in December 2003
PRIVACY IN THE OFFICE ENVIRONMENT
By Joel Lewitz, PE
Understanding the sound and the silence: applications
of sound masking in open- and closed-plan environments,
with possible HIPAA requirements.
Applications of sound masking to open-plan offices are well
documented. Recent projects emphasize the application of
sound masking to closed-plan environments. There are similarities
and differences, which we’ll cover here.
Of the many definitions of
noise, the one that best applies to the office environment
is, “unwanted sound.” A good example is the
conversation I had with a facility manager who had just
moved into an open-plan office designed with a sound-masking
system. There had been a power failure and the sound-masking
system went off. She said, “Suddenly it got real noisy.”
I thought for a moment, because I knew that the background
noise level had just dropped from about 47dBA to 37dBA.
This is a subjective reduction of 10dB or about half as
loud as when the sound masking was operational. Of course,
she meant that she was suddenly aware of all the conversations
from distant workstations. The intelligibility and distraction
were the noise to her. The level of unwanted sound had gone
How to Reduce Noise?
How do we reduce noise and
achieve privacy and freedom from distraction? Let’s
review the basics of noise control. Applying one or more
of these six techniques can alleviate most noise problems.
Elements of Noise Control:
1. Control the noise at the source.
2. Move the source and receiver farther
3. Enclose the source.
4. Enclose the receiver.
5. Mask the signal at the receiver.
6. Active noise control (noise cancellation).
For any noise problem, increasing
the number of elements that are brought to bear will increase
the success of the result. Trying to do the whole job with
just one element will prove costly, ineffective, unacceptable
or simply not feasible.
Fortunately, in the office
environment, typically we can apply the first five elements.
If this happens and the work is done properly, we will be
successful. The Talker-Path-Listener concept takes into
account the major variables associated with speech privacy.
In terms of our balanced approach to noise control and speech
privacy, we can:
• Control the noise at the source by controlling the
talker voice level (Element 1): There are limits to this
element. Many consultants are using raised voice, rather
than normal voice levels in their speech-privacy analysis.
We also have to consider speakerphones or amplified sound
in videoconference rooms as a source. If the person is a
“shouter,” he will never have privacy unless
extreme measures are taken, such as moving him to a soundproof
office. This partially incorporates Elements 2 and 3, but
is still rather impractical.
• Move the source and receiver farther apart, enclose
the source, enclose the receiver. All apply to some form
of space planning, architectural design, partition type
and dimensions and materials selection in the source-to-receiver
path (Elements 2, 3 and 4). There are limits to these elements,
which often relate to cost, but because we are applying
three of the six noise-control techniques, this is a very
important part of the successful solution.
• Mask the signal at the receiver with masking sound:
Privacy modeling software has become more elegant and powerful
recently. What is evident from the modeling tools is how
powerful an element sound masking really is. Furthermore,
when the project is finished, and increased privacy is desired,
it is far easier to increase the masking a few decibels
than it is to increase the partition height a few feet.
There are also limitations.
We could make it sound like Niagara Falls in the office.
The privacy would be perfect, but no one would want to be
there, let alone try to work.
Regarding Element 6, active
noise cancellation cannot be applied to office privacy yet.
We do not have the necessary computer-processing power at
Path considerations differ
between the open- and closed-plan environments. These physical
variables have costs associated with them that often limit
their effectiveness. We worked on a project where the company
president dictated that all offices would be open plan,
including his own. A glance at the space plan revealed that
the closest workstation to the president’s was 50
feet away in any direction. There was no question that he
had confidential speech privacy. In this project, distance
attenuation was used to achieve confidential privacy for
one receiver. The cost of office space prohibits allocating
2500 square feet to each worker’s cubicle.
Open Plan Offices
Variables associated with
speech privacy in open-plan offices are well documented.
1(pdf) shows the typical open plan and the schematic
sound transmission path between talker and listener. Architect,
space planner and facility managers have control over the
variables that will determine the interzone attenuation
of unwanted sound between talker and listener. These include:
• Distance between work stations
• Workstation partition height and width
• Workstation partition sound transmission class (STC)
• Ceiling reflection attenuation
• Vertical surface path attenuation.
