Speech Intelligibility Index (SII) 0.2 or less gives employees speech privacy and blocks most acoustical distractions.

Sound has to be controlled in an open-plan office to create good acoustical conditions for the occupants. Without distance or floor-to-ceiling walls and doors, office designers need to use a combination of methods to increase speech privacy and reduce acoustical distractions:

• isolate sources and receivers by blocking sight lines between co-workers, equipment, etc.;
• absorb reflected sound;
• reduce diffracted sound;
• provide neutral background noise.

After a 4-year study, the COPE researchers compiled design strategies to help designers make use of all of these methods and create normal privacy (SII 0.2). Each technique is more effective when combined with others; good acoustical conditions are created by an interaction of design elements.

Elements of Open-plan Office Acoustics
In an open-plan office, designers have to isolate noise sources and receivers using the available design features:

• Ceiling and floor
• Partitions (partial-height screens)
• Workstation and Occupant Orientation
• Lighting fixtures
• Noise Masking System

All of these features play a role in the acoustical design strategies and should be used to control sound propagation. Good acoustical conditions can be created only if all workstation elements are at or near optimum levels. The Acoustical Design of conventional open-plan offices evaluates the change in speech privacy created by a change in each element, and makes it clear that few trade-offs are possible. ^[1]

Note: It is impossible to block all direct, reflection, diffraction, and transmission paths in an open-plan office because cubicle occupants move around their workstations, changing the conditions constantly. It is most effective to create good conditions where they are required: seated, standing, at desk, in front of computer, etc.

Ceiling and Floor Absorption

• Make the ceiling sound absorbent to reduce reflections. The ceiling is the largest bare surface in an open-plan office. It extends across the entire open-plan office, making it possible for sound to reflect into every workstation. COPE researches conducted experiments in mock-up open-plan offices and found that an open-plan office ceiling should have a Sound Absorption Average (SAA) rating close to 1.0 ^[2]. Noise and Vibration Control Engineering Principles and Applications suggests at least SAA of 0.9 ^[3]. Absorptive ceilings broken by baffles or coffers, may be even less reflective than unbroken ceiling planes ^[4].

  

  Figure shows mean SII values for 5 ceiling absorption values (in terms of SAA) and for both open grille lights over the centre of each workstation and a flat lens light over the separating panel. The 2.74 m by 2.74 m workstations were constructed of 1.52 m high panels faced with 50 mm absorbing foam. (Taken from COPE Report: Measurement of Sound Propagation between Mock-up Workstations)

• Place light fixtures, baffles, air supply diffusers, return grilles, and any other ceiling elements carefully to avoid sound reflection. Avoid spots that reflect sound directly from one cubicle to another, because non-absorbent elements mounted in the ceiling can compromise ceiling absorbency.
• Consider higher ceilings if the ceiling is also very absorbent. Higher ceilings have only a small effect on sound propagation but can contribute to speech privacy in some situations.
• Consider carpeting to absorb reflections and to reduce some occupant noises (typing, squeaky chair, etc.). The floor is as large as the ceiling and also reaches into all workstations. Sound propagation paths using the floor are generally blocked by partitions, desks, cabinets, chairs, and people, but carpeting can reduce the reflections that do get through. Carpet and furniture also reduce any problems created by gaps between the floor and partitions. COPE found that gaps of up to 50 mm do not affect the SII rating when the floor is carpeted ^[1]. Even gaps of 100mm are acceptable with carpeting, according to Noise and Vibration Control Engineering Principles and Applications ^[3].

Partition Material

Partitions block direct and transmitted sound. Sound transmission loss (given in STC) indicates a partition's ability to block sound. If the partition is not massive or heavy enough, it will not attenuate the sound passing through.

Use partitions with as high a Sound Transmission Class (STC) as possible. The higher the STC, the less sound will travel through the partition into the neighbouring workstation. Canadian Building Digest-139 states that partitions should be at least 1/4 pound per square foot ^[4]. Theoretically, this give the partition an STC of approximately 13; however, STC of 15 would give a good safety margin. Screens higher than 6 ft should have an STC of at least 20 ^[5]. The Acoustical design of conventional open-plan offices suggests that STC 20 is an acceptable minimum. Based on example situations, STC 15 does not provide adequate speech privacy, especially if there are other design problems in the room ^[10].

