Acoustics and office Design

Acoustics and office Design

Acoustics and office Design

April 30 2019

Acoustics and Office Design

We are surrounded by sound at all times. Some sounds are pleasant and soothing; others
annoying and distracting.
Sound affects the way we feel and behave. In the working environment, sound is particularly
critical. Complaints in the office often involve the lack of speech privacy, high noise levels and
the distraction of, overheard extraneous conversations. These complaints risk becoming more
pronounced when traditional office walls are replaced by partial height panels, which allow
sound to circulate more freely throughout a space.
In the open plan office, the acoustical performance of panels, ceilings, floors and walls must be
controlled if speech privacy is to be maintained, and the overall noise level is compatible with
the intended use of the space. (Conference areas, executive offices and general office areas
each have different sound level and privacy criteria).
Open plan offices can be a difficult acoustic environment to design for. The ceiling is often the
only surface you can use for any form of acoustic control.

However, with the increasing use of exposed concrete, even this surface is becoming off-limits.
This paper looks at some of the acoustic issues in open plan offices and considers how ceilings
can accommodate these various requirements
For many years the accepted Reverberation Time of an enclosure has been 60 dB. Whilst there
have been many proposed modifications to better accommodate specific conditions, this
formula has proved very useful and still forms the basis of most acoustic design work carried
out today. However, given the wide range of building types and the activities taking place
inside them, the problems faced by modern acoustic designers are often very different from
those found in a traditional lecture theatre. In the case of open plan offices, the acoustic
landscape is all too often a neglected aspect of a building’s interior.

Open Plan Offices

When it comes to the acoustic design of open-plan offices, what is ideally required is perfect
communication out to a specific distance, for example twice that from one side of a desk to the
other, and then zero intelligibility beyond that. Obviously people need to be able to
communicate clearly, however other people’s conversations can be an annoying source of
distraction – especially when they are on the telephone.

Distraction

Research has shown that it can take around 10-15 seconds for someone to refocus their
attention on a task once they have been momentarily distracted. The human voice has the
greatest distraction potential of any other source, apart from sirens and high pitch discordant
sounds. Moreover, it has information content which the brain finds difficult to ignore and will
process, leading to periodic and long term attention diversion. Obviously this is not an issue
unless you really have to concentrate on something which is why; in general, executive and
managerial groups have been shown to be more disturbed by noise than clerks and secretarial
staff.

The amount of space per person is also an important influence on sound levels in offices as
there is a tendency for people to raise their voices when the room they are in is more densely
occupied, particularly when talking on the telephone. Overcrowding produces tension and
therefore a heightened awareness of the surrounding environment. Areas of between 8.5 m²
and 14 m² per person are a satisfactory range, with around 10 m² being preferable). However,
below 12.6 m² there will often be a small number of people who still feel overcrowded.

Dealing with the Ceiling

Open plan offices usually have relatively low ceilings and large floor areas, which means that
the surrounding walls play a negligible role in the sound field. As well, internal partitions tend
to be low level and relatively lightweight, so they too have limited effect. With hard-wearing
carpeted floors being all but mandatory, the only real variable within the office that the
designer has left to play with is the ceiling.

Given cost constraints and the need to accommodate services, ceilings in most office buildings
provide nothing more than a large expanse of relatively lightweight acoustic tile. Whilst there
are many high quality and well-designed tiles available, they can only do so much to solve
these problems. Moreover, with the increasing use of chilled beams and natural ventilation

Systems that require exposed thermal mass for night-cooling, there simply may not be enough
surface area left in the ceiling to rely solely on good absorption. However, with some careful
design, even a highly reflective exposed concrete ceiling can provide excellent acoustics, and a
more successful and interesting solution than just highly absorptive tiles.
With a flat ceiling, there really is nothing to stop sound reflecting off into the distance. Whilst
partitions, furniture and planting can offer some obstruction at low level, sound traveling
outwards and upwards can continue relatively unimpeded, as shown in Figure1.
As a result, the sound field in an office is not the same as in a large open volume in which
propagating sound obeys the inverse square law – falling off at around 6dB per doubling of
distance. Instead, the open plan space tends to create a tunneling effect, which means there
can be areas where the sound reduction is as little as 3dB per doubling of distance.

