The invention provides an acoustical attenuator comprising: a porous material comprised of particles sintered and/or bonded together at their points of contact, having at least a portion of pores continuously connected, wherein said porous material has an interstitial porosity of about 20 to about 60 percent, an average pore diameter of about 5 to about 280 micrometers, a tortuosity of about 1.25 to about 2.5, a density of about 5 to about 60 pounds per cubic foot, a modulus of about 12,000 pounds per square inch or above, wherein said porous material has at least one through hole and wherein said interstitial porosity, average pore diameter, density and modulus values are for the porous material in the absence of any through holes, wherein the average diameter of the through hole is greater than the average pore diameter.

TECHNICAL FIELD 
This invention involves methods of attenuating sound which use perforated 
acoustical attenuators, acoustical systems which incorporate such 
perforated acoustical attenuators, and the perforated acoustical 
attenuators themselves. 
BACKGROUND OF THE INVENTION 
The prior art teaches that acoustical barrier materials should be 
non-porous, massive and limp in order to be effective. A common 
misunderstanding is that sound absorbing materials also are good 
acoustical barrier materials. But, acoustical barrier materials have the 
opposite property from acoustical absorbing materials, i.e., barriers are 
highly reflective to sound, and may not absorb it. Acoustical barriers are 
ineffective when they are placed over an area which is not a significant 
noise source or path. In order to provide a noticeable improvement (3 dB 
reduction in sound level), the treated area must be the source or path of 
half the acoustical energy of the targeted noise. 
U.S. Pat. No. 3,802,163, (Riojas) issued Apr. 9, 1974, discloses discs 
useful as filters for exhaust gases in a muffler. The discs can be steel 
mesh, expanded metal, asbestos, fiberglass, perforated coke, and 
combinations thereof. The purpose of Riojas is to reduce the impurities in 
automobile engine exhaust. 
U.S. Pat. No. 3,898,063, (Gazan) issued Aug. 5, 1975, discloses a combined 
filter and muffler device having replaceable ceramic filter elements 
therein. The filter elements can be a molded ceramic having apertures 
which are cylindrical, or pie shaped, or holes that pass completely 
through the element. The muffler is designed such that fluids entering the 
filter are forced to exit out through the ceramic filter walls. 
U.S. Pat. No. 4,435,877, (Berfield) issued Mar. 13, 1984, discloses a noise 
muffler for a vacuum cleaner constructed of flexible open cell foam 
inserts. Where the foam extends across the opening where working air 
flows, the foam has a plurality of relatively large perforations so that 
large particles pass through the foam barrier thus preventing plugging of 
the foam cells. 
Holes cut into acoustical barrier materials, to provide for ventilation, 
structural supports, electrical wiring, control cabling, and the like, 
degrade the performance of the barrier. In order to regain the acoustical 
performance that was obtained prior to making the holes, the barrier 
materials may be modified by providing sealant materials to eliminate the 
acoustical leaks caused by the holes. Of course, when the holes are made 
to provide ventilation, methods other than sealing must be used to regain 
acoustical barrier performance. One approach is to provide additional 
ducts with baffles. Additionally, the baffles may be provided with sound 
absorbing materials. 
SUMMARY OF THE INVENTION 
We have discovered an attenuator comprised of a class of acoustic materials 
perforated with through holes showing performance that degrades 
surprisingly little. This class of acoustical materials is characterized 
by the acoustical materials' modulus, porosity, tortuosity, average pore 
diameter, and average density. By reducing the degree of degradation of 
performance due to holes being cut, the need for compensating 
modifications is minimized. 
The acoustical attenuator of the invention comprises: 
a porous material comprised of particles sintered and/or bonded together at 
their points of contact, having at least a portion of pores continuously 
connected, wherein said porous material has an interstitial porosity of 
about 20 to about 60 percent, an average pore diameter of about 5 to about 
280 micrometers, a tortuosity of about 1.25 to about 2.5, a density of 
about 5 to about 60 pounds per cubic foot, a modulus of about 12,000 psi 
or above, wherein said porous material has at least one through hole and 
wherein said interstitial porosity, average pore diameter, density and 
modulus values are for the porous material in the absence of any through 
holes, wherein the average diameter of the through hole is greater than 
the average pore diameter. 
Surprisingly the perforated acoustical attenuator of the invention provides 
sufficient ventilation while still providing a good level of sound 
attenuation. 
The invention also provides a method of using an attenuator as an 
acoustical barrier in an ambient medium. 
The invention also provides an acoustical system comprising a sound source 
and the attenuator. The sound source may be within an enclosure comprising 
the attenuator, or outside of such an enclosure. 
The acoustical attenuators of the invention have a wide variety of 
applications including but not limited to the following: office equipment 
including but not limited to computers, photocopiers, and projectors; 
small/large appliances including but not limited to refrigerators, dust 
collectors, and vacuum cleaners; heating/ventilation equipment including 
but not limited to air conditioners; sound equipment including but not 
limited to loudspeaker cabinets. 
The attenuator of the invention is particularly useful in applications 
requiring both stiffness and flexural strength sufficient to be 
self-supporting. In these applications, practice of the invention achieves 
the goals of self support, air flow, and acoustical performance through 
the use of only a single material.

DETAILED DESCRIPTION OF THE INVENTION 
ACOUSTICAL MATERIAL 
A variety of acoustical materials can be used in the attenuator of the 
present invention. The acoustical material is preferably an acoustical 
barrier material. 
As examples, types of useful acoustical materials are shown in FIGS. 1A and 
1B, as described in U.S. patent application Ser. No. 07/819,275, (Whitney 
et al.), incorporated herein by reference. 
As shown in FIG. 1A, a particular acoustical material 10 which can be used 
in the attenuator of the invention comprises non-fibrous particles 11 
sintered together at points of contact 12 leaving interstitial voids 
between particles 13, the acoustical material subsequently being provided 
with at least one through hole to provide the attenuator of the invention. 
The acoustical material itself and the attenuator made therefrom is capable 
of operating within an ambient medium 14. Typically the ambient medium 
comprises air, but it can comprise other gases, such as hydrocarbon 
exhaust gases from a gasoline or diesel engine, or some mixture of air and 
hydrocarbon exhaust gases. 
The particle 11 can made from an inorganic or polymeric material. It can be 
hollow or solid. An average outer diameter in the range of about 10 to 
about 500 microns is suitable. Hollow particles, preferred for their light 
weight, may have a wall thickness (difference between inner and outer 
average radii) of about 1-2 microns. The preferred particles have average 
outer diameters of approximately 20 to 100 microns, more preferably about 
35 to about 85 microns, and in these preferred particles the wall 
thickness is not critical if it is less than the outer diameter by at 
least by an order of magnitude. 
The material through which through holes are subsequently made is made of 
particles 11 which form between themselves voids 13 which have a 
characteristic pore diameter which may be measured by known mercury 
intrusion techniques or Scanning Electronic Microscopy (SEM). Results of 
such tests on the materials used in the practice of the invention indicate 
that a characteristic pore diameter of about 25 to 50 microns is preferred 
for applications in air. 
Alternatively, and independently, the acoustical material, before the 
addition of through hole(s), may be characterized by a porosity of 20 to 
60 percent, preferably 35 to 40 percent (in determining porosity, any 
hollow particles are assumed to be solid particles) as measured by known 
mercury intrusion techniques or water saturation methods. 
Additionally, the acoustical material may be characterized by a tortuosity 
of about 1.25 to about 2.5 prior to the addition of the through hole(s), 
preferably about 1.2 to about 1.8. 
For this invention, before the addition of through hole(s), an attenuation 
of sound by the acoustical material is comparable to mass law performance 
over substantially all of a frequency range of 0.1 to 10 kHz. 
