Sound absorption laminate

A sound absorption laminate comprises a porous insulation substrate, such as, a thermoplastic glass or polymeric fiber blanket or a foamed polymeric resin sheet and a facing sheet with a high air flow resistance. The facing sheet is adhered to a surface of the porous insulation substrate to augment the acoustical properties of the substrate. With the facing sheet the air flow resistance of the laminate is greater than the air flow resistance of the substrate and the laminate exhibits a higher sound absorption coefficient than the sound absorption coefficient of the substrate. Thus, the laminate exhibits better sound absorption properties than the substrate and is suitable for sound absorption applications for which the substrate alone would not be suitable.

BACKGROUND OF THE INVENTION 
The present invention relates to the use of facings, having high air flow 
resistances, to enhance the sound absorption properties of certain porous 
insulation materials and especially, those insulation materials having low 
air flow resistances. By laminating the high air flow resistance facings 
to these particular porous insulation materials, these porous insulation 
materials exhibit sound absorption characteristics normally provided by 
more costly fibrous insulation materials of greater thickness, higher bulk 
density and/or smaller average fiber diameter and foam insulation 
materials having smaller cells and pores. 
Porous insulation materials such as thermoplastic glass or polymeric fiber 
blankets and polymeric foams are used in many applications to enhance the 
sound absorption performance of various products and systems. Typical 
applications include: acoustical wall panels, ceiling panels and office 
partitions; automotive headliners and hoodliners; liners for heating, 
ventilating and air conditioning systems; appliance insulation; and 
similar applications. 
The sound absorption characteristics of these porous insulation materials 
is a function of the acoustic impedance of the material. The acoustic 
impedance is a complex quantity consisting of frequency dependent 
components called, respectively, acoustic resistance and acoustic 
reactance. The acoustic reactance of these porous insulation materials is 
governed largely by the thickness of the product and, to a much lesser 
extent, by the mass per unit area of an air permeable facing or film which 
may be applied over the surface of the porous insulation material. The 
acoustic resistance of the porous insulation material is governed by the 
air flow resistance of the porous insulation material. 
The ratios of the acoustic reactance and the acoustic resistance to the 
characteristic impedance of air determines the normal incidence sound 
absorption coefficient. For a given value of the acoustic reactance ratio, 
there is an optimum value of the acoustic resistance ratio which will 
provide the maximum sound absorption. Since the reactance ratio of a 
porous insulation material is determined largely by the thickness of the 
porous insulation material, the most effective way of controlling the 
sound absorption properties of a porous insulation material is by 
adjusting the acoustic resistance ratio. In the past, the acoustic 
resistance ratio has been adjusted by changing the physical properties of 
the porous insulation materials. In fibrous insulations, such as glass 
fiber insulations, the average fiber diameter of the insulation has been 
decreased, the bulk density of the insulation has been increased, and the 
binder content of the insulation has been increased. In polymeric resin 
foam insulations, the average pore or cell size of the insulation material 
has been decreased. While these physical modifications increase the 
acoustic resistance ratio of these insulation product, the cost of 
producing these products is also increased. 
SUMMARY OF THE INVENTION 
The present invention uses a thin, coated or uncoated, semi-porous paper, 
fabric or perforated film facing of controlled air flow resistance to 
increase the air flow resistance of an underlying porous insulation, such 
as, a glass or polymeric fiber insulation or a polymeric foam insulation, 
having an acoustic resistance ratio less than the optimum acoustic 
resistance ratio for optimum sound absorption. The acoustic reactance 
ratio of the laminate formed by applying the facing to the porous 
insulation substrate is not materially different from the acoustic 
reactance ratio of the porous insulation substrate. However, the increased 
air flow resistance of the laminate (formed by applying the facing to the 
porous insulation substrate) relative to the air flow resistance of the 
porous insulation substrate, results in an acoustic resistance ratio for 
the laminate which is greater than the acoustic resistance ratio of the 
porous insulation substrate. Accordingly, the sound absorption properties 
of the laminate are superior to those of the porous insulation substrate. 
The benefits of the present invention are most dramatic when such facings 
are applied to low cost, thin, lightweight fibrous insulations made with 
large diameter fibers and thin, lightweight, polymeric foam insulations 
having large cells and pores. The air flow resistance provided by such 
insulations is frequently too low to provide adequate sound absorption for 
many applications. By increasing the air flow resistance of these low cost 
porous insulation materials through the use of controlled air flow 
resistance facings, the sound absorbing properties of these porous 
insulation materials are improved so that these low cost insulations can 
be used for more demanding applications previously requiring the use of 
more expensive insulation materials. 
However, the greater the thickness and/or bulk density of a porous 
insulation material, the greater the air flow resistance of the material. 
