Atlas AA-SMG Quick start guide

Sound Masking Systems
by Ashton Taylor, Hoover & Keith Inc. for Atlas Sound
A technical guide to achieving effective
speech privacy in open-plan offices
and other environments

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . 4
WHAT IS SOUND MASKING?
THE ECONOMIC BENEFITS OF SOUND MASKING 4
DEFINITION OF TERMS
(ALSO SEE APPENDIX A) . . . . . . . . . . . . . . . . . . 4
PURPOSE OF THIS PAPER . . . . . . . . . . . . . . . . . . 4
PART 1 - A DISCUSSION OF
SOUND MASKING
APPLICATIONS FOR SOUND MASKING SYSTEMS . 5
Open-Plan Offices . . . . . . . . . . . . . . . . . . . . . . 5
Medical Examination Rooms . . . . . . . . . . . . . 5
Confidential Offices . . . . . . . . . . . . . . . . . . . . . 5
Court Rooms . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Buildings near Major Roads, Railroads,
& Airports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Personal Masking Units . . . . . . . . . . . . . . . . . 6
Security Systems . . . . . . . . . . . . . . . . . . . . . . . 6
WHEN SOUND MASKING SHOULD NOT BE USED . . 6
Unrealistic Client Expectations . . . . . . . . . . . 6
Rooms Requiring Very Low Ambient Noise . . 6
Space Used by Sight-Impaired People . . . . . . 6
Space Used by Hearing-Impaired People . . . . 6
BENEFITS OF MASKING TO THE END USER . . . 7
Cost-Effective Speech Privacy . . . . . . . . . . . . 7
Increased Productivity . . . . . . . . . . . . . . . . . . 7
Flexibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
PART 2 - THE SOUND MASKING
ACOUSTICAL ENVIRONMENT
THREE STEPS TO SUCCESSFUL SOUND MASKING
1 - Attenuate the Direct Sound . . . . . . . . . . . . 8
2 - Reduce Sound Reflections . . . . . . . . . . . . . 8
3 - Raise the Ambient Sound Level
Using Sound Masking . . . . . . . . . . . . . . . 8
Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
A BASIC SOUND MASKING EXAMPLE . . . . . . . . 8
EVALUATING THE ACOUSTICAL ENVIRONMENT . . . 9
ATTENUATION OF DIRECT SOUND . . . . . . . . . . 9
Orientation of Talker . . . . . . . . . . . . . . . . . . . .10
Screens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Sound Transmission Class . . . . . . . . . . . . . . .10
Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
REDUCTION OF REFLECTED SOUND ENERGY 13
Ceiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Absorption Ratings . . . . . . . . . . . . . . . . . . . . .13
Noise Reduction Coeffcient . . . . . . . . . . . . . . .13
Articulation Class . . . . . . . . . . . . . . . . . . . . . .13
Lighting Fixtures . . . . . . . . . . . . . . . . . . . . . . .14
MASKING LOUDSPEAKERS AND THE CEILING .14
Special ceiling tiles . . . . . . . . . . . . . . . . . . . . .14
Sound leaks . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Boots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
OTHER CAUSES OF UNWANTED
REFLECTIONS . . . . . . . . . . . . . . . . . . . . . . . .15
AMBIENT NOISE . . . . . . . . . . . . . . . . . . . . . . . . . .17
PART 3 - THE BASIC ELECTRONIC
SOUND MASKING SYSTEM
CONCEPT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
Don’t Tell the Employees? . . . . . . . . . . . . . . .18
Self-Contained Masking Units . . . . . . . . . . . . . . .18
Single-Channel vs Multi-Channel Masking . .18
Basic Electronics . . . . . . . . . . . . . . . . . . . . . . .18
SOUND MASKING AND BACKGROUND MUSIC
OR PAGING . . . . . . . . . . . . . . . . . . . . . . . . . . .19
BASIC SYSTEM ELECTRONICS . . . . . . . . . . . . . .19
Masking Sound Generator . . . . . . . . . . . . . . .19
Equalizer . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
PART 4 - MULTI-CHANNEL MASKING,
BACKGROUND MUSIC AND PAGING
TWO (AND MORE) CHANNEL MASKING . . . . . .21
Zone Level Controls . . . . . . . . . . . . . . . . . . . .22
Amplified Monitor Panel . . . . . . . . . . . . . . . . .22
BACKGROUND MUSIC . . . . . . . . . . . . . . . . . . . . .22
PAGING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
Paging Sound Level . . . . . . . . . . . . . . . . . . . . .23
Paging Equalizers . . . . . . . . . . . . . . . . . . . . . .23
Part 1
Index

