ARCHITECTURAL
ACOUSTICS
In this second article in a three-part series on Architectural Acoustics, Mr. Ian Campbell, DA(Edin.), ARIBA, MASA, outlines some of the basic techniques and methods by which architects and builders can identify potential noise sources. Identification is the first step towards avoiding noise problems.
2. Graphs and figures to
check design requirements
PREVENTION being better than cure, the identification of potential noise sources during design will obviate the costs and frustrations of remedying noise problems after a project has been completed.
In order to accomplish this preven- tion, one must learn first what are all the sources of noise associated with the project and second what are the means of controlling the noise pro- duced. But before proceeding to these two subjects it is important that the reader understands the terminology employed in acoustics (see opposite page).
Noise sources
The initial consideration in the analysis of noise control requirements is to define possible noise sources, the transmission paths of these sources, and the listeners. The paths constitute 'the system'.
A number of noise sources are pre- sent both inside and outside all build- ing projects. The requirement is to assess their effect on the environment being created.
1. ADDING NOISE LEVELS. Sup- pose we wish to add together two 70 db noise levels. The result will not be 140 db, as these noise levels have to be converted to relative energy levels and then added algebraically. The follow- ing graph eases the problem to a great
extent.
12
dù TO BE ADDED TO LOWER HUMBER
-2
O
2
t
10
DIFFERENCES IN db BETWEEN TWO LEVELS BEING
ADDED
CRAPH NO.1
EXAMPLE
-
Calculate the result- ant noise level when 70 db, 70 db and 76 db are combined. Step 1 differ- ence between the two lower levels is 0 from table add 3 db = 73 db. Step 2 – difference between 73 and 76 db is 3 :. from table add 5 db to 73 = 78 db. To add many levels, this operation is performed as many times as is neces-
sary.
2. TRANSPORTATION NOISE. Each type of transportation vehicle has its own particular noise level and can reach 100 dba or more at a distance of 3 ft. It is possible for a person to be affected by the noise as far away as 300 ft. A higher noise level can be tolerated when the noise source is not continuous. The following graph esti- mates the increased tolerance related to the percentage of time involved.
NOISE DURATION IN % OF TIME
-025
어
25
10
15
10
30
db TO 61 ADDED TO MAXIMUM ALLOWABLE NOISE LEVEL WHEN NOISE IS NOT CONTINUOUS.
GRAPH NO. 2
EXAMPLE It can be seen from the graph that when the noise is present for only 6 per cent of the time, 10 db can be added to the accepted design level.
3. AUTOBUSES. These vehicles are a prime source of annoyance, especial- ly if the noise source is present for a considerable amount of time, i.e. heavy traffic. The following graph is an estimate of the noise level gene- rated by the average autobus.
SOUND PRESSURE LEVEL IN DBA FOR
AUTOBUS AT 20 TO 40 APU
FELT 15 ALTERS 3
CRAPH NO 3
30
100
10
30
300 100
1050
300
82
4. HELICOPTERS. Another type of transportation which is likely to cause noise problems more frequently in the future.
SOUND PRESSDEI LAVEL IN NBA FOR HELICOPTER
(AVERAGE • TWIN ROTOR)
10
FEET 160
METERS 30
300
100
1000
3000 1,000
10000
3.000
300
LINE OF SIGHT DISTANCE TO HELICOPTER GRAPH NO. 4
Far East BUILDER, July 1970