Sound Absorption Method and Apparatus

The invention relates to a method for absorbing sound in a clean-room environment and the apparatus, sound-absorbing panel, for carrying out that method; and more particularly relates to a sound-absorbing panel that is lightweight, washable and an effective sound absorber of acoustical energy in a selected range of frequencies.

Noise generated in a clean-room environment, a room for use as for example in food processing, needs to be attenuated in order to allow for minimizing the background noise to at least within the limits specified by the Occupational Safety and Health Administration (OSHA). Numerous devices have been developed to effectively absorb sound, acoustical tile and other perforated sound absorbing material are examples. The problem of absorbing sound in a clean-room environment requires that the sound absorbing panel preferably have few areas in which organic growth can be supported and must be easily cleanable. While solutions to this problem have been found, U.S. Pat. No. 4,301,890 to Zalas is illustrative, such honeycomb diaphragm type panels are inherently expensive and still have the problem of puncture, since the membrane is simply stretched over the honeycomb area as a drumhead would be stretched over a drum body.

Heretofore, many different other methods have been utilized in an effort to make porous material suitable as sound absorber for clean-room, sanitary enrironments. These methods include inter alia the covering of traditional porous type acoustical ceiling tile with washable plastic surfaces. Of course these methods were unsuccessful since the acoustical attenuation features of the tile were seriously degraded by the coating process.

Therefore, there is a need for an inexpensive acoustical absorption panel suitable for clean-room environment in that it be washable and preferably hermetically sealed against the environment while still retaining full acoustical absorption characteristics within the specific acoustical frequency range to be absorbed.

I have developed a sound-absorbing panel for use in a clean-room environment which comprises a fiberglass core of specific thickness that is hermetically sealed within a continuous membrane, the membrane being preferably waterproof and substantially smooth to allow washing, the membrane being attached to at least one of the broad surfaces of the fiberglass core by means of an adhesive disposed in a continuous pattern of closed geometric figures, the places of attachment of the membrane to the fiberglass core defining the edges of said geometric figures. In this way, the membrane occupying the central portion of each of the geometric figures is free to act as a small independent diaphragm, the entire surface of the panel comprising a series of such independent diaphragms. Further, the size of the geometric figures is controlled in order that the resulting diaphragms will have lower order natural frequencies of vibration which correspond to the frequency range of the sound wave energy to be absorbed.

Another aspect of the present invention resides in the provision of a method for absorbing sound within a clean room environment comprising the steps of disposing on at least one interior surface of said clean room a sound absorbing panel, said panel having a fiberglass core of specific thickness, the core being hermetically sealed within a continuous membrane, the membrane being washable, non-porous, and substantially smooth to allow washability, the membrane being attached to at least one of the broad surfaces of the fiberglass core in a continuous pattern of closed, planar geometric figures, the places of attachment of the membrane to the fiberglass core defining the edges of said geometric figures. In this way, the membrane occupying the central portion of each of the geometric figures is free to act as a small, independent diaphragm, the entire surface of the panel comprising a series of such independent diaphragms. Further, the size of the geometric figures is controlled in order that the resulting diaphragms will have lower order natural frequencies of vibration which correspond to the frequency range on the sound wave energy to be absorbed. The sound absorbing panel is disposed in such a way on the interior surface of the room that the side having the continuous pattern of closed geometric figures faces the interior of the room.

An additional aspect of the present invention resides in modifying the sound-absorbing panel above described by the disposition of a perforated planar material between the membrane and the fiberglass core, the perforated material providing the site at which the membrane is attached, the perforated planar material in turn being attached preferably by means of an adhesive to the surface of the fiberglass core. The perforations in the perforated planar material correspond to the geometric figures sought to be produced and are specifically controlled in size for the above described reasons.

FIG. 1 is a prospective view of a sound-absorbing panel according to the present invention.

FIG. 2 is an enlarged corner detail plan view of the sound-absorbing panel of FIG. 1.

FIG. 3 is an elevational sectional view taken along line 3--3 of FIG. 2.

FIG. 4 is an enlarged corner detail plan view of an alternative embodiment of an acoustical panel according to the present invention.

FIG. 5 is an elevational sectional view along line 5--5 of FIG. 4.

FIG. 6 is a graph, the Random Incidence Absorption Coefficient versus Sound Wave Frequency, for a sound-absorbing panel according to the present invention.