PART 5 - MASKING LOUDSPEAKERS
AND SELF-CONTAINED MASKING UNITS
MASKING LOUDSPEAKERS . . . . . . . . . . . . . . . . .24
Upwards Loudspeaker Orientation . . . . . . . .24
Downwards Loudspeaker Orientation . . . . . .25
Horizontal (Sideways)
Loudspeaker Orientation . . . . . . . . . . . . . . .25
In-Ceiling Placement . . . . . . . . . . . . . . . . . . .25
Valuable Masking Loudspeaker Features . . .25
SELF-CONTAINED MASKING UNITS . . . . . . . . . .26
PART 6 - COMMISSIONING THE
MASKING SYSTEM
LEVEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27
Connecting Spaces . . . . . . . . . . . . . . . . . . . . .27
Setting the Level During System Adjustment 27
Gradually Adjust to Final Level . . . . . . . . . . .27
MASKING SPECTRUM . . . . . . . . . . . . . . . . . . . . .28
Ideal Masking Sound Spectrum . . . . . . . . . . .28
Masking Spectrum 1 . . . . . . . . . . . . . . . . . . . .28
Masking Spectrum 2 . . . . . . . . . . . . . . . . . . . .29
Masking Spectrum 3 . . . . . . . . . . . . . . . . . . . .29
A Comparison of All Three Masking Spectra .30
EQUALIZING THE SYSTEM . . . . . . . . . . . . . . . . .30
The Equalization Process . . . . . . . . . . . . . . . .30
Using an Octave-Band Equalizer for . . . . . . .
Troubleshooting . . . . . . . . . . . . . . . . . . . . . . .31
COVERAGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
TEST EQUIPMENT . . . . . . . . . . . . . . . . . . . . . . . .31
PART 7 - PREDICTING PRIVACY IN THE
MASKING ENVIRONMENT
. . . . . . . . . . . .32
ARTICULATION INDEX AND PRIVACY
CATEGORY DEFINITIONS . . . . . . . . . . . . . . . . .32
Marginal Privacy . . . . . . . . . . . . . . . . . . . . . . .32
Normal Privacy . . . . . . . . . . . . . . . . . . . . . . . .33
Confidential Privacy . . . . . . . . . . . . . . . . . . . .33
Total Privacy . . . . . . . . . . . . . . . . . . . . . . . . . .33
PREDICTING SPEECH PRIVACY . . . . . . . . . . . . . .33
PART 8 - CASE HISTORIES
MASKING IMPROVES SPEECH PRIVACY
IN A QUIET SPACE . . . . . . . . . . . . . . . . . . . . . . .34
BOOTS REDUCE HOT SPOTS PROBLEM . . . . . .34
PROBLEMS RESULTING FROM UNINSTALLED
BOOTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34
LEAKY LUMINAIRES CAUSE HOT SPOTS . . . . . .34
MASKING LOUDSPEAKERS TAPPED TOO LOW . .35
COMPLICATED SYSTEM . . . . . . . . . . . . . . . . . . .35
MEDICAL SUITE MASKING TEST . . . . . . . . . . . .35
MEDICAL PROFESSIONAL BUILDING MASKING .36
MASKING IMPROVES PRIVACY IN A
PASTOR’S OFFICE . . . . . . . . . . . . . . . . . . . . . . .36
MASKING AND UNWANTED REFLECTIONS
IN A PSYCHIATRIST’S OFFICE . . . . . . . . . . . . .36
CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . . . .37
APPENDIX A - DEFINITIONS . . . . . . . . . .38
APPENDIX B - WORKSHEET . . . . . . . . . .40
GENERAL INSTRUCTIONS . . . . . . . . . . . . . . . . . .40
Entering the Data . . . . . . . . . . . . . . . . . . . . . .40
Calculating the Speech Level at the Listener 40
Calculating the Articulation Index . . . . . . . . .40
DETAILED WORKSHEET INSTRUCTIONS
Section A Instructions . . . . . . . . . . . . . . . . . . .41
Section B Instructions . . . . . . . . . . . . . . . . . . .41
Section C Instructions . . . . . . . . . . . . . . . . . . .42
Section D Instructions . . . . . . . . . . . . . . . . . . .43
Section E Instructions . . . . . . . . . . . . . . . . . . .44
Section F Instructions . . . . . . . . . . . . . . . . . . .45
Section G Instructions . . . . . . . . . . . . . . . . . . .45
Section H Instructions . . . . . . . . . . . . . . . . . . .46
Section I Instructions . . . . . . . . . . . . . . . . . . .46
Section J and Section K Instructions . . . . . . .47
Section L Instructions . . . . . . . . . . . . . . . . . . .47
SOUND-MASKING, OCTAVE-BAND,
ARTICULATION- INDEX WORKSHEETS
WORKSHEET EXAMPLE 1 -
OPEN-PLAN ENVIRONMENT . . . . . . . . . . . . .48
Part 1 - No Speech Privacy . . . . . . . . . . . . . . .48
Part 2 - Add Masking Sound . . . . . . . . . . . . . .48
Part 3 - Substitute 6-Foot-High Partition
Screens . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49
Part 4 - Move Workstations Farther Apart . . .49
Part 5 - Install a High Articulation Class
(AC) Ceiling . . . . . . . . . . . . . . . . . . . . . . . . .50
Summary and Conclusions . . . . . . . . . . . . . . .50
WORKSHEET EXAMPLE 2 - A WALLED SPACE
Part 1 - No Masking Sound . . . . . . . . . . . . . . .51
Part 2 - Add Masking Sound . . . . . . . . . . . . . .51
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52
No part of this white paper may be copied or used without the written permission of Atlas Sound.
© 2000 Atlas Sound

