Noise in buildings may take many paths. The following figure illustrates the possible paths.
As discussed in
the perception of
sound levels, the human hearing system has
different sensitivities at different frequencies. This means that
the perception of noise is not equal at all frequencies. Noise with significant
measured levels (in dB) at high or low frequencies will not be as annoying
as it would be when its energy is concentrated in the middle frequencies.
In other words, the measured noise levels in dB will not reflect the
actual human pereception of the loudness of the noise.
A specific circuit is added to the sound level meter to
correct its reading in regard to this concept. This
reading is the noise level in dBA. The letter A is added
to indicate the correction that was made in the
The following table displays A-weighted sound levels for some
||Car 65 mph at 25'
||Light traffic at 100'
||Quiet residential (daytime)
||Quiet residential (nighttime)
||Sewing machines at 3'
TL, STC and IIC
The transmission loss (TL) for a partition and the noise reduction in the room are defined in
the following drawing and demonstrated in the program NoiseControl2D.
Sound transmission class (STC) is a single number used to
characterize the air-borne isolation properties of a
partition. The STC is determined from the measured TL of
a partition at different frequencies. These measured
values are then compared with standardized STC contours as shown in the "examples" page.
Simple rules apply in choosing the
appropriate contour. For example: STC for painted concrete
block depends on its weight, but will be in the range of
45-47 for 8", and of 47-51 for 12".
The STC of the composite partition (wall and window for
example) is not the sum of the STC of its components. We have
first to calculate the TL for the composite wall (taking
into account the surface area of the components) then find the
STC rating. STC and compsite STC calculations is provided in NoiseControl2D program. Sketch of the calculations is provided in the "examples" page
Impact isolation class (IIC), like STC, is another
single-number rating system for a solid-borne noise
(floor-ceiling structure). The higher the IIC rating, the
more efficient the construction will be in attenuating
the impact sound within the frequency range of the IIC.
For example: 6" reinforced concrete slab (75
lb/sq ft) has IIC 34, and about 37 when linoleum is added
to the floor.
Community Noise (Leq
Ldn, L10, L50,
Community noise constantly changes its level and
duration. It can reach 50 dBA changes in short time.
The following are some of the A-weighted
quantities used in measuring the effects of environmental
Equivalent sound level (Leq)
is a steady-state sound that has the same energy and A-weighted level
as the community noise over a given time interval.
Day-night averaged sound level (Ldn) is
the 24-hour Leq obtained after addition of 10
dBA to the sound levels from 10 P.M. to 7
10 percentile-exceeded sound level (L10)
L90 can be used) is the A-weighted sound level
happened at 10% or more of the time of the measurement (or 50%, 90% in case of L50,
There are many other different quantities used in the
community noise measurements (such as Ln and CNEL).
The criteria of evaluating
the community noise can be given in any of these
quantities. These criteria include more details for
studying the community noise, and they depend on the local
regulation and the type of the noise.
Room Noise (NC, RC, NCB)
Noise in buildings is more stable (over time) than outside
community noise. The
maximum acceptable background noise level generated by
the mechanical systems in buildings is usually specified in terms of
average A-weighted sound levels, NC, RC, or NCB.
The noise criteria (NC) values are determined from
the measurements of the octave-band sound levels in an occupied
room when the air-conditioning system is on. The measured values are then compared to standard NC
The room criterion (RC) is mostly used for acoustical design
of HVAC systems. The RC criteria take into account the noise components
at the lowest and the highest frequencies. The measurement values should be
taken in an unoccupied room.
Another criterion called balanced noise criterion
(NCB) has been recently standardized and has concepts similar to
NC and RC.
The sound level measurements for NCB should be taken in an occupied room.
The RC and NCB
ratings include procedures for checking different factors such as the rumble compliance
(excessive noise at frequencies below 500 Hz) and the hiss compliance (excessive noise at
frequencies above 1000 Hz).
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