It was discovered by Zalas, U.S. Pat. No. 4,301,890, that a suitable sound-abosorbing panel could be fabricated by placing a membrane in contact with a honeycomb core material. The primary feature of that particular panel was that the sizing of the honeycomb compartments in conjunction with the material properties of the membrane, produced a flexible diaphragm at each honeycomb chamber in which the lower order natural frequencies of vibration of the diaphragm were substantially similar to both (1) the natural low order frequencies of vibration of the diaphragm in combination with the cavity in the honeycomb core and (2) the lower order standing wave frequency of sound in the cavity itself. In this way, the acoustical absorption properties of the panel were due to the hysteretic damping associated with the flexing of the membrane rather than simply the absorption of the sound in a fibrous material. Lower order frequencies of both the natural frequency and standing wave natural frequency type include the fundamental frequency and about the first ten harmonies of the fundamental frequency. Thus, the membrane, and more particularly, each of the individual diaphragms associated with the individual honeycomb chambers acted together as the primary sound-absorbing elements. The present invention provides a fibrous material core with a membrane stretched over that core, the membrane being attached to the core itself, or indirectly to the core via an intermediary perforated planar material, in such a way that only portions of the membrane are actually attached to the core, and those points of attachment define the edges of a continuous pattern of closed, planar geometric figures. The central portion of each of these geometric figures is not attached to the core or to any intermediate material and is free to act as an individual, independent diaphragm. The presence of the fibrous material, preferably a fiberglass core, immediately adjacent to each of the vibrating diaphragms does not interfere with the free vibration of the diaphragm, and therefore, it was discovered that by selectively attaching the membrane to the fiberglass core utilizing a purposefully sized geometric pattern, that properties similar to those derived from a membrane/honeycomb sound absorbing panel could be achieved in a much more inexpensive manner.

The sizing of the geometric figures, be they hexagons, circles, or other similar closed geometric figures, may be sized in order to "tune" the panel to selectively absorb sound waves in the range of acoustical or sound wave energy that is sought to be absorbed. The sizing of these geometric figures requires that the sound band to be absorbed or attenuated be known and also the fundamental properties of the membrane material must be well known. Additionally, it has been found that the thickness of the fiberglass core material plays a significant part in the overall effectiveness of the acoustical panel. In this way, the low order natural frequencies of the diaphragms in the membrane and the low order natural frequencies of the membrane/material thickness and also the low order standing wave natural frequencies of sound in that material thickness must be substantially similar, and lie within the sound wave energy range to be absorbed. The equations for computing these various frequencies are similar to those previously described by Zalas. The membrane natural frequency is given by:

The combination of membrane/material thickness natural frequency is given by:

The material thickness standing wave natural frequency is given by:

Where:

f.sub.m =the membrane natural frequency in Hertz;

t=membrane thickness in meters;

.beta.=1.02, 1.47, 1.88, 2.01, . . . factors as determined for a circular plate of fixed perimeter

a=membrane radius in meters (the radius of a circle having an area equal to the cross-sectional area of a diaphragm within the membrane);

E=elastic modulus of the membrane in Newtons per square meter;

.rho.=membrane density in kilograms per cubic meter;

.sigma.=Poisson's ratio of membrane;

f.sub.e =combination membrane/material thickness natural frequency in Hertz;

M=equivalent surface mass of membrane in kilograms per square meter;

M.sub.1,M.sub.2 =surface masses, respectively, of the two membranes in kilograms per square meter;

d=material thickness in meters (fiberglass core thickness); and

f.sub.s =frequency between first quarter wave and first half-wave standing wave resonance of sound in the material thickness in Hertz.