Closed Plan Offices
Concerns for speech privacy
are not limited to the open-plan office environment. Congress,
as part of a broad healthcare reform, enacted the Healthcare
Insurance Portability and Accountability Act of 1996 (HIPAA),
which addresses accountability for privacy in terms of protection
of written or electronic healthcare information. Under the
oral communications section, healthcare providers must also
provide reasonable safeguards to ensure that conversations
with or about a patient are private. The awareness of the
need for speech privacy in healthcare facilities with enclosed
offices such as exam rooms and doctor’s offices has
increased in recent years.
In the corporate environment,
companies are moving into “spec” buildings,
which typically have ceiling height partitions between offices.
The “standard partition” usually cannot provide
confidential privacy such as might be required for attorneys
or HR-related activities.
2 (pdf) shows the typical closed-plan sound transmission
paths between talker and listener. These include:
• Ducted connections to supply air grilles
• Lightweight ceiling with open joints
• Recessed light fixtures
• Open plenum return air grilles
• Partition-to-ceiling joints
• Glazing and edge leaks at window mullions
• Recessed receptacles and light switches in the same
• Gaps under base plate.
As with the open-plan path
variables, the closed-plan variables are numerous. Adding
to the complexity is the importance of detailing the sound-rated
construction. Penetrations in sound-rated construction,
such as pipes, ducts, conduit and boxes, must be caulked
and sealed airtight. Small cracks and gaps can allow a lot
of sound to penetrate a perfectly good sound-rated partition,
seriously compromising privacy.
3 (pdf) illustrates four common partition conditions
for enclosed offices and the approximate STC rating for
each. [See also Table A.]
Compare Figure 3 with the
subjective privacy criteria in Figure
4 (pdf). The ceiling height partition with mineral fiber
ceiling tile, at STC 36, is just barely into the low end
of the moderate privacy range. This condition is commonly
found in office buildings and healthcare facilities. If
the quality of the construction is not perfect, the STC
rating will deteriorate to below STC 35. Without sound masking,
there will be no speech privacy. This condition will not
be compliant with HIPAA guidelines unless sound masking
is introduced into the design.
Once the source noise level
has been attenuated over the path between talker and listener,
it is the ratio of this signal to the background noise level
that determines the degree of speech privacy. This signal-to-noise
ratio is at the heart of both the subjective perception
and objective measurement of speech privacy.
The Articulation Index (AI)
was developed in response to the need to quantify how much
intelligibility is in a noisy environment. For example,
this would be important in a noisy airplane cockpit, particularly
if the pilot were trying to communicate with the control
tower or crew. AI is computed by summing the contributions
of the signal-to-noise ratio in each of 15 1/3 octave frequency
bands from 200Hz to 5000Hz. Before the arithmetic sum is
taken, the 15 signal-to-noise ratios are weighted against
how much speech intelligibility is carried in that particular
frequency band. The 1/3 octave band that is assigned the
highest weighting is centered at 2000Hz. According to the
standard, it carries the most intelligibility. Close behind
is the 1600Hz 1/3 octave band.
Real-world applicability of
the standard can be demonstrated easily if you have a band-limiting
filter through which you can play a voice recording. If
you listen only to the frequencies between 1782Hz and 2245Hz
(the 1/3 octave band centered at 2000Hz), it will sound
like an old transistor radio from the 50s—but intelligibility
will be fairly good. In the 1/3 octave band centered at
5000Hz, you will hear a few “s”s and “t”s,
but you won’t understand a single word. Likewise,
down at 200Hz, you will hear a few low booms, but there
will be nothing that contributes to intelligibility of the
AI goes from 0 to 1, where
zero is no articulation (full privacy) and one is full articulation
(no privacy). In the early 60s, efforts were made to correlate
AI’s objective measurement to subjective privacy criteria,
as shown in Table B.
Because the highest AI results
in the least privacy, another related scale was required
to give the higher privacy the higher value. This is the
Privacy Index, or PI. PI is 100X(1-AI). Furthermore, the
categories were reduced for simplicity. Today, the generally
accepted speech privacy definitions are listed in Table
5 (pdf) illustrates a typical range for masking noise
spectra. Most consultants use some form of this curve. A
discussion of the characteristics of this curve will greatly
enhance its application in the field.
