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22 TRG-TRC007-EN
notes
period two
Sound Perception and
Rating Methods
The following steps describe how to calculate an NC rating.
1 Plot the octave-band sound-pressure levels on the NC chart.
2 The 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.
NC
NC
-
-
15
15
NC
NC
-
-
20
20
NC
NC
-
-
25
25
NC
NC
-
-
30
30
NC
NC
-
-
35
35
NC
NC
-
-
40
40
NC
NC
-
-
45
45
NC
NC
-
-
50
50
NC
NC
-
-
55
55
NC
NC
-
-
60
60
NC
NC
-
-
65
65
70
70
50
50
40
40
30
30
20
20
10
10
sound pressure, dB ref 20
sound pressure, dB ref 20
m
m
Pa
Pa
80
80
60
60
octave
octave
-
-
band frequency, Hz
band frequency, Hz
63
63
125
125
250
250
500
500
1,000
1,000
2,000
2,000
4,000
4,000 8,000
8,000
Noise Criteria (NC) Curves
NC
NC
-
-
39
39
Figure 27