SEPTEMBER 2016 \ BUILDING DIALOGUE \
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(i.e., Composite Sound Transmission Class and,
more recently, Composite Outside to Inside
Transmission Class).
STCc is an acoustical calculation of the overall
ability of a building element to block exterior
noise to inside rooms. The individual STC rat-
ings of the roof, exterior walls, doors and win-
dows are analyzed and based on the square foot
of each, a composite sound isolation is calcu-
lated. The calculations include the frequencies
between 125 and 4,000 hertz, which are speech
frequencies. It doesn’t work well if the noise in-
cludes lower frequencies (<100 Hz) caused by
trucks, train engines, aircraft flyovers, etc.
OITCc is a more recent ASTM test standard (E-
1332) also used to rate the sound isolation per-
formance of the exterior of a building. It was ad-
opted because it includes frequencies down to
80 Hz. This better represents aircraft, truck and
rail noise because it is weighted more to lower
frequencies. OITCc should be used if the residential project is
located near sources having significant low-frequency noise
content. However, OITC is more difficult to use since OITC
ratings for many building elements are not available.
It is important to select the appropriate method to evalu-
ate a project. If you apply OITCc analysis to a project where
there is no significant low frequency noise impact on
the building, the results can lead to higher costs due to
overdesign. Conversely, if you use STCc for the building
analysis and there are low-frequency noise sources near-
by, complaints are likely.
It is important to first evaluate the location and then mea-
sure exterior noise levels at the project site to determine the
appropriate method to use in the analysis and to determine
how to reduce noise inside the living spaces. If the project is
in a quiet suburb, STCc probably applies. On the other hand,
a project located near highways or rail lines should probably
be analyzed using OITCc.
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ELEMENTS
Noise Reduction
Sound Pressure
Noise Source / Location
Level (in dBA)
Subjective Description
Quiet Suburban Residential area at night 40
Faint
Quiet Urban area during day
50
Moderate
Conversational Speech at 3 ft.
50 to 60
Moderate to Loud
Highway Traffic
60 to 70
Moderate to Loud
Shouting Speech at 3 ft.
75 to 80
Loud
Busy Street Curbside
75 to 80
Loud
Heavy Truck Noise at 20 ft.
85 to 90
Very Loud
Siren at 10 ft.
90 to 95
Very Loud
Train Horn at 50 ft.
90 to 95
Very Loud
Construction Site with Pneumatic Drilling 90 to 100
Deafening
