Atlas AA-SMG, Sound Masking Systems Technical Manual
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 confidentiality is important.
Sound masking can also reduce the distraction caused by traffic, office machinery and
other unwanted sounds. Because this benefit is limited to situations where the unwanted sounds are of relatively low level, however, 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 telephone. Many times each day, an agent will
ask a prospective client for financial and
health information. The insurance company 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 privacy 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 insulating 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 electronics of a successful sound masking system and provides case histories as illustrations. Appendix A contains definitions of
sound masking and acoustical terms.
Appendix B is a useful sound masking worksheet 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 consultant 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 loudspeaker. 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, “marginal” refers to an unacceptable level of
speech privacy. “Normal” speech privacy is
acceptable for open-plan office environments. “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 barriers. Sound masking completes the environment by adding speech privacy.
Compared to the completely open “typing
pool” concept, each employee has a comfortable working zone with both visual and
speech privacy.
Medical Examination Rooms
Medical examination rooms are often small
(perhaps 100 square feet) and close together. 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 personnel 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 conjunction with less costly construction techniques, 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 existing structures, may involve significant
acoustical problems or building layout
issues. In these cases, Atlas Sound recommends the services of a qualified acoustical
consultant.
Court Rooms
Sound masking can be useful in a courtroom 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 loudspeakers 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 completely 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 commonly 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 windows 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 partition 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 construction 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 guaranteed, 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 ambient noise. Instead, retain a qualified
acoustical consultant to help with acoustical
solutions.
Space Used by Sight-Impaired People
Masking sound and an absorbent environment 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 understand 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 usually 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 costeffective solution that allows a building
owner or leasee to retain the flexibility of an
open-plan office.
Confidential privacy, without sound masking, requires multiple-layer walls, from the
floor to the deck above the ceiling, combined with special sound-isolation doors,
door seals and careful caulking of all penetrations of the wall to stop sound leaks.
This kind of construction can be very costly.
In contrast, masking sound allows confidential 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 workspaces. Even when working in a private
office, employees may hear noises and conversations 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 performing necessary job functions.
With sound masking, the open office gains
the speech privacy of individual private
offices yet retains the flexibility of the openplan concept. Just move partitions to add or
delete offices, combine offices into a conference 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 unwanted sound. The last step, adding masking
noise, masks the remaining unwanted
sound in such a way as to create speech privacy 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 represents the level (as a graph) of speech and
the dotted line represents the room or background 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 reflected sound energy, and the masking loudspeakers 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-tonoise 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 ceiling 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 acoustical consultant can help when an evaluation
suggests that problems are inevitable.
SOUND PRESSURE LEVEL re 20 µPa,
FIG. 2 - This two-part graph illustrates the concept 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 contribute 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.
dB
60
A
50
Reflected
Sound
Speech-
to-Noise
Ratio
40
Talker
A
Direct
Sound
B
Masking Loudspeakers
Speech Sound
Level
Room
Sound
Level
Room Sound
Level
Speech
Sound
Level
30
20
10
60
50
40
30
20
10
Background
1000 2000 4000 8000250 50063 125
Talker sound:
Reduced level
Background sound:
Raised level
1000 2000 4000 8000250 50063 125
OCTAVE-BAND CENTER FREQUENCY,
Hz
B
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 Aweighted 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 orientation 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.
Page 10
SOUND PRESSURE LEVEL re 20 µPa,
dB
80
70
60
50
4000
8000125 250
2000
1000
500
OCTAVE-BAND CENTER FREQUENCIES,
Hz
0°
90°
180°
90°
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 barrier. 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 marginal 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 hardsurfaced work area. However, screens can
have their upper surface (no more than the top
1-foot) made of glass for visual openness.
(a)
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.
(b)(c)
Screen
6′ high
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
FIG. 6 - Examples of good and bad layouts for workstations in open-plan spaces.
Reduction of Reflected Sound Energy
Page 12
Direct, uninterrupted
path (talkers face each
other)
Longer
uninterrupted
path
Only
uninterrupted
path
Screens (to interrupt
path to opposite
workstation)
Separation distance,
6 ft.
PLAN VIEW
Ceiling
The ceiling in an open-plan office affects
speech privacy more than any other acoustical element. A hard ceiling reflects sound
from one workstation to another, bypassing
the sound barrier provided by the workstation 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 perfect (total) absorption. We often think of
this as one square foot of an open window.
“Absorption coefficients” rate the absorptivity of a surface between 0.00 (perfect reflector) 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 higher than 1.00 are sometimes given for very
highly absorptive materials. This is an artifact 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 articulation class rating is the sum of the attenuations (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 openplan office space. Because it measures
effectiveness in real-world conditions, articulation class is the preferred rating method
for ceiling tile. Select ceiling tile products
with AC ratings of 200 or more for openplan 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 frequencies between workstations, “short-circuiting”
the acoustic privacy provided by the workstation 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, fluorescent fixtures utilize flexible electrical
conduit and can be moved to a new position without re-wiring. Figure 7 shows
good and bad placement of fluorescent fixtures.
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 without 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 necessary in a correctly-designed masking system. In fact, they can cause problems.
There are always small sound leaks in the
ceiling. With normal ceiling tile the masking 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
FIG. 7 - Speech frequencies reflect off the flat
lenses of ceiling fluorescent fixtures. If the fluorescent fixtures are mounted over workstation
partition screens, this reflected sound can
reduce speech privacy.
Page 14
Bad
Good
Light Fixture Light Fixture
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 existing 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 frequency absorption than thin, direct glue-down
carpets. Carpeting also reduces the irritation 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
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
Return
Air Ret urn
Grille
Wood / gyp.
Walls or Hard
Screens
Wood / Metal
Chairs
Tile Floor
Return
Air Return
Boot
Boot
Air Ret urn
Grille
Plenum Air
Air Return
Wood Shelves
Hard Space
Plan View
Upholstered
Chairs
Absorptive
Wall Pa nels
or Screens
Carpet Floor
(padded)
Wood Shelves with Absorptive Panels
Soft Space
Plan View
faces such as shelf covers and drawer faces.
Of course, workstation partition screens
must be highly absorptive.
Hard walls, doors and windows can seriously 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 barrier and reflect speech sound into an adjacent workstation (see the previous discussion 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 workstation 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 covering (special fabrics for interior finish use),
and a mounting system. Standard acoustical wall panels come in 2´x 2´, 2´x 4´, and
4´x 4´ sizes and in 1 inch and 2 inch thicknesses. 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
FIG. 10 - Walls, doors, windows and curtain walls can reflect sound into adjacent workstations.
Page 16
Sound
Absorbing
Baffles
Glass Window
Acoustical
Wall Panel
Glass Window Hard Wall
Extend
Screen to
wall
PLAN VIEW
Ambient Noise
To the extent possible, keep building and
office equipment noises below the level of
the masking system. The heating, ventilating, 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 conference area in an open-plan office environment. 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
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