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Technical Note (National Research Council of Canada. Division of Building Research), 1965-04-01
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Donato, R. J.
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DIVISION OF BUILDING RESEARCH
NATIONAL RESEARCH COUNCIL OF CANADA
'fE
C
1HIN II CAlL
No.
439NOTE
PREPARED BY R. J. Dona to CHECKED BY TDN APPROVED BY NBH
April 1965
PREPARED FOR
SUBJECT
General Distribution
Transmission Loss on a Double-Stud Wood Party Wall
The wall tested was for use in Britain and so its specifica-tion should meet that described in British Building Research Staspecifica-tion
Digest
88.
This differs from the standard normally used in NorthAmerica in that 16 third-octave bands are used extending from 100 cps
to 3200 cps. Our experimental technique at the moment only permits
us to take measurements at half-octave intervals, so it has to be assumed that the transmission loss is not affected by reducing the
interval. Since the British criterion is a straight averaging one,
with no weighting factors to be employed ｾｴ different frequencies,
this assumption seems reasonable.
The or iginal requ irement was for each face of t he wall to be
a double layer of
3/8
in. thick gypsum board. For our own interesta subsidiary experiment was made using only one layer on each face. Finally, because it was found that the original design fell just short of the British requirements, a third layer was added to one face.
Description of the Wall
The overall wall was 10 ft. wide and
8
ft. high and wasmounted in one of the NRC sample mounting racks. The panels were
nailed to 1
518
in. x3
518
in. wooden studs, the studs being 90 in.long and 16 in. apart. Each row of studs was terminated at the '
top and bottom of the wall by a 1
5/8
in. x3 5/8
in. wooden plate.In turn each plate at the top was nailed to another 1
518
in. x3 5/8
in. plate, and the two at the bottom to a 15/8
in. x9
in. plate;the two rows of studs were thus separated by 1
3/4
in. ｾｩｮ･ｲ｡ｬ woollayers, 2 in. thick, were stapled to the outer faces of each set of
studs. All joints and nail-heads were taped. In addition the joints
..
- - - -- - -
2
-were secured to the studs with two-inch ring nails 8 in. apart. The total weight of the double-layer wall per unit square foot of surface was 9.5 lb., the individual weights being:
Results Wood frame Gypsum board Mineral wool - 240 lb. 480 lb. 40 lb.
The test was performed according to the requirements of
ASTM E90-6lT, apart from the-extension to 100 cps. The results for
face panels one on each side, two on each side, and three on one side
and two on the other are given in Figure 1 and Table I. The STC
rating for all three curves is surprisingly the same and equal to 50. The divergences from the British standard for House Grade party wall
are shown in the ｦｩｾｲ･Ｎ The wall with one panel on each side does
not meet the standard and neither does the specified wall with two
panels on each side. However, in this latter case the departure is
small; the British standard allows a total deficiency of 23 dB in the
one-third octave bands, ｷｨ･ｲｾ｡ｳ here we have 28 dB. The addition of
one more thickness of panel to one face improves the low-frequency performance sufficiently that the wall now fulfills the requirements. Comments
It is interesting, and a trifle startling, to see the effect
the addition of an extra panel can make. Theory indicates that at most
the transmission loss would vary as the fourth power of the mass. Doubling the mass would increase the transmission loss by 12 dB, and
increasing it in the ratio 5:4 would increase the transmission loss
about
4
dB. At the lower frequencies we find an overall change nearthe calculated 16 dB in going from a total number of gypsum board
layers of two to five. The intermediate curve for a total of four
layers is situated roughly between the other two to fit the theory. Further, the theory predicts this mass dependence to hold at the high frequency region which is not what we observe from the curves.
There is a transmission loss minimum in the region 300-500 cps, hinted at in the thinner wall but pronounced for the thicker
one. Ignoring the effect of the mineral wool layers the smallest
calculated value for this minimum of the transmission loss is about
660 eps; To explain the lower observed value we would have to assume
that the velocity of sound between the two faces of the wall is reduced
by the mineral wool layer. This, of course, is if we take the wool to
be a set of capillaries.
As an interesting sidelight all three observed curves would
be rated on the STC system as 50. Thus although their low-frequency
characteristics are vastly different the presence of one minimum falling in a critical range of frequencies can completely dominate the overall rating.
•
TABLE I
TRANSMISSION LOSSES FOR DIFFERENT NUMBER OF GYPSUM BOARD LAYERS
FREQUENCY TRANSMISSION LOSS IN dB
IN c/s
CASE
1
CASE2
CASE3
100
27.0
31.0
125
28.0
35.0
43.0
175
34·0
43.0
48.0
250
42.0
47.0
55.0
350
44·0
46.0
46.0
500
45.0
46.0
46.0
700
49.0
52.0
52.0
1000
54·0
56.0
55.0
1400
5800
60.0
59.0
2000
63.0
65.0
66.0
2800
64·0
65.0
65.0
4000
57.0
60.0
59.0
CASE 1: One layer of
3/8
in. gypsum board on each face.CASE
2:
Two layers of3/8
1n. gypsum board on each face.CASE
3:
Two layers of3/8
1no gypsum board on one face, three•
"
7° .50 cO D ＨＯ｜ｾＴＰ (/\ o oJ ｾＳＰ (jj (/\ ::t ｾ 20 « cr I-10 / / ,/ / / ＬＮＭＭｾ ,,,,, B',,
'-A / , / /A OWE. LAY£R -[-Ye,"GYPSUMSOARD ON /!.ACJI FACE..
8 TWO LAYE.R5ofＮＳｾＢ GYPSUM BOARD ON EACI/
rACE-c rlVo LAYE.R5 -f%/GYPSUM BOARO
ON ON£. FACE-JTIIRE.£. LAyE.RS
ON THE. OTHeR D BRlnSJI JlOUSE. GRADE.
PARTY WALL. REQUIReMENTS
FIGURE... 1
SOUNI7 ｔｾＢｎｓｍｉＵｓｉＰｎ LOSS of DOUBLE.. - STUI' \"ALL \lfITH ｖａｾｉｏｕｓ ｓｕｾｆａＨＬｅＮＮ ｔｾｅＮＢｔｍｅＮｎｔｓ