 

Figure 1: The effect of a flat ceiling on sound reflection. Low ceiling height plus the large open
floor space can create an acoustic tunneling effect resulting in increased noise spread and
loudness

Some well-designed acoustic ceiling tiles make use of incidence angles to reflect useful sounds
whilst at the same time absorbing unwanted reflections. Those sounds that go straight
upwards are useful as they can be quickly reflected back down quite near to the speaker,
which is desirable as this helps increase sound levels for those nearby. Those sound that travel
closer to the horizontal strike the ceiling much further away and at a much lower incidence
angle, making them unwanted reflections.

 

Figure 2: An example of an acoustic ceiling tile that reflects vertical sounds whilst absorbing
those arriving at grazing angles.
If, as shown in Figure 2, the ceiling consists of areas of flat reflective horizontal surfaces, set at
different heights, then highly absorptive material can be placed on the vertical surfaces that
separate them. This ensures those sounds that would ordinarily travel quite far are instead
absorbed by the ceiling, whilst those that travel near to vertical are reflected straight back
down. As shown in Figure 3, this can be a good first step in reducing more distant levels.

 

Figure 3, this can be a good first step in reducing more distant levels.

Figure 3: Through the judicious location of absorptive and reflective surfaces on the ceiling, it is
possible to reflect useful sounds whilst absorbing potentially problematic ones.
Taking this same process and scaling it up makes it possible to design a ceiling configuration
that promotes acoustic privacy. Figure 4 shows just one example of how this might be done.
Whilst there are many other configurations that would work similarly, their main feature is the
division of the ceiling area into boxed sections.

Figure 4: Division of the ceiling into boxed sections (mirroring divisions in activity from the
spaces below) can further help to reduce the spread of noise in an open plan environment.

In this example the vertical elements that provide the separation are made highly absorptive to
reduce unwanted reflections traveling into adjacent sections. Also, the top of each section is
made highly reflective and angled slightly to ensure that sound generated within each space
remains as much as possible within that section exclusively.

In considering what is and what is not good open plan acoustics, it is important to appreciate
the difference between “hearing” and ‘understanding”. The muffled sounds from an adjacent
work space generally are not as distracting if they cannot be understood. Of course, if they
cannot be understood, you have confidential speech privacy even though some degree of
distraction may exist. The overall ambient sound level is also important.
We create a quiet environment by dealing with the paths that sound takes between the
sources (equipment and people) and the receivers (people).
In the office, people and equipment are sound sources. People also are receivers of that sound
which is transmitted.
There are three paths by which sound travels: a direct path which is the straight line between
the source and receiver; a reflected path which occurs as sound bounces off various surfaces;
and a defracted path which involves sound bending over the top and around the sides of
partitions. The control of sound in an open office requires consideration of all three paths.

 

Blocking Sound
To stop the direct path of sound, we erect barriers (system wall panels) which stop sound from
passing through. The STC, or Sound Transmission Class, of a partition measures the ability of a
barrier to stop sound from passing through it. A material of an STC of 21 will prevent 21
decibels from passing through.
The most sound reduction that can be expected between workstations is 21 decibels, because
sound wills defract, that is, bend over the top and around the side of partial height partitions.
An STC of more than 21 is not generally an advantage.

 

Absorbing Sound
Acoustically absorbent panels absorb rather than reflect sound. The reflection of sound off
hard surfaces is called reverberation. The absorption of sound, on the other hand, actually
refers to energy conversion.
Sound is created when something resonates i.e. pushes against the air and retracts creating
waves of dense and rarefied air–fluctuations in air pressure.

 

Sound is a form of energy, and energy cannot be destroyed. It can be dissipated as it spreads
out over distance or converted into another form of energy. Acoustical panels convert sound

 

energy into mechanical energy. As sound waves impact the material, the material responds by
ǀiďƌatiŶg. Those ǀiďƌatioŶs ;͞ǁiggles͛Ϳ aƌe theŶ dissipated as a ŵiŶute aŵouŶt of heat. The
ability of a material to convert sound energy to mechanical energy is measured in a test that
provides the Noise Reduction Coefficient (NRC). An NRC of 70 means that a material absorbs
70 percent of the sound that hits it. The reciprocal, 30 percent, is returned.
The ability of a material to absorb sound determines its acoustical capabilities. The most
effective sound reduction in a office environment is achieved when the higher frequencies of
human speech, those that lend intelligibility, are those which are absorbed. These are often
referred to as the articulate speech frequencies.
The N‘C is a siŵple aǀeƌage of the ŵateƌial͛s aďsoƌptioŶ of sound at frequencies of 250, 500,
1,000 and 2,000 Hertz (Hz). Hertz cycles per second (CPS), and frequency all refer to the
number of fluctuations per second which determines the pitch of a sound.
Our ears are more sensitive to certain frequencies – it is no accident that these frequencies are
the same as those of human speech. It is the higher frequencies of human speech (1,000 Hz
through 3,000 Hz) that provide intelligibility. It is these frequencies which must be considered
most closely in developing an acceptable open plan environment. There is also a test which
provides an Articulation Index (AI) This test measures the intelligibility of sound (usually the
human voice) by giving a higher value to the articulate speech frequencies.

