Trane TRG-TRC007-EN Sound Perception and Rating Methods, period two, Noise Criteria NC Curves

period two

Sound Perception and

Rating Methods

notes

Noise Criteria (NC) Curves

	80
mPa	70
		NC-39						NC-65
20	60
								NC-60
ref
								NC-55
dB	50
								NC-50
pressure,
	40							NC-45
								NC-40
								NC-35
sound	30
								NC-30
	20							NC-25
								NC-20
	10							NC-15
		125	250	500	1,000	2,000	4,000	8,000
	63

octave-band frequency, Hz Figure 27

The following steps describe how to calculate an NC rating.

1Plot the octave-band sound-pressure levels on the NC chart.

2The highest curve crossed by the plotted data determines the NC rating.

Although the NC curves are popular and easy to use, they do have a few shortcomings. Specifically, they do not account for the tonal nature and relative magnitude of each octave band. Figure 27 shows octave-band data measured in an open-plan office space and plotted on an NC chart. The resulting value, NC- 39, is generally considered to be acceptable for this type of environment. Notice that this NC value is set by the 63 Hz octave band and the sound drops off quickly in the higher octave bands.

In this particular example, sound generated by the air-handling unit travels through the ductwork, breaks out through the duct walls, and radiates into the office area. To achieve the desired NC level, two layers of sheet rock were added to the exterior surface of the duct to block the low-frequency sound. Unfortunately, because high-frequency sounds are much more easily attenuated than low-frequency sounds, the upper octave bands are now over- attenuated.

Although an objective analysis deems the resulting NC-39 sound level acceptable in this type of open-plan office space, most listeners in the space would probably perceive this unbalanced spectrum as having an annoying rumble.

22 TRG-TRC007-EN