NTi Audio XL2 Sound Level Meter - Manual (Page 107)

107
Noise Curves
Room Noise Criteria RNC
(in accordance with ANSI S12.2-2008)
The RNC method is used to determine noise ratings when
the noise from HVAC systems at low frequencies is high, and
which is also suspected of containing sizeable fluctuations or
surging. It essentially represents a rumble criterion. The RNC
curves also provide a procedure that reduces the result essen-
tially 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.
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 exceed the criterion for
moderately noticeable rattle.