Civil Engineering Reference
In-Depth Information
frequencies.
To be acceptable, the “loudness” of noise from mechanical systems
must not prevent people from hearing sound they want to hear. Addition-
ally, the “quality” of mechanical noise must not be intrusive or annoying.
Therefore, the maintenance staff must be concerned about both the level
of mechanical noise and its quality.
In occupied environments, sound is measured on the basis of the
“A-scale.” Since people do not hear low frequency or high frequency
sound as well as mid-range frequency, A-scale sound meters adjust the
measured levels in each of 9 frequency bands to give more authority to
the midlevel ranges. The sound level at each of these 9 frequency bands
is then “added” to yield a total in terms of
decibels,A-scale(dBA)
. Noise
levels measured using the A-scale represent the filtering of sound that
replicates the human hearing response. This is the most commonly used
descriptor to quantify the relative loudness of various types of sounds
with similar or differing frequency characteristics.
The World Health Organization (WHO), recommends that the level
of noise in occupied spaces should not exceed those specified in Table 4-8.
Table 4-8. WHO Maximum Allowable Background Noise Levels
————————————————————————————————
To
Max.NoiseLevelandDuration
————————————
Avoid
Hrs
————————————————————————————————
Hearing impairment 85-100 1
————————————————————————————————
Annoyance 50-55 16
————————————————————————————————
Disturbance of communication/lack
of speech intelligibility 35 All
————————————————————————————————
Sleep disturbance 30 8
————————————————————————————————
dBA
Air Filters and Cleaners for Particulates
The first step in reducing exterior air particulates and pollutants is
to evaluate outdoor air intakes and, if necessary, relocate them to reduce
the potential for outdoor pollution. ASHRAE recommends that the fol-
lowing criteria be used for outdoor air intakes:
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