A sound masking system emits low-level,
non-distracting masking noise designed to
reduce speech intelligibility and thereby
improve speech privacy. This improvement
in speech privacy can be of great value in
open-plan offices, doctors’ examination
rooms and other environments where confi-
dentiality is important.
Sound masking can also reduce the distrac-
tion caused by traffic, office machinery and
other unwanted sounds. Because this bene-
fit is limited to situations where the unwant-
ed sounds are of relatively low level, howev-
er, speech privacy is the focus of most
sound masking systems.
A typical sound masking system consists of
a masking noise generator, an equalizer,
one or more power amplifiers and a group
of special loudspeakers installed above a
dropped ceiling. Well-designed room
acoustics are an important component of a
successful masking system.
The Economic Benefits
of Sound Masking
The economic benefits of sound masking
vary from application to application but can
be significant. Consider a large insurance
company selling life insurance over the tele-
phone. Many times each day, an agent will
ask a prospective client for financial and
health information. The insurance compa-
ny must maintain a reasonable degree of
confidentiality for this kind of information.
Yet, if the agents work in a traditional open
office environment, the lack of speech pri-
vacy makes it nearly impossible to achieve
this goal.
One way to provide speech privacy would
be to construct a private office for each
agent. Yet, as anyone who has ever slept in
a cheap motel room knows, even doors and
walls do not guarantee privacy! A truly
“private” office must include sound insulat-
ing walls, sealed doors and baffles in the
air-handling ducts — not a low-cost
solution.
A lower cost solution is an open plan office
with well-designed acoustics and a sound
masking system. This kind of environment
can achieve normal speech privacy while
maintaining the flexibility of the open plan
office. As a side benefit, the sound masking
system will reduce the distraction of
unwanted sounds like office machinery and
traffic, enabling the insurance agents and
other office workers to maintain a higher
level of productivity.
Purpose of this Paper
This paper discusses the acoustics and elec-
tronics of a successful sound masking sys-
tem and provides case histories as illustra-
tions. Appendix A contains definitions of
sound masking and acoustical terms.
Appendix B is a useful sound masking work-
sheet that can help estimate the degree of
privacy achievable in a new or retrofitted
system.
Although it is detailed and accurate, this
paper cannot make the reader into a sound
masking expert. For this reason, Atlas
Sound recommends that architects, building
owners and systems contractors seek the
assistance of a qualified acoustical consult-
ant when contemplating the design and
installation of a sound masking system.
Introduction and Executive Summary
What is Sound Masking?
Page 4

Applications for Sound Masking
Systems Open-Plan Offices
Definition of Terms (also see Appendix A)
In this paper, the term “talker” refers to a
person. The term “speaker” refers to a loud-
speaker. The term “listener” refers to anyone
hearing sounds, whether or not they intend to
hear those sounds.
“Marginal”, “normal” and “confidential”
speech privacy are subjective terms that are
discussed more completely in the section
entitled “Predicting Privacy in the Masking
Environment”. In general, however, “mar-
ginal” refers to an unacceptable level of
speech privacy. “Normal” speech privacy is
acceptable for open-plan office environ-
ments. “Confidential” speech privacy is
desirable for confidential conference rooms,
psychiatrist’s and lawyer’s offices and other
highly confidential environments. Modern
open-plan office environments function as a
group of independent offices in a single
large open space. Movable screens between
offices act as both acoustical and visual bar-
riers. Sound masking completes the envi-
ronment by adding speech privacy.
Compared to the completely open “typing
pool” concept, each employee has a com-
fortable working zone with both visual and
speech privacy.
Medical Examination Rooms
Medical examination rooms are often small
(perhaps 100 square feet) and close togeth-
er. The low-cost construction used for these
rooms provides walls and doors for visual
privacy but offers very limited speech privacy.
In fact, it is not uncommon to hear and
understand every word of a conversation
between a doctor and patient in adjacent
examination rooms! This can be very
inhibiting for the patients. Sound masking
can create effective speech privacy in these
rooms at a lower cost than construction
improvements alone.
Confidential Offices
Psychiatrists, lawyers, law enforcement per-
sonnel and marriage or school counselors
all require confidential privacy in their
offices. This privacy can be achieved with
construction techniques alone. However,
the required sound isolating walls, doors,
and windows can be very expensive. The
alternative of sound masking, in conjunc-
tion with less costly construction tech-
niques, can achieve the required privacy at
a lower overall cost.
Some environments, such as psychiatrists’
offices, may require an extremely high
degree of privacy. Other situations, in exist-
ing structures, may involve significant
acoustical problems or building layout
issues. In these cases, Atlas Sound recom-
mends the services of a qualified acoustical
consultant.
Court Rooms
Sound masking can be useful in a court-
room when the judge needs to have a
private conference with lawyers and
prosecutors at the bench. Equip the judge’s
microphone with a mute switch that also
engages sound masking through loudspeak-
ers located over the audience and the jury.
Part 1
A Discussion of Sound Masking
Page 5

Buildings near Major Roads,
Railroads, and Airports
In most buildings, it is not feasible to com-
pletely mask higher-level noises like those
from heavy trucks, trains, or aircraft.
However, sound masking can soften the
impact of these noises. If a client wants
masking to cover up these sounds, make
sure their expectations are not too high. In
most cases, the intruding sounds will still be
audible after masking is installed. However,
masking will minimize the startle effect
because the sound level changes less.
Personal Masking Units
Personal masking units, which are com-
monly sold as sleep aids, offer a selection of
masking sounds and other pleasant sounds
like breaking surf, babbling brooks, train
clickity-clack, rain, waterfall, and church
bells. Do not confuse these units with the
self-contained masking units (described
later in this paper) which are designed for
professional use in offices. Other than this
brief discussion, personal masking units are
not covered in this paper.
Security Systems
Specialized masking systems emit high
intensity masking sound outside the win-
dows and doors of top-secret conference
rooms in buildings that require extremely
high levels of security. These systems are
not covered in this paper.
When Sound Masking
Should Not Be Used
Unrealistic Client Expectations
A successful masking system requires careful
coordination of an acoustical ceiling, office par-
tition screens, absorptive furniture, overall
building acoustics and the electronic sound
masking system. Yet, some clients, having
heard about a “miracle” at another facility, may
expect electronic sound masking alone to solve
their problems.
Educate these clients about the limits of sound
masking and about the acoustical and construc-
tion requirements. If the client is unwilling to
make necessary acoustical or construction
improvements, tell them clearly that only the
electronic functionality of the system is guaran-
teed, not the acoustical results.
Rooms Requiring
Very Low Ambient Noise
The acoustic echo cancellers, used in audio
and video teleconferencing systems, work
best in rooms with very low ambient noise.
Thus, masking sound is not a good way to
maintain voice privacy or to mask unwanted
noises in teleconferencing rooms or in other
environments which require very low ambi-
ent noise. Instead, retain a qualified
acoustical consultant to help with acoustical
solutions.
Space Used by Sight-Impaired People
Masking sound and an absorbent environ-
ment can hide the aural clues used by the
visually impaired to sense their immediate
surroundings.
Space Used by Hearing-Impaired People
Masking sound can impair the ability of
people with acute hearing loss to under-
stand speech, especially in situations where
face-to-face communication is not possible.
Page 6

