Basic Documentation

Page 2 of 12 Siemens Industry, Inc.
Document No. 149-979
were developed to quantify various loudness levels
of typical ventilation relate
d sound in rooms. The NC
curves extend over the eight standard octave bands
and only represent ventilation related sound. They
do not include other sounds that might also occur in
rooms such as speech, work related sounds, office
machines, etc.
Now Come the Complications and
Confusion
After the NC curves became an accepted reference
in the ventilation industry, the acoustical gurus
1
came up with another set of sound curves referred
to as the Room Criterion (RC) curves. The RC
curves were touted as being superior to the NC
curves because, among other things, the RC curves
1. ASHRAE Technical Committee 2.6 - Sound and Vibration
Control.
extend down to 16 Hz. (Yet, the RC curves only
extend up to 4000 Hz and not 8000 Hz as do the NC
curves.) Soon thereafter, these sound guru's came
up with another set of curves called the NCB Sound
Criterion curves and more recently the RC Mark II
Room Criterion curves. They now claim the RC Mark
II Room Criterion curves are the preferred way of
representing ventilation related sound, even though
they admit these curves are complicated to
understand and use.
Based upon this continuously changing sound scene
it is not surprising that many designers are reluctant
to delve into the subject of ventilation related sound.
Incidentally, when asked, several of the sound gurus
reluctantly admitted that the NC curves will very
likely remain the industry's preferred reference for
room related ventilation sound.