 

In evaluating an NRC rating for open plan acoustics, it is important that the higher absorption
coefficients are at the higher frequencies (the articulate frequencies of human speech).
Two different materials may have the same NRC but, the one which absorbs more of the
intelligible (higher) speech frequencies is a more effective material for controlling sound in an
office environment. In fact, the reflection of low frequencies may be an advantage in that it
permits an ambient sound level that can reinforce the background masking sound.
Masking Sound
It is important to have background masking sound to overwhelm that sound which is not
absorbed or blocked by acoustical panels as well as that sound which is detracted. That sound
can be masked artificially with electronically produced sound evenly distributed throughout a
space, usually through speakers above the ceiling. It cannot be masked with the low frequency
sounds of speech, heating, ventilating and air conditioning equipment, or other background
sound.

Effective sound masking has the following characteristics:
* It is uniform throughout the space – Ŷo ͞hot spots͟ oƌ ͞dead spots.͟
* It is the correct volume – loudeƌ thaŶ ǁhat you doŶ͛t ǁaŶt to heaƌ ďut, Ŷot so loud as to
interfere with the conversations you want to have. One should never feel compelled to
speak over it or strain to listen.
* It has the ĐoƌƌeĐt toŶal Ƌualities. A ͞huŵŵ͟ Ŷot a ͞hiss.͟ It is Ŷeǀeƌ oďtƌusiǀe – it
disappears and may be thought to be the normal background sound of a well-designed
HVAC system.
* It is not noticed when it is on but, is missed if turned off. (You never turn it off!)
Background music is not effective as masking. It is another specific signal which may or
may not be a distraction – depending on the individual and /or the task.

To be effective, the masking level should be 4 to 10 decibels louder than the level of incoming
intrusive speech from adjacent work stations. See our page on Sound Masking for more
information.

Putting the pieces together

In an open plant office, the STC (Sound Transmission Class) and NRC (Noise Reduction
Coefficient) must be balanced to achieve good speech privacy, while background sound levels
are comfortably maintained.
The combination and interaction of the STC and NRC is measured as the Noise Isolation Class
(NIC). A furniture system with a NIC of 21 will reduce sound by approximately 21 decibels
between work stations. Developed in the 1970’s, this criteria has fallen into somewhat of non-use; probably though lack of understanding of it by those who should be using it in the design
and evaluation of space. Each component in the open plan can be tested under these criteria;
however, an educated evaluation of traditional properties (NRC, STC, etc.) can suffice.
It is important to recognize that all of the components in an open plan office – ceiling, walls,
floor and partitions – contribute to or detract from the overall acoustical performance of the
space. Each component can only compensate so much for the poor performance of another
component. When too many compromises are made, the overall performance suffers.

NO ONE COMPONENT CAN DO THE JOB. IT CAN ONLY DO IT JOB!͟

While carpeting reduces some airborne sound; its greatest contribution is in the reduction of
“foot-fall” sound. Carpeting stop sound from starting – impact noise. It͛s use on walls is not
recommended for noise control.
Performance criteria for open plan offices include:
Ceilings NRC .75 or higher
Partitions STC 21 or higher
Partitions NRC .70 or higher
Walls NRC .80 or higher
Floor (Carpeted) NRC .15

 

Designing

Spacing between people also is important and should be maximized with the layout of the
office.  “Lines of sight” between people should also be minimized. If you can see someone, you
haven’t dealt with the direct path of sound. Separate noisy activities from quiet ones.
Flat lighting fixture lenses should be avoided or replaced. From a purely acoustical perspective,
the best lighting in the ceiling is: no lighting in the ceiling, ambient or indirect lighting.
Conclusion
Obviously the ceiling geometry by itself cannot prevent all sound from escaping into adjacent
areas. However, when faced with a situation where the ceiling has a number of different roles
to play, it is still possible to provide an acceptable acoustic landscape in an open plan office
that maintains the privacy, and therefore the productivity, of its occupants.