Benefits of Masking
to the End User
Cost-Effective Speech Privacy
Normal (not confidential) privacy can usual-
ly be achieved with floor-to-ceiling walls
between workspaces. However, sound
masking allows normal privacy to be
achieved in an open-plan office with simple
partitions between cubicles. This is a cost-
effective solution that allows a building
owner or leasee to retain the flexibility of an
open-plan office.
Confidential privacy, without sound mask-
ing, requires multiple-layer walls, from the
floor to the deck above the ceiling, com-
bined with special sound-isolation doors,
door seals and careful caulking of all pene-
trations of the wall to stop sound leaks.
This kind of construction can be very costly.
In contrast, masking sound allows confiden-
tial privacy to be achieved with normal
building partitions that extend from floor to
ceiling.
Increased Productivity
Without sound masking, employees in an
open-plan office must deal with constant
audible distractions, including office
machinery noises, traffic noises and clearly
heard conversations from adjacent work-
spaces. Even when working in a private
office, employees may hear noises and con-
versations coming from adjoining offices or
hallways.
With sound masking, these noises will be
less irritating and the conversations, while
still audible, will be unintelligible and
therefore much less distracting.
Flexibility
Without sound masking, the open-plan
office is little more than an old-fashioned
typing pool with partitions. Noises and
clearly audible conversations from nearby
cubicles distract workers and limit their
productivity. Lack of speech privacy may
even inhibit some employees from perform-
ing necessary job functions.
With sound masking, the open office gains
the speech privacy of individual private
offices yet retains the flexibility of the open-
plan concept. Just move partitions to add or
delete offices, combine offices into a confer-
ence area or to create an open space for use
as a break-room or file-room area. In most
cases, lighting and air ducts, which are
located in the ceiling, need not be moved.
Also, in a well-planned open-office space,
it’s easy to reconfigure electrical, telephone,
fax and computer connections.
Page 7

Three Steps to Successful
Sound Masking
Carefully planned acoustics, combined with
masking sound, make it possible to achieve
the goal of increased speech privacy
between workstations.
There are three steps to successful
sound masking:
1.
Attenuate the Direct Sound
“Direct sound” from a talker reaches a
listener by the shortest path without
being reflected by any object.
2.
Reduce Sound Reflections
Reflected sound from a talker reaches a
listener after being reflected from one or
more hard objects.
3.
Raise the Ambient Sound Level Using
Sound Masking
Sound masking adds low-level background
noise to reduce the speech-to-noise ratio
and reduce intelligibility.
Discussion
It’s not always necessary to take all three
steps to achieve a desired level of speech
privacy. In private offices, for example,
floor-to-ceiling walls may attenuate the
direct sound enough to achieve normal
speech privacy.
In open-plan offices, however, even normal
speech privacy requires all three steps. Use
absorptive furniture and screens (partitions)
to attenuate the direct sound and reduce
unwanted reflections. Use acoustical ceil-
ings to further reduce reflections between
adjacent office spaces. Sound masking
completes the job by adding a low level of
random electronic noise to mask the
remaining unwanted sounds.
In effect, the first two steps, which involve
acoustics alone, reduce the level of unwant-
ed sound. The last step, adding masking
noise, masks the remaining unwanted
sound in such a way as to create speech pri-
vacy and reduce distractions.
A Basic Sound Masking Example
Figure 1 illustrates these concepts. Part A
shows a poorly-designed open-plan office
environment. There is no barrier to reduce
the direct sound level between the talkers
and the listener, the hard ceiling reinforces
the direct sound with reflections, and the
low level of background sound does not
mask the speech. The dashed line repre-
sents the level (as a graph) of speech and
the dotted line represents the room or back-
ground sound level. Notice that the room
level is much lower than the speech level.
In Part B, the screen attenuates direct
sound, an absorptive ceiling reduces reflect-
ed sound energy, and the masking loud-
speakers in the ceiling plenum add masking
sound. The result is effective (normal)
speech privacy.
Figure 2 introduces the concept of sound
masking in octave bands. The solid line in
Part A shows the octave-band sound levels
of a talker as heard at a nearby workstation.
The dotted line in Part A shows quiet back
Part 2
A Discussion of Sound Masking
Page 8

