Noise Paths
Noise in buildings may take many paths. The following figure illustrates the possible paths.

Noise Levels
As discussed in the perception of sound levels, the human hearing system has different sensitivities at different frequencies. Therefore, the perception of noise is not same for 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 perception of the loudness of the noise. A specific circuit is added to the sound level meter to correct its reading according 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 measurement.


The following table displays A-weighted sound levels for some common noises:

Small office	Large office	Car 65 mph at 25'	Light traffic at 100'	Quiet residential (daytime)	Quiet residential (nighttime)	Sewing machines at 3'
50-55 dBA	60-65 dBA	70-80 dBA	50-60 dBA	40-50 dBA	30-50 dBA	95-100 dBA

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 NoiseControl3D.

Sound transmission class (STC) is a single number used to characterize the air-borne isolation properties of a partition.

STC is determined from the measured TL of a partition at different frequencies. These measured values are then compared with standardized STC contours. Simple rules apply when 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 composite STC calculations are provided in NoiseControl3D program.

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, L90)
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 noise:

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 adding 10 dBA to the sound levels from 10 P.M. to 7 A.M. 10 percentile-exceeded sound level (L10) (L50, L90 can be used) is the A-weighted sound level have happened at 10% or more of the time of the measurement (or 50%, 90% in case of L50, L90).

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 curves.

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).