In open-plan offices or open-plan office environments, it requires a lot of acoustic optimization to gain acceptance for the new office landscape, especially if the employees used to be housed in individual offices. Dipl.-Ing. Thomas Plötzner, specialist in environmental acoustics at OWA, outlines the (spatial) acoustic challenges in his article and compares them with the current and new (still in the design phase) normative specifications.
The goal regarding room acoustics of multi-person offices is to enable trouble-free work at all workplaces. In these rooms, the “human voice” is at the same time the largest source of interference, which not only reduces well-being, but is also responsible for diminished performance. This has been proven in numerous scientific studies. Up to 54% of employees in offices report noise as sometimes the most severe disruptive factor in their work , which also has a negative effect on job satisfaction. In one study , 99% of respondents stated that their concentration was impaired by noise. Especially conversations between employees and telephone rings are perceived as disturbing [1, 2]. These findings on subjectively reported disruptive effects are supported by studies that demonstrate negative effects of office noise on various cognitive functions such as concentration or memory [3, 4, 5, 6].
It is therefore clear that background noise and background speech in particular, is not only perceived as annoying or disturbing, but actually impairs performance. The common assumption that noise effects depended solely on the level of background noise cannot be maintained, especially with the focus on office activities. Studies have shown that even background noise at a level of only 40 dB (A) can cause performance degradation. The decisive factor in this context is not the level, but the intelligibility of the background speech  (see Figure 01).
In addition, noise effects on cognitive performance, subjective sensation or physiological states do not necessarily coincide and should therefore be considered separately [7, 8].
This complexity of noise effects on people in office landscapes has so far been insufficiently considered by the rules of technology. Accordingly, problems often arise, even if multi-person offices are planned and executed according to generally accepted rules of technology and thus supposedly correctly.
Multi-person offices are characterized by the antagonism between good speech intelligibility and absence of interference. These two qualities are contradictory – high speech intelligibility causes a high interference potential and vice versa. It must be accepted that there can be no confidentiality in multi-person offices. For confidentiality, individual offices are mandatory. It is necessary to find a compromise between good intelligibility and low interference potential. For this purpose, an acoustic planner has three options:
– to attenuate the room with sound-absorbing measures
– to decouple individual zones or even workstations acoustically by means of sound shields
– and to sound the room additionally (so-called masking)
The use of only one of the above-mentioned means is generally insufficient for any multiple use, which can even worsen the room acoustic situation. At least the first two measures must be considered.
2. Recommendations according to generally recognized rules of technology
The recommendations to room acoustics of multi-person offices according to the generally accepted rules of technology do not directly address speech intelligibility, but rather the measures they determine, e.g. reverberation time, sound absorption coefficient or equivalent sound absorption area of room surfaces and sound shields, dimensions of the latter and (technical) noise level.
Room attenuation can be described, for example, by the reverberation time (T60). It is defined as the time during which the sound pressure level in a room decays by 60 dB after a sound source has been switched off. The sound absorption coefficient (α) indicates how much sound energy is absorbed by a surface and therefore no longer reflected back into the room. The equivalent sound absorption area (A) is the product between the sound absorption coefficient and the area of the component (e.g. false ceiling). Usually, only technical noise levels (Lp) are considered.
This means that the noise from infrastructure (TGA, IT, copiers, etc.) in the office and the noise penetrating from outside (e.g. road traffic) are determined. The noises generated by people during their work are not considered! All of the sizes mentioned are frequency-dependent.
In  a sound level of max. 55 dB(A) is recommended for predominantly cognitive activities in the office. It refers to the outdated Workplace Ordinance. In view of the new findings, see above, that even low but substantial interference levels caused by humans reduce performance and well-being, this recommendation has become less important. On the other hand, the recommendation in  that the noise levels caused by technical building equipment systems in multi-person offices should not exceed 40 dB(A) makes sense. Very accurately  describes that the reverberation time in large multi-person offices is not a suitable criterion for the assessment and interpretation of room acoustics. It recommends an absorption area to volume ratio (A/V) of 0.3 to 0.35 m-1 (corresponds to T = 0.5 s ± 10 %) in order to reduce reverberation and sound levels of distant speakers.  even states that multi-person offices cannot be overattenuated at all. In view of the new findings under 1. this statement is questionable. If the above-mentioned recommendations on room attenuation and technical noise levels are complied with, it can be assumed that
– the reverberation is low, but the speech intelligibility is high
– near speakers are clearly perceptible
– and the distant speaker are perceived quietly but intelligibly
Sound barriers must be used to reduce the disturbing influence of office workers on each other. In , a minimum screen height of 1.5 m is recommended in order to achieve a sound level reduction between adjacent workplaces of the order of 5 – 10 dB. In practice, large zones are successfully acoustically insulated with at least 1.8 m high screens and individual workstations with 1.6 m high screens. Of course, cabinet walls can also assume the acoustic function of sound shields. For sound barriers to function well, the ceiling must be sound-absorbing.
