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26 TRG-TRC007-EN
notes
period two
Sound Perception and
Rating Methods
If we plot the acoustical data for our example office space on the RC chart, we
find that it results in a rating of RC-31(R). The SIL is 31 and the sound-pressure
levels in the 63 Hz and 125 Hz octave bands are above line D, indicating a
rumble characteristic of the sound.
This time, our objective and subjective analyses lead to the same conclusion.
Although the space is quiet enough, the background noise is perceived as
having a rumble. A sound spectrum that falls in the RC neutral category would
be judged as excellent by most observers. It is this conformity of analysis
results that makes the RC rating method a better tool than the other single-
number descriptors for specifying acoustical requirements indoors. Despite
the advantages of the RC rating system, it is less widely used than other single-
number descriptors.
Finally, accurate determination of sound-power levels for the 16 Hz and 31.5 Hz
octave bands requires a very large reverberant room. Most HVAC equipment
manufacturers do not provide sound data in these two octave
bands due the cost of constructing such a large test room and the difficulty in
qualifying it. This makes it difficult to predict sound-pressure levels in these
octave bands.
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
31.5
31.5
63
63
125
125
250
250
500
500
1,000
1,000
2,000
2,000 4,000
4,000
RC
RC
-
-
25
25
RC
RC
-
-
30
30
RC
RC
-
-
35
35
RC
RC
-
-
40
40
RC
RC
-
-
45
45
RC
RC
-
-
50
50
A
A
B
B
Room Criteria (RC) Curves
C
C
D
D
E
E
RC
RC
-
-
31 (R)
31 (R)
Figure 30