For porous insulation materials of a certain thickness and/or density, the 
air flow resistance of the porous insulation material, alone, provides the 
insulation with an acoustic resistance ratio at or above the optimum 
acoustic resistance ratio for optimum sound absorption. For these porous 
insulation materials, increasing the air flow resistance, by applying a 
facing to the porous insulation material will only degrade the sound 
absorption properties of the insulation material. Thus, the present 
invention is directed to the use of thin, coated or uncoated, semi-porous, 
facings, only, on those porous insulation materials where the air flow 
resistance of the facing, when combined with the air flow resistance of 
the porous insulation material, forms a faced porous insulation laminate 
with superior sound absorption properties to those of the porous 
insulation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a sound absorption laminate 12 comprising a porous 
insulation substrate 14 and a controlled porosity facing 16 adhesively 
bonded to one surface of the porous insulation substrate. While the porous 
insulation substrate 14 can be a relatively high density substrate, the 
porous insulation substrate 14 is typically a low cost, thin, lightweight, 
large diameter fiber, fibrous insulation material, such as, glass fiber 
insulation having a bulk density of less than two pounds per cubic foot, 
or a low cost, thin, lightweight polymeric foam insulation material having 
large cells and pores. The controlled porosity facing 16 is a thin, coated 
or uncoated, semi-porous paper, fabric or perforated film facing having a 
controlled air flow resistance which is selected to add to the air flow 
resistance of the underlying substrate 14 whereby the sound absorption 
properties of the laminate 12 are improved over the sound absorption 
properties of the substrate 14, alone. The appropriate facing for a 
particular substrate 14 is selected as follows. 
The sound absorption performance of the porous insulation substrate 14 is a 
function of the acoustic impedance of the substrate 14. The sound 
absorption performance of the sound absorption laminate 12 is a function 
of the acoustic impedance of the laminate 12. The acoustic impedance of 
the porous insulation substrate 14 is a function of the acoustic 
resistance and the acoustic reactance of the substrate. The acoustic 
impedance of the sound absorption laminate 12 is a function of the 
acoustic resistance and the acoustic reactance of the laminate. The 
acoustic reactance of the porous insulation substrate 14 or the sound 
absorption laminate 12 is governed largely by the thickness of the 
substrate or the laminate. The acoustic resistance of the porous 
insulation substrate 14 or the sound absorption laminate 12 is governed 
largely by the air flow resistance of the substrate or the laminate. The 
ratios of the acoustic reactance and the acoustic resistance to the 
characteristic impedance of air, .rho.c, for the porous insulation 
substrate 14 or the laminate 12 determine the sound absorption 
characteristics of the substrate or the laminate. 
The angle of incidence of sound waves with respect to the surface of the 
porous insulation substrate 14 or the sound absorption laminate 12 also 
affects the degree of sound absorption achieved by the substrate or the 
laminate. For the purposes of illustration, the following discussion deals 
only with the normal (90.degree. angle of incidence) sound absorption 
properties of a porous insulation substrate or a laminate. However, the 
concept of enhancing the sound absorption characteristics of a porous 
insulation substrate, through the application of a high air flow 
resistance facing to the substrate, applies to products intended to absorb 
sound in both normal incidence and/or diffuse (random incidence) sound 
fields. The following calculations of the normal incidence acoustical 
properties or characteristics of porous insulation substrates 14 and 
laminates 12, as well as, the procedures for estimating the desired air 
flow resistance characteristics of the high air flow resistance facing to 
be applied to the substrate are intended to be used only as a first 
approximation. 
Once the value of the optimum air flow resistance for a facing to be 
applied to a given porous insulation substrate has been estimated, the 
actual value of the optimum air flow resistance for a facing to be applied 
to a given porous insulation substrate, to maximize the sound absorption 
properties of the laminate formed by applying the facing to the substrate, 
is best determined experimentally using a range of various air flow 
resistance facings over the particular substrate. Optimizing such a sound 
absorption laminate system for normal incidence sound absorption 
properties can be accomplished by using either the standing-wave or the 
two-microphone impedance tube methods described in ASTM methods C-384 and 
E-1050, respectively. Optimizing the random incidence sound absorption of 
a particular sound absorption laminate system can be accomplished 
experimentally by using the reverberation room test method described in 
ASTM method C-423. 
