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Defining the Acoustic Environment of (semi-)open Plan Offices Acoustic Measurements leading to Activity Based Design for retrofit Buildings

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Before creating a new activity based design in retrofit buildings the existing acoustic environment is being carefully measured. Not only the acoustic conditions of the existing building are being measured on the field of room acoustics and building acoustics (according to ISO 3382 for room acoustics, ISO 3382-3:2012 for room acoustics in open plan offices and NEN 5077 for sound insulation), as a very important input for defining the acoustic environment the actual behaviour of people is being measured as well. During a representative week of working hours, the sound levels are being monitored at different locations in the open plan office. To gather information about the character of the sound, sound fragments are as well being recorded based on a trigger level. Defining the acoustic environment of (semi-)open plan offices based on measurements provides a good starting point for redesigning a diversity of office environments. Often the new design leads to activity based office plans, where the different activities are carefully projected in a (semi-) open plan office. Practical measurement data of office noise levels are being presented and analysed.
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Defining the Acoustic Environment of (semi-)open Plan Offices
Acoustic Measurements leading to Activity Based Design for retrofit Buildings
ir. Sara Vellenga-Persoon, ir. Theodoor Höngens,
M+P consulting Engineers, Aalsmeer, The Netherlands
corresponding author: SaraVellenga@mp.nl
ABSTRACT
Before creating a new activity based design in retrofit buildings the existing acoustic environment is being
carefully measured. Not only the acoustic conditions of the existing building are being measured on the field of
room acoustics and building acoustics (according to ISO 3382 for room acoustics, ISO 3382-3:2012 for room
acoustics in open plan offices and NEN 5077 for sound insulation), as a very important input for defining the
acoustic environment the actual behaviour of people is being measured as well. During a representative week of
working hours, the sound levels are being monitored at different locations in the open plan office. To gather
information about the character of the sound, sound fragments are as well being recorded based on a trigger
level.
Defining the acoustic environment of (semi-)open plan offices based on measurements provides a good starting
point for redesigning a diversity of office environments. Often the new design leads to activity based office
plans, where the different activities are carefully projected in a (semi-) open plan office. Practical measurement
data of office noise levels are being presented and analysed.
1. INTRODUCTION
Working in an office environment includes many different activities. Communication on the
phone, social interaction and meetings produce not only sound, but are also in need of a good
speech intelligibility and therefore in need of good room acoustics. Difficult performance
tasks implicate different acoustic conditions. Areas with a more silent environment are
needed so there’s a minimised level of distraction from surrounding activities. In the same
office environment there is also a need for areas with a high level of interaction for the
purpose of teamwork. Because the different activities ask for specific acoustic conditions, the
existing acoustic environment is first being mapped before creating a new office environment
in an existing retrofit building.
2. STATE OF ART
The ISO 3382-3 [1] norm provides guidelines for measuring building characteristics for open
plan offices. An important statement is that concentration and privacy start to improve rapidly
where the speech transmission index falls below 0,50. The negative effects of speech on work
performance disappear if the STI is below 0,20. This statement is reinforced by Jahncke,
Hongisto and Virjonen where the effects of speech intelligibility have been studied for
different office tasks. This work demonstrates that attempts to minimize speech intelligibility
will yield increases in cognitive performance with a varying degree, depending on the type of
focus task.
The corresponding distance to a STI below 0,50 is defined as the distraction distance rD. The
corresponding distance to a STI below 0,20 is defined as the privacy distance rP. This
distance is the distance between the receiver and the speaker. The distraction distance is
defined based on STI measurements combining the receiver levels of speech Lp,A,S and the
levels of background noise levels Lp,A,B. The built environment of the office without the
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existing background noise levels can be defined by the spatial decay rate of A-weighted SPL
of speech D2,S. The roomacoustic properties of an open plan office can be defined by
measurements as well as by predictions using computer modelling , Error: Reference source
not found. Keränen and Hongisto present a regression model for predicting the spatial decay.
Their research has shown an acceptable prediction accuracy for most practical purposes.
2.1 Office noise levels
However, the actual behaviour of people and how many people are talking at the same time is
often not taken into account in defining the acoustic environment. The effects of unattended
speech of performance and subjective distraction have recently been studied by the Finnish
Institute of Occupational Health and the University of Turku . The speech conditions differed
in terms of the degree of absorption, screen height, desk isolation and the level of masking
sound. The actual sound level of the unattended speech is not taken in account as such, but
can be regarded as varied in the distance to the receiver (2 to 6 meter). For all situations the
distraction was rated higher for the nearby speech as opposed to the speech heard from a
further distance. However, this result has to be seen as a combination of a level increase and
an increase of intelligibility and cannot be used as an evaluation of purely the effect of the
sound levels. In 2005 Chigot presented an overview of 11 abstracts on the topic of effects of
sound in offices - subjective experience versus objective assessment . Besides parameters as
‘satisfaction with privacy’ the sound level is often mentioned in relation to an increase of
subjective workload and a decrease of cognitive performance in memory tasks . As an
important comment on the research of Tang , Chigot mentions that LA,eq, 5 min correlates the
best with human auditory sensation.
