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2.4.4 Building acoustics
The sound absorption coefficient of a material is
α = (1 − r), where
r, the sound energy reflection coefficient, is the ratio of sound
energy reflected from the surface of the material to that incident upon it.
Values for a specific material depend upon frequency and upon the angle of
incidence of the sound. When the sound field is approximately diffuse the
corresponding quantity is denoted by αs; this may be determined
in accordance with BS 3638:1987. The values in the table overleaf have been
taken from Evans and Bazley (1978) which predates the current standard.
Absorption depends on mounting and other details of construction and the
following values should be regarded only as typical.
Reverberation absorption
coefficients
|
Material |
Thickness
(mm) |
Frequency/Hz |
|
125 |
250 |
500 |
1000 |
2000 |
4000 |
|
Acoustic
plaster . . . . . . . . . . . . |
13 |
0.15 |
0.20 |
0.35 |
0.60 |
0.60 |
0.50 |
|
Acoustic tiles (perforated
fibreboard) . . |
18 |
0.10 |
0.35 |
0.70 |
0.75 |
0.65 |
0.50 |
|
Asbestos
(sprayed). . . . . . . . . . . |
25 |
0.10 |
0.30 |
0.65 |
0.85 |
0.85 |
0.80 |
|
Brickwork . . . . . . . . . . . . . . |
— |
0.02 |
0.02 |
0.03 |
0.04 |
0.05 |
0.07 |
|
Carpet
(Axminster) . . . . . . . . . . |
8 |
— |
0.05 |
0.15 |
0.30 |
0.45 |
0.55 |
|
Carpet on
underlay . . . . . . . . . . |
14 |
— |
0.05 |
0.20 |
0.40 |
0.60 |
0.65 |
| Curtain
(velour,
draped) . . . . . . . |
— |
0.14 |
0.35 |
0.55 |
0.72 |
0.70 |
0.65 |
|
Glass fibre
(resin-bonded) . . . . . . |
25 |
0.10 |
0.25 |
0.55 |
0.70 |
0.80 |
0.85 |
| Glass
wool
(uncompressed) . . . . . |
25 |
0.10 |
0.25 |
0.45 |
0.60 |
0.70 |
0.70 |
|
Mineral
wool . . . . . . . . . . . . |
25 |
0.10 |
0.25 |
0.50 |
0.70 |
0.85 |
0.85 |
|
Polystyrene, expanded (rigid backing) |
13 |
0.05 |
0.05 |
0.10 |
0.15 |
0.15 |
0.20 |
|
Polystyrene, expanded (on 50 mm battens) |
13 |
0.05 |
0.15 |
0.40 |
0.35 |
0.20 |
0.20 |
|
Polyurethane foam (flexible) |
50 |
0.25 |
0.50 |
0.85 |
0.95 |
0.90 |
0.90 |
|
Snow . . . . . . . . . . . . . . . |
25 |
0.15 |
0.40 |
0.65 |
0.75 |
0.80 |
0.85 |
|
Wood panelling (oak, on 25 mm battens) |
13 |
0.20 |
0.10 |
0.05 |
0.05 |
0.05 |
0.05 |
Insulation against airborne
noise. The sound reduction index (R) is the
ratio of sound energy incident on a partition to that which is transmitted
through it, expressed in decibels (BS 2750: 1980). The values vary with
frequency and angle of incidence; for comparative purposes a single value
(Rw) can be derived from the data over the frequency range
100–3150 Hz according to BS 5821: Part 1: 1984. Although mass is the main
determinant of R above the resonant frequency, the relationship is not a
simple one, due to ‘coincidence’ effects associated with flexural
waves in the partition (Fahy 1985). For single, homogeneous partitions,
however, the mean value of R over the range 100–3150 Hz can be
estimated to be approximately Ravg =10 + 15
log10(m) dB, where m is the surface mass density in kg
m−2. For some types of partition Rw ≈
Ravg + 3 dB. The table opposite gives laboratory values of
R and Rw obtained at the Building Research
Establishment and the Building Test Centre of British Gypsum Ltd (Dr L. C.
Fothergill, private communication).
