ARI White Paper
NC Levels – What they Mean and What They Do Not Mean
Many engineers rely on the NC tables in manufacturers’ catalogs to give them an
estimate of noise levels in their proposed finished spaces as well as a comparison
between manufacturers. This is a relatively easy process for the consultant or the
contractor because this is a single number comparison, and these numbers are easily
found in manufacturers’ catalogs.
There is only one standard for predicting occupied space sound levels. It is ARI
Standard 885-98 (with 2002 Addendum), “Procedure for Estimating Occupied Space
Sound Levels in the Application of Air Terminals and Air Outlets”. You will find in
Appendix E, on the last page of the standard, a list of recommended deductions that
can be used for charting catalog performance in predicting NC average values in
unknown conditions. These are the criteria used in manufacturers’ catalogs to calculate
the cataloged NC values.
Since NC values are single numbers, they do not reflect which octave band is setting
the value (where the noise peaks). This can still cause some confusion. Units peaking in
the 3rd or 4th octave bands will produce a different sound quality in the room from those
peaking in the 2nd octave band. They will also add noise differently to the other noise
levels in the space, which will change the type of noise heard by the occupant. It is
impossible to determine rumbly from hissy or neutral spectrums using only NC values.
That is why other options such as RC values are sometimes used. RC values are
averaged numbers with quality descriptors. For a complete description of these ratings,
see ARI Standard 885. The Standard also describes why RC values are not practical for
rating equipment because of their respective averaged values. You can download a free
copy of ARI Standard 885-98 (with 2002 Addendum) at www.ari.org.
It is important to note that the deductions in Appendix E of ARI Standard 885 are typical
of a large number of averaged conditions. They are described in detail in the Standard.
However, this does not mean that they are relative to your job. They certainly are not
specific to your job. They may be similar, and they may have no similarity at all. That is
what ARI Standard 885 is all about: how you can reasonably predict the room sound
levels of a particular room in a particular building with the proper known construction
and finishing details. This could never be done in a manufacturer’s catalog because
these conditions are unknown at the time the catalog is printed. While Appendix E may
be typical of an average office space, it is specific to none. Consequently it can only be
used as general criteria.
It is also important to note that ARI certification has nothing to do with NC levels or any
other room sound level predictions. There are no ARI certified NC values. ARI
certifies only sound power levels measured in a reverberant test chamber for variable
air volume terminals . The attenuation values listed in Appendix E are applied to the
sound power data that are generated per ASHRAE Standard 130-1996, “Methods of
Testing for Rating Ducted Air Terminal Units” and ARI Standard 880-98 (with 2002
Addendum), “Air Terminals” to arrive at the predicted sound pressure levels in the
space. Sound power levels derived by ARI Standard 880 and ASHRAE Standard 130
are used as a starting point before the attenuation values are deducted. It is the sound
power levels measured under ARI Standard 880 and listed in the catalog that are ARI
certified data. You can download a free copy of all ARI certified ratings at www.ari.org.
The only way to compare performance between manufacturers using their respective
catalogs is to look at their sound power levels as generated in reverberant test
chambers. Those are the sound power levels by octave band, which are listed in all the
catalogs. This is more difficult than single number comparisons, but it is the only way to
compare without testing each unit in a fixed mock up environment. It is also the only
way to add the sound pressure levels contributed by other pieces of equipment such as
air handlers, ducts, dampers, terminal devices, etc. (This addition process by octave
band is also described in ARI Standard 885.) Using these added numbers, the resultant
sound pressure levels for the room can then be plotted on an RC chart to predict the
room sound pressure level and quality for the space being evaluated.
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