In 1978, interesting information was published about office noise levels in the Acoustical
Designing in Architecture . A comprehensive survey of the noise in several thousand
locations was conducted by the Bell Telephone Laboratories in order to determine typical
noise conditions indoors and outdoors. The noise levels are a combination of three broad
classifications: people, machinery and outdoor sources. For 45 per cent of the business
locations people were the predominant source of noise, followed by machinery in 25 per cent
of the locations and outdoor sources in 30 per cent of the locations.
2.2 Office types, acoustical conditions and performance
Modern day offices or not designed and used according to a standard format with fixed
workplaces in cellular offices. Because of new ways of work, based on more flexibility, new
office environments are being realised in existing buildings as retrofit projects. These days it
is seldom found that new offices are being built, so new office environments can be created
from scratch. In The Netherlands a lot of the existing office buildings are made ready for
refurbishment within the retrofit building. The pattern of the office lay-out and the use of
workspaces is no longer set as a regular pattern with fixed working spaces within cellular
offices.
De Croon, Sluiter, Kuijer and Frings-Dresen state in that conventional and innovative office
concepts can be described according to three dimensions: 1. the office location (e.g. telework
office versus conventional office), 2. the office lay-out (e.g. open lay-out versus cellular
office), 3. the office use (e.g. fixed versus shared workplaces). A systematic review of
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literature between 1972 and 2004 provides strong evidence that working in open workplaces
reduces privacy and job satisfaction. Limited evidence is available that working in open
workplaces intensifies cognitive workload and worsens interpersonal relations. Close distance
between workstations intensifies cognitive workload and reduces privacy and desk-sharing
improves communication.
In 2009 the Finnish Institute of Occupational Health and the University of Turku in Finland
performed a longitudinal study during relocation on the effects of the acoustic environment
on work in private office rooms and open plan offices . The aim was to determine how the
perceived work environment - especially acoustic environment - and its effects differed in
private office rooms as opposed to open-plan offices. The article states that the results suggest
that the open plan office is not recommended for professional workers.
3. EXPERIMENTS
3.1. Equivalent sound levels in modern day offices
In nowadays offices we have collected a lot of data of noise levels in our measurements in
open plan office floors. During a representative week of working hours, the equivalent noise
levels LA,eq,5min have been measured in different buildings with different types of workplaces.
All measurements were conducted in open plan offices (> 10 desks) for one working week
from 9 to 5. The data for offices with mixed tasks was collected from 8 different office
buildings with 2 or 3 monitoring positions per building (in total 20 measurement positions).
The remaining data in the histogram was collected from 1 or 2 buildings for the specific
office tasks (engineering, programming, governmental advisors) with 2 or 3 monitoring
positions per office building.
To make a comparison of the sound levels through history the measured data is presented in
figure 1. We conclude that the levels of modern day offices are substantially lower compared
to those in the seventies of the twentieth century. The old fashioned loud typing machinery
and hard acoustic environments will probably have a cause in this.
Figure 1. Noise level data (mean value -/+ standard deviaton), comparison data 1978,
United States of America, D.F. Seacord [1] and data 2014, The Netherlands, M+P
In figure 2 the noise levels in modern day offices in The Netherlands measured by M+P are
presented in a histogram. The curve of the histogram shows the curve of a normal
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distribution. The main characteristic is that all measurements have a mean value of 50 to 51
dB(A). The difference is especially noticed in the standard deviation. Specific office tasks
like computer programming and engineering tasks show a smaller standard deviation (3-4
dB) compared to the mixed tasks (5 dB) as shown in figure 1 and 2 for modern day offices.
Figure 2. Histogram noise level in modern day offices measurement data M+P
3.2 Designing (semi-) open plan offices in retrofit buildings
It is important to define the existing office environment before starting a new office floor
design. The variation can be found in office design, room acoustics and building acoustics
and the activities of the workers. In table 1 variations with associated parameters and choices
are stated for defining the (acoustic) environment based on the experience of engineering of
M+P for Dutch offices.
Table 1. Defining the variations with associated parameters for defining the (acoustical)
environment
Variation Choices Acoustical parameters
Office design LAY-OUT open / semi-open / closed -
USE permanent versus flexible
(desk-sharing)
-
TYPE mixed or activity based -
Measurements
room acoustics and
building acoustics
ABSORPTION/
AVOID
REFLECTIONS
ceilings, wall panels,
furniture, interior elements
reverberation time T
spatial decay D2,S
STI
ROOM
INSULATION
walls, ceilings, floors,
facades, doors, windows
sound insulation DnT,A
SCREENS screens, walls, rooms,
cabinets
spatial decay D2,S
STI
INSTALLATIONS ventilation principles,
masking systems
background noise Lp,A,B
STI/SNR
Measurement
behavioural acoustics
- - sound level (Lmax, Leq)
recording wave-files
(defining sound source,
type of sound)
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Based on the measured noise levels in modern day office buildings the needed distances have
been calculated for the design of new office environments. Supposed are a background noise
level of 40 dB(A), a signal to noise ratio for speech of 3 dB and a spatial decay of 8 dB.