Sound Reduction Indices (dB)
|
|
Type of element |
|
|
1 |
Octave Band Centre Frequency/Hz
|
|
3 |
|
Rw
dB |
|
100 |
125 |
160 |
200 |
250 |
315 |
400 |
500 |
630 |
800 |
1000 |
1250 |
1600 |
2000 |
2500 |
3150 |
|
1 |
Thermal double glazing |
— |
22 |
23 |
19 |
17 |
21 |
26 |
28 |
30 |
32 |
33 |
33 |
32 |
30 |
28 |
30 |
33 |
30 |
|
|
(6-12-6), timber frame |
|
|
|
|
|
|
|
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|
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|
|
|
|
|
|
|
|
2 |
As 1, plus 4 mm secondary pane |
— |
35 |
30 |
31 |
27 |
32 |
38 |
40 |
44 |
47 |
49 |
49 |
44 |
41 |
43 |
43 |
45 |
42 |
|
|
spaced 150 mm, reveal lined |
|
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with absorbent |
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|
3 |
Brick, 100 mm thick, no finish |
200 |
33 |
35 |
33 |
34 |
34 |
33 |
36 |
39 |
42 |
44 |
46 |
48 |
49 |
51 |
52 |
53 |
44 |
|
4 |
Lightweight aggregate |
260 |
15 |
13 |
15 |
15 |
14 |
15 |
17 |
17 |
19 |
20 |
22 |
23 |
27 |
28 |
31 |
37 |
22 |
|
|
blockwork 215 mm thick, no |
|
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|
finish (permeable block) |
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|
5 |
As 4 with plaster finish both |
280 |
36 |
31 |
33 |
35 |
38 |
42 |
47 |
49 |
52 |
55 |
59 |
61 |
63 |
65 |
66 |
67 |
51 |
| |
sides |
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|
6 |
Two leaves 100 mm blockwork, |
390 |
28 |
27 |
31 |
33 |
35 |
35 |
41 |
42 |
47 |
52 |
55 |
59 |
63 |
66 |
69 |
70 |
46 |
|
|
50 mm cavity, no finish |
|
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|
7 |
As 6 with plaster finish both |
410 |
41 |
37 |
37 |
38 |
38 |
41 |
46 |
49 |
54 |
59 |
63 |
67 |
72 |
76 |
79 |
82 |
52 |
|
|
sides |
|
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|
8 |
Two leaves 100 mm aerated |
133 |
35 |
40 |
44 |
44 |
43 |
42 |
42 |
41 |
44 |
49 |
53 |
58 |
61 |
64 |
66 |
71 |
50 |
|
|
concrete blockwork, 50 mm |
|
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cavity, no finish |
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9 |
As 8 with plaster finish both |
153 |
37 |
42 |
46 |
43 |
38 |
38 |
40 |
43 |
47 |
51 |
56 |
60 |
66 |
68 |
71 |
76 |
49 |
|
|
sides |
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|
10 |
Floor–18 mm chipboard |
— |
19 |
15 |
20 |
25 |
28 |
29 |
34 |
37 |
37 |
42 |
43 |
44 |
44 |
42 |
40 |
41 |
38 |
|
|
194 × 45 mm joists at 450 mm |
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centres, 12.5 mm plasterboard |
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ceiling |
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|
11 |
As 10 but 600 mm joist spacing |
— |
17 |
26 |
32 |
38 |
35 |
35 |
37 |
37 |
41 |
45 |
47 |
49 |
51 |
49 |
45 |
40 |
42 |
|
12 |
12.5 mm plasterboard sheet |
11.8 |
21 |
21 |
23 |
25 |
25 |
26 |
28 |
28 |
29 |
31 |
33 |
34 |
35 |
35 |
29 |
26 |
31 |
|
13 |
12.5 mm plasterboard each side |
|
17 |
18 |
27 |
37 |
34 |
27 |
35 |
33 |
37 |
37 |
42 |
46 |
48 |
49 |
46 |
38 |
38 |
|
|
of 75 mm × 38 mm timber studs |
|
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14 |
As 13 with 25 mm mineral wool |
|
13 |
15 |
28 |
35 |
31 |
32 |
37 |
37 |
42 |
43 |
47 |
50 |
53 |
53 |
46 |
41 |
40 |
|
|
in cavity |
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|
15 |
As 14 with no layers of |
|
24 |
24 |
33 |
42 |
35 |
37 |
42 |
44 |
47 |
48 |
51 |
54 |
57 |
58 |
52 |
48 |
46 |
|
|
plasterboard each side |
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| 16 |
Two leaves of 12.5 mm + 19 mm |
|
43 |
47 |
55 |
56 |
59 |
64 |
67 |
71 |
72 |
72 |
75 |
80 |
83 |
84 |
86 |
86 |
70 |
|
|
plasterboard on metal studs, |
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separated by 250 mm cavity |
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with 100 mm mineral wool |
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References
BS 2750: Part 3: 1980 Laboratory measurements of airborne sound
insulation of building elements, equivalent to ISO 140/111–1978.
BS 3638: 1987 Measurement of absorption in a reverberation room,
equivalent to ISO 354-1985.
BS 5821: Part 1: 1984 Rating the sound
insulation in buildings and of building elements, equivalent to ISO
717/1-1982.
E. J. Evans and E. N. Bazley (revised 1978) Sound Absorbing
Materials, NPL ISBN 0 9504496 3 6.
F. Fahy (1985) Sound and
structural vibration, Academic Press.
D.R.Bacon
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