detractfrom' the desired effect of'a'smooth ceiling. Other proposals have also been advanced forsolving this problem including the use ofsound-abs'orbing 'louvers suspended from the ceiling and betweenwhich light fixtures may be disposed to illuminate 'the room Such proposals are also subject to the above-mentioned and other disadvantages.

A very satisfactory solution to this problem, however, is disclosedinto-pending application Serial No. 483,467 of Jordan]; Batfucli,- entitled Light-Transmitting Acoustic Absorber andMethodfi-arid filedfJ anuary'Z l, 1955. This rigid light-transmitting member, as of plastic, having openings covered by resistive sound-absorbing material, such as fibrous layers and the like. It is sometimes desirable, however, to utilize the rigid light-transmitting plastic or other member by itself. As before explained, this has not heretofore been susceptible of accomplishment in view of the fact that such material, while rigid enough to serve as a carrier member, is highly reflective to sound energy. While it has been known that openings in such a carrier member present an impedance to sound energy of various frequencies passing therethrough, it has not heretofore been considered feasible to producean acoustic impedance with such a structure that can render the member an effectiveacoustical absorbing device in and of itself. To the contrary, perforated carrier members have usually been backed by an air space containing a wall or the like that is provided with sound absorbing material. 1

lt has been discovered, however, that through proper selection of the size of the openings and the number of openings per unit surface area of a carrier member of a predetermined thickness, the actual openings themselves may provide considerable acoustic impedance that imbues the member with a new inherent characteristic of wide-band acoustic-energy absorption. The carrier member may then be utilized as a unitary device to serve both as a light-transmitting member and as an acousticabsorbin'g member without the necessity for additional layers of sound-absorbing. material or sound-absorbing linings'dispos'ed upon surfaces" such as walls, ceilings, or th'elike. H V H Ari object of the present invention, therefore, is to provide such a unitarylight-transmitting acoustically absorbent carrier member.

An additional object is to provide anew and improved acoustic absorberf,

Other and further objects willrbe explained hereinafter and, willbe more particularly pointed out in the appended claims. l

The invention will now be more fully described in connectionwith the accompanying drawing, Fig. l of which is a perspective view of a preferred embodiment of the inventionadaptedfor use as a false ceiling; Fig. 2 is a fragmentary section, taken upon the line 2-4 of Fig. 1, looking in the direction of the arrows, but drawn upon an enlarged scale;

Fig. 3 is a graph illustrating the design considerations underlying the present invention;

Fig. 4 is afur'ther. graph illustrating the performance of the invention; and

Fig. 5 is a fragmentary view of a modification.

Conventional fluorescent or other light sources or fixtures 1 are shown suspended at 3 from a fixed ceiling 5. Depending from the fixed ceiling 5, below the light fixtures 1, are a plurality of struts 7 that are terminally secured by vertical projections 11 to vertical elements 12 of horizontally extending supporting beams 9, illustrated as,T-shaped in cross section. Between the horizontal surfaces 13 of adjacent T-shaped beams 9 are supported members 15 preferably of light-transmitting, acoustically absorbing material constructed in accordance with the present invention, and. thus maintained in spaced Y relation with the ceiling 5.

solution to' th'e problem nvolves theuse of 'a light-transmittingsound absorbiiig member comprising a relatively j The member 15 comprises a relatively rigid light-transmitjting carrier member having, a plurality of spaced openings, perforations or apertures 17 passing therethrough between the upper and lower surfaces of the member. The term perforations is hereinafter employed in its broad sense to embrace any kind of opening or aperture. Where it is desired that the light from the sources of illumination 1 be diffusely shielded or shaded from the space-below the carrier member 15, the

spanner member 15 may be translucent. If transparency is desired, on the other hand, the member 15 may be transparent. Many other desired esthetic effects may be obtained through corrugation, embrossing or otherwise decorating the carrier member 15, as desired. Among useful materials for the carrier 15 are plastics such as styrene, polystyrene, polyester resins, and relatively rigid polyvinyl plastics such as polyvinyl chloride and acetate, known as Vinylite, to mention but a few. It is, of course, well known that these and other plastic materials can be fabricated to provide light-transmission by diffusion, translucency or transparency, to any desired degree. In addition, it is also well known that such materials can be fabricated with the necessary perforations 17 and with any desired degree of corrugation, embossing or other decorative or ornamental surfacing.