The figures are best understood by reference to FIGS. 1,2, and 3 which depict a perspective view of a sound-absorbing panel in accordance with the present invention details and sections of the same. A fibrous material 10, preferably fiberglass with a density of 3.5 to 4 lbs. per cubic foot (56 to 64 kilograms per cubic meter), is enclosed by the combination of membrane 12 on its upper extensive surface and a similar membrane 14 along its lower extensive surface, membranes 12 and 14 being sealed together by a seal 16 which is preferably hermetic sufficient to prevent the intrusion of air, water or foreign material into the fiberglass core area. It will be apprciated that membranes 12 and 14 may be a single, continuous membrane that is disposed to fully enclose the core. Enclose means to fully surround the core in a three-dimensional way. The membrane material is preferably 1.5 to 2.0 mil (0.38 to 0.51 centimeter) thick polyurethane as for example, the material marketed by Norwood Industries under the name Korel (a registered trademark of Norwood Industries). The specified polyurethane material has an elastic modulus of 1.times.10.sup.9 Newtons per square meter, a density of 1.24.times.10.sup.3 killograms per cubic meter, and a Poisson's ratio of 0.4. Therefore, to estimate the size of the closed geometric figure to produce a diaphragm capable of absorbing wide sound wave energy range centering on 1000 Hz. to about 2000 Hz., the usual sound range audible to the human ear and often caused by equipment operating in food processing area, that diameter should be approximately 1/2 inch (12.7 mm.). The membrane 12 on the upper extensive surface of the fiberglass core is attached at the edges of the geometric figures to be formed preferably by means of an adhesive. Reference to FIG. 2 will show the attachment areas 20 and the open diaphragm areas 22. An adhesive may be applied either to the fiberglass surface as by means of a stencil or other such means or to the reverse side of the membrane 12 again in the appropriate pattern allowing for the closed diaphragms to be appropriately formed. FIG. 2 indicates the geometric shape as being closed independent hexagon shapes, while reference to FIG. 4 shows a circular pattern in which closed, independent circular shaped diaphragms 46 are formed while the intermediate area 44 is the attachment area.

Reference to FIGS. 4 and 5 together will describe an alternative embodiment of the sound-absorbing panel. A fiberglass core 30 is sealed within a membrane 32 across the upper extensive surface of said fiberglass core and a membrane 34 over the lower extensive surface of the fiberglass core, the two membranes 38 and 34 being sealed together at seal 36 to prevent the intrusion of air, water and dirt. A perforated planar material 38 has been inserted between membrane 32 and fiberglass core 30. The perforated planar material has been perforated in this case with circles the size of the desired diaphragms 46. The remaining material in the perforated planar material provides the surface areas for gluing at 44. An adhesive 40 has been applied to the upper surface of the perforated material 38 that it may adhere to the membrane 32 producing the diaphragms 46 while a similar adhesive 42 has been applied to the opposite face of perforated material 38 so that the perforated material may adhere to the upper extensive surface of the fiberglass core 30.

It will be appreciated that the membrane may be attached to both the upper extensive surface and to the lower extensive surface of the fiberglass core either by directly applying the membrane to the fiberglass core material as described in FIGS. 1-3 or by utilization of an intermediate perforated material as illustrated by FIGS. 4 and 5. The utilization of both sides of the fiberglass core material would allow for acoustical interception on either face of the sound absorbing panel. The presence of the membrane along the lower extensive surface, whether it be adhered in a geometric pattern, or loosely applied and maintained in position by means of the hermetic seal, provides a backup mass to the panel which is necessary in order to allow the fiberglass core to participate in the sound absorption by the panel. The primary sound absorbing element remains the diaphragms on the upper extensive surface of the panel.

Turning now to FIG. 6 which is a graph of the Random Incidence Absorption Coefficient, a quantification well known in the art, versus Sound Wave Frequency for a panel utilizing a continuous pattern of closed geometric figure diaphragms according to the present invention wherein the average diameter of the diaphragms measured either as the diaphragm of a circle or as the circular equivalent of a polygonal figure is approximately 1/2 inch (12.7 mm.). It can be seen that the graph centers on approximately 1000 Hz. to 2000 Hz., retaining relatively high absorption over a wide range of sound wave energy frequencies, (250 Hz. to 8000 Hz.). This confirms the results of the previous calculations.

It will be appreciated that the fibrous core material of such an acoustical panel need not be fiberglass but may be of other fibrous materials of various densities. Fiberglass was chosen to illustrate the various embodiments of this invention since it is readily available and relatively light weight for easy handleability. It will also be appreciated that various other membrane materials may be utilized provided they have the features of washability, nonporosity and being relatively smooth so as not to hide organic growth in a clean room environment. Additionally, it will be appreciated that other diaphragm diameters may be utilized depending upon the target range of sound energy sought to be absorbed, or attenuated.

It will be appreciated that numerous changes and modifications may be made in the above described embodiments of the invention without departing from the scope thereof. Accordingly, the foregoing description is to be construed in an illustrative and not in a limitative sense, the scope of the invention being defined solely by the appended claims.