An example of commercially available acoustic material useful herein is the 
POREX(R) X-Series of porous plastic materials available from Porex 
Technologies Corp., Fairburn, Ga. 
Examples of suitable inorganic particles include but are not limited to 
those selected from the group consisting of glass microbubbles, 
glass-ceramic particles, crystalline ceramic particles, and combinations 
thereof. Examples of suitable polymeric particles include but are not 
limited to those selected from the group consisting of polyolefin 
particles, such as, polyethylene, and polypropylene; polyvinylidene 
fluoride particles; polytetrafluoroethylene particles; polyamide 
particles, such as, Nylon 6; polyethersulfone particles, and combinations 
thereof. 
Glass microbubbles are the most preferred particles 11, especially those 
identified by Minnesota Mining and Manufacturing Company as SCOTCHLITE.TM. 
brand glass microbubbles, type K15. These microbubbles have a density of 
about 0.15 g/cc. 
As shown in FIG. 2, an alternative to sintering is binding together the 
particles 11 at their contact points 12 with a separate material 20, known 
as a binder, but not so much binder 20 as would eliminate voids 13. 
Typically this may be done by mixing the particles 11 with resin of binder 
20, followed by curing or setting of the resin. 
If used, the binder 20 may be made from an inorganic or organic material, 
including ceramic, polymeric, and elastomeric materials. Ceramic binders 
are preferred for applications requiring exposure to high temperatures, 
while polymeric binders are preferred for their low density. 
Alternatively the binder can be of the same material as the particles. For 
example, polymeric particles may be treated such that they bond to 
themselves with only slight deformation. 
However, some polymers and elastomers may be so flexible that the 
acoustical material is not sufficiently stiff to perform well. Thus, the 
acoustical material must have a density of about 5 to about 60 lbs/cubic 
ft., preferably about 5 to about 40 lbs/cubic ft., and most preferably 
about 5 to about 15 lbs/cubic ft., and a Young's Modulus of 12,000 p.s.i. 
or above. If the modulus is too low sound attenuation becomes poor. Such 
materials will have suitable acoustical performance and also be 
self-supporting, making them suitable for use as structural components of 
enclosures. 
Nonetheless, many polymeric binders are suitable, including epoxies, 
polyethylenes, polypropylenes, polymethylmethacrylates, urethanes, 
cellulose acetates and polytetrafluoroethylene (PTFE). 
Suitable elastomeric binders are natural rubbers and synthetic rubbers, 
such as the polychloroprene rubbers known by the tradename "NEOPRENE" and 
those based on ethylene propylene diene monomers (EPDM). 
Other suitable binders are silicone compounds available from General 
Electric Company under the designations RTV-11 and RTV-615. 
Additionally, the acoustic barrier material described hereinabove can be 
further processed to form a useful barrier material as described in 
copending concurrently filed, U.S. patent application Ser. No. 08/185,598, 
Scanlan et al., "Starved Matrix Composite" incorporated by reference 
herein by: 
(a) forming an article having a matrix microstructure with a surface 
available for coating from a mixture comprising ceramic particles and an 
organic polymer binder; 
(b) pyrolyzing the article of step (a) to carbonize the binder while 
retaining the matrix microstructure of the article; and 
(c) depositing a coating selected from the group consisting of silicon 
carbide, silicon nitride, and combinations thereof on at least a portion 
of the surface of the microstructure of the article to form the acoustic 
material. 
For this embodiment, preferably, the binder is an epoxy resin, phenolic 
resin, or combination thereof. The method can further include applying a 
second organic binder to the article prior to step (b). 
The silicon carbide, silicon nitride, or combination thereof, is preferably 
deposited by chemical vapor deposition. 
According to Scanlan et al., preferably, composite parts according to the 
Scanlan, et al. invention are prepared by mixing filler particles with a 
resin binder and other (optiona)l desired additives in a twin shell 
blender. After mixing for a time sufficient to blend the ingredients, the 
mixture is poured into a mold having a desired shape. To promote removal 
of the composite part from the mold, the mold is preferably treated with a 
release agent such as a fluorocarbon, silicone, talcum powder, or boron 
nitride powder. The mixture is then heated in the mold. The particular 
temperature of the heating step is chosen based upon the resin binder. In 
the case of epoxy and phenolic resins, typical temperatures are about 
170.degree. C. For large parts or parts having complex shapes, it is 
desirable to ramp the temperature up to the final temperature slowly to 
prevent thermal stresses from developing in the heated part. 
According to Scanlan, et al., after heating, the composite part is removed 
from the mold. If desired, additional resin can be applied to the 
composite part (e.g., by dipping or brushing). Preferably, this resin is 
different from the resin in the initial mixture. For example, where the 
resin in the initial mixture is epoxy resin, an additional coating of 
phenolic resin may be applied to the composite part. The composite part is 
then heated again. 
According to Scanlan, et al., once the part is removed from the mold, the 
composite part may be further shaped by machining or used as is. For 
example, the part can be sectioned into discs or wafers. The part can also 
be provided with holes or cavities. The composite part is then placed in a 
furnace (e.g., a laboratory furnace) provided with an inert (e.g., 
nitrogen) or reducing gas (e.g., hydrogen) atmosphere to pyrolyze the 
binder. Typically the pyrolysis is carried out at atmospheric pressure. 
The particular pyrolysis temperature is chosen based upon the binder. For 
epoxy and phenolic binders, typical pyrolysis temperatures range from 
500.degree. to 1000.degree. C. The composite part is loaded into the 
furnace at room temperature and the furnace temperature then ramped up to 
the final pyrolysis temperature over the course of a few hours (a typical 
ramp cycle is about 2.3 hours). 
According to Scanlan, et al., during pyrolysis, the starved matrix 
microstructure is preserved and the binder is converted into carbonaceous 
material. The carbonaceous material typically covers the surfaces of the 
ceramic filler particles and forms necks between adjacent particles, 
thereby producing a carbonaceous matrix throughout the part. This 
carbonaceous matrix forms part of the surface available for coating with 
silicon carbide or silicon nitride. It is further expected that some of 
the particles will have portions where no carbonaceous material is 
covering them due to the way in which the binder coats them and forms 
between them. The uncoated surface of these particles can be coated with 
silicon carbide and/or silicon nitride as well. Generally, however, it is 
preferred that at least 50% (more preferably, at least 90%) of the surface 
available for coating be provided with carbonaceous material. 
According to Scanlan, et al., following pyrolysis, the composite part is 
removed from the furnace for coating with silicon carbide, silicon 
nitride, or combinations thereof. The coating can be formed from solution 
precursors such as polysilazanes dissolved in organic solvents. Moreover, 
in the case of silicon carbide, the coating can be formed by reaction of 
molten silicon metal with carbon from the carbonaceous matrix of the 
pyrolyzed composite part. However, it is preferred to deposit the coating 
by chemical vapor deposition (CVD) of gaseous precursors at reduced 
pressures according to techniques well-known in the art. 
The acoustical material which is used in forming the attenuator of the 
invention may optionally further comprise one or more functional additives 
including but not limited to the following: pigments, fillers, fire 
retardants, and the like. Preferably, the material of the invention 
comprises sintered particles and/or bonded particles with no additives. 
The material of U.S. patent application Ser. No. 07/819,275 comprises 
hollow microbubbles having average outer diameters of 5 to 150 micron, 
bound together at their contact points to form voids between themselves. 
The acoustical barrier material has an air flow resistivity of 
0.5.times.10.sup.4 to 4.times.10.sup.7 mks rayl/meter, and an attenuation 
of sound comparable to mass law performance. Since air flow resistivity 
depends independently on the porosity of the material and the void 
volumes, the acoustical barrier material can be characterized by either a 
porosity of from 20 to 60 percent; or a void characteristic diameter 
within an order of magnitude of the viscous skin depth of the ambient 
medium. 