An unperforated or improperly perforated carrier member 15, however, would not, of itself, have utility as an effective acoustic absorber. On the contrary, it would reflect wide frequency ranges of acoustic energy impinged thereupon to a considerable degree. Were such a carrier member 15 utilized in themanner shown in Fig. 1, therefore, acoustic energy rising upward in the room would merely reflect back downward into the room from the unperforated portions of the member 15. That acoustic energy which entered improperly designed perforations 17, moreover, would pass through the same into the bounded airspace region between the member 15 and the fixed ceiling 5. This energy would be reflected back downward from the ceiling 5 and some of it would pass back through the perforations 17 into the room below. While such a carrier member could provide the required light-transmission and could be sulficiently rigid to be suspended as a false ceiling between the beams 9, it could not thus serve to quiet the sounds in the room below the ceiling.

In accordance with the present invention, however, it has been determined that if a proper number of openings or perforations is provided per unit area of the member 15, and if the openings are of a proper size consistent with a proper carrier member thickness, the member will become startlingly transformed into an effective acoustic absorber. The reason for this appears to be as follows. The openings or perforations 17 present, in general, the main path for the acoustic energy to pass from the lower surface to the upper surface of the carrier member 15. If the perforations are properly designed, as the acoustic energy is forced to pass through the openings 17 of the carrier member 15, the acoustic energy will be dissipated in view of the viscous friction and other impeding effects of the openings or perforations. If the openings are of the size ordinarily used in other types of sound absorbing ceilings, however, such as openings of the order of A of an inch in diameter, more or less, relatively closely spaced from one another, then the resistance presented to the incident sound energy in the audible frequency range is too low to be effective to dissipate sound energy through the action of the perforations alone. Unless a particular relation between opening size and the number of openings per unit area is employed for carrier members of predetermined thickness, the carrier members 15 are incapable of acting as effective wide-frequency-band absorbers.

The general relation between the specific flow re sistance of the material 15 to acoustic energy, represented by the letter R: half the cross-dimension of the openings 17, assumed for convenience to be substantially circular and represented by the symbol r; the thickness d of the material 15; and the number n of perforations or openings 17 per square inch of surface of the member 15, is substantially as follows:

The quantities r and d in Equation 1 are expressed in units of thousandths of an inch, termed mils. It has been determined that substantially optimum acoustic absorption is produced when the flow resistance of the member 15 is a factor 0 of substantially 1.8 times the characteristic impedance of air to a plane acoustic wave; i.e. that R==l.8 pc=76 rayls, expressed in c.g.s. units, where p is the air density and c the velocity of the acoustic waves. The corresponding optimum relationship between the interdependent variables r, n and d has been found to be:

(2) 4 log r=3.39log n/d The optimum design curve represented by Equation 2 is plotted at A in Fig. 3. For a member 15 having a ratio of number of holes per square inch to thickness of the member 15, n/d=10, the optimum perforation crossdimension 2r should be substantially 8 mils. A larger ratio n/d=l00, would require performations of smaller cross-dimension of substantially 4 /2 mils. A small ratio n/d=0.1, on the other band, would necessitate the utilization of much larger perforations of substantially 24 /2 mils in cross-dimension.