ground sound levels typical in an open-plan
office. Thus, Part A shows a high speech-to-
noise ratio in every octave band resulting in
high articulation and no speech privacy.
Part B shows a lower speech-to-noise ratio
and a more desirable level of speech privacy
achieved with partitions, absorptive surfaces
and masking sound.
Evaluating the Acoustical Environment
In existing spaces, it may not be possible to
improve the acoustics by installing absorptive
partitions and furnishings, improving the ceil-
ing or applying new interior finishes. In new
spaces, the building owner or lessee may have
very specific ideas about building decor which
limit the ability to optimize the
acoustics.
It is always important, however, to be able
to evaluate the acoustical environment and
provide advice to a prospective client. The
acoustical information in this section and
the worksheet in Appendix B are designed
to aid that process and help avoid some
common pitfalls. Again, a qualified acousti-
cal consultant can help when an evaluation
suggests that problems are inevitable.
Direct
Sound
Reflected
Sound
Speech-
to-Noise
Ratio
Speech Sound
Level
Room
Sound
Level
Masking Loudspeakers
Room Sound
Level
Speech
Sound
Level
A
B
60
0
40
30
20
10
60
0
40
30
20
10
SOUND PRESSURE LEVEL re 20 µPa,
dB
1000 2000 4000 80002 0 0063 12
1000 2000 4000 80002 0 0063 12
OCTAVE-BAND CENTER FREQUENCY,
Hz
A
B
Talker
Background
Talker sound:
Reduced level
Background sound:
Raised level
FIG. 2 - This two-part graph illustrates the con-
cept of sound masking by showing octave-band
sound levels of a talker and background sound
before (Part A) and after (Part B) acoustical
improvements and sound masking are installed.
Page 9
FIG. 1 - In Part A, direct sound from the talker
and reflected sound off a hard ceiling con-
tribute to poor speech privacy. In Part B, an
absorptive ceiling and screen reduce the direct
and reflected sound level, and masking sound
provides effective (normal) speech privacy.

Attenuation of Direct Sound
The direct sound is speech from a talker
that arrives directly at the ear of a listener
without being reflected. Figure 3 shows the
direct peak sound levels for male and
female talkers at a distance of one meter.
FIG. 3 - Octave-band speech peak sound levels
for male and female talkers at a distance of 1
meter. The solid curves are for male talkers
with normal (lower curve) and raised voices
(upper curve). The dashed lines are for female
talkers with normal and raised voices. The
heavier solid curve is the ANSI S3.5 standard
voice level.
Orientation of Talker
Speech sound level varies as a talker turns
away from a listener. Speech levels are
highest during face-to-face conversation
where the talker is “on axis” (0∞) with the
listener. As the talker turns away, the A-
weighted sound level at the listener is
reduced by approximately 1.5 dB for each
30
º ( the talker is off axis from the listener
(see Figure 4).
The head orientation of the listener with
respect to the talker makes little difference
in terms of received level, and is therefore
unimportant in sound masking calculations.
For speech privacy calculations, assume
that the talker is on-axis with the listener
(worst case) unless the talker/listener ori-
entation is fixed.
FIG. 4 - This polar plot shows the relative level
from a talker versus angle. The speech level at a
listener’s position decreases by approximately
1.5dB for every 30º the talker is off-axis from
the listener. The orientation of the listener’s
head is unimportant in speech level calculations.
Screens
The partitions between work areas in an
open-plan office are called screens.
Because these screens function as sound
barriers, they must be designed to attenuate
the sound passing through them and they
must be tall enough to provide a barrier to
sound passing over them. Finally, screens
must be absorptive enough to prevent sound
build-up within each workstation. Figure 5
illustrates these concepts.
80
70
60
0
1000 2000 4000 800012 2 0 00
OCTAVE-BAND CENTER FREQUENCIES,
Hz
SOUND PRESSURE LEVEL re 20 µPa,
dB
90°
0°
180°
90°
Page 10

Sound Transmission Class
Sound transmission class (STC) is a standard
way to specify the attenuation of sound through
a wall, an open-plan office screen or other bar-
rier. A higher STC is better. A screen with a
high STC rating will attenuate the sound more
than a screen with a low
STC rating.
STC values for typical gypsum board office
walls are 30 - 35. Very thick and massive
wall constructions may have STC values of
60 or more. Open-plan office screens
should have an STC value of at least 20.
However, once the STC exceeds 25, the
sound passing over the screen becomes the
limiting factor. Thus, most commercially
available screens have STC ratings between
20 and 30.
Diffraction
Even if the ceiling is non-reflective, sound
can pass above a screen by a process known
as “diffraction”. Lower-frequency sounds
will diffract over a screen of a given height
more easily than higher-frequency sounds.
Fortunately, the higher-frequency sounds
are the most important for speech privacy
and this suggests that a screen higher than
a tall person’s mouth level should be high
enough to block diffraction of the most
important speech frequencies.
Following this line of thinking, a 4-foot high
barrier, which is barely above the level of a
seated person’s mouth, provides only mar-
ginal attenuation between workstations, a
5-foot high barrier provides adequate
attenuation if the ceiling and walls are very
absorptive, and a 6-foot high barrier usually
provides good attenuation.
For best results, the screen should be at
least 3 times as wide as it is high although
that implies 15-foot to 18-foot cubical widths
which is often impossible. Ideally, the
bottom of the screen should make direct
contact with the floor. The maximum
acceptable gap along the bottom of a screen
is 1 inch.
Screens must be absorptive to prevent
sound build-up in an individual workspace.
A workspace surrounded by absorptive
screens can be 5 to 6dB quieter than a hard-
surfaced work area. However, screens can
have their upper surface (no more than the top
1-foot) made of glass for visual openness.
Screen
6′ high
(a)
(b)(c)
Page 11
FIG. 5 - Screens should (a) be high enough to
reduce sound passing over them, (b) provide a
good barrier to sounds passing through them,
and (c) absorb incident sound.

