Health, Wellbeing and Productivity in Offices
Research Note: Acoustics
RESEARCH NOTE - ACOUSTICS
We are particularly grateful to Trevor Keeling, Paul Appleby, Guy Newsham, Derek Clements-Croome, Ashak Nathwani and Phil Hampshire for their efforts in leading the production of this research note.
Introduction
Acoustics is often one of the lead causes of dissatisfaction with the office environment (Kim and de Dear, 2012). Sound (or noise) in a building can be characterised by its magnitude, measured on the log scale of decibels (dB), sound power and sound pressure levels (SWL and SPL). Noise has been defined by the World Health Organization as ‘unwanted sound’. Key to a room’s acoustic performance is the noise that finds its way in from outside and from neighbouring spaces such as plant rooms, via openings or transmitted through the fabric, structure or ductwork; noise generated within the space; and the influence of the space upon these. Because acoustics is so much a property of the geometry and surfaces of the room remediation can be costly. Although sometimes simple and cost effective remediation is possible such as raising background noise levels to mask more annoying speech sounds and reconfiguring space use.
The acoustic performance of a building can be specified in terms of physical design elements, such as the use of Accredited Construction Details for compliance with UK Building Regulations, or the amount of sound absorptive material per floor and ceiling surface. It can also be both specified and measured in terms of the properties of the acoustic environment such as background noise levels, noise rating/criteria, and sound level difference between offices, reverberation time, speech intelligibility or privacy factor. Finally the subjective experience of the occupants can be assessed in terms of satisfaction, acceptability, perception of privacy, acoustic quality, annoyance and loudness.
The characteristics of sound and noise transmitted within a building can have a number of effects on its occupants, including:
• Prolonged and high noise levels that can increase stress
• At the extreme of noise exposure hearing can be temporarily or permanently damaged • Speech intelligibility: i.e. the ability to hear and understand what someone says • Ability to have a private conversation
• Cognitive performance and concentration • How a space feels – i.e. its acoustic quality
Impact on occupants
Noise exposure in an office environment rarely if ever is at a level that can cause acoustic trauma or hearing damage. For example the Noise Exposure Limit in the UK is 87 decibels (dBA) of average exposure over a day or week (ref). This compares with typical background levels in an office of 40 to 50 dBA.
Prolonged noise at lower levels doesn’t damage hearing but can increase a person’s stress levels. For instance studies of those living close to transport hubs show them to have higher levels of blood pressure and stress hormones (Evans et al, 1998). Short bursts of noise can cause pulse amplitude to decrease and dilation of the pupils (Croome, 1977). These limits are especially relevant in people's homes, but also have some bearing on workplaces.
External noise sources impacting on a site will frequently be a primary consideration for acoustic design. Common sources of external noise pollution are transport and activities associated with noisy activities such as markets, factories and entertainment. A common problem with offices that rely on openable windows for ventilation is the ingress of noise and pollution from traffic and often the decision to mechanically ventilate a building can be driven by the external noise and pollution levels.
Any type of window when open will reduce the sound level entering the building by between 10 and 15dB compared with the level at the external surface of the window. If that window were located 20m from the edge of a motorway, for example, the 18h average of noise levels at the building façade would typically be around 77dB, which corresponds to between 62 and 67dB for someone sitting close to an open window. This compares with the ‘reasonable conditions for study and work requiring concentration’ recommended for an office space in BS8233 (2014) as an LAeq,T of 50dB, where LAeq,T is the average weighted sound pressure level in the room in dBA over time T.
The level of distraction an individual experiences will depend upon the task they carry out, the acoustic environment and personal cognitive characteristics (Sörqvist, 2010). A study by Banbury and Berry from 1998 'Disruption of office-related tasks by speech and office noise' found that, where habituation was not possible, there was up to 66% drop in performance for a 'memory for prose' task when participants were exposed to different types of background noise (Banbury and Berry, 1998). A follow-up study found that 99% of people surveyed across two offices reported that their concentration was impaired by office noise such as unanswered phones and background speech (Banbury and Berry, 2005).
