THE IMPORTANCE OF GOOD ACOUSTICS within offices and its link to worker productivity has been known since the 1940s. It can take up to 30 minutes for a person to regain full concentration on a task after being interrupted (DeMarco & Lister 1987) and 50 percent of office workers surveyed indicated that noise stops them from being as productive as possible (GSA, 2011).
Loss of productivity is not the only issue that should concern the architect or designer. The other key acoustic issue in offices is the provision of confidentiality. Whether it is for the meeting rooms of a corporate law firm or simply a small meeting room where difficult discussions occur, confidentiality is a key requirement for any office. Therefore the aim is to design spaces which allow for a balance between achieving good levels of confidentiality and reducing disturbance.
9.1.1 Disturbance vs. confidentiality
If we attempt to control noise disturbance, particularly from external and fixed mechanical sources producing too low a level, we run the risk of creating an environment in which activity within the office becomes more disturbing and where providing confidentiality is increasingly difficult to achieve.
Figure 6.2 should be referred to when considering the balance between controlling ambient noise, i.e., the level of steady noise such as external traffic and the level of acoustic insulation provided by separating partitions. If the background noise level is decreased, the insulation value of the partition must be increased to avoid the possibility of speech in an adjoining room being understood and/or heard. If the insulation value of the partition is low, then higher non-intrusive ambient noise levels would be required to mask speech from an adjoining room.
To adjust the effects of ambient noise, we can:
alter the level of noise break-in from outside
control noise from fixed mechanical equipment and machinery associated with the building
reduce or increase the level of reverberation within a room
introduce masking noise to cover disruptive ambient sound or create more non-disruptive ambient sound
This chapter aims to provide guidance on achieving speech privacy and reducing disturbance for the office worker.
9.2.1 Performance specifications
Appendix B details suitable ambient noise levels, suitable control levels for fixed mechanical equipment, and suggested maximum reverberation times for each space. It should be noted that for many spaces a range is given for ambient and NC/NR (Noise Criterion, US; Noise Rating, UK) levels. The aim is not to create an entirely silent space but to allow for the inclusion of some non-intrusive steady noise, which can help to act as a masking source for more disturbing noises (e.g. loud speech, sudden sounds, etc.).
9.2.2 Sound insulation between spaces
Airborne sound insulation performance and impact sound insulation performance levels for offices are also given in Tables 8.3 and B.1 respectively). Requirements may differ depending upon the core activity within the office or the building type with which it is associated.
9.2.3 Control of speech noise in open-plan and large spaces
The speech transmission index, as detailed in Chapter 5, provides a performance target within a space, rather than between spaces, and in particular in open-plan or very large office spaces. The nature of the space will dictate the relevance of the speech transmission index (STI). For example, in open-plan call centers the aim would be for a low STI, in order to reduce the disturbance of a conversation by one operator affecting another operator. Conversely, for rooms where training activities take place it is preferable to have a high STI so occupants can clearly understand the spoken word. Table 9.1 outlines the subjective STI levels and references them with some suitable room types.
Table 9.1 Recommended speech transmission indices by open-plan or large room type-speech overheard between workstations or listeners (Irish, 2012)
STI | Subjective intelligibility | Subjective privacy | Suitable room type |
0–0.3 | Bad | Good | Call center |
0.3–0.45 | Poor | Reasonable | Call center |
0.45–0.6 | Fair | Poor | Interactive open-plan office |
0.6–0.75 | Good | Very poor | Seminar room Board room |
Normal speech from one person at 3 ft (1 m) would be around LAeq 57 dB, while raised speech would be around LAeq 65 dB. Within open-plan offices, speech disturbance can be controlled to a reasonable standard where the noise level from speech reduces to below LAeq 48 dB at a distance of 13 ft (4 m). This distance can be reduced when the workers face away from each other. Therefore
Two workers face-to-face, 13 ft (4 m) apart, normal speaking: < 48 dB criteria achieved.
Two workers back-to-back, 6 ft (2 m) apart, normal speaking: < 48 dB criteria achieved.
At closer distances, or with higher speech levels, additional treatments would be required, e.g., barriers or noise masking.
9.2.4 Noise from maximum noise events, rain noise, and lift/elevator noise
The ambient noise criteria given in Appendix B provide indicative standards for anonymous noise break-in and adequate control levels for most office room types. With regards to maximum noise levels from sudden or impulsive external noise sources or noise from rain or lift/elevators, guidelines are given in Table 9.2.
Table 9.2 Control levels for various noise sources – offices
Room | External sources | Rain noise | Lift/elevator noise |
LAmax(f) dB | NR/NC | LAmax(f) dB | |
Open-plan office | 55 | 50 | 55 |
Meeting room | 45 | 45 | 30 |
Reception | 55 | 50 | 50 |
Small office | 50 | 45 | 40 |
In instances where there is a preference for natural ventilation, the LAeq and LAmax levels for external sources should be relaxed by +5 dB.
9.2.5 Internal doors and openings
Table 9.3 suggests insulation levels for office doors and links them to possible door types and room locations.
Table 9.3 Suggested door specifications
Door type | Expected performance rating STC/Rw dB | Suitable location |
Glazed door no gasket or frameless 6–8 mm glass | 25–28 | Reception area, informal meeting, storage, open-plan office |
Glazed door with frame and/or gasket 6–8 mm glass | 28–30 | Reception area, informal meeting, withdrawn workspace, open-plan office |
Timber/metal door (light 4 4mm @ 27 kg/m2) | 30 | Withdrawn working, small offices, toilets, staff rooms |
Timber/metal door (heavy 54 mm @ 29 kg/m2) | 35 | Seminar rooms, confidential meeting rooms, mechanical and server rooms |
9.2.6 Flanking via glazing mullions
Sound transmission via external glazing elements between office spaces is possible. With regards to horizontal transmission, mullion specification should be set as follows:
group office space to group offices spaces: min DnF,n 45 dB
group to withdrawn or private office space: min DnF,n 53 dB
withdrawn/private office to withdrawn/private office: min DnF,n 53 dB.
The basic performance requirement can usually be achieved with the inclusion of an expanding spray foam or foam pad within a boxed mullion. The enhanced performance requirement can usually be achieved with the inclusion of two foam pads separated by a 100 mm gap within the boxed mullion.
9.2.7 Noise masking
It is often necessary to introduce background noise into an office space to improve speech privacy and reduce disturbance, particularly within open-plan offices. Electronic noise-masking systems can be used to help achieve required continuous, unobtrusive background noise levels that mask or cover intrusive sound.
Open-plan spaces: masking levels of between LAeq 45–48 dB are considered optimal.
Enclosed spaces: masking levels of between LAeq 38–42 dB are considered optimal.