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Sound Transmission Loss
Transmission loss or what is commonly called noise insulation is the reduction
in the amount of sound energy passing through decks, bulkheads, walls, floors,
roofs etc. and is a property of the element as a whole. It is expressed in
decibels (dB).
Noise may be due to airborne or impact sound and both must be taken into
account where appropriate. Impact sound is a very important factor for example
in intermediate floors, whilst airborne sound would be the predominant factor
in separating walls and partitions.
Particular attention should be paid, when designing elements of construction,
to the ways in which sound may penetrate at the edges of the element, or
through doors, windows etc. This ‘flanking noise transmission’ can be a
problem if it is not taken into account at the design stage.
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Airborne and impact
noise transmission |
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Flanking noise
transmission |
Noise Absorption
Noise absorption is the reduction in sound energy obtained when the sound is
reflected from a surface. In general, the harder the surface, the more sound
is reflected.
Noise absorption is expressed as a factor between 0 and 1.0. If a surface
absorbs no sound, its coefficient of absorption is 0. If 100% of the sound is
absorbed, the coefficient is 1.0 and if 50% then 0.50 and so on.
The structure of the mineral wool makes it an ideal product for use as a sound
absorber, with characteristically high coefficients over a wide frequency
range.
The test used is the reverberation room method which gives a good rating
assessment. Due to room differences and inherent material variations, measured
absorption co-efficients show wide variations and it is not practical to place
reliance on differences in co-efficients of less than 0.15.
When used as a sound absorbent lining within a room, the sound reduction that
can be obtained usually ranges between 5 and 10 dB. The actual reduction
depends on the room details etc, and an assessment by an acoustic consultant
should be obtained.
An appreciable airspace incorporated behind the acoustic slabs improves the
low frequency sound absorption co-efficient.
Absorption Co-efficients For Some Mineral Wool Products
| Material |
Thickness |
Mounting |
|
|
H |
z |
|
|
| |
mm |
|
125 |
250 |
500 |
1000 |
2000 |
4000 |
| 32kg Glass |
50 |
Direct |
0.15 |
0.50 |
1.00 |
1.00 |
1.00 |
1.00 |
| 32kg Glass |
75 |
Direct |
0.39 |
0.66 |
0.92 |
1.00 |
1.00 |
1.00 |
| 45kg Rock |
50 |
Direct |
0.15 |
0.60 |
0.90 |
0.90 |
0.90 |
0.85 |
| 45kg Rock |
75 |
Direct |
0.25 |
0.60 |
1.00 |
1.15 |
1.15 |
1.05 |
| 60kg Rock |
50 |
Direct |
0.25 |
0.65 |
1.05 |
1.10 |
1.10 |
0.95 |
| 60kg Rock |
75 |
Direct |
0.55 |
1.10 |
1.20 |
1.15 |
1.15 |
1.05 |
| 60kg Rock |
100 |
Direct |
0.56 |
1.00 |
1.05 |
1.07 |
1.00 |
1.00 |
| 100kg Rock |
50 |
Direct |
0.35 |
0.85 |
1.10 |
1.10 |
1.15 |
1.10 |
The absorption co-efficients shown above are typical figures
that can be achieved by mineral wool products.
They have been obtained from a comprehensive range of measurements made over a
number of years.
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