In a laboratory study, Balazova et al. (2008) found that the speed of text typing and false detections of mistakes in a proof-reading task were affected by the acoustic exposure, indicating that tasks requiring processing of words may be affected by office noise. An analysis set out in a 2012 paper from the same department at the Technical University of Denmark revealed that a number of previous studies had found that ‘the performance of some tasks, particularly those requiring … concentration or … dealing with the processing of words, were affected by office noise…’ (Toftum et al, 2012). These Danish researchers found a significant difference between perceptions of the effect of office noise on task performance and the actual performance impact.
Perceptual load theory suggests that individuals can filter out task-irrelevant distractors when performing under high levels of ‘perceptual load’, but fail to do so when under low levels (Lavie, 1995). This suggests that unpredictable noises, meaningful noise and complicated tasks are key risk factors in noise distraction (McCoy and Evans, 2005).
Similarly Janhcke (2012) concluded that ‘open-plan office noise can have a negative impact on fatigue, motivation and performance’, the degree of which depends on the cognitive processes required.
Background noise levels should be sufficient to drown out unwanted distraction but not too loud to cause stress. In open plan offices a lot of extraneous foreground noise is expected, therefore a minimum unoccupied background noise level (Laeq,T)of 45dB is recommended; whereas for a private/cellular office a lower level is possible (40dB) (Wright, 2003). Indeed alternative servicing arrangements, such as the use of chilled beams rather than fan coil units (FCU)s, can sometimes reduce background noise levels to below target levels at which point it may be desirable to add broad spectrum background noise (white noise) to help bring it back up. This is called ‘sound masking’ and generally employs a white noise generator linked to speakers located at high level in the office.
There are many times in work when the ability to hear what another person says is critical. This is governed by the volume of the background noise, the volume of the speaker, what is being said, and the reverberation time and path characteristics of the room. A person’s ability to understand what another is saying is called speech intelligibility. People are able to understand speech even when it is quieter than background noise levels, for instance a signal to noise ratio (defined as difference between speech and background noise level in this case) of -4dB allows about 75% of spoken words to be understood (see Figure 1) (Long, 2006).
The reverberation time of a room is principally effected by the absorption characteristics of the room surfaces and the volume of the space. Surface treatments that are good for the thermal properties of a building may not be so good for reverberation. Typically exposed concrete soffits that are chosen for their thermal mass properties will require additional acoustic treatments to ensure a suitable reverberation time and a conducive acoustic environment. In these cases it is necessary to add acoustic absorbing material, often suspended from the ceiling or incorporated into luminaires.
Figure 1: results of intelligibility test. The ability to understand what is being said depends upon the signal to noise ratio and the content of the message (Long, 2006)
There are some times when not being overheard is imperative. Privacy between offices and between an office and an occupied space requires good sound insulation and moderate background noise to mask intruding speech. In cellular office layouts the minimum acceptable sound insulation between two offices is about Dw = 38 dB, where Dw is the weighted level difference between rooms. Where privacy is important the minimum sound insulation should be Dw = 48 dB although even then it is possible that voices will be heard, but the conversation will not usually be understood. Where the indoor ambient noise level is low it may be necessary to design for higher insulation values. As a rough guide, speech will be audible but not intelligible if Dw + LAeq,T> 75. Where privacy is viewed to be critical, or where the room is adjacent to a noisy space such as a kitchen, the area should comply with an enhanced privacy index of Dw + LAeq,T> 85. However good design practice is for noisier areas to be grouped together and placed away from quieter areas where workers need to concentrate or not be disturbed. Speech intelligibility is expressed in terms of the Speech Transmission Index (STI), as set out in the International Standard, ISO 3382-3: 2012, on a scale of 0 to 1.0, where an STI of 1.0 is perfectly intelligible. The distance at which intelligibility has dropped such that
privacy is assured is known as the ‘privacy distance’ and corresponds to an STI of 0.2, whilst distraction is possible up to a distance where STI is 0.5.
There is a conflict between providing the need for privacy and the desire for openness and communication. The balance of the one versus the other will change depending not only upon the type of organisation that is housed in a building but also the different tasks that a person might carry out over a day. There are several design options that can help negotiate this trade-off including personal measures such as the use of headphones, and organisational approaches, such as providing a range of different work spaces and allowing staff flexibility in their use. Part height partitions or cubicles made from acoustically absorptive materials can be used to reduce sound transmission and improve acoustic privacy between workstations, although to be effective they need to be of a height that will also cut off access to a view, which may have a negative effect on performance (see …..).