The acoustical barrier material of U.S. Ser. No. 07/819,275 comprises a 
plurality of lightweight microbubbles, bound together at their contact 
points by any convenient method. 
According to U.S. Ser. No. 07/819,275 preferred microbubbles are made from 
a ceramic or polymeric material. An average outer diameter in the range of 
5 to 150 microns is suitable. Preferred microbubbles may have a wall 
thickness (difference between inner and outer average radii) of 1-2 
microns. The preferred microbubbles have average outer diameters of 
approximately 70 microns, and in these preferred microbubbles the wall 
thickness is not critical if it is less than the outer diameter by at 
least by an order of magnitude. 
The hollow microbubbles form between themselves voids which have a 
characteristic void diameter, which may be measured by known mercury 
intrusion techniques. Results of such tests on the materials used in U.S. 
Ser. No. 07/819,275 indicate that a characteristic void diameter of about 
25 to 35 microns is preferred for applications in air. 
According to U.S. Ser. No. 07/819,275, this range of values provides 
preferred acoustical performance because the characteristic void diameter 
approximates the viscous skin depth of the ambient medium (which depends 
only on the viscosity and density of the medium, and the incident 
frequency of the sound). For example, the viscous skin depth of air varies 
from 200 micron at 0.1 kHz to 70 micron at 1 kHz to 20 micron at 10 kHz. 
Thus, the acoustical barrier material of U.S. Ser. No. 07/819,275 may be 
characterized by a characteristic void diameter within an order of 
magnitude of the viscous skin depth of the ambient medium; an air flow 
resistivity of 0.5.times.10.sup.4 to 4.times.10.sup.7 mks rayl/meter, 
preferably 7.times.10.sup.5 mks rayl/meter; and an attenuation of sound by 
the material comparable to mass law performance. 
Alternatively, and independently, the acoustical barrier material of U.S. 
Ser. No. 07/819,275 may be characterized by a porosity of 20 to 60 
percent, preferably 40 percent (in determining porosity, the hollow 
microspheres are assumed to be solid particles); an air flow resistivity 
of 0.5.times.10.sup.4 to 4.times.10.sup.7 mks rayl/meter, preferably 
7.times.10.sup.5 mks rayl/meter; and an attenuation of sound by the 
material comparable to mass law performance. 
For U.S. Ser. No. 07/819,275 an attenuation of sound is "comparable to mass 
law performance" when it is not less than 10 dBA below the theoretical 
performance predicted by either the field incident or normal incident mass 
law, over substantially all of a frequency range of 0.1 to 10 kHz, other 
than coincidence frequencies. 
For example, the normal incident mass law predicts that the transmission 
loss, in decibels, is 
EQU 20 log (.omega.m/2.rho.c) 
where 
.omega. is the (angular) frequency of the incident sound, 
m is the mass per unit area of the acoustical barrier, 
.rho. is the density of the ambient medium, 
c is the speed of sound in the ambient medium. 
Coincidence frequencies are those regions of the acoustical spectrum where 
the acoustical barrier is mechanically resonating such that the acoustical 
impedance of the barrier as a whole is equal to that of the ambient 
medium, i.e., perfect transmission will occur for waves incident at 
certain angles. Such frequencies are determined only by the thickness and 
mechanical properties of the acoustical barrier. 
For U.S. Ser. No. 07/819,275 glass microbubbles are the most preferred 
lightweight microbubbles, especially those identified by Minnesota Mining 
and Manufacturing Company as "SCOTCHLITE" brand glass microbubbles, type 
C15/250. These microbubbles have density of about 0.15 g/cc. Screening 
techniques to reduce the size distribution and density of these 
microbubbles are not required, as they have only minimal effect on 
acoustical performance (in accordance with mass law predictions). 
According to U.S. Ser. No. 07/819,275, an alternative to sintering is 
binding together the microbubbles at their contact points with a separate 
material, known as a binder, but not so much binder as would eliminate 
voids. Typically this may be done by mixing the microbubbles with resin of 
binder, followed by curing or setting. 
If used, the binder may be made from an inorganic or organic material, 
including ceramic, polymeric, and elastomeric materials. Ceramic binders 
are preferred for applications requiring exposure to high temperatures, 
while polymeric binders are preferred for their flexibility and lightness. 
According to U.S. Ser. No. 07/819,275, some polymers and elastomers may be 
so flexible that the acoustical barrier is not sufficiently stiff to 
perform well. Preferably, the acoustical barrier is additionally 
characterized by a specific stiffness of 1 to 8.times.10.sup.6 
psi/lb-in.sup.3, and a flexural strength of 200 to 500 psi as measured by 
ASTM Standard C293-79. Such barriers will have suitable acoustical 
performance and also be self-supporting, making them suitable for use as 
structural components of enclosures. 
According to U.S. Ser. No. 07/819,275, many polymeric binders are suitable, 
including epoxies, polyethylenes, polypropylenes, polymethylmethacrylates, 
urethanes, cellulose acetates and polytetrafluoroethylene (PTFE). Suitable 
elastomeric binders are natural rubbers and synthetic rubbers, such as the 
polychloroprene rubbers known by the tradename "NEOPRENE" and those based 
on ethylene propylene diene monomers (EPDM). Other suitable binders are 
silicone compounds available from General Electric Company under the 
designations RTV-11 and RTV-615. 
BARRIER MATERIAL I OF U.S. SER. NO. 07/819,275 
To manufacture the acoustical barrier material, Minnesota Mining and 
Manufacturing Company "SCOTCHLITE" brand glass microbubbles, type C15/250, 
having density of about 0.15 g/cc and diameters of about 50 micron were 
mixed with dry powdered resin of Minnesota Mining and Manufacturing 
Company "SCOTCHCAST" brand epoxy, type 265, in weight ratios of resin to 
microbubbles of 1:1, 2:1 and 3:1. The microbubbles were not screened for 
the 1:1 and 3:1 mixtures, but both screened and unscreened microbubbles 
were used in 2:1 mixtures. The resulting powder was sifted into a wood or 
metal mold and cured at 170.degree. C. for about an hour. 
The cured material had a density of about 0.2 g/cc. The void characteristic 
diameter was about 35 micron. The air flow resistivity was 10.sup.6 mks 
rayl/meter, and porosity was about 40% by volume; each of these values is 
approximately that of packed quarry dust as reported in the literature. 
The flexural strength ranged up to 500 psi depending on resin to bubble 
ratio. The composite did not support a flame in horizontal sample flame 
tests. 
Three types of acoustical characterization were performed on the material. 
First, impedance tube measurements determined the sound attenuation of the 
material in dB/cm. The results of these measurements are independent of 
sample geometry (shape, size, thickness). Three types of samples were 
measured and compared to 0.168 g/cc and 0.0097 g/cc "FIBERGLASS" brand 
spun glass thermal insulation (Baranek, Leo L., Noise Reduction, 
McGraw-Hill, New York, 1960, page 270), and also to packed quarry dust 
(Attenborough, K., "Acoustical Characteristics of Rigid Fibrous Absorbents 
and Granular Materials," Journal of the Acoustical Society of America, 
73(3) (March 1983), page 785). 
The acoustical attenuation of a sample prepared with a 1:1 weight ratio of 
resin to hollow microbubbles was between 0.1 and 10 dB/cm over a frequency 
range of 0.1 to 1 kHz, comparable to the attenuation of each of the other 
three materials (roughly 0.3 to 5 dB/cm). 