Because we are trying to mask
speech, we need masking in the frequencies that carry most
of the intelligibility. The AI weighting and other studies
tell us that this is in the middle frequencies, from about
250Hz to 4000Hz. We don’t need energy below 250 and
above 4000 to mask intelligibility because there is no intelligibility
to mask. Energy above 4000Hz will only make the masking
system sound hissy. (Figure 5 doesn’t have a lower
limit above 4000Hz.)
Below 250Hz and above 4000Hz,
we want sufficient energy to create a smooth curve and make
the sound more natural and acceptable. Figure 5 shows a
masking spectrum down to 125Hz. This is the lowest practical
capability of a cost-effective masking speaker. Some consultants’
masking spectrum goes down to the 63Hz octave band, but
at that frequency there has to be some contribution from
the mechanical system for the energy in that band.
Another aspect of Figure 5
is the upper and lower limit. First, the requirement for
sound masking will vary depending on the contribution to
signal-to-noise ratio from other variables such as voice
level and path attenuation. Also, the privacy criteria may
vary. Nevertheless, practically speaking, masking systems
typically are not set higher than about 51dBA. For enclosed
offices, the upper limit would fall about 5dB to 46dBA because
the office is a more confined space and the other elements,
such as a ceiling height partition between talker and listener
contribute more to the overall equation than a partial height
screen such as found in cubicles.
The dBA level does not tell
the whole story. The spectrum must also be correct. We were
called into a facility with a masking system that still
had privacy problems. The masking system was operating at
46dBA, which should have been sufficient for this office.
However, most of the 46dBA was above 2kHz. The rest of the
spectrum was below the preferred masking curve. The speech
frequencies lacked sufficient masking sound to achieve adequate
It’s Not White, Not Pink
White noise is a random noise
that contains an equal amount of energy per frequency band:
that is, 100-200, 800-900, 3000-3100, etc.
Pink noise, by definition,
has an equal amount of energy per octave. The bands 0-200,
800-1600 and 3000-6000 all contain the same amount of energy.
An octave band is a band of frequencies in which the upper
band limit is nearly two times the lower band limit (within
2%). For example, the octave band centered at 1000Hz is
roughly from 707Hz to 1414Hz. The octave band center frequencies
we are familiar with are set by standard and are shown on
the abscissa of Figure
Pink Noise would be a straight
line on the octave band graph paper in
Figure 6. White noise, when plotted on octave band paper,
goes up 3dB per octave.
It’s not even an NC curve. Beranek’s NC curves
and Blazier’s RC curves were intended to establish
HVAC system design goals. We are attempting to follow mechanical
system noise spectra for reasons of subjective acceptability,
but we neither need nor want the higher frequency component
of the NC curve that typically results from air flow through
Beware: Mimicking of the mechanical
system can backfire. In one project, an open-plan office
lacked privacy simply because it was too quiet. After a
masking system was installed, we asked the facility manager
if the occupants felt the privacy had improved. He said
the privacy was excellent and there were no complaints about
the masking system. However, one person complained that
the office was too cold.
Confidential privacy criteria
for many sectors of business, commerce and healthcare organizations
(see HIPAA at www.hipaa.org)
are bringing sound masking requirements into more closed-plan
office projects. Awareness and attention to the differences
between the physical characteristics of the work environment
and correct sound masking design, tuning, balance and adjustment
for both open and closed plan will turn the sound into the
silence successfully for privacy-sensitive environments.
|Full height partition
|Ceiling height partition with mineral fiber ceiling
|Ceiling height partition with fiberglass batts laid
over the top of the partition
|Ceiling height partition with a plenum barrier at
|0.00 – 0.05
|0.05 - 0.20
|0.20 – 0.35
|0.35 – 0.50
|0.50 – 0.65
|0.65 – 1.00
|95 – 100
|80 – 95
|60 - 80
|Less than 60
Founding principal of Lewitz and
Associates, Inc., Joel Lewitz has more than 30 years of
experience in acoustics and sound-system design. A member
of Sound & Communications’ Technical Council,
he holds M.S.E. (Electrical Engineering) and B.S.E. (Electrical
Engineering) degrees from the University of Michigan.