For normally incident sound, the sound absorption of a substrate or sound 
absorption laminate may be estimated from the following relationship: 
##EQU1## 
Thus, as shown by Equation 1, for a given value of the acoustic reactance 
ratio (x/.rho.c) which is governed mainly by the thickness of the porous 
insulation substrate 14 or sound absorption laminate 12, there is an 
optimum value of the acoustic resistance ratio (r/.rho.c) which will 
provide the largest normal incidence sound absorption coefficient for the 
porous insulation substrate or the sound absorption laminate 12. Where the 
fibrous insulation substrate 14, due to its thickness, bulk density and/or 
fiber diameters, or the polymeric foam insulation substrate 14, due to its 
thickness and/or pore and cell size, already has a sufficient air flow 
resistance to optimize the value of the acoustic resistance ratio, the 
lamination of a thin, semi-porous facing to the substrate will not enhance 
the sound absorption characteristics of the substrate and the facing 
should not be applied for acoustical purposes. However, where the porous 
insulation substrate does not have the required air flow resistance to 
optimize the value of the acoustic resistance ratio for the particular 
substrate, a thin, semi-porous facing laminated to the substrate can 
increase the air flow resistance of the substrate to optimize the value of 
the acoustic resistance ratio and thereby optimize the sound absorption 
coefficient. 
To determine whether or not a facing can be laminated to a porous 
insulation substrate 14 to improve the acoustical performance of the 
substrate, the acoustic resistance ratio of the unfaced substrate, the 
acoustic reactance ratio of the unfaced substrate and the optimum acoustic 
resistance ratio for the substrate should be calculated. If the value of 
the acoustic resistance ratio of the unfaced substrate is already 
sufficient to maximize the sound absorption properties of the substrate, 
there is no need to apply a facing to the substrate for acoustical 
purposes. If the value of the acoustic resistance ratio of the porous 
insulation substrate is insufficient to maximize the sound absorption 
properties of the substrate, the additional air flow resistance to be 
provided by a facing to achieve the optimum acoustic resistance ratio is 
determined and the appropriate facing is selected. 
Equations 2, 3, and 4, below, can be used to determine the acoustic 
resistance ratios and the acoustic reactance ratios for the porous 
insulation substrate 14 and the sound absorption laminate 12. These 
equations enable one to determine whether or not a facing can enhance the 
acoustical performance of a porous insulation substrate and, if the 
acoustical performance can be enhanced, the additional air flow resistance 
required from a facing to optimize the acoustic resistance ratio. 
For plain glass fiber insulations without a facing, the acoustic resistance 
of the substrate (r.sub.s) may be either measured directly by ASTM method 
C-522 or estimated on the basis of the following empirical relationship: 
##EQU2## 
The acoustic resistance ratio of a laminate of the glass fiber insulation, 
with a facing applied, is calculated as follows. The acoustic resistance 
(r.sub.s) of the glass fiber insulation substrate is either measured or 
computed as in Equation 2, above. The additional acoustic resistance 
provided by the facing (r.sub.f) is added to the acoustic resistance of 
the glass fiber insulation substrate to obtain the acoustic resistance of 
the sound absorption laminate. This sum is then divided by the 
characteristic impedance of air (.rho.c) to obtain the acoustic resistance 
ratio (r.sub.L /.rho.c) of the sound absorption laminate as follows: 
##EQU3## 
The acoustic reactance ratio (x/.rho.c) used in the calculation of the 
normal incidence sound absorption coefficient can be approximated by the 
following expression: 
##EQU4## 
At a given frequency, the optimum acoustic resistance ratio for a sound 
absorption laminate, r.sub.L /.rho.c, will be approximately equal to the 
following expression: 
EQU r.sub.L /.rho.c=[(1+(x/.rho.c).sup.2 ].sup.0.5 Eq. 5 
In order to determine whether or not the addition of a high flow resistance 
facing will improve the normal incidence sound absorption provided by a 
particular porous insulation substrate at a given frequency, the acoustic 
resistance ratio (r.sub.s /.rho.c) of the substrate is measured (ASTM 
C-522) or computed (Eq. 2) and the acoustic reactance ratio (x/.rho.c) is 
computed (Eq. 4). If the magnitude of the acoustic resistance ratio 
(r.sub.s /.rho.c) of the porous insulation substrate is numerically less 
than the optimum value computed from Equation 5, the application of a high 
flow resistance facing to the porous insulation substrate will likely 
improve the normal incidence sound absorption of the substrate. If the 
magnitude of the acoustic resistance ratio of the porous insulation 
substrate is numerically equal to or larger than the optimum value 
computed from Equation 5, the application of a high flow resistance facing 
to the substrate will likely reduce the normal incidence sound absorption 
provided by the substrate alone. 
This procedure can be repeated over a range of frequencies in order to 
determine whether or not a high air flow resistance facing will be 
beneficial and, if so, approximately what value of flow resistance is 
required for the facing. 
The desired value of acoustic resistance ratio for the facing material to 
be applied over a particular substrate will then be the difference between 
the acoustic resistance ratio of the porous insulation substrate and the 
optimized value of the acoustic resistance ratio for the substrate. Thus, 
the acoustical properties of thin, low density, porous insulation 
materials can be upgraded through the use of the appropriate thin, coated 
or uncoated, semi-porous paper, fabric or perforated film facing. 