The attenuation for a sample prepared with a 2:1 weight ratio of resin to 
unscreened hollow microbubbles was between 0 and 12 dB/cm over the same 
frequency range, while the other three materials showed attenuations of 
0-3 dB/cm over the same range. For a 2:1 weight ratio using screened 
hollow microbubbles, the attenuation decreased somewhat in the 0.2 to 0.4 
kHz range, but rapidly increased to over 14 dB at 1 kHz. 
Second, insertion loss measurements according to SAE J1400 were made using 
panels inserted in a window between a reverberant room containing a 
broadband noise source and an anechoic box containing a microphone. The 
panel sizes were 55.2 cm square and up to 10.2 cm thick. These results are 
strongly dependent upon geometry. 
The acoustical barrier panels comprising hollow microbubbles were about 
10.2 cm thick and had mass of about 19.8 kg. By comparison, gypsum panels 
of 1.59 cm thickness (common in the building industry) had mass of about 
16.3 kg. A lead panel had mass of 55 kg. 
Over the 0.1 to 10 kHz frequency range, the panel comprising microbubbles 
performed somewhat better than the gypsum panel. In particular, at 160 Hz, 
the insertion loss through the panel comprising microbubbles was 10 dB 
greater than that through the lead panel, despite having only 36 percent 
of the mass. 
As compared to theoretical performance, the panel comprising microbubbles 
exceeded mass law predictions except: between about 0.25 kHz and about 0.4 
kHz, but by less than 10 dB throughout the range; at 0.8 kHz, but again by 
less than 10 dB; and from about 3 kHz to 10 kHz, but this is due to a 
coincidence frequency range centered about 6 kHz. 
Third, insertion loss measurements were made with boxes containing a 
broadband noise source, using a microphone and a frequency analyzer. The 
roughly cube-shaped boxes ranged in size from 41 to 61 cm on a side. These 
results are strongly dependent upon geometry. 
A box made from the acoustical barrier material comprising microbubbles and 
a box made from gypsum were constructed so that each had the same total 
mass, about 52.8 kg, despite different wall thicknesses. Thus, the box 
made from material comprising microbubbles had walls about 10.2 cm in 
thickness, and the box comprising gypsum had walls about 1.6 cm in 
thickness. 
The attenuation by the box made from the acoustical barrier material 
comprising microbubbles exceeded mass law performance over the entire 
frequency range from 0.04 kHz to 1 kHz, and was no less than 10 dB less 
than mass law performance over substantially all of the frequency range of 
1 kHz to 8 kHz. 
Below 1 kHz and above 2 kHz, the box made from the acoustical barrier 
material comprising microbubbles performed generally about 10 dB better 
than the box made from gypsum. 
BARRIER MATERIAL II OF U.S. SER. NO. 07/819,275 
A piece of acoustical barrier material was manufactured as described in 
Barrier Material I of U.S. Ser. No. 07/819,275 from "SCOTCHCAST" brand 
epoxy resin type 265 and "SCOTCHLITE" type C15/250 glass microbubbles, 
blended in weight ratios ranging from 2:1 to 1:1 and thermally cured to 
form rigid structures ranging from about 4.8 mm to 15.9 mm in thickness. 
Several 3.5 cm diameter cylinders of material were cut and shaped such 
that the cylinders fit snugly into the muffler housing of a "GAST" air 
motor, model number 2AM-NCC-16, which had approximately the same inner 
diameter as the outer diameter of the cylinder. The cylinder replaced a 
conventional muffler, namely two #8 mesh screens supporting between 
themselves a dense non-woven fiber of about 13 cm thickness. 
THROUGH HOLE(S) 
As indicated previously, the attenuator of the invention comprises an 
acoustical material having one or more through holes. By "through holes" 
is meant openings traversing the acoustical material such that the through 
holes are capable of connecting high pressure and low pressure surfaces 
(when there is flow of ambient medium) and/or are capable of connecting 
high sound intensity and lower sound intensity surfaces of the acoustical 
material. The number and size of the through holes can vary. Typically, 
sufficient through holes are present to provide the desired air flow rate 
for a particular use, such as ventilation. Moreover, sufficient through 
holes are present such that about 0.10 to about 90 percent of the total 
acoustical material surface area (without through holes) contains through 
holes. If less than 0.1 percent of the total acoustical material surface 
area (without through holes) contains through holes the flow 
characteristics approach that of the acoustical barrier material without 
holes. If greater than 90 percent of the total acoustical material surface 
area (without through holes) contains through holes the structural 
integrity of the material can be compromised and acoustical benefits are 
negligible. Preferably, the total acoustical material surface area 
(without through holes) contains about 0.5 to about 50 percent through 
holes for reasons of maximizing air flow and sound attenuation, most 
preferably about 0.9 to about 25 percent for reasons of ease of 
manufacturing and to further maximize sound performance. 
The acoustical material can contain any number of through holes. However, 
the total percentage area covered by the through holes may be held 
constant by varying the hole diameter. If only several through holes are 
present which have very large diameters, the sound attenuation may be 
diminished. If a very large number of through holes are present which have 
small diameters the back pressure may rise appreciably when compared to 
the case of a few larger holes. Typically, a sufficient number of through 
holes having a sufficient diameter is selected such that the air flow and 
sound attenuation is good for a particular application. This invention 
provides an unexpectedly broad range of flexibility to achieve these sound 
and back pressure targets when compared with non-porous perforated 
substrates. Preferential attenuation of high frequency sound was 
unexpectedly attained with an increasing number of through holes as 
demonstrated by Example 9 in samples greater than or equal to 4 inches in 
thickness. 
The diameter of the through hole(s) is application dependent and can range 
from just greater then about the average pore diameter of the acoustical 
material to much greater than the thickness of the attenuator, subject to 
the other limitations disclosed hereinabove. For a large number of 
applications, the diameter of the through hole(s) range from about 1/64 
inch to about 6 inches, typically, from about 1/16 inch to about 2 inches. 
If the diameter of the through hole is less than about 1/64 inch the back 
pressure may increase greatly. The through holes need not be all the same 
diameter. Typically, the through holes are all of the same diameter for 
ease of machining. 
The length of the through hole is typically the same as the thickness of 
the acoustical material although it can differ if the through hole is not 
both straight and perpendicular through the material. It is foreseeable 
that the paths of the through holes may be other than straight (twisted or 
curved for example). It is believed that such through holes would result 
in a material that also functions well for its intended purpose. This is 
particularly useful when application design limits the barrier material 
thickness. The length of the through hole depends upon the intended 
application of the acoustical material as well as the thickness of the 
acoustical material. It has been observed that when the hole length is 
about 1/2" or greater pressure drop through attenuators comprising porous 
barrier materials is lower than for non-porous substitutes. If the hole 
length is less than about 1/2", resistance to ambient flow through the 
attenuator approaches that of a nonporous material provided with similar 
through holes. 
The ratio of hole length to diameter can vary depending upon the attenuator 
application. Typically, however, the length to diameter ranges from about 
1:1 to about 100:1 for reasons of good air flow and sound attenuation. If 
the length to diameter ratio is greater than about 100:1, back pressure 
may substantially increase. If the length to diameter ratio is less than 
about 1:1, sound attenuation may diminish. 
The shape of the through holes can vary. The through hole can take a 
variety of shapes including but not limited to the following: circular, 
elliptical, square, slits, triangular, rectangular, etc. and combinations 
thereof. Typically, the holes are circular for ease of machining. A cross 
section of the hole may vary but is typically constant also for ease of 
machining. 
The pattern of the through holes can vary. The pattern can be symmetrical 
or asymmetrical. It is preferable that the through holes be relatively 
evenly distributed for reasons of uniform air flow. If the through holes 
are all concentrated in one location of the material structural integrity 
may be compromised. In some circumstances it is desirable to concentrate 
the through holes in one location in the material; in its intended use the 
attenuator will only receive incident air at that location. In that 
portion of the attenuator it is best that the through holes are uniformly 
distributed. 