The overall feel of a room is often characterised by its reverberation time and sound absorption. These define how bustling a space will be and how crisp noises sound. Certain soundscapes, such as more natural rather than urban ones, can be restorative (Payne, 2013) and buildings can be too quiet (Clements-Croome, 2013). Some studies also indicate people find certain types of music pleasant in work environments (Sundstrom and Sundstrom, 1986) and that music can reduce stress for visitors to hospitals (Staricoff, 2004).
Vibration may be one of the mechanisms by which noise is transmitted through a structure from plant, construction machinery or road traffic, for example. It may impact on the human body through contact with a vibrating surface and individuals may be sensitive to vibration levels that are barely above levels of perception. Humans are more sensitive to vibration that is transmitted from one side of the body to the other or from back to front, rather than from foot to head. Also typically humans are three times more tolerant to long-term exposure to vibration in an office environment compared with at night in their homes and six times more tolerant if the exposure is intermittent (CIBSE, 2006; BSI, 1992; ISO, 1997).
Control and energy/resource use
The link between energy and acoustic performance of a building bridges indoor air quality, ventilation, thermal performance through the design of the building façade and structure. A building that is designed to be naturally ventilated provides a link with external noise sources which can in themselves be distracting. If windows have to be closed because this distraction becomes excessive then ventilation may be inadequate and pollution levels rise.
As discussed above the hard surfaces associated with thermally massive buildings result in high reverberation times and greater risk of noise discomfort.
Geographical and cultural differences
The amount of noise associated with office activities varies considerably, not only with the nature of the activity itself, but also with cultural differences between countries.
Innovations
A ceiling manufacturer conducted studies in which the ceiling systems were replaced with absorbent equivalents and sound masking systems. Employees in a number of companies were surveyed prior to and following these retrofits. The workers indicated that ‘freedom from auditory distractions was the most important feature in efficiently and effectively accomplishing their work tasks’. 80 percent of workers believed they would be more productive if their workspace provided more acoustical privacy and in cases where distractions from noise were reduced a 25 percent increase in the perceived quality of the work
environment was reported, with a 27 percent reduction in stress and a 20 percent increase in productivity (American Society of Interior Designers; Armstrong World Industries, Inc.; DynaSound, Inc.; Milliken and Co.; Steelcase, Inc, 2005).
Should electric vehicles become more common in the future traffic pollution and noise may be so reduced that natural ventilation could become more feasible.
Case study: University of Exeter Forum
University of Exeter built a new forum as an exciting versatile space to be used for large events for guest speakers and smaller more collegiate events. In this case the large span, lightweight roof that was required to bring daylight into the space also posed an acoustic risk, due to the potential high noise levels during rainfall. The project team took advantage of the technique of auralisation so that they could understand the noise impact of different design options. This led to the roof design using a combination of timber panels, ETFE and glass so that an efficient balance between acoustic environment and daylight potential was achieved.
Figure 2: the forum at Exeter University. The roof design was balanced to provide maximum daylight but also limit noise from rainfall.
Quick, clear communication was critical to this new build office for a banking client, especially on the trading floors. To achieve the acoustic requirements the design focused on surface finishes to promote comfortable room acoustics, separating walls and floors to control noise spill, suitable facades to ensure noise control and ventilation equipment to comply with legislative and internal noise level requirements.
Case study: Marks & Spencer the Carmine fit out
Marks & Spencer’s modern new office formed an integral part of their plan A environmental strategy. It included a considerable plant room essential to meeting the building's low carbon energy demands. The acoustic design focussed on partitioning and selection of appropriate noise control for plant, so that occupants are barely aware of the systems keeping the building functioning.
References
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Banbury S. and Berry DC. (2005) Office noise and employee concentration: Identifying causes of disruption and potential improvements. Ergonomics 48, pp 25-37
Balazova I, Clausen G. Rindel JH. Poulsen T. and Wyon DP. (2008) “Open-plan office environments: A laboratory experiment to examine the effect of office noise on human perception, comfort and office work performance.” Proceedings of Indoor Air 2008, Paper ID 703. Copenhagen, Denmark
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