ASHRAE Journal - July 2013 - 20

It’s not unusual to see equipment schedules where individual
rooms are shown with various maximum allowable sound levels
like NC22, NC23, NC25, NC28, etc. This typically occurs when
the designer takes room sound levels from a manufacturer’s
product selection program rather than provide the actual desired
maximum room sound level. When scheduling NC levels, it’s
standard practice to specify various rooms in increments of 5 NC
points. There’s an audible difference between NC35 and NC40,
whereas it’s impossible to distinguish between NC35 and NC36.
Sound levels for rooms in a building should always be selected to be appropriate for the purpose of the room. Here are
some typical NC levels for ofﬁce building spaces:
• NC30: Maximum sound level for a conference rooms;
• NC35: Maximum sound level for private ofﬁces;
• NC40: Ideal sound level for open ofﬁce areas with cubicles; and
• NC45: Maximum sound level for hallways, restrooms,
utility spaces, etc.
Conference rooms are typically the quietest rooms because
they should provide an ideal environment for communication
and speech clarity. Private ofﬁces should provide supervisors
with a quiet place for writing, talking on the phone or meeting with employees. Employees in cubicles can work more effectively when a slightly higher background level blocks distractions from nearby coworkers and provides speech privacy.
Hallways and utility rooms are transitory spaces and, therefore, are not typically sound sensitive.
It’s helpful to remember that equipment need not be located
over the space being served. For instance, if a fan-powered
terminal is needed to supply a large conference room, but the
radiated sound level is likely to be an issue, it may be advisable to locate the air terminal over a non-critical space such as
a hallway or utility room.

Specifying Maximum Allowable Sound Power Levels
Perhaps the best way of ensuring that project sound levels
will be achieved is to specify the maximum allowable sound
power levels for each device on the schedule. A good way to
start this process is to select a desired sound level in terms
of RC. Since RC takes into account not only the loudness of
a sound spectrum but also looks at tonal imbalances, an RC
with a neutral spectrum can be speciﬁed. Table 15 in AHRI
885 provides recommended RC levels for various types of
rooms.
Once a suitable RC level has been selected, refer to Table 14
in AHRI 885 to get the sound pressure levels by octave band that
match the appropriate RC level. By limiting tonal imbalances in
low and high frequencies, rumble and hiss can be prevented. Establish the maximum allowable sound pressure levels required to
maintain a neutral spectrum by adding 5 dB to octave bands 2–3
and adding 3 dB to octave bands 5–7. These are now the maximum sound pressure levels that must be maintained. Sound path
attenuations must be determined for both radiated and discharge
sound in order to calculate the maximum allowable sound power
levels. Add the total attenuation of each path by octave band to
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ASHRAE Journal

the maximum sound pressure levels to determine the maximum
allowable radiated and discharge sound power levels.
The RC method was developed to rate the sound performance
of a system as whole. When using RC as described previously to
achieve room sound levels, other possible sources of noise such
as air handlers and other equipment are not taken into account.

Sound Paths
Radiated sound paths typically involve a ceiling/space effect for sounds originating in a ceiling plenum or a space effect
for sound sources located within a room. Ceiling/space effects
are estimated based on ceiling materials in accordance with
ASHRAE Research Project RP-755, Transmission of Sound
from Air Terminal Devices through Ceiling Systems. Ceiling/
space effects can be found in AHRI Standard 885, Table D14.
The results assume that the plenum is at least 3 ft (0.9 m) deep,
at least 30 ft (9 m) wide or lined with insulation and that there are
no signiﬁcant openings directly below the sound source. Space
effects are estimated based on room volume and the distance
from the sound source to the listener using the Schultz equation:
Space Effect = (25) – 10 log (r) – 5 log (V) –3 log (f)
Where:
r = Shortest distance from sound source to listener, ft (m)
V = Room volume, ft3 (m3)
f = Octave band center frequency, Hz
Discharge sound paths are more complicated and often include attenuation due to ductwork, end reﬂection, ﬂex duct,
air outlet devices and space effect. The attenuation effects of
ductwork are largely dependent upon whether internal lining is
present or absent. Great care must be exercised when designing
unlined ductwork to prevent noise issues related to air velocity.
End reﬂection loss (ERL) occurs when air suddenly expands
as it exits an air outlet, causing an impedance mismatch. This
results in some amount of sound energy traveling back towards
the source rather than reaching the occupied space.
Air outlets are the ﬁnal elements in a discharge sound path.
These outlets could be grilles, registers or diffusers serving occupied spaces. Larger spaces may be served by multiple outlets,
but so long as the outlets are spaced far enough apart, there’s no
need to calculate an additive effect. As a general rule, if outlets
are selected for a sound level of NC30 or less and at least 10 ft
(3 m) apart, the sound levels should not be additive.
As stated previously, outlets produce their highest sound levels in the speech interference bands. Therefore, outlets should
be selected for sound levels low enough so as not to create a
poor speaking environment. For an outlet to be inaudible and
not contribute, it should be selected at least 10 dB (or roughly
10 NC points) lower than the desired room sound level.
All of these sound path attenuations and additional information regarding sound path estimation can be found in AHRI
Standard 885. Like all AHRI publications, it is available for free
download at their website www.ahrinet.org. AHRI also has a
ashrae.org

July 2013

ASHRAE Journal - July 2013

Table of Contents for the Digital Edition of ASHRAE Journal - July 2013