Another aspect of the invention is an acoustical system comprising a source 
of sound, radiating in the direction of the acoustical attenuator. In a 
typical acoustical system, it is sufficient to simply place the acoustical 
attenuator between the sound source and the listener, but for additional 
attenuation of sound, the acoustical attenuator substantially (or even 
completely) surrounds either the sound source or the ear of the listener. 
For example, as shown in FIG. 4, an open box 40 (such as an open-faced 
enclosure for a loudspeaker 41) could be constructed using the acoustical 
attenuator. 
Another application would be headphones having ear enclosures constructed 
from the acoustical attenuator, since the ear enclosures would "breathe" 
in a passive manner, and thus provide improved comfort for the listener. 
In many applications, such a system can be acoustically sealed, relying on 
the porosity of the acoustical attenuator itself to allow air and moisture 
to escape from the enclosure directly through the attenuator. 
Thus, for example, a sealed noise reduction enclosure could be provided for 
a piece of machinery mounted on a base. The acoustical attenuator could be 
partially lined with acoustical absorbing material. 
Muffler Applications 
One particularly preferred acoustical system utilizes the acoustical 
attenuator as a muffler. In this application, the acoustical attenuator 
has allowed gasses to readily pass through the muffler. 
Structural Applications 
It is possible to use the acoustical attenuator described above without a 
separate supporting assembly, i.e., as a structural component. Large 
volume enclosures may be made from panels, blocks, or sheets of 
attenuator. 
Such panels are formed so that each panel has a portion of an interlocking 
joint. Such interlocking panels are especially useful in forming 
acoustically sealed enclosures. 
TEST METHODS 
The following test methods were used to measure the various test results 
reported in the examples. 
Back Pressure and Sound Pressure Level 
Back pressure and sound pressure level of a sample were tested at various 
flow rates on a laboratory flow bench. A sample holder in the shape of a 
box was connected to a laboratory pressurized air line by means of metal 
tubing at one face or end of the box and the sample to be tested was 
affixed to the opposite end of box. A 12 inch by 12 inch surface area of 
the sample was exposed to the incoming air. The temperature of the inlet 
air was measured with a thermometer. A gauge pressure sensor was placed in 
line between the air inlet and the sample to measure the build-up of back 
pressure from the sample. 
Measurement of sound pressure level (i.e., noise level) was accomplished by 
means of a Bruel and Kjaer Dual-Channel Portable Signal Analyzer Type 2148 
(commercially available from Bruel and Kjaer, Naerum, Denmark) positioned 
1 meter from the center of the sample surface at an angle of 45 degrees 
from the direction of the sound source. Each measurement was the result of 
a single reading point. The air flow rate was set at the desired level and 
once the air flow rate level was stable, the sound pressure level reading 
was taken. The units of measurement were in dBA, which refers to an 
A-weighted decibel scale. 
Back pressure (measured in inches of H.sub.2 O) was the pressure difference 
across the sample (i.e., the pressure at the inlet minus the pressure at 
the outlet). Flow was measured in standard cubic feet per minute (scfm). 
Low values of back pressure and sound pressure level are desirable. 
Young's Modulus 
Young's Modulus for each sample was calculated (roughly according to ASTM C 
623) as follows: 
The weight and dimensions of the sample were measured and used to calculate 
the density of the sample. Care was taken to assure that the measured 
frequency corresponded to the first bending mode. An accelerometer and an 
instrumented impact hammer were connected to a frequency analyzer to 
measure frequency response function of various points on the sample. The 
frequency response function was analyzed using the modal analysis program 
"Star Modal", Version 4, commercially available from GenRaid/SMS Inc., 
Milpitas, Calif., to determine natural frequency and modal shapes of the 
sample. A numerical analysis (finite element modelling) was performed to 
calculate the theoretical first bending mode. The measured dimensions and 
density values were input to the model, and a value for Young's modulus 
was assumed. The theoretical first bending frequency from the finite 
element model was compared to the actual first bending mode from the 
measurement. The purpose of this step is to determine how to adjust the 
initial Young's modulus value; if the theoretical frequency was below the 
actual measured frequency, Young's modulus was increased, and vice versa. 
The above step was repeated until the theoretical first bending frequency 
from the finite element model agreed with the actual first bending mode 
from the measurement. Young's modulus was the latest or last value used in 
the finite element model and is reported in pounds per square inch (psi). 
ABBREVIATIONS 
The following abbreviations are used herein: 
______________________________________ 
Abbreviation Definition 
______________________________________ 
SPL Sound Pressure Level 
BP Back Pressure 
AFR Air Flow Rate 
DEG Degrees (angular) 
Dia. Diameter 
dBA A-weighted decibel 
scfm Standard cubic feet per minute 
L/D Length of hole/diameter of hole 
Wall Surface Area = pi .times. diameter of hole .times. number 
of holes .times. length of holes 
______________________________________ 
EXAMPLES 
This invention is further illustrated by the following representative 
Examples, but the particular materials and amounts thereof recited in 
these Examples, as well as other conditions and details, should not be 
construed to limit this invention. All parts and percentages are by weight 
unless otherwise indicated. 
EXAMPLE 1 
In this Example, the benefit of the through holes coupled with the 
acoustical barrier material porosity is demonstrated. 
Two samples of the acoustical material of this example were prepared as 
follows: Minnesota Mining and Manufacturing Company SCOTCHLITE.TM. brand 
glass microbubbles, type K15, having a density of about 0.15 g/cc and 
diameters of about 50 microns were mixed with dry powdered resin of 
Minnesota Mining and Manufacturing Company SCOTCHCAST.TM. brand epoxy, 
type 265, in weight ratios of resin to microbubbles of 2:1. The resulting 
powder was sifted into a mold, vibrated by mechanical means to settle the 
loose powder and facilitate the release of any trapped air, and cured at 
170.degree. C. for up to about 4 hours depending on the block size. The 
cured blocks were then cut if necessary to the desired test size and 
thickness. 
The cured material would have a density of about 0.2 g/cc based on 
historical measurements. The pore characteristic diameter would be about 
35 microns. The porosity would be about 40% by volume. The Young's modulus 
was about 60,000 pounds per square inch. This material was designated as 
"ACM-1". One of the thus prepared samples was further treated by coating 
one of its faces with a two-part liquid epoxy such that the surface was 
sealed and the surface pores were filled in. Next, 265 through holes of 
1/8 inch diameter were drilled perpendicular to the major attenuator 
surface in an evenly spaced square array pattern (grid pattern) over the 
12 inch by 12 inch face of the each sample. The sample thickness was 2 
inches. In this Example, hole length was equivalent to the sample 
thickness. The samples were then tested for sound pressure level back 
pressure according to the test methods outlined hereinabove. 
The sound pressure level (SPL) in dBA, the back pressure (BP) in inches of 
water, and the air flow rate (AFR) in scfm are reported in Table 1 below. 
TABLE I 
______________________________________ 
Epoxy Coated Vs. Uncoated ACM 
Uncoated Attenuator 
Epoxy Coated Attenuator 
Flow 2651/8" Dia. Holes 
2651/8" Dia. Holes 
Rate Pressure SPL Pressure SPL 
(scfm) (Inches of H.sub.2 O) 
(dBA) (Inches of H.sub.2 O) 
(dBA) 
______________________________________ 
5 0 5.0 0 5.1 
10 0 54.4 0.1 55.2 
15 0.1 56.8 0.1 57.7 
20 0.1 58.1 0.2 59.3 
25 0.2 60 0.3 61.6 
30 0.2 62.3 0.4 63.4 
35 0.3 63.5 0.5 64.9 
40 0.4 65.2 0.5 66.3 
45 0.4 66.5 0.7 67.7 
50 0.5 67.7 0.8 68.4 
55 0.6 69.1 1 70.2 
60 0.7 70.1 1.1 71.2 
65 0.8 71.8 1.3 72.4 
70 0.9 73 1.5 74 
75 1.1 74.5 1.7 75.3 
80 1.2 75.4 1.9 76.2 
85 1.4 76.4 2.1 77 
90 1.5 77.4 2.4 78.1 
95 1.7 78.5 2.7 78.9 
______________________________________ 
It can be seen from the data that the porosity of the barrier material 
reduces the pressure drop and produces better sound attenuation. 
