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Resources
» Myths & Realities
» Painted Foam |
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Acoustic Myths and Realities:
Painted Foam
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Long before
Newton was “occasioned by the falling of an
apple” the common assumption was that a two
pound object fell twice as fast as a one pound
object (Aristotle). Galileo contradicted
Aristotle. After he dropped a few things from
the Leaning Tower of Pisa and had them reach the
ground simultaneously, what everybody
knew-to-be-true changed.
Following this tradition, we tested the
assumption that painted acoustical foam would
not absorb sound as well as natural, unpainted
acoustical foam. It didn’t work that way,
although we still read that it does. Paint is
applied by spraying in multiple, light surface
coats to avoid sealing the surface or saturation
of the foam, producing no significant change in
performance. |
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The following lab results show that the NRC
values for 3-inch and 4-inch acoustical,
anechoic wedge foam are identical. (Before
rounding to nearest 0.05, 4-inch painted foam
measured 0.01 above the natural!) The slight,
and insignificant, degradation in test
performance for 2-inch painted acoustical foam,
0.04 - 0.05 (rounded), could have been caused by
the differences between the acoustical
characteristics of the natural (unpainted) foam.
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Sound Absorption tests performed by
International Acoustical Testing
Laboratories, Inc.,
January 1980 according to ASTM
C-423-77
SOUND ABSORPTION COEFFICIENTS
1/3 Octave Band Center Frequency
(Hz)
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Dept. |
125 Hz |
250 Hz |
500 Hz |
1K Hz |
2K Hz |
4K Hz |
NRC* |
NRC Actual |
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2"N |
0.08 |
0.25 |
0.61 |
0.92 |
0.95 |
0.92 |
0.70 |
0.68 |
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3"N |
0.14 |
0.43 |
0.98 |
1.03 |
1.00 |
1.00 |
0.85 |
0.86 |
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4"N |
0.20 |
0.70 |
1.06 |
1.01 |
1.01 |
1.00 |
0.95 |
0.95 |
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2"S |
0.08 |
0.27 |
0.55 |
0.82 |
0.90 |
0.91 |
0.65 |
0.64 |
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3"S |
0.14 |
0.45 |
0.98 |
0.99 |
1.00 |
1.00 |
0.85 |
0.86 |
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4"S |
0.23 |
0.75 |
1.05 |
1.03 |
1.00 |
1.00 |
0.95 |
0.96 |
N = Natural Foam
S=Fire Retardant Overspray
Test Procedure:
Sixty-four square feet of
material in an 8' x 8'
configuration was placed flat on
the floor, near but not at the
center of a 17' x 20' x 15' high
reverberation chamber. The
perimeter edges of the material
were taped ot the floor.
Testing was carried out in
strict accordance with ASTM:
C423-77 test procedure.
Precision at ninety percent
confidence limits for the
absorption coefficients was ±
0.03 at 125 Hz, less than ±
0.015 at 250,500, and 1000 Hz,
and less than ± 0.025 at 2000
and 4000 Hz.
*Noise Reduction Coefficient
(NRC) values are reported per
ASTM C423-77 to the nearest
0.15. Actual values in the
INTEST reverberation room are
shown for reference. The
difference between the sprayed
and unsprayed 2" sample could
have been caused by the
differences between the
acoustical characteristics of
the natural foams.
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Note: Contrary to
cartoons and folklore, Newton was never hit on
the head by an apple, he just noticed it and
wondered why things fall. The Leaning Tower of
Pisa cannot be proven as the “drop site” in
Galileo’s experiment.
*NOTE:
IF YOU WISH TO REFER TO THESE TESTS, PLEASE
CREDIT ACOUSTICS FIRST® CORPORATION AS THE
SOURCE. WE CANNOT AUTHORIZE USE OF THE "INTEST"
NAME BEYOND OUR REPRODUCTION OF THIS ORIGINAL
PRINTED SUMMARY. |
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