FIGS. 2.1 to 2.6 and 3.1 to 3.6 illustrate how the use of a thin, 
semi-porous facing can enhance the sound absorption characteristics of 
thin, low density, porous glass fiber insulation materials. FIGS. 2.1 to 
2.6 show the calculated normal incidence sound absorption coefficients for 
frequencies from 100 to 5,000 Hz. for one-half inch nominal thickness 
faced and unfaced glass fiber insulation, comprising fibers having a mean 
fiber diameter of 4.7 microns, at bulk densities of 0.5, 1.0, 1.5, 2.0, 
4.0 and 6.0 pounds per cubic foot. FIGS. 3.1 to 3.6 show the calculated 
normal incidence sound absorption coefficients for frequencies from 100 to 
5,000 Hz. for one inch nominal thickness faced and unfaced glass fiber 
insulation, comprising fibers having a mean fiber diameter of 4.7 microns, 
at bulk densities of 0.5, 1.0, 1.5, 2.0, 4.0 and 6.0 pounds per cubic 
foot. As illustrated in FIGS. 2.1 to 2.6, a facing can greatly enhance the 
sound absorption characteristics for one-half inch thick glass fiber 
insulation for densities up to approximately 4.0 pounds per cubic foot. 
Above a density of 4.0 pounds per cubic foot, the flow resistance of the 
porous insulation has already reached the optimum value and a facing does 
not augment the acoustical properties of the glass fiber insulation. As 
shown in FIGS. 3.1 to 3.6, a facing can greatly enhance the sound 
absorption characteristics for one inch thick glass fiber insulation for 
densities up to approximately 1.5 pounds per cubic foot. Above a density 
of 1.5 pounds per cubic foot, the flow resistance of the porous insulation 
has already reached the optimum value and a facing does not augment the 
acoustical properties of the glass fiber insulation. Thus, FIGS. 2.1 to 
2.6 and 3.1 to 3.6 illustrate that the present invention, through the use 
of relatively inexpensive, thin, semi-porous facings can upgrade the 
performance of particular inexpensive, porous insulations whereby such 
inexpensive insulations can be used for more demanding applications 
previously requiring the use of more expensive insulation materials. 
In FIGS. 2.1 to 2.6 and 3.1 to 3.6, the optimum acoustic resistance value 
for a particular insulation of a given thickness and bulk density was 
calculated by iteratively computing the normal incidence sound absorption 
coefficient for that thickness and density as a function of r.sub.f. The 
value of r.sub.f which provided the highest average normal incidence sound 
absorption coefficient for the frequencies of 250, 500, 1,000 and 2,000 
Hz. was taken as the optimum air flow resistance value for the facing. The 
frequencies of 250, 500, 1,000 and 2,000 Hz were selected for determining 
the optimum air flow resistance for the facing because sound absorptive 
materials are normally specified on the basis of the single number Noise 
Reduction Coefficient (NRC). The NRC is computed on the basis of the 
average random incidence sound absorption at those four frequencies. It is 
assumed that the random incidence sound absorption coefficients will rank 
in order with the normal sound incidence coefficients as computed for 
FIGS. 2.1 to 2.6 and 3.1 to 3.6. 
This assumption was verified using Manville one-half inch nominal 
thickness, 1.5 pound per cubic foot, EXACT-O-COTE glass fiber insulation 
with and without a high air flow resistance SNOWWEB fibrous fabric facing 
imbedded in an acrylic coating applied to one side of the glass fiber 
insulation. With the use of the SNOWWEB facing, the air flow resistance of 
the sound absorption laminate was approximately 740 mks rayls while the 
air flow resistance of the glass fiber insulation, alone, was only 
approximately 360 mks rayls. The average normal incidence sound absorption 
coefficient for the frequencies of 250, 500, 1,000 and 2,000 Hz. was 
increased from 0.29 to 0.37 and the random incidence noise reduction 
coefficient (NRC) was increased from 0.55 to 0.60 even though the flow 
resistance for the laminate was below the theoretical optimum value of 
approximately 1,250 mks rayls. To applicants' knowledge, a 0.60 NRC value 
was not previously possible to attain in an one-half inch thick glass 
fiber insulation product at a bulk density of approximately 1.5 pounds per 
cubic foot. 
In describing the invention certain embodiments have been used to 
illustrate the invention and the practice thereof. However, the invention 
is not limited to these specific embodiments as other embodiments and 
modifications within the spirit of the invention will readily occur to 
those skilled in the art on reading this specification. Thus, the 
invention is not intended to be limited to the specific embodiments 
disclosed, but is to be limited only by the claims appended hereto.