EXAMPLES 2-3 
These Examples show the effect of varying the through hole number, length 
to diameter ratio, and wall surface area while holding the percent open 
area and sample thickness constant. 
The barrier material used in these Examples was ACM-1 prepared according to 
Example 1 above. A plurality of through holes was drilled in the samples 
in the same pattern as Example 1 and the samples were tested as in Example 
1. Example 2 had a percent open area of 1.23%. Example 3 had a percent 
open area of 2.26%. 
The number of through hole(s), diameter (D) of through holes, AFR, SPL, and 
BP are given in Table II below. 
TABLE II 
______________________________________ 
1 Hole 11/2" Dia. 
4 Holes 3/4" Dia. 
2" Thick 2" Thick 
Flow Rate 
Pressure SPL Pressure SPL 
(scfm) (Inches of H.sub.2 O) 
(dBA) (Inches of H.sub.2 O) 
(dBA) 
______________________________________ 
5 0 56.3 0 54.2 
10 0 62.5 0.1 62.6 
15 0.1 67.3 0.1 62.8 
20 0.2 69.2 0.2 63.8 
25 0.3 70.7 0.4 67.9 
30 0.4 72.2 0.5 68.6 
35 0.5 73.3 0.6 69.8 
40 0.7 74.9 0.8 71.2 
45 0.9 76 1.1 72.4 
50 1.1 76.7 1.3 73.2 
55 1.3 77.9 1.6 74.6 
60 1.5 78.5 1.8 75.4 
65 1.8 79.9 2.1 76.9 
70 2.1 81 2.5 78.5 
75 2.4 82.7 2.8 79.3 
80 2.7 83.3 3.2 80.3 
85 3 84.2 3.6 81.3 
90 3.4 85 3.9 81.9 
95 3.8 86.3 4.4 82.8 
______________________________________ 
36 64 Holes 144 
Holes 1/4" Dia. 
3/16" Dia. Holes 1/8" Dia. 
2" Thick 2" Thick 2" Thick 
Flow Pressure Pressure Pressure 
Rate (Inches SPL (Inches 
SPL (Inches 
SPL 
(scfm) 
of H.sub.2 O) 
(dBA) of H.sub.2 O) 
(dBA) of H.sub.2 O) 
(dBA) 
______________________________________ 
5 0 51 0 49.4 0.1 50.3 
10 0.1 55.9 0.1 54.9 0.1 53.3 
15 0.1 57.2 0.2 56 0.2 54.7 
20 0.2 57.8 0.3 56.8 0.4 55.8 
25 0.4 61.1 0.4 58.8 0.6 57.2 
30 0.5 62.9 0.6 60.3 0.7 59.1 
35 0.7 63.9 0.8 62.1 1 60.9 
40 0.9 65.5 1 63.5 1.3 62.4 
45 1.1 66.7 1.3 65.3 1.6 63.7 
50 1.4 67.5 1.6 66.3 2 65 
55 1.4 67.4 1.9 67.9 2.5 66.7 
60 2 70.2 2.2 69.1 2.9 67.9 
65 2.3 71.2 2.5 70.2 3.4 69.1 
70 2.6 72.6 2.9 71.5 4 70.4 
75 3.1 74.2 3.3 72.6 4.6 71.6 
80 3.4 74.6 3.7 73.9 5.1 72.7 
85 3.8 75.9 4.1 74.6 5.7 73.6 
90 4.3 77.1 4.5 75.6 6.4 74.5 
95 4.8 77.8 5.1 77 7.2 75.7 
______________________________________ 
Example 3 
Same Thickness Same % Open Area Varied L/D 
265 Holes 1/8" Dia. 
170 Holes 5/32" Dia 
2" Thick 2" Thick 
Flow Rate 
Pressure SPL Pressure SPL 
(scfm) (Inches of H.sub.2 O) 
(dBA) (Inches of H.sub.2 O) 
(dBA) 
______________________________________ 
5 0 50 0 50.7 
10 0 54.4 0 55.2 
15 0.1 56.9 0.1 57.2 
20 0.1 58.1 0.1 58.8 
25 0.2 60 0.2 61.1 
30 0.2 62.3 0.2 62.8 
35 0.3 63.5 0.3 64.2 
40 0.4 65.2 0.3 66.1 
45 0.4 66.5 0.4 67.5 
50 0.5 67.7 0.4 68.5 
55 0.6 69.1 0.5 69.7 
60 0.7 70.1 0.6 71.1 
65 0.8 71.8 0.7 72.4 
70 0.9 73 0.9 73.9 
75 1.1 74.5 1.9 74.9 
80 1.2 75.4 1.1 76.3 
85 1.4 76.4 1.2 77 
90 1.5 77.4 1.3 78.1 
95 1.7 78.5 1.5 78.5 
______________________________________ 
118 Holes 3/16" Dia. 
1060 Holes 1/16" Dia. 
2" Thick 2" Thick 
Flow Rate 
Pressure SPL Pressure SPL 
(scfm) (Inches of H.sub.2 O) 
(dBA) (Inches of H.sub.2 O) 
(dBA) 
______________________________________ 
5 0 51.6 0.1 30 
10 0 57.5 0.1 52.9 
15 0.1 57.8 0.2 55 
20 0.1 59.2 0.3 56.6 
25 0.2 61.4 0.4 58.1 
30 0.2 63.1 0.5 39.8 
35 0.3 64.7 0.5 61.7 
40 0.4 66.2 0.7 62.9 
45 0.4 67.9 0.8 64.5 
50 0.5 68.8 0.9 65.7 
55 0.6 70.6 1.1 67.1 
60 0.7 71.5 1.3 68.9 
65 0.7 73 1.4 70 
70 0.9 73.9 1.6 71.3 
75 1 75.5 1.8 72.5 
80 1.1 76.5 1.9 73.5 
85 1.3 77.3 2.1 74.9 
90 1.4 78 2.3 75.3 
95 1.6 79.4 2.6 76.4 
______________________________________ 
It can be seen from the data that when the percent open area was held 
constant, smaller numbers of larger holes and associated changes in wall 
surface area and length to diameter ratios led to lower back pressures and 
higher noise levels. Conversely, larger numbers of smaller holes and 
associated changes provided for increased noise attenuation but with 
greater back pressure. 
EXAMPLE 4 
This Example showed the effect of varying the through hole(s) patterns. 
In this Example, the ACM-1 barrier material as prepared in Example 1 was 
used. Three 2 inch thick samples were made and 144 through holes having a 
1/8 inch diameter were drilled into them, each having a different pattern. 
The patterns were the evenly spaced array (grid pattern) of Example 1, a 
series of corner to corner relatively evenly spaced holes in a double 
rowed (3/8 inch row spacing) "X" pattern (X), centered on the sample, and 
2 concentric circles (circle) of diameters of 43/4" and 101/2" 
respectively, from relatively evenly spaced holes. The samples were then 
tested for SPL and BP. 
Test results along with the flow rate is given in Table III. 
