User manual

109
Noise Curves
It essentially represents a rumble criterion. The RNC curves
also provide a procedure that reduces the result essentially
back to the NC curves when systems are well designed and
acoustically well-behaved. Following the RNC specification, the
XL2 measures the octave-band sound pressure level every 100
ms, followed by processing to determine the applicable room
noise criterion (RNC) curve. The minimum measurement time
is 20 seconds.
Preferred Noise Criteria PNC
(in accordance with ASA 1971)
PNC curves represent a more stringent method based on an
extension of the basic Noise Criteria system. They have been
used in the past to judge the acceptability of ventilation and
other background broadband noise. PNC curves are less often
used than Noise Criteria curves because they are more strin-
gent at lower frequencies than the Noise Criteria curves, but
also because the latest (2008) version of Noise Criteria curves
also includes an extended frequency range somewhat mitigat-
ing the original reasons for PNC.
Room Criteria RC
(in accordance with ANSI S12.2-1995)
The RC criterion curves are a system for use in the design
of heating, ventilating, and air-conditioning (HVAC) systems in
office buildings, dwelling units, etc., where the desired mid-
frequency levels are in the range of 25 to 50 dB. Each RC crite-
rion curve bears a rating number equal to the level at 1000 Hz.
Spectrum classification
Neutral spectrum (N): The levels at 500 Hz and below do not •
exceed the RC curve corresponding to a sound level spec-
trum by more than 5 dB; and the spectrum levels in Band
1000 Hz and higher do not exceed the corresponding RC
curve by more than 3 dB.
Rumble (R): Excessive noise in low-frequency band•
The level in one or more of the octave bands at and below
500 Hz exceeds the RC curve corresponding to a spectrum
by more than 5 dB.
Hiss (H): Excessive noise in high-frequency bands•
The level in one or more of the octave bands at and above
1000 Hz exceeds the RC curve corresponding to a spectrum
by more than 3 dB.
Vibration and rattle (RV): The level in one or more of the oc-•
tave bands from 16 Hz through 63 Hz exceeds the criterion
for moderately noticeable rattle.