While Equation 2 would, in theory, hold for any size perforation, it has been found, in practice, that there are upper and lower commercially useful limits. The lower limit of perforation cross-dimension is determined not only by the feasibility of effecting minute perforations in the member 15, but, also, by the susceptibility of the perforations to clogging by dust, dirt and other particles,

The clogging difficulty may, in part, be mitigated against by blowing air through the perforations as, for example, with the aid of a blower duct in the space between the member 15 and the ceiling 5. A lower limit in perforation cross-dimension of substantially 2 mils is shown in Fig. 3. The upper limit, on the other hand, is determined by the appearance of an esthetically unattractive pinholing effect that results when the perforations are sufliciently large to permit viewing of the light fixtures therethrough. It so happens that poor high-frequency acoustic-energy absorption occurs, also, when the perforations are too large. In Fig. 3 accordingly, an upper limit in the perforation cross dimension of about 40 mils is illustrated.

V The invention is also of considerable utility in regions on either side of the optimum relationship of Equation 2 and curve A. The invention is inoperative, however, both too far to the right, in the region of ordinary perforated acoustic facings, and too far to the left, in the region of sound impervious materials. Limiting practical values of flow resistance of substantially 0:0.18 and substantially 6:18 times the characteristic impedance of air presented to a plane acoustic wave may usefully be employed, providing values of substantially R=7.6 and R=760 c.g.s. rayls, respectively. The limiting curves of 0:0.18 and 0:18 are shown at B and C, respectively, in Fig. 3. The equations represented by the respective curves B and C are given as follows:

3 4 log r=2.391og n/d and (4) 4 log r=4.391og n/d If the numerical constant in Equations 2, 3, and 4 be represented by the letter K, then the above range, shaded in Fig. 3, involves values of K of from substantially 2.39 to substantially 4.39.

The close correspondence between theoretical analysis and experimental result is illustrated in Fig. 4 in connection with the absorption characteristic of a rigid perforated translucent plastic vinyl sheet having substantially the parameters d=l0 mils, r=3 mils, n=225 and R= rayls, and mounted about eight inches from the ceiling 5. The absorption coeflicient of this sheet is plotted along the ordinate, in Fig. 4, as a function of the acoustic-energy frequency, plotted along the abscissa. The dash-line curve iicient and the solid line curve represents actual measurements, 'of. acoustic-energy absorption as a function of frequency. It will be observed that a high degree of absorption is attained'in the all-important relatively wide-band region of the low, intermediate and relatively high acoustic frequencies normally encountered in rooms and the like. The absorption coefficient is within the range of approximately 0.55 to 0.85 over frequencies ranging from approximately 200 cycles to 2000 cycles. A considerable degree of absorption in the very-high frequency range is produced even up to 9000 cycles, while the low-frequency absorption drops below about 100 cycles.

Some degree of improvement in the acoustic-energy absorption characteristic of members constructed in accordance with the present invention has been noted, however, in the relatively low-frequency range as a result of the reaction upon the air in the space between the member and the ceiling 5 caused by the vibration of the member 15 in response to such low frequencies. Such operation has been found to occur, for example, with a mil vinyl sheet 15 of flow resistance equal substantially to 3 c, spaced about 2 /2 inches below a fixed ceiling 5 and secured at its edges by tape. The resonant frequency of the air space between the member 15 and the ceiling 5 was about 1400 cycles. Improved absorption in the lowand mid-frequency ranges of from about 250 to about 900 cycles was obtained. It is important, though, that the real part or the resistance; of the mechanical impedance presented to the air or other fluid medium by the member 15, due either to the members fiexural resistance or to the resistance introduced by the method of securing the member 15 at its edges, be not too small as otherwise the member 15 will move without energy dissipation at the low frequencies and thus without sound absorption.

In general, the maximum absorption occurs when the acoustic mass reactance presented by the holes is made small, as by utilizing relatively large perforations, or by using a large number of perforations, or by employing a thin sheet, all within the criteria of Equations 1 through 1 fmu=% cycles per second where L is the depth of the air space between the member 15 and the ceiling 5, expressed in units of feet. The frequency of this peak can be lowered, if desired, by in creasing the acoustic mass reactance of the holes, but such a decrease in the peak frequency is obtained only at the expense of lowering the degree of absorption, especially at frequencies away from the peak frequency. Since, moreover, the mass reactance increases with increasing frequency, a sheet which is suitable for use with a deep air space for low-frequency absorption may not operate well with a shallow air space as a high-frequency absorber.