3. Optimization of the acoustic situation
In the following, the three methods for acoustic room conditioning of open-plan offices are quantified:
3.1 Sound attenuation/absorption
Sound absorbers can be used in various places with regard to the acoustic design of the office space, in principle four groups can be distinguished:
Acoustic ceilings represent the largest group of sound absorbing materials; in most cases there is sufficient space available for acoustic optimization of these elements. In the case of thermally activated components, lamella systems or sails can be integrated, which further guarantee the thermal effect.
In general, in combination with ceiling absorbers, this is the most effective solution, as it is generally advantageous to attenuate the three room dimensions, since the sound also propagates on all sides.
Textile floor coverings contribute both to sound insulation between rooms within a building and to room acoustic optimization and reduction of noise levels.
Partitions, desk elements, cupboards or even room-in-room systems with absorbent surfaces can be used for room acoustic planning.
3.2. Sound insulation
The aim of sound shielding is to reduce levels appropriately and provide adequate protection against unintentional monitoring of conversations on neighbouring workplaces. In general, the following applies to the shielding effect: it is even better,
– the higher the canopy is compared to the room height
– the higher the absorption coefficient of the ceiling is
– the wider and higher the sound screen is
– the closer the shield connects to limiting components
– the closer it is to the speaker and listener
– and the higher the sound absorption of the screen is
The screens must be at least as wide as the table group (two tables with side furniture). Between individual workstation groups, screens should be arranged as close as possible to the speakers. It is advisable to use storage furniture between the individual table groups. Screens along the long sides of tables placed side by side are equally suitable. A screen should be absorbent on both sides (Fig. 03).
For rooms with a high communication load, it is possible that the above measures for sound absorption and shielding are no longer sufficient to reduce the intelligibility of conversations at neighbouring workplaces to a sufficient degree. Then a masking noise must be introduced . This process is adaptive, i.e. the noise level adjusts itself depending on the load in the room. The maximum masking level should not exceed 42 dB(A). A suitable masking spectrum is shown in the following diagram. Before masking systems are accepted, a phase of familiarisation and adjustment should be planned (Fig. 04).
4. Quantification of multi-person offices
DIN EN ISO 3382-3  defines new acoustic parameters for the quantification of multi-person offices:
– Deflection distance, rD, distance from the speaker at which the speech transmission index (STI) falls below 0.50.
– Confidentiality Distance, rP (optional) – distance from the speaker at which the Speech Transmission Index (STI) falls below 0.20.
– Spatial decay rate of A-weighted sound pressure level of speech, D2,S.
– A-weighted sound pressure level of speech at a distance of 4 m, Lp,A,S,4 m.
Most open-plan offices have poor or inadequate acoustic conditions. Typical singular values in offices with poor acoustic conditions are e.g. D2,S < 5 dB, Lp,A,S,4 m > 50 dB and rD > 10 m.
Open-plan offices with good acoustic conditions are rare, but an example of target values could be D2,S ≥ 7 dB, Lp,A,S,4 m ≤ 48 dB and rD ≤ 5 m.
5. Planning and construction expenditure
The planning and structural expenditure for room acoustic measures in multi-person offices varies depending on the room acoustic classes that describe the respective suitability for use. In EVDI 2569 , the planning effort in the room acoustics classes is indicated as follows:
Room acoustics class A requires very extensive and highly effective room acoustic measures for room attenuation and reduction of sound propagation. An improvement of the room acoustic conditions beyond class A is not possible while maintaining an open office structure. With this high level of planning and construction effort, room acoustics class A is well suited for call centres and other rooms with communication-intensive uses. Room Acoustics Class B is considered to be well suited for sales, construction or administration rooms with an expenditure classified as medium. Room acoustics class C requires little planning and structural effort for effective room acoustic measures. The usage recommendations describe it as suitable for administrative rooms as well as for sales and construction (Figure 05 and Table 01, 02 and 03).
6. Conversation killer open-plan offices!?
A current study  on the influence of the workplace on human cooperation shows how the change from the individual office to the open-plan office changes the interaction of employees. The change reduces direct communication between colleagues by around 70 %, and at the same time 56 % more e-mails and 67 % more electronic instant messages were sent, which increased in volume by 75 %.
It seems that open office environments do not lead to more communication, but rather to a kind of defensive reflex on the part of employees. The researchers concluded that multi-person offices “over-stimulate”, i.e. too much information and distraction lead to the opposite result: productive exchange is reduced rather than encouraged. Physical proximity through fewer barriers, fewer doors, everything should be open, is therefore overrated as a “communication promoter”!
However, open office landscapes can only be designed in such a way that effective and efficient work is possible through clever interaction between office structure / organisation and acoustic parameters.
This text has been published at: https://www.dbz.de/artikel/dbz_Raumakustik_in_offenen_Buerolandschaften_3340579.html
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