Commercial Architecture May 2019 - 20

▲

FEATURE

| acoustics
workstation size, and furnishings. Some studies used mockups of actual cubicles in

FIG. 1: OPEN-OFFICE SPEECH PRIVACY

open spaces. Other studies used sophisticated acoustical-analysis software, based
on the image sources technique.

0.40

Of the office-design features studied, it was found that ceiling absorption,
screen-wall height, and workstation plan size have the largest effects on speech

0.35

privacy in open offices. The most significant noise paths are those that reflect

Speech privacy (SII)

sound from the ceiling and diffract sound over the separating screen-wall. However, only a very limited range of these parameters will lead to acceptable speech

0.30

privacy.
The ceiling is a critical element in any open office. There are no obstacles to

0.25

prevent sound from reaching the ceiling and being reflected down into adjacent
cubicles. The absorptive properties of the ceiling can have a large effect, but

0.20

speech-privacy values are only substantially reduced for quite highly absorbing

Range of
acceptable
speech privacy in
open offices

0.15
0.10

ceiling tiles. For a wide range of medium- and low-absorption ceiling tiles (NRC
0.50 to 0.80), acceptable speech privacy is not achievable and not much influenced
by the ceiling absorption because too much sound is still reflected off these
low-performing panels.

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

Ceiling absorption (NRC or SAA)
Figure 1: In this figure, speech privacy (vertical axis) improves with lower speech intelligibility index (SII)
values. Once ceiling SAA/NRC is 0.90 and higher, acceptable speech privacy can be achieved in open offices.
Each incremental increase in SAA/NRC above 0.90 results in an appreciable improvement in speech privacy.

The main conclusion about ceiling absorption after years of intense investigation is that a minimum ceiling absorption for acceptable speech privacy is NRC/
SAA 0.90. In practice, it would be better to have an even more absorptive ceiling
than this to compensate for any limitations or absence of the other important
design parameters.
SEEING NRC 0.90 DIFFERENCE

The foundational studies conducted by the National Research Council of Can-

FIG. 2: SOUND-INTENSITY PROBE SCANS

ada 18 years ago have been corroborated more recently by research presented at
InterNoise 2018 ("Look, Do You See the Noise Leaking Through that Ceiling?"
Madaras, G.S., InterNoise 2018, Chicago).
A sound-intensity probe was used to scan an acoustic-ceiling system with panels
of different absorption-performance levels of NRC 0.60 to 0.95 while loud, broadband noise was being played in the space under it. A high-definition camera and
analysis software tracked the probe location and the sound intensity levels it measured. These location-specific sound-intensity data were then processed into color
sound maps, which were overlaid onto the digital image of the ceiling.

Reference ceiling

NRC 0.60

Yellow and red colors in Fig. 2 (at left) indicate loud noise reflecting off the
acoustic ceiling. Blue indicates noise being absorbed by the acoustic ceiling. Red
areas are mostly caused by noise reflecting off the hard, painted-metal, plaque-style
air diffuser and from light fixtures. Note that the open return-air grille on the right
side of the images (blue) acts as an effective sound absorber because the noise
passes through the opening into the plenum and is not reflected back. The base
question is: At what NRC rating does an acoustic ceiling stop behaving like a
reflector (red and yellow) and behave more like an effective absorber (blue)? Based
on the series of images in Fig. 2, the answer is NRC 0.90.

NRC 0.75

NRC 0.85

DESIGNING SPACES

Implementing the required level of sound absorption overhead does not mean that
every open office must have a standard acoustic ceiling from wall to wall. There
are numerous materials, systems, and combinations thereof that can provide the
necessary amount of sound absorption.
While a standard acoustic wall-to-wall ceiling might not be desired as the aesthetic for all open-office spaces, when appropriate, the solution should be obvious
at this point. Select a ceiling panel with an NRC of 0.90 or higher. Stone wool and
NRC 0.90

NRC 0.95

Figure 2: Ceiling panels with NRC ratings ranging from 0.60 to 0.95 reflect varying amounts of noise (red
and yellow) when they are below NRC 0.90, but absorb most noise (blue) at NRC 0.90 and higher.

MAY 2019

COMMERCI A L A RCHI T EC T UREM AG A ZINE.COM

glass-fiber ceiling panels can easily achieve this level of performance, and higher,
without a large incremental cost increase, compared with lower-performing panels.
Wet-felted mineral-fiber panels are not able to achieve NRC 0.90.
If a contiguous ceiling is acceptable, but a different or higher-grade finish is

Commercial Architecture May 2019

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