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64 TRG-TRC007-EN
notes
period five
Review
Period Two discussed how the human ear perceives sound. As a selective
sensory organ, the human ear is more sensitive to high frequencies than to
low frequencies. The sensitivity of the human ear at a particular frequency also
changes with loudness.
Many single-number rating systems have been developed over the years and
each has its advantages and drawbacks. This clinic focused on some of the
more commonly used rating systems, including A-weighting, Noise Criteria
(NC), and Room Criteria (RC). A-weighting is typically used to describe the
sound in outdoor environments, and for determining whether or not hearing
protection is required in certain industrial environments. Noise Criteria and
Room Criteria are used to describe the sound in indoor environments. The RC
method has the added feature of describing the character, or quality, of the
sound, as well as its loudness. However, because most HVAC equipment
manufacturers do not have sound-power data for the 16 Hz and 31.5 Hz octave
bands, it is difficult to predict the sound-pressure levels in these octave bands.
Finally, both sound-power levels and sound-pressure levels can be described
using either full or one-third octave bands. Octave-band sound-power data is
commonly used for describing the sound generated by HVAC equipment. It can
also be used to describe the sound in either indoor or outdoor environments.
Review—Period Two
I A-weighting
K Outdoor environments
and hearing protection
I Noise criteria (NC)
K Indoor environments
I Room criteria (RC)
K Indoor environments
K Also describes sound
character
I Octave-band data
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
25
25
30
30
35
35
40
40
45
45
50
50
RC
RC
chart
chart
Figure 70