TABLE III 
__________________________________________________________________________ 
1441/8" Holes 2" Thick Varied Hole Patterns 
Concentric 
Grid Pattern X-Pattern Pattern (2 Circles) 
Flow Pressure Pressure Pressure 
Rate (Inches of 
SPL (Inches of 
SPL (Inches of 
(scfm) 
H.sub.2 O) 
(dBA) 
H.sub.2 O) 
(dBA) 
H.sub.2 O) 
SPL (dBA) 
__________________________________________________________________________ 
5 0.1 50.3 
0.3 0.3 0.1 50.3 
10 0.1 53.3 
0.1 55.7 
0.1 55.2 
15 0.2 54.7 
0.2 57.3 
0.2 55.9 
20 0.4 55.8 
0.3 59 0.3 57.5 
25 0.6 57.2 
0.5 61 0.5 59.1 
30 0.7 59.1 
0.6 62.6 
0.6 60.8 
35 1 60.9 
0.9 64.2 
0.8 62.9 
40 1.3 62.4 
1.1 65.7 
1 64.1 
45 1.6 63.7 
1.4 66.8 
1.3 65.9 
50 2 65 1.7 68 1.6 66.5 
55 2.5 66.7 
2.1 69.2 
2 68.4 
60 2.9 67.9 
2.5 70.6 
2.4 69.1 
65 3.4 69.1 
2.9 71.8 
2.8 70.5 
70 4 70.4 
3.3 72.9 
3.2 70.2 
75 4.6 71.6 
3.8 74.3 
3.7 73 
80 5.1 72.7 
4.3 75.1 
4.2 74.4 
85 5.7 73.6 
4.8 76.2 
4.7 75.1 
90 6.4 74.5 
5.3 77 5.1 75.7 
95 7.2 75.7 
6.1 78.4 
5.8 76.9 
__________________________________________________________________________ 
From the data it can be seen that the through hole pattern has an effect on 
the sound performance and back pressure of the attenuator. 
EXAMPLE 5 
In this Example, various types of porous materials were used. 
The porous materials used were ACM-1, prepared according to Example 1 and 
porous polyethylene (commercially available under the trade designation 
"Porex X-4930" from Porex Technologies, Fairburn, Ga.). The "Porex X-4930" 
had a density of 31.9 lb/ft.sup.3, a Young's modulus of 31,200 psi, and 
would have a pore diameter of about 10 micrometers to about 40 
micrometers. The 12 inch by 12 inch by 0.24 inch thick sample weighed 290 
grams. The ACM-1 sample was 0.25 inch thick. Both samples had 144 through 
holes of 1/8 inch diameter drilled in them in the grid pattern of Examples 
1 and 4. The samples were tested as in Example 1 for SPL and BP. Test 
results and AFR are given in Table IV below. 
TABLE IV 
______________________________________ 
.25" 
Flow X-4930 W/1441/8" Holes 
ACM-1 W/1441/8" Holes 
Rate Pressure Pressure SPL 
(scfm) 
(inches of H.sub.2 O) 
SPL (dBA) (inches of H.sub.2 O) 
(dBA) 
______________________________________ 
5 0 55.9 0 56.5 
10 0.1 61.5 0 61 
15 0.2 64.7 0 64.3 
20 0.3 66.1 0.1 66.1 
25 0.4 68.6 0.2 67.8 
30 0.5 69.8 0.2 70.1 
35 0.6 71.4 0.5 71.5 
40 0.8 72.7 0.4 73.3 
45 1 73.8 0.5 75 
50 1.2 74.7 0.6 75.8 
55 1.4 76 0.7 77.2 
60 1.6 77.1 0.8 78.1 
65 1.8 78.6 1 79.5 
70 2.1 80.1 1.1 80.9 
75 2.3 80.9 1.2 81.9 
80 2.6 82.3 1.4 82.8 
85 2.8 83.1 1.5 83.6 
90 3 84.2 1.7 84.5 
95 3.4 85.4 1.9 85.8 
______________________________________ 
EXAMPLE 6 
In this Example, another type of porous material was used to prepare an 
attenuator of the invention. A comparative attenuator was prepared from a 
non-porous material. 
The porous material, designated ACM-2, was prepared according to Example 1 
except that aluminosilicate spheres (commercially available under the 
trade designation "Z-Light W1600" from Zeelan Industries, St. Paul, Minn.) 
were used in place of the K15 glass bubbles and the type 265 epoxy resin 
was blended with the Z-Light W1600 in a 1:6 by weight resin to particle 
ratio. The resulting block was 123/4 inches by 123/4 inches. The ACM-2 had 
a density of 28.8 lb/ft.sup.3, Young's modulus of 218,000 psi, and a % 
porosity of about 35%. The non-porous material was aluminum which had a 
density of about 171 lb/ft.sup.3. Both samples were 1/2 inch thick and had 
144 through holes of 1/8 inch diameter drilled through them in the grid 
pattern of Examples 1 and 4. The samples were tested as in Example 1 for 
SPL and BP. 
Test results and flow rate are given in Table V below. 
TABLE V 
______________________________________ 
ACM-2 Aluminum 
1441/8" Holes 1441/8" Holes 
Flow Rate 
Pressure SPL Pressure SPL 
(scfm) (inches of H.sub.2 O) 
(dBA) (inches of H.sub.2 O) 
(dBA) 
______________________________________ 
5 0 52.4 0 51.6 
10 0.1 57 0 55.3 
15 0.1 59.3 0.1 58.6 
20 0.2 61.1 0.2 59.9 
25 0.4 63.5 0.3 62.4 
30 0.5 65.3 0.5 64.7 
35 0.6 66.9 0.6 65.9 
40 0.7 68.5 0.7 67.9 
45 0.9 70.3 0.9 69.9 
50 1.1 71.1 1.1 70.7 
55 1.3 72.5 1.3 72.7 
60 1.5 73.6 1.6 73.3 
65 1.7 75.1 1.8 74.5 
70 1.9 76.4 2.1 75.6 
75 2.1 77.6 2.4 76.9 
80 2.4 79.6 2.6 78.1 
85 2.6 79.6 2.9 78.8 
90 2.9 80.5 3.3 79.9 
95 3.2 81.3 3.5 80.3 
______________________________________ 
From the table it can be seen that the sound performance of aluminum and 
the attenuator of the invention are comparable which is not expected on a 
mass law basis. Additionally, the attenuator of the invention has lower 
back pressure. 
EXAMPLE 7 
In this Example, a porous material was used to prepare an attenuator of the 
invention and compared to a comparative attenuator prepared from a 
non-porous material. 
The porous material used was ACM-1, prepared according to Example 1. The 
non-porous material was particle board. All samples were 3/4 inch thick 
and had 265 through holes of 1/8 inch diameter drilled in them in the grid 
pattern of Examples 1 and 4. The weight of the ACM-1 sample was 506.2 
grams and the weight of the particle board was 1,525.9 grams. The samples 
were tested as in Example 1 for SPL and BP. Insertion loss was measured 
according to the following: the sound pressure level was measured 
according to Example 1 with no sample in place, i.e., an open box. Then 
the sound pressure level was measured with the sample in place in the 
holder. The difference between the sound pressure level for no sample and 
the sound pressure level with sample in place was the insertion loss. 
Test results and flow rate are given in Table VI below. 
