ATTENUATION thì dò
10
20
10
FRET METERS I
GRAPH на 10
10
3
30
100
30
300
100
L000
300
EXAMPLE — The noise level at 3ft. from the source is 90 db. It will be 90-20 or 70 db at 30ft. from the source.
2.TRANSMISSION LOSS OF A PARTITION. In a single partition weight (mass law) is the determining factor in its ability to resist noise penetration. The following graph re- presents an averaging of a partition's ability taken over the frequencies of 125, 250, 500, 1000, 2000 and 4000 CPS. NOTE: For each doubling of the weight there is an increase of only 4 to 5 dbTL and if the noise contains a predominance of low frequency com- ponents or narrow bands of noise add a 5-10 db safety factor.
TRANSMISSION LOSS IN Gb.
40
30
30
›
GRAPH No. 11
6010246 20
A 6 8100146 200
WEIGHT OF PARTITION IN LBS/SQ.FT.
EXAMPLE - The TL of a 101b/$ parti- tion is approximately 36 db. To gain an extra 4 to 5 db will require a 20lb/§ partition.
3. TRANSMISSION LOSS OF A DOUBLE WALL STRUCTURE. The single partition's sound transmission loss is limited by the weight factor. A double partition of equal weight to a single partition can achieve a 5-15 db increase in TL. This benefit is derived by having the two wythes isolated from each other and by the addition of surface applied treatments (isolated plaster diaphram, large cavities with absorbent treatments, etc.).
It is advisable to seek the help of an acoustical consultant on the use of complex forms, as all bounding parti- tions have to be considered in order to achieve a balanced control. Any weak
link destroys the value of the complex construction.
4. TRANSMISSION LOSS OF A COMPOSITE WALL. Most walls in building designs include doorways or windows and/or openings and general ly the wall TL exceeds that of the doors and windows. The following graph provides a means for computing the transmisson loss of a composite
structure.
(IND) TL OF WALL- TL OF DODE,WINDOW OR OPENING
•
30
a
OF TOTAL 4RSA OF WALL SCCURES
345478 10 15 30 BS TO BE SOBTRACTER FEDA YL OF WALL FOR TL OF
GRAPH no. 12
COMPOSITE BARRIER.
NOTE. The attenuation of noise by the barrier does not include the re- duction due to distance.
6. NOTES ON NOISE CONTROL. Air-tightness is the essential factor in achieving effective noise control. Un- less care and supervision are adequate during the construction phase much of the value of the noise control con- struction will be lost.
It is essential to ensure that all joints, meeting points and changes of construction at doors, windows, ducts, pipes, conduits and abutting of walls to floors, etc. be properly sealed. Where movement is likely to take place, replace rigid jointing by flexible packing such as fiberglass, oakum, soft board or caulking, or augment by raked jointing with caulking material as pointing.
As an example of the effect of cracks, open joints etc: If a wall of 100 sq. ft. with a TL of 50 db has 1 sq. 500 in. or say 0.1% opening, then by graph No. 12, where the line of 50 db crosses the curve of 0.1% gives a loss of 19 db. Therefore the wall overall value is cut to 31 db. This is an extreme example but it demonstrates the importance of air-tightness.
EXAMPLE - A wall of 250 sq. ft. with a doorway of 25 sq. ft. The wall has a TL of 45 db and the door a TL of 25 db. The difference in TL is 20 db and the percentage of doorway to wall is 10%. Where the 20 db line cuts the 10% curve gives 11 db. Therefore the transmission loss of the composite wall is 45-11-34 db.
5. ACOUSTICAL SCREENS. Acoustical screens can attenuate noise by 10-15 db as long as the following factors are met.
i) TL of the wall is at least 15 db. ii) There are no openings to reduce its effectiveness.
iii) The ratio of distance between listener and barrier, and barrier and noise source exceeds 8 to 1.
iv) The ratio of distance between end of barrier and noise source, and noise source and barrier is 2 to 1 or more. If this ratio cannot be met use a partial enclosure.
t
KLISTENER
SECTION
ALINE OF
SIGHT
NOISE SOURCE
SCREEN
2+
NOISE SOURCE
PLAN
7. LOCATION. It cannot be over- stressed that the planned placement of noise producing equipment can con- siderably reduce noise control require- ments. Further to this the next im- portant step in early design is the planned placement of environments re- quiring above average noise protection in relation to noise source internal and external to the project.
8. COMMON PROBLEMS EX- AMINED.
EXAMPLE No.1
COOLING TOWER
TERRACE
In this example the noise source is the cooling tower and the listener is the terrace and restaurant.
H.P. of cooling tower 100. Distance from source to listener 75ft. Allowable noise level at terrace 50 dba
Noise level of cooling tower at 3ft.
Attenuation with distance
of 100ft.
=
= 105 dba
= 30 dba
= 75 dba – much too high
:. noise level at terrace
=
COOLING TOWER
EXAMPLE NO. 2
Far East BUILDER, July 1970
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