Layout
Simple layout changes can often improve
speech privacy in an open-plan office. And,
even though these changes will disrupt
daily routine in an existing space, clients
with severe privacy problems are usually
willing to comply. In general, an effective
layout means avoiding these problems:
* Adjacent workstations closer than 10 feet
(16 feet preferred)
* Workstation openings directly across from
each other (line of sight)
* Side-by-side openings of two adjacent
workstations
* Desks facing each other on each side of a
screen (see page 12).
* Openings near windows or building
curtain wall (external perimeter)
* Openings to a common corridor or other
area with an opposite hard wall
Figure 6 shows poor and improved layouts
for open-plan workstations.
POOR LAYOUTS IMPROVED LAYOUTS
PLAN VIEW
Direct, uninterrupted
path (talkers face each
other)
Longer
uninterrupted
path
Only
uninterrupted
path
Screens (to interrupt
path to opposite
workstation)
Separation distance,
6 ft.
FIG. 6 - Examples of good and bad layouts for workstations in open-plan spaces.
Reduction of Reflected Sound Energy
Page 12

Ceiling
The ceiling in an open-plan office affects
speech privacy more than any other acousti-
cal element. A hard ceiling reflects sound
from one workstation to another, bypassing
the sound barrier provided by the worksta-
tion screens. This problem is worse when
the angle of reflection is between 40º and
60º. For this reason, open-plan offices
should always have absorptive ceilings.
Absorption Ratings
The unit of absorption is the sabin. One
“sabin” (in the US customary measurement
system) is equal to one square foot of per-
fect (total) absorption. We often think of
this as one square foot of an open window.
“Absorption coefficients” rate the absorptivi-
ty of a surface between 0.00 (perfect reflec-
tor) and 1.00 (perfect absorber) and are
written as two-decimal numbers.
Specifications for typical interior finish
materials provide absorption coefficients in
octave bands. Absorption coefficients high-
er than 1.00 are sometimes given for very
highly absorptive materials. This is an arti-
fact of the testing procedure since it is
impossible to absorb more than 100% of the
incident sound.
Noise Reduction Coeffcient
Ceiling tile absorption is rated with an
acoustical descriptor called the “noise
reduction coefficient” (NRC) which is an
average of the absorption coefficients of the
250-Hz, 500-Hz, 1000-Hz, and 2000-Hz
octave bands, rounded to the nearest 0.05.
Typical 3/4-inch thick mineral fiber ceiling
tile has an NRC value between 0.50 and 0.70
but normal speech privacy in open-plan
office environments commonly requires
1-inch thick compressed fiberglass ceiling
tiles with an NRC value of 0.90 or more.
Articulation Class
“Articulation Class” (AC) is a new rating for
acoustical performance. A material’s articu-
lation class rating is the sum of the attenua-
tions (in dB) of the 15 third-octave bands
from 200 Hz to 5000 Hz.
Articulation class is measured between a
source (talker) workstation and a receiver
(listener) workstation in an actual open-
plan office space. Because it measures
effectiveness in real-world conditions, artic-
ulation class is the preferred rating method
for ceiling tile. Select ceiling tile products
with AC ratings of 200 or more for open-
plan offices. If a ceiling tile product does
not have an AC rating, use the NRC rating.
Page 13

Lighting Fixtures
Typical ceiling-mounted fluorescent lighting
fixtures have flat plastic lenses flush with the
ceiling. These fixtures reflect speech frequen-
cies between workstations, “short-circuiting”
the acoustic privacy provided by the worksta-
tion partition screens. To avoid this problem,
do the following:
* Best — use indirect lighting in the work
station and eliminate fluorescent ceiling
fixtures.
* Good — use parabolic lens or open grid
lighting fixtures and avoid placement over
workstation partition screens.
* Marginal — use flat lens fluorescent fix-
tures but avoid placement over screens.
When a client is unwilling to spend the
money to replace flat lens lighting fixtures
with parabolic lens types, ensure that the
flat lens fixtures are not located over
workstation partition screens. Often, fluo-
rescent fixtures utilize flexible electrical
conduit and can be moved to a new posi-
tion without re-wiring. Figure 7 shows
good and bad placement of fluorescent fix-
tures.
Masking Loudspeakers
and the Ceiling
Sound masking loudspeakers are usually
installed above the ceiling. Thus, the
ceiling in an open-plan office must be
capable of passing masking sound with-
out excessive attenuation.
Special ceiling tiles
Foil-backed ceiling tile may be specified to
diffuse the masking sound above the ceiling.
High transmission loss tile may also be
specified for sound masking. However,
these special tile types are not really neces-
sary in a correctly-designed masking sys-
tem. In fact, they can cause problems.
There are always small sound leaks in the
ceiling. With normal ceiling tile the mask-
ing sound coming through these leaks is
low in level and generally not a problem.
If, however, the masking sound level is
increased to force sufficient masking sound
through high transmission loss ceiling tiles,
then the masking sound eminating from the
leaks may increase to the point that it
becomes audible and distracting.
High transmission loss ceiling tiles can
increase speech privacy between standard
walled offices when masking is not provided.
Sound leaks
Although small ceiling leaks may not be a
problem, it’s best to avoid all leaks to the
extent possible. The first place to look for
sound leaks is the return air system.
In a typical open-plan space, room air
returns to the building mechanical system
through a ceiling plenum (the space
between the ceiling and the deck). The air
gets into the plenum through air return
grilles installed directly in the ceiling.
These grilles provide an open door for
masking sound to leak into the office space
below. Beneath these grilles, the masking
sound will be louder and more high-pitched
Light Fixture
Light Fixture Light Fixture
FIG. 7 - Speech frequencies reflect off the flat
lenses of ceiling fluorescent fixtures. If the fluo-
rescent fixtures are mounted over workstation
partition screens, this reflected sound can
reduce speech privacy.
Page 14
Bad
Good