These values are set as quite representative for modern Dutch office design as seen as in
figure 3.
Figure 3.Modern modern day office in The Netherlands (semi-open, activity based)
4. RESULTS
For open plan office environments we recommend a semi-open structure which provides
some screening and divides different areas in the working space. This results in a
corresponding spatial decay of about 8 dB assuming that acoustic absorption is provided in
ceiling and/or wall absorption. Another possibility is to create zones varying from silent to
more interactive. Activity based work provides the possibility of reducing the design distance
between work departments (working groups) as seen in figure 4. The bars corresponding to
specific office tasks show a smaller standard deviation. Because of this a much smaller design
distance is required. To achieve a distraction distance rD (STI < 0.50 for 98% in time, a design
distance is needed of about 23 meters in a mixed tasks office. In a specific task office this
distance decreases to about 17 meter.
Figure 4.Needed distance for STI to drop below 0,50 (distraction distance rD) for different
design criteria (98% of time corresponding to mean value+2*st.dev. 85% of time
corresponding to mean value+1*st.dev. or 50% of time corresponding to the mean value of
measured noise levels)
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REFERENCES
[1] International Organization for Standardization. ISO 3382-3:2012 Acoustics – Measurement of room
acoustic parameters – Part 3: Open plan offices, Geneva, Switzerland.
Jahncke H., Hongisto V., Virjonen P. (2013). Cognitive performance during irrelevant speech: Effects of speech
intelligibility and office-task characteristics, Applied Acoustics 74, 307-316, faculty of Engineering and
Sustainable Development of University of Gävle, Gävle, Sweden, Finnish Institute of Occupational
Health, department of Psychology of University of Turku, Turku, Finland.
Persoon S.A., Höngens Th. (2013). Modeling acoustics as a powerful design tool for open plan offices,
proceedings IBPSA 2013, Aalsmeer, The Netherlands.
Keränen J., Hongisto V. (2013). Prediction of the spatial decay of speech in open-plan offices, Applied
Acoustics 74, 1315-1325, Indoor Environment Laboratory of Finnish Institute of Occupational Health,
Turku, Finland.
Haapakangas A., Hongisto V., Hyönä J., Kokko J., Keränen J. (2014). Effects of unattended speech on
performance and subjective distraction: The role of acoustic design in open-plan offices, Applied
Acoustics 86, 1-16, Indoor Environment Laboratory of Finnish Institute of Occupational Health,
department of Psychology of University of Turku, Turku, Finland.
Chigot, P. (2005). Effects of sound in offices – subjective experience vs. objective assessment, Facilities, Vol. 23
Iss: 3/4, 152 – 163, Ecophon, Hyllinge, Sweden.
Jackson T. S., Irrelevant speech, verbal task performance, and focused attention: A laboratory examination of the
performance dynamics of open-plan offices. (1999), The Sciences and Engineering, Vol 60(6-B): 2997.
Kjellberg A., Landstroem U., Tesarz M., Soederberg, L. et-al (1996). The effects of nonphysical noise
characteristics, ongoing task and noise sensitivity on annoyance and distraction due to noise at work,
Journal of Environmental Psychology, Vol 16(2): 123-136.
Tang S. K. (1997). Performance of noise indices in air-conditioned landscaped office buildings, Journal of the
Acoustical Society of America, Vol 102(3): 1657-1663.
Knudsen V.O., Harris C.M. (1978). Acoustical designing in architecture, Acoustical Society of America, United
States of America.
De Croon E., Sluiter J., Kuijer P.P., Fings-Dresen M. (2005). The effect of office concepts on worker health and
performance: a systematic review: a systematic review of the literature, Ergonomics 48:2, 119-134,
Coronel Institute for Occupational and Environmental Health, Academic Medical Center, Research
Institute Amsterdam for Health and Health Care Research (AmCOGG), Amsterdam, The Netherlands.
Kaarlela-Tuomaala A., Helenius R., Keskinen E., Hongisto V. (2009). Effects of acoustic environment on work
in private office rooms and open-plan offices – longitudinal study during relocation, Indoor Environment
Laboratory of Finnish Institute of Occupational Health, department of psychology of University of
Turku, Turku, Finland.
Pop, C.B., Rindel, J.H. (2005). Speech privacy in open plan offices, Proceedings of Inter-Noise 2005, Rio de
Janeiro, Brazil.
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... When the signal to noise SNR drops under 3 dB the sound intelligibility rapidly decreases, assuming the speech level as the signal and the background noise level interferes as noise. With a background level of about 40 dB a D2,S of about 8 dB is needed for an open plan office with mixed activities according to the experience of M+P (see also [7]). Figure 1a ...
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Acoustical designing in architecture
  • V O Knudsen
  • C M Harris
Knudsen V.O., Harris C.M. (1978). Acoustical designing in architecture, Acoustical Society of America, United States of America.