Chapter hvac engineering Fundamentals: Part 1 Introduction

20.5.1
Noise criteria curves
Noise criteria curves were the standard for many years, and they de-
fine acceptable limits for sound pressure level in each octave band.
Figure 20.4 shows the standard NC curves. The actual environment
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492
Chapter Twenty
Figure 20.4
NC curve. (
SOURCE
: Copyright 1997, American Society of Heating, Refrig-
erating and Air Conditioning Engineers, Inc., www.ashrae.org. Reprinted by permission
from ASHRAE Handbook,
1997 HVAC Fundamentals,
Chap. 7, Fig. 4.)
must not exceed the specified curve at any point, but can be at any
level below the curve. The resulting sound may be too quiet in some
frequencies. A higher sound power level is acceptable at lower fre-
quencies. These curves emphasize the fact that high frequencies sound
louder than low frequencies when sound power levels are equal.