and the masking sound coverage will be
uneven. These are very undesirable results.
Lighting fixtures with open grid diffusers
can cause similar problems.
Other Causes of
Unwanted Reflections
Ceilings aren’t the only source of reflected
sound problems in an open-plan office. As
illustrated in Figure 9, hard floors and walls
and even office furniture can contribute to
unwanted reflections.
Boots
To prevent leaks in the ceilings of new
buildings, install a length of fiberglass duct
(called a boot) at each return air register.
Figure 8 shows a return air register before
and after the installation of a boot. In exist-
ing spaces, the sound masking contractor
can fabricate boots. Use four 2’ x 4’ ceiling
tiles (matching the tiles in the ceiling) set
on end to form a 4’ high vertical boot that is
2’ x 2’ in section. Attach the tiles together
with duct tape. Maintain the full opening
area (typically four square feet), especially if
the ceiling to deck distance is short (do not
“pinch” air between the boot and the deck).
Open-plan offices must be carpeted. Thick
padded carpets provide more voice frequen-
cy absorption than thin, direct glue-down
carpets. Carpeting also reduces the irrita-
tion of footfall noises.
Choose absorptive office furniture including
cloth-covered and thickly padded chairs
(avoid leather chairs). If possible, select
office furniture with absorption on its sur-
Plenum Air
Return
Air Return
Grille
Air Return
Boot
Plenum Air
Return
Air Return
Grille
Air Return
Boot
Hard Space
Plan View
Wood / gyp.
Walls or Hard
Screens
Wood Shelves
Wood / Metal
Chairs
Tile Floor
Sof Space
Plan View
Carpet Floor
(padded)
Absorptive
Wall Panels
or Screens
Upholstered
Chairs
Wood Shelves with Absorptive Panels
FIG. 8 - Install boots above open return air ducts
in ceiling plenums.
FIG. 9 - Use absorptive office furnishings and
thick, padded carpet to reduce unwanted
reflected sound.
Page 15

faces such as shelf covers and drawer faces.
Of course, workstation partition screens
must be highly absorptive.
Hard walls, doors and windows can serious-
ly degrade speech privacy in both open-plan
spaces and in standard offices. Any hard,
flat vertical surface such as a fixed wall,
movable wall (curtain wall), window, or
door can bypass the workstation screen bar-
rier and reflect speech sound into an adja-
cent workstation (see the previous discus-
sion of ceiling lighting fixtures). Figure 10
shows wall reflections and some possible
solutions.
Sometimes, the best way to solve reflection
problems is to change the room layout so
that sound (speech) coming from one work-
station can’t reflect into openings in another
workstation. When room layout changes
aren’t possible, add absorption to reduce the
level of the reflected sound.
For walls, add the kind of acoustical wall
panels that have an absorptive core material
(usually rigid fiberglass board), a cloth cov-
ering (special fabrics for interior finish use),
and a mounting system. Standard acousti-
cal wall panels come in 2´x 2´, 2´x 4´, and
4´x 4´ sizes and in 1 inch and 2 inch thick-
nesses. Options include custom artwork or
logo design, impact-resistant core material,
and alternate mounting methods.
The outside wall in a glass building (the
“curtain wall”), reflects sound between
nearby open-plan workstations, reducing
speech privacy. Acoustical wall panels
could attenuate this reflected sound but
would also block incoming light. One way
to solve this problem is to install acoustic
wall panels at 90oto the curtain wall as
shown in Figure 10.
Hard Wall
Glass Window
Sound
Absorbing
Baffles
PLAN VIEW
Extend
Screen to
wall
Acoustical
Wall Panel
Glass Window Hard Wall
FIG. 10 - Walls, doors, windows and curtain walls can reflect sound into adjacent workstations.
Page 16

Ambient Noise
To the extent possible, keep building and
office equipment noises below the level of
the masking system. The heating, ventilat-
ing, and air conditioning (HVAC) system
makes a sound similar to an electronic
masking sound. However, the level and
spectrum will be different from workstation
to workstation and, in many buildings, the
system cycles on and off.
Acousticians use one of two descriptors to
rate HVAC system noise: Noise Criteria (NC)
or Room Criteria (RC). Since the masking
sound will be approximately RC 40, the
HVAC sound should be no higher than RC
35 or NC 35. Evaluate the office equipment
and building noise in an existing space by
measuring the octave band sound levels
with the HVAC system operating and office
equipment being used. Ensure that each
octave band sound level is 5 dB lower than
the corresponding masking sound octave
band level (See “Masking Spectrum” in Part
6) for the 250-Hz through the 4000-Hz
octave bands. Then add electronic masking
sound to raise background sound levels
high enough to mask voices, but not so
high that people subconsciously raise their
voices.
It’s okay to put a general-purpose confer-
ence area in an open-plan office environ-
ment. Highly private conference rooms,
however, must be traditional separate
spaces with high STC wall partitions that
extend from the floor to the deck above the
ceiling, sealed heavy doors and no sound
leaks. These conference rooms may still
benefit from reduced levels of masking
sound. For teleconferencing, use very
absorptive interior finishes, very high STC
walls, and no sound masking.
Page 17

