ASHRAE Journal - December 2020 - 53

COLUMN ENGINEER'S NOTEBOOK

Noise Criteria (NC)
It all starts with relative quiet, and noise criteria (NC)
is our ﬁrst important metric in the tool box (Figure 1). NC
is a target noise level for listening, learning and acoustic
comfort. NC is a curve that deﬁnes a collection of upper
limits by octave bands, such that if mechanical equipment has a tonal peak above the curve in any frequency,
it will be noticeable and thus irritating because it is not
" smoothed " into relative background noise levels consistent with the neighboring frequencies.
Improper NC selection may also impact the ability for
a youth to hear low-level vowels or speech consonants
that deﬁne intelligibility and allow focus. An appropriate NC level can improve learning and preserve the ability to understand a foreign dialect. NC is not dBA (the
decibel). dBA calculates noise levels in each octave and is

FIGURE 1 NC graph for a quiet church or performing arts hall defined by the highest octave band value above the NC line as noted.

Noise Criteria (NC) Rating: NC-27

90
80
70

Sound Pressure Level (dB)

misunderstood vocabulary of acoustics. This includes
the deﬁnition of noise itself, deﬁned as " sound that is
unwanted. "
Acoustics is, therefore, challenging to the A/E/C community because its terminology can be confusing. This
is exacerbated by the acoustical industry where a wealth
of terms seem to change regularly. Product marketing
further touts more limited prescriptive solutions that
can add confusion for product selection. Therefore, it is
becoming the norm for the A/E industry to incorporate
an acoustical engineer to sort out and apply acoustic
terms for noise abatement and acoustic comfort.
A basic acoustic vocabulary is the starting point. Acoustic
fundamentals can be grouped into a simple list and can
be illustrated by the ANSI/ASA S12.60 standard. That standard was an early effort to summarize acoustics for classrooms into a simple design exercise. That standard was
subsequently extended into LEED® acoustic fundamentals. This improved acoustical awareness in the A/E ﬁeld.
Now acoustic fundamentals are regularly addressed by
the A/E industry. This is particularly true for mechanical
designers where the 2017 ASHRAE Handbook-Fundamentals,
Chapter 8, and 2019 ASHRAE Handbook-HVAC Applications,
Chapter 49, review the fundamentals of noise and vibration control. These chapters are good guidelines.
The simplest list of acoustic design starts with four signiﬁcant fundamentals that are almost always considered
for any type of project. The acoustic metrics are noise
criteria (NC), room reverberation (RT60), sound transmission class (STC), and impact insulation class (IIC).

NC-70
NC-65

60
50

NC-60
NC-55

NC-50
NC-45

NC-40
30

NC-35

27
23

NC-30
NC-25
NC-20

125

NC-15
250
500 1000 2000 4000 8000
Octave Band Center Frequency (Hz)

a measure of perceived sound level weighted for human
hearing. NC predicts perceived noise annoyance.
The NC value of any space predicts how annoying
the space is perceived to be. It is best if measured by a
quality calibrated instrument. Phone apps might work
for very rough estimates, and not for code compliance.
Phone app platforms are not calibrated for low-frequency nor quiet noise readings. A quiet worship space
must provide the ability to hear low-level speech consonants or soft music to be considered contemplative even
under audience ambient noise. An open ofﬁce space can
have higher background noise, but must not have noise
so high that conversations can't be understood.
Recommended and typical NC values for design can be:
* NC-20: commercial broadcast studio/music performance space;
* NC-25: worship/music facility venues/collaborative
speech in large rooms;
* NC-30: quiet executive ofﬁces/lecture spaces/small
conference room;
* NC-35 to 37: highest-level noise for classrooms
(ANSI S12.60);
D ECEM BER 2020

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ASHRAE Journal - December 2020

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