Open-plan office acoustics
Open-plan offices have become quite common in recent years, mainly due to reduction of costs and due to facilitating collaboration between coworkers. However, open-plan office design allows noise to propagate more freely, causing distraction. Thus, the acoustic design of the space must be kept in mind. The standard ISO 3382-3 [1] (ISO 3382 part 3 for open plan offices) defines the important parameters and target values for appropriate open-plan office acoustics.
Noise in open-plan offices
The foremost source of noise in open-plan offices comes from telephone calls and discussions between co-workers. Conversations are particularly disturbing because of the information content. Speech recognition notoriously distracts much more than other noises of a similar level. However, if numerous people are speaking at the same time at a sufficient distance , we are no longer able to recognize what they are saying, and the noise generated has a beneficial masking effect on speech nearer to us.
Acoustic parameters
The most important parameters to control open-plan office acoustics are the amount of sound absorption material (usually in the ceiling), the use of screens, and the background noise. Whereas room acoustics are usually characterized by the reverberation time, ISO 3382-3 introduces different acoustic parameters. These are the spatial decay rate of noise for a typical speech spectrum, and speech intelligibility as a function of distance. These, in turn, help calculate two important measurements: privacy and distraction distances.
The ISO standard for measurements of open-plan office acoustics
The international standard ISO 3382-3:2012 [1] defines a number of measurable room acoustic parameters for the objective evaluation of the open-plan office acoustics. The method is intended for furnished rooms, i.e. the room model must include tables, bookshelves, screening elements etc., but with no people present. An omni-directional sound source is used for the measurements, which includes seven octave bands from 125 Hz to 8000 Hz. The results can be used to evaluate room acoustical properties in open plan offices, and it may be used for comparison of alternative solutions in design or acoustical treatment.
Measuring in accordance with this standard, four acoustic parameters are mandatory:
- distraction distance rD.
- spatial decay rate of A-weighted sound pressure level (SPL) of speech, D2,S.
- A-weighted SPL of speech at 4 m, Lp,A,S,4 m.
- average A-weighted background noise level, Lp,A,B.
In the revised second edition of the standard, which is expected in 2021, an additional parameter will be included: the comfort distance rC defined from the spatial decay rate as the distance from a point source where the SPL of speech is decreased to 45 dB.
Distraction distance
Distraction distance refers to the distance where speech is only partly intelligible, but mostly contributes to the background noise. The ideal solution is to bring together people who want to share information, preferably within the distraction distance, and to keep further away those belonging to other groups.
The distraction distance is defined as the distance from a source within which the speech transmission index (STI) ≥ 0.5. The distraction distance is a particularly interesting parameter because it considers most of the important acoustic variables, namely the amount of absorption, the effect of screens, the spatial attenuation and the masking from background noise. The background noise in the empty office must be applied for measurements in accordance with ISO 3382-3, but the standard opens for additional calculation of results using other kinds of background noise, e.g. that from human activity in the office.
The figure above is an example of an open-plan office. One of the workstations is chosen as the centre of a circle with radius equal to the distraction distance rD = 7 m. Thus, other persons in positions inside the circle can be assumed to cause distraction when they are talking, while speech from persons outside the circle create a masking babble noise. The distraction distance should be as short as possible to minimize the number of distracting sound sources and maximize the number of remote masking sound sources.
This model explains why acceptable acoustical conditions in open-plan offices may better be obtained in large rooms. If the office is too small, there may be only distracting sources and no masking sources. The model also suggests that wide offices may be better than long, narrow offices.
An optimization problem
Designing open-plan office acoustics is a complicated task. Since the main problem is distraction by speech between other people, it is not a simple one-dimensional noise problem that can be solved by a sufficiently high damping of the room. If the reverberation time is very short, the remote voices are heard with high clarity and thus the amount of distraction from speech is high. A long reverberation time, on the other hand, leads to a very noisy environment, which is also disturbing and annoying. Studies in laboratory simulations have shown that excessive sound absorption may have a negative impact on occupants’ perception of noise, acceptability of noise and performance of office work [2]. Similarly, with the background noise: It should be neither too low nor too high. People may be concerned by silence as much as with the noise [3].
Effects of absorption and screens
In order to look deeper into the problem, three versions of an office have been simulated [4]. Office 1 has an acoustic ceiling and no screens; reverberation time 0,5 s. Office 2 has a concrete ceiling and reverberation time 1,0 s. Office 3 is like office 1 but with additional absorbing baffles and screens. The SPL as function of distance is seen in the figure below for the three offices.