Note: It is important to make sure that the partitions have no holes in them. Gaps for electrical cords and outlets must be covered to prevent noise from getting through holes.

Partition Height

• Use higher partitions better attenuate sound. Partitions can also block vertical diffraction paths if they are high. The higher the partitions (greater angle of diffraction), the quieter the sound becomes as it travels over. Noise and Vibration Control Engineering Principles and Applications ^[3] and The Acoustical Design of conventional open-plan offices ^[1] suggest a minimum height of 1.7m (5.6 ft). ASTM document "Standard Guide for Open Office Acoustics and Applicable ASTM Standards" states that partitions lower than 1.5m (60 in) do not provide adequate speech privacy. However, partition heights greater than 2m (80 in) offer smaller and smaller improvements ^[6]. Breaking the line of sight between occupants can provide acoustic privacy where it is needed most, either seated or standing.

  

  Figure shows mean SII values for 3 workstation panel heights and for 3 different workstation plan sizes. The ceiling tile was H-B at a ceiling height of 2.74 m (9 ft) and there was a flat lens light over the separating screen. (Taken from COPE Report: Measurements of Sound propagation between Mock-up Workstations)

• Consider using very high partitions to isolate work areas from each other, instead of using them to create individual cubicles. Very high partitions are not often used in open-plan offices because they can be aesthetically displeasing, and can block electric light and daylight.
• Make free-standing partitions reasonably wide to reduce horizontal diffraction. Noise and Vibration Control Engineering Principles and Applications recommends a minimum width of 1.8m (5.9 ft) ^[14]. In general, however, horizontal diffraction is not possible with modular furniture.
  
• Use an absorbent ceiling and high partitions TOGETHER to attenuate sound, because the combination has the greatest effect on open-plan office acoustics. Changing the partition height by a small amount has a much greater effect on the SII rating when the ceiling is very absorbent.

Note: High partitions create lighting and aesthetic problems. Please refer to COPE LIGHTING and COPE WORKSTATION DESIGN for further information.

Partition Absorption

• Use absorbent partitions to attenuate reflected sound as it bounces off partitions. Noise and Vibration Control Engineering Principles and Applications ^[3] and Canadian Building Digest-139 ^[4] suggest that partitions should have an SAA rating of at least 0.8 or be covered in a material with SAA 0.7. If a partition is covered on both sides with a material having SAA 0.7, it can be given a total SAA of O.8. The Acoustical design of conventional open-plan offices recommends SAA 0.9 for partitions ^[1].
• If it is not possible to make all the partitions absorbent, then partitions between workstations and all parallel partitions must be absorbent to stop reflections bouncing off a back wall and over the partition.
• Consider covering partition edges with absorptive material to attenuate some diffracted and reflected sound.

Note: Absorbent materials are fibreglass, acoustical open-cell foams, or other porous, dense materials. Sound gets trapped in the air pockets, but cannot travel easily because of the density.

Lighting Fixtures and Placement
Light fixtures can reflect sound and make ceilings less absorbent. Both the light fixture type and placement are important. COPE researchers investigated two positions for ceiling-mounted lighting: over the centre of the workstation (CW) and over the separating partition between two workstations (OS) ^[2]. (The indirect light fixtures tested were mounted on the partitions.)

• Flat lens lights (prismatic-lensed luminaires) are undesirable because they reflect sound. COPE researchers found that they created a hard surface that reflected sound easily from one workstation into the other when placed over the separating partitions (OS). Over the centre of the workstations (CW), the reflection paths are less direct.
• Open-grille lights (parabolic-louvered luminaires) scatter sound. Over separating partitions (OS), they tend to scatter the sound away from the occupants. Over the centre of the workstation (CW), they tend to scatter the sound directly towards occupants.

