Commercial Architecture May 2019 - 19

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FEATURE

| acoustics

absorption and sound masking. People who fail to imple-

sound-absorbing ceilings and other overhead systems.

careful with the units of area. There are Sabins and met-

ment these two components in their designs typically

* Noise-reduction coefficient: NRC is the most fre-

ric Sabins, depending on how area was calculated. Note

find themselves scrambling soon after occupancy to add

quently used and specified metric to quantify the

that ASTM has phased out the use of the word Sabin, but

them in some manner or another.

sound-absorption capabilities of a material or surface. It

product manufacturers still rely on and report it.

generally ranges from 0.0 to 1.0 in 0.05 increments. High-

* Ceiling attenuation class: CAC is a metric that applies

OVERHEAD SOUND ABSORPTION

er values indicate more sound absorption. It can be cate-

to acoustic ceiling panels, but is a measure of panel capac-

Here is a short review of the fundamental terminolo-

gorized as good at 0.70+, better at 0.80+, and best at

ity to prevent sound traveling from one enclosed room

gy used to describe/specify acoustic performance of

0.90+. NRC is the average of the one-third-octave band

over an interior partition and through an open ceiling

absorption coefficients at 250, 500, 1,000, and 2,000 Hz.

plenum into another room. CAC does not apply to open

NRC is used when the acoustic concern is multiple sound

offices. It should not be included in specifications for ceil-

waves reflecting around inside a room at random angles

ing panels in large open spaces without walls.

and potentially interfering with speech intelligibility
and/or overall loudness and comfort. NRC does not

STANDARDS, GUIDELINES,

apply to three-dimensional absorbers hung free in space,

RATING SYSTEMS

such as baffles, clouds, and islands.

The General Services Administration (GSA), Washing-

* Sound-absorption average: SAA is very similar to

ton, Facilities Standards for The Public Buildings Service

NRC and is in a rather long process of replacing NRC.

(P-100) requires ceilings over open offices areas to be

Whether or not SAA will eventually replace NRC com-

NRC 0.90 or higher for 100% of the space. This infor-

pletely is yet to be determined. The main differences

mation can be found in Table 3.1 on p. 101 of the 2018

include the use of all one-third-octave band absorption

version.

coefficients in the average as opposed to just the four

According to "Sound Matters: How to achieve acous-

included in NRC. This accounts for potential perfor-

tic comfort in the contemporary office," a related GSA

mance anomalies at specific frequencies that NRC does

document, published in December 2011: "Open work-

not include. SAA is also rounded to the nearest 0.01 as

spaces require acoustical treatment on a significant por-

opposed to NRC's 0.05 rounding. Performance differenc-

tion of the surfaces in the space to absorb noise from

es near 10% can round to the same NRC rating, not so

people and equipment. The more absorptive the material

with SAA. Manufacturers do not yet typically report

added to the open space and the higher the acoustical

SAA, nor do architects specify it.

performance rating of the material, the more acoustically

* Articulation class (AC), NRC are highly correlated:

comfortable the environment will be. Two surfaces are

The values generally range between 150 for hard, sound-re-

key contributors to absorption. High-quality acoustic

flecting

sound-

ceiling material is typically the most significant contribu-

absorptive surfaces. A goal value for open offices is 180 or

surfaces

and

210

for

highly

tor to sound absorption. Similarly, walls may be treated

higher, which correlates with an NRC of 0.90. The values

with acoustic material, either applied to a surface or inte-

do not carry any units and, therefore, are hard for many

gral with the wall finish."

people to understand. AC is used when the main acoustic

The WELL Building Standard (Comfort section 80,

concern is sound waves from a single, fixed source reflect-

p. 130, v. 1) requires that the ceiling over open office spac-

ing at a specific angle off the ceiling over a cubicle wall

es be NRC 0.90 or higher for the entire surface area,

from one workstation to another and preventing speech

exclusive of light fixtures and air devices. Complying

privacy. Because AC is so correlated with NRC and the

with this criterion improves the functioning of the car-

values are hard to relate to, architects generally prefer

diovascular, endocrine, and nervous systems of building

NRC in their specifications even though AC would tech-

occupants.

nically be the better metric to specify. Some manufacturers report AC as well as NRC.

WHY NRC 0.90?

* Sabins: Sabins are the unit of sound absorption. When

While it is known that good acoustic design of open

the NRC of a material is multiplied by the area of the

offices requires high-performing sound absorption of

material, the result is Sabins of absorption. For example,

NRC 0.90 or higher, the question of "why" might still

10 sq. ft. of a material with an NRC rating of 0.90 provides

linger in the minds of some architects and specifiers. An

9 Sabins of absorption. NRC cannot be used for three-

abbreviated answer to this question is because exhaustive

dimensional absorbers hung free in space, such as islands,

and conclusive research has shown it to be necessary.

clouds, and baffles. Sabins of absorption are used instead.

In the early 2000s, the National Research Council

Some manufacturers incorrectly report NRC values

Canada (see references at the end of this article) method-

for these types of absorbers. As a result, some architects

ically isolated and tested a dozen physical features of

specify them incorrectly as well. More technically savvy

open offices relative to speech privacy between worksta-

manufacturers will report Sabins of absorption by fre-

tions. Those features included ceiling absorption and

quency for their products. When dealing in Sabins, be

height, screen-wall absorption and height, light fixtures,
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Commercial Architecture May 2019

Table of Contents for the Digital Edition of Commercial Architecture May 2019