The shorter reverberation time in office 1 compared to office 2 results in lower SPL, as could be expected, but the spatial decay rate D2,S remains the same. The addition of screens and baffles in office 3 yields increased spatial decay rate and lower SPL in remote positions, but not in the nearest position.
The simulated STI as function of distance is seen in the figure below for the three offices.
The distraction distance rD is the distance where the regression line crosses STI = 0,5. It is about 12 m in office 1 but decreases to about 8 m in office 2 and in office 3. So, this parameter indicates that the office 1 with short reverberation time is worse than the other cases, either with less absorption or with screens and increased attenuation.
With the longer reverberation time in office 2, STI is low in the positions close to the sound source because of the more reverberant sound. However, in remote positions the background noise is more important for the STI because the signal-to noise level is lower. In this office the sound is loud, but because of the reverberation the intelligibility is low, and thus the distraction is also low.
In office 3 the distraction distance is also short, but for another reason; when the sound level is reduced by screens and baffles the background noise is more important for STI except in the nearest positions. So, a greater part of the speech is masked by background noise.
A simple parameter study
To illustrate the interrelationship between some of the open-plan office acoustic parameters, the result of some computer simulations is shown in the figure below. The A-weighted background noise level was 35 dB, the reverberation time was either 1,0 s or 0,4 s. The simulations were made without screens and with screens of three different heights [5].
The preferred conditions are with rD as short as possible and Lp,A,S,4m as low as possible. Increasing screen height is efficient to reduce rD, but has very limited influence on Lp,A,S,4m. A short RT is necessary to bring down Lp,A,S,4m. However, the shorter RT tends to increase the distraction distance, especially without screens. This explains why it is necessary to optimise the reverberation time; it should neither be too long nor too short.
Annoyance and distraction distance
Data from surveys of many offices have been collected, and the possible relation between the percentage persons highly disturbed by noise (HD) and the acoustical parameters has been analysed [6]. No correlation was found with the spatial decay rate D2,S while good correlation was found with the distraction distance (r = 0.79) and with A-weighted SPL of speech at 4 m (r = 0.71). The table below is derived from [6] and gives examples of the relationship with estimated percentage persons highly disturbed by noise (HD).
HD (%)
Lp,A,S,4m (dB)
rD (m)
10
20
30
40
50
60
45,6
46,5
47,4
48,3
49,1
50,0
5,6
7,5
9,3
11,2
13,1
15,0
Acoustical measurements or simulations in ODEON
The measurement method for open-plan office acoustics can be reproduced in ODEON, making it possible to compare measured vs. simulated results. In a simulated procedure, the positions of sources and receivers must correspond to actual workstations in the furnishing plan. When doing measurements, it is necessary to create a dummy room with sources and receivers. In this way the relative distances between sources and receivers are automatically extracted from the dummy-room. Of course, it is also possible to create a more accurate model of the office in question, but it is not necessary for doing the measurements.
Two application notes are available for either simulations or measurements in accordance with ISO 3382-3 using the ODEON software.
References
[1] ISO 3382-3 (2012). Acoustics – Measurement of room acoustic parameters – Part 3: Open plan offices. International Organization for Standardization, Geneva, Switzerland.
[2] I. Balazova, G. Clausen, J.H. Rindel, T. Poulsen, D.P. Wyon (2008). Open-plan office environments: A laboratory experiment to examine the effect of office noise and temperature on human perception, comfort and office work performance. Proceedings of Indoor Air 2008, 17-22 August 2008, Copenhagen, Denmark.
[3] V. Acun & S. Yilmaez (2018). A grounded theory approach to investigate the perceived soundscape of open-plan offices. Applied Acoustics 131, 28-37.
[4] J.H. Rindel (2012). Prediction of acoustical parameters for open plan offices according to ISO 3382-3. Paper 3aAA, Proceedings of Acoustics 2012, Hong Kong, PRC, 13-18 May 2012.
[5] J.H. Rindel (2018). Open plan office acoustics – a multidimensional optimization problem. Proceedings of DAGA 2018, 19-22 March 2018, Munich, Germany.
[6] B.J. Mueller, A. Dickschen, N. Martin (2020). How reliable are ISO 3382-3:2012 measurements to predict employee satisfaction with acoustics in open space offices? Preliminary results of multiple measurements. Proceedings of Forum Acusticum, 7-11 December 2020, Lyon, France.