The electronic sound masking system creates
a “blanket” of background noise carefully
controlled in level, spectrum, and coverage.
Masking sound should not call attention to
itself in any way. It should merely seem to be
part of the general building noise. In fact, if
people are unaware that a masking system is
in operation, they usually believe they are
hearing the ventilation system.
Concept - Don’t Tell the Employees?
One of the early rules of sound masking
installations was “Don’t tell tell the employ-
ees that we just installed sound masking”!
Many believed the employees would com-
plain about headaches or other maladies, or
that the masking system was some type of
corporate manipulation. Of course, these
concerns were unfounded. Masking simply
reduces the speech-to-noise ratio and mask-
ing sound is no more harmful than any
other low-level mid-frequency sound.
Today, partly because of the popularity of
personal masking units, this early rule no
longer applies. In fact, it is difficult if not
impossible to “sneak” a masking system into
an existing office space. It is better to tell
employees about the masking system and
sell them on its benefits.
Self-Contained Masking Units
Large sound-masking systems may cover
entire floors or even entire office buildings.
Small systems may cover only one office or
workstation. For these small systems, with
only a few loudspeakers, consider self-con-
tained masking units. These self-contained
devices have a built-in masking sound gen-
erator, simple equalizer, small amplifier,
and loudspeaker. Generally, self-contained
units use local (workstation) AC power. In
some cases, they can utilize a circulated DC
power supply.
Single-Channel vs Multi-Channel Masking
For budget reasons, masking systems com-
monly use a single generator, equalizer, and
power amplifier. However, a two-channel,
or even a multi-channel masking system
has a distinct performance advantage.
In a single-channel system, all masking
loudspeakers have the same coherent
signal. As employees walk out of the cover-
age of one loudspeaker into the next, they
hear phase cancellations between the two
loudspeakers. This “phase shift” sound
draws unwanted attention to the masking
system (ventilation sounds would not pro-
duce this effect). Two-channel systems
minimize this problem by connecting adja-
cent loudspeakers to separate masking gen-
erators. Multi-channel systems reduce this
problem to negligible levels.
Basic Electronics
For larger systems, with many masking
loudspeakers, economics dictate a central
rack of equipment containing the masking
sound generators, equalizers, and ampli-
fiers. To enhance security, terminate all
cables inside the rack and close and lock
the rack doors to prevent tampering with
Part 3
The Basic Electronic Sound Masking System
Page 18

the equipment. Ensure the rack has
adequate ventilation for uninteruppted
usage 24 hours a day, 365 days a year.
For an existing space, include the cost of an
electrical subcontractor to provide dedicat-
ed AC circuits hardwired into the rack.
Consider an uninterruptible power supply
(UPS) to prevent system shutdown during
brief power outages or brownouts.
Sound Masking
and Background Music or Paging
Background music and paging systems
normally use loudspeakers installed in holes
in the ceiling, facing downwards to provide
intelligible, clear sound to the listeners.
Sound masking systems normally use loud-
speakers installed above the ceiling tiles,
facing upwards or sideways to randomize
the distribution of the masking sound.
Following these suggestions will make the
most of a combined system. A combined
masking and paging system usually involves
compromise in performance to one system
or the other. However, it is not impossible.
Background music and paging take place at
a higher level than masking. Thus, in a
combined system, choose a higher power
amplifier and loudspeakers and tap the
loudspeakers at a higher level. Always use
separate equalizers for the masking sound
and the background music and/or paging.
Do not allow the masking sound to be
ducked or attenuated during a page. Never
combine masking with a life-safety system.
Basic System Electronics
A basic masking system includes a masking
sound generator, an equalizer, a power
amplifier and one or more loudspeakers.
Figure 11 is the wiring diagram for a basic
masking system. Electronically, basic sound
masking systems are among the simplest
types of sound systems.
Masking Sound Generator
The electronic masking generator (noise
generator) is the heart of the masking sys-
tem. Pink noise (equal energy per octave)
is the most common masking noise. In rare
situations, a white noise generator (equal
energy per hertz) may benefit the system.
Choose a generator that is rack-mounted,
AC powered and produces a stable noise
signal.
An ideal masking noise generator produces
true random noise that never repeats.
Digital noise generators generate a pseudo-
random signal that repeats every so often.
Choose either a true random noise genera-
tor (analog) or a digital noise generator
Noise
Gen.
Equalizer Amplifier
Masking
Loud-
speakers
FIG. 11 - Wiring diagram of a basic sound
masking system.
Page 19

with a pseudo-random sequence of at least
several seconds. Test equipment noise gen-
erators usually repeat too frequently to be
acceptable for sound masking.
Some masking sound generators have
computer controls that gradually reduce
the normal daytime masking sound to a pre-
set nighttime level. This reduction usually
begins just after normal office hours and
slowly takes place over one to two hours.
Then, one to two hours before the office
reopens, the masking sound level gradually
ramps back up to the normal level. The
level change is usually on the order of 6 dB.
However, in some circumstances, masking
sound is more critical during quiet after-
hours times.
Equalizer
For sound masking, use a third-octave
equalizer with included high and low pass
filters, interpolating filter interaction
response and overall shaping filters.
Interpolating filters allow a boost or cut at a
frequency between two adjacent third-
octave frequencies by the relative settings of
the adjacent filter controls. Alternately, use
a parametric equalizer. The best paramet-
rics have control over the complete audio
range in each filter. Other signal processing
devices such as delays, crossovers, and
notch filters are not normally required for
masking systems.
Amplifier
Use high quality professional or commercial
grade power amplifiers with 70-volt outputs
for sound masking. The ability to run con-
tinuously year in and year out is much more
important in a masking amplifier than good
audio performance.
Page 20