  

  Figure shows mean SII values for 6 lighting configurations (including no lights) with H-B ceiling tiles at a ceiling height of 2.44 m. The 2.74 m by 2.74 m workstations were constructed of 1.52 m high panels faced with 50 mm absorbing foam. (Taken from COPE report: Measurements of Sound propagation between Mock-up workstations)

  
• Indirect luminaires may reflect sound towards occupants, depending on their shape. The fixtures tested during the COPE investigations had rounded bodies and caused some reflection. Different shapes might create different effects, and smaller fixtures might reflect less sound. The COPE researchers created their own "privacy panels" to stop sound reflection off the curved indirect light fixture. This is a creative way to improve speech privacy.

Note: The choice and placement of lighting fixtures also affects lighting issues such as glare, visibility, and luminance levels. Please refer to COPE LIGHTING for more information.

Air Supply and Return Grilles
Place ceiling-mounted air supply diffusers and return grilles carefully. They can reduce ceiling absorption if they are misplaced. They can also create sound masking noise "hot spots" (areas in which the masking noise sounds much louder). Avoid placing diffusers and grilles directly over workstations or in areas where they are likely to reflect sound into neighbouring workstations.

Note: The placement of return and supply grilles is very important for air distribution and proper ventilation. Please refer to COPE Ventilation, and consult a ventilation expert to ensure that air grilles are appropriately placed to maintain air quality.

Workstation Design and Orientation
The workstation design in an open-plan office should screen occupants from noise sources (machinery or other occupants).

• Use large workstations to increase speech privacy and reduce acoustical distractions because sound diminishes over distance. Distance is especially important if the partitions are low.
• Design workstations to isolate occupants from noise sources (co-workers, office equipment, corridors, etc.). This generally involves breaking sight lines between sources and receivers. For example, sound will travel easily across a corridor into another workstation if the openings face each other.
• Place workstation openings to avoid reflection paths. Sound reflection is very like light reflection. Imagine a billiard ball bouncing off rails, or using a mirror to see around corners; sound will bounce like the ball or mirror reflection. These visualization techniques can help identify sound reflection paths in an open-plan office. Carefully arranging the workstation openings and making vertical surfaces absorbent reduces these reflection paths.
• Consider covering vertical surfaces with an absorbent material. Reflections bounce off vertical surfaces, like furniture and pillars. Noise and Vibration Control Engineering Principles and Applications recommends a material with at least SAA 0.7 or more ^[3].
• Within workstations, orient occupants to face away from each other: Place computers and telephones in absorbent corners away from neighbouring occupants. (This technique is ineffective if the partitions are not absorbent.) Speech sounds are directional and are much louder at the front of a person.
• Place occupants close to the partitions to reduce diffracted sound. If the occupant is close to the partition, the angle of diffraction is very great and little sound will bend over the barrier. However, this technique is less effective if the partitions are low, thin, or non-absorbent and if the workstation is small.
• Ask occupants to keep partition-hung elements to a minimum or to place them on surfaces where they are least likely to reflect sound directly into neighbouring workstations. Hanging many items on partitions can marginally reduce partition sound absorption, especially if the items have hard, reflective surfaces. Things like partition-hung shelving, white or chalk boards, picture frames, etc, cover the partition's absorptive surface with a reflective surface.
• Consider the sound reflects that windows may cause. Additional panels should block reflections bouncing off windows that extend into multiple workstations. Window reflections can also be prevented by using parabolic louvers (window blinds) to scatter sound and maintain the view and sunlight provided by the window. Windows can also have recessed or tilted window baffles to reduce acoustic reflections; however, these window solutions involve significant renovation. Surfaces parallel to windows should be absorbent to stop reflections bouncing off windows and over the partitions.
• Orient computer screens away from other occupant to avoid reflections that bounce off the screen. These reflections are difficult to avoid; however, the person sitting at the computer usually blocks much of the sound.

Note: COPE Workstation Design Strategies has additional layout recommendations that can increase occupant satisfaction with the open-plan office.

Sound Masking System
A sound masking system is a method of creating neutral background noise by adding simulated ventilation sound to the office. Background noise reduces the intrusion of intermittent noises by covering up (masking) more distracting speech sounds and by reducing the contrast between the quiet office and any noise.