While the invention has been illustrated and described in connection with its application as a false ceiling for a room or other space, it is to be understood that this is by way of illustration only, and that the invention may be used in any other place where it is desired to employ its acoustical absorption properties, such as along a wall, a floor or any other surface, including even as lamp or lighting shades or fixtures. It is also to be understood that the absorber need not be formed in the substantially planar form illustrated. It may be fabricated in any desired shape or, after fabrication, it may be post-formed into the desired configuration. The perforations or openings may be integrally formed or they may be subsequently punched as with the aid of a plurality of needles and the like. While it is preferred that the perforations be similar and substantially uniformly distributed, deviations can, in practice, be tol a d. in which event the values of the var ou parame e s o Equa 1 th ou 4 m ybe ave a e alues,- lu v ew of its P o io for acoustic ,resistiyity through the use of the proper number and s zeo a Pa sa es orperfc at o moreover, the

invention is inherently extremely useful in systems where air distribution is effected, such as, for example, in heating, ventilating or air-conditioning systems. The use of the invention in such systems, in addition to providing light-transmitting and acoustic-absorbing properties, provides for draft-free ventilation with no unsightly diffusers or other apparatus to mar the appearance of the ceiling or other surface. If the room in which the present invention is to be utilized were employed for such purposes as painting, or other operations where particles could normally plug the perforations 17, a steady flow of air through the perforations 17 could be effected in order to guard against the depositing of dirt and other particles within the perforations 17, as previously indicated.-

It is also to be understood that the invention is useful for its acoustic-absorbing properties alone, as in many applications where light-transmission is not required. The member 15 may then be opaque, if desired. The carrier medium may also be formed as shown in Fig. 5 by weaving together, as in a cloth, small threads or rods 20 of material, such as plastic. These threads may or may not be translucent, depending upon the desired end. They can readily be formed by saturating fibrous glass threads with a plastic material, such as polyester resin. The perforations or openings in the medium, as previously described, would in this case be formed by the interstices 22 between the threads.

Further modifications will occur to those skilled in the art and all such are considered to fall within the spirit and scope of the present invention as defined in the appended claims.

What is claimed is:

1 For use in association with a wall surface or the like, a structure for absorbing sound waves, comprising a perforated sheet supported adjacent and spaced from said wall surface, said sheet having substantially the following relationship between the thickness d of the sheet, expressed in thousandths of an inch, the number n of perforations per square inch of surface of the sheet and the half-cross-dimension r of the perforations, expressed in thousandths of an inch:

Where k is a constant lying within the range of from substantially 2.39 to substantially 4.39 and wherein the cross-dimension of the perforations lies within the range of from substantially 2 to substantially 40 thousandths of an inch, the sound waves being constrained to pass through said sheet to gain access to the space between said sheet and said wall surface, and the sheet per se having the property of high acoustic resistance over a wide band of acoustic frequencies.

2. The structure of claim 1, wherein k equals 3.39.

3. The structure of claim 1, wherein said sheet is light-transmitting.

4. The structure of claim 1, wherein said sheet is a planar thin plastic sheet.

5. The structure of claim 1, wherein said sheet cornprises a plurality of small units integrated together, said perforations comprising openings between said units.

6. The structure of claim 5, wherein said units are plastic.

References Cited in the file of this patent UNITED STATES PATENTS 1,726,500 Norris Aug. 27, 1929 2,096,233 Ericson Oct. 19, 1937 (Other references on following page) 8 UNITED STATES PATENTS FOREIGN PATENTS 2,159,488 Parkinson May 23, 1939 866,183 Germany Feb. 9, 1953 2,506,951 Deane May 9, 1950 1 2,656,004 Olson Oct. 20, 1953 OTHER REFERENCES 2,659,808 Beckwith Nov. 17, 1953 5 Journal of the Acoustical Society of America, vol. 23

2,703,627 DEustachio Mar. 8, 1955 pp. 533540, September 1951.