TABLE VI 
______________________________________ 
Particle Board - 
3/4" Thick with ACM-1 - 
265 Holes 3/4" Thick with 265 Holes 
Flow Insertion Insertion 
Rate Pressure Loss Pressure Loss 
(scfm) (Inches of H.sub.2 O) 
(dBA) (Inches of H.sub.2 O) 
(dBA) 
______________________________________ 
5 0.60 13.3 0.45 12.9 
10 0.70 15.6 0.60 13.3 
15 0.70 14.1 0.65 14.2 
20 0.75 16.4 0.75 16.3 
25 0.75 16.5 0.75 16.5 
30 0.80 17.0 0.75 16.6 
35 0.95 16.9 0.80 16.7 
40 1.10 17.3 0.85 16.4 
45 1.15 18.2 0.95 18.0 
50 1.20 19.1 1.10 19.0 
55 1.45 17.3 1.20 17.3 
60 1.70 17.6 1.20 17.3 
65 1.75 17.3 1.40 15.8 
70 1.85 17.2 1.50 16.8 
75 2.15 16.9 1.60 16.8 
80 2.40 17.1 1.75 16.9 
85 2.50 16.2 1.85 16.3 
90 2.70 17.1 2.10 16.2 
95 2.80 17.3 2.20 16.9 
100 3.15 17.3 2.40 15.8 
______________________________________ 
From the table it can be seen that the attenuator of the invention provides 
better overall sound performance by providing comparable insertion loss 
values and better back pressure performance with less mass when compared 
to particle board. This data along with that from Example 6 shows that the 
porous material shows a pressure drop benefit when the hole length is 
greater than about 1/2 inch. 
EXAMPLE 8 
In this Example, a porous barrier material of varying thickness and number 
of through holes was used to prepare an attenuator. 
The porous materials used was ACM-1, prepared according to Example 1 in 
varying thicknesses. A plurality of 1/8 inch diameter holes was drilled in 
each sample in the grid pattern of Examples 1 and 4. The samples were 
tested as in Example 1 for SPL and BP. 
Each sample was tested over the air flow range of 5 to 100 scfm and the 
differences in SPL and BP among the samples were approximately the same 
over the range of 20-100 scfm. Test results for 60 scfm air flow are given 
in Table VII below. 
TABLE VII 
__________________________________________________________________________ 
1.23% Open Area 2.26% Open Area 
5.34% Open Area 
144 Holes 263 Holes 625 Holes 
Thickness 
Pressure 
SPL Pressure 
SPL Pressure 
SPL 
(Inches) 
(Inches H.sub.2 O) 
(dBA) 
(Inches H.sub.2 O) 
(dBA) 
(Inches H.sub.2 O) 
(dBA) 
__________________________________________________________________________ 
1 2.919 71.8 
1.047 75.4 
0.804 80.1 
2 3.933 68.9 
1.48 71.4 
0.804 75.5 
4 4.864 65.9 
1.819 66.7 
0.888 70.4 
6 5.202 65.1 
1.903 66.3 
0.888 68.5 
__________________________________________________________________________ 
From the table it can be seen that the attenuator of the invention shows 
the following trends with regard to sample thickness, number of holes, and 
percent open area. As thickness of the sample increases, both back 
pressure and sound attenuation increase. As number of holes and the 
percent open area increases, back pressure and sound attenuation decrease. 
EXAMPLE 9 
In this example, the sound performance of an attenuator made from porous 
material with varying number of through holes versus frequency was 
determined. 
The porous material used was ACM-1, prepared according to Example 1. Three 
samples of 6 inch thickness were prepared and drilled with 144, 265 or 625 
through holes of 1/8 inch diameter, in the grid pattern of Examples 1 and 
4. 
Each of the samples was tested for SPL as outlined in Example 1 except that 
frequency in Hertz was measured instead of air flow rate. 
SPL values and frequency are given in Table VIII below. 
TABLE VIII 
______________________________________ 
Frequency (Hz) 
144 Holes 265 Holes 625 Holes 
______________________________________ 
31.5 18.27 18.46 23.54 
40 22.34 20.48 24.74 
50 22.91 23.33 19.92 
63 31.96 32.43 29.84 
80 25.59 25.05 24.46 
100 24.39 24.04 25.07 
125 29.61 29.00 28.64 
160 33.18 33.89 33.32 
200 38.59 38.17 39.22 
250 42.92 45.15 49.65 
315 41.98 44.9 50.63 
400 41.53 44.14 48.75 
500 55.01 59.71 64.86 
630 51.36 51.83 57.83 
800 55.43 57.01 59.34 
1000 47.53 47.95 51.57 
1250 52.40 54.00 55.93 
1600 49.98 52.77 54.16 
2000 51.27 50.89 50.99 
2500 51.88 52.80 53.81 
3150 50.99 50.87 52.88 
4000 50.82 50.12 49.91 
5000 53.83 53.57 52.96 
6300 56.65 65.21 55.41 
8000 57.38 56.73 55.69 
10000 52.63 52.75 51.43 
______________________________________ 
These data show the unexpected affect of greater noise attenuation at 
frequencies 4000 Hertz and above with increasing number of holes. 
Loudspeaker Example 
A loudspeaker cabinet was constructed from the attenuator of the invention. 
In the case of a loudspeaker cabinet the combined electrical, mechanical 
and pneumatic interactions resulted in a resonant magnification and 
redirection of sound. The cabinet was constructed of the same type of 
material as ACM-1 (prepared according to Example 1) with one inch 
thickness, mass of 3.97 kilograms and one inch hole spacing. The holes on 
the top were in an array 8.times.13, on the sides 8.times.19 and on the 
back 13.times.19. 
The cabinet interior dimension, was 13".times.19".times.8". All through 
holes were 1/8" in diameter. The loudspeaker cone used was an Audio 
Concepts type AC8, LaCrosse, Wis. Its direct current impedance was 4.8 
Ohms. 
Two types of test were performed on the cabinet: off-axis simulated free 
field response tests and impedance tests. 
Off-axis simulated free field response is termed the horizontal polar 
response. Polar response measurements were made for 45 degree increments 
in azimuth around the cabinet at angles normal to the front of the cabinet 
of 0, 45, 90, 135 and 180 degrees (deg). Acoustic responses were made in 
1/3 octave bands with center frequencies starting at 20 Hertz and ending 
at 20000 Hertz. A Bruel and Kjaer 2144 real time analyzer was used with 
input from a Bruel and Kjaer 4135 microphone. Data was collected with the 
microphone in the horizontal plane of the center of the loudspeaker cone 
and one meter distant from it. A Bruel and Kjaer 1402 pink noise source 
was used as a sound source. Pink noise is defined as noise having equal 
energy in each 1/3 octave band of interest. The pink noise was amplified 
by a Crown Com-Tech 800 before being fed into the loudspeaker. Testing was 
performed in an anechoic chamber. 
Impedance data was collected for the same cabinet. Impedance is the 
combined effect of a speaker's electrical resistance, inductance and 
capacitance opposing an input signal. It varies with frequency and is 
measured in ohms. The Audio Concepts type AC8 loudspeaker was used. A 
Bruel and Kjaer WB1314 noise source generator was used to drive the 
loudspeaker. A 1000 Ohm resistor in series with the loudspeaker created a 
constant current circuit and the frequency response voltage across the 
loudspeaker terminals was measured with a Bruel and Kjaer 2148 dual 
channel analyzer from zero to 400 Hertz in 1/2 Hertz steps. A calibration 
was carried out with a 10 Ohm resistor replacing the series combination of 
1000 Ohm resistor plus loudspeaker. The loudspeaker response in free air 
was measured. Then the loudspeaker was mounted in the loudspeaker cabinet 
and the cabinet's response was measured. 
The resonant frequency for the loudspeaker in free air was at 33.5 Hertz 
while the cabinet resonated at 30.5 Hertz. The cabinet resonance was 
shifted down in frequency from the free air case because the holes yielded 
a dynamic mass increase, which lowered the resonant frequency. The net 
effect of having holes in the cabinet was to produce a particular type of 
ported or vented loudspeaker cabinet. 
While this invention has been described in terms of specific embodiments it 
should be understood that it is capable of further modification. The 
claims herein are intended to cover those variations one skilled in the 
art would recognize as the equivalent of what has been done.