NTi Audio XL2 Sound Level Meter - Manual (Page 106)

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Noise Curves
Noise Curves (optional)
The Spectral Limits Option extends the XL2 with the Noise
Curves measurement function. A noise curve may be used to
characterize room noise or other environments. Various stan-
dardized international noise curves are the most widely used
means for evaluating background sound in buildings, and other
facilities such as transit facilities, as well as in other indoor/
outdoor spaces. Residual noise in buildings may be generated
from both environmental sources (e.g. outside traffic) and sys-
temic sources (e.g. heating, ventilating, and air-conditioning
(HVAC) systems; or other machinery in use). It is also frequently
necessary to measure residual noise curves prior to expected
changes in advance of construction or prior to other expected
environmental noise changes.
Noise Rating NR
(in accordance with ISO 1996)
Noise rating (NR) is a graphical method for assigning a single
number rating to a noise spectrum. It can be used to specify
the maximum acceptable level in each octave band of a fre-
quency spectrum, or to assess the acceptability of a noise
spectrum for a particular application. The method was originally
proposed for use in assessing environmental noise, but it is
now used frequently for describing noise from mechanical ven-
tilation systems in buildings. To make a rating, the measured
noise spectrum is superimposed on a family of NR contours;
the NR of the spectrum corresponds to the value of the first
NR contour that is entirely above the spectrum.
Noise Criteria NC
(in accordance with ANSI S12.2-2008 and -1995)
The NC rating of a spectrum is designated as the value of the
highest NC curve “touched” by the measured octave-band
spectrum. The designating number for any NC curve is, ap-
proximately, its speech interference level (SIL); the average of
the levels in the 500, 1000, 2000 and 4000 Hz octave bands.
The XL2 Analyzer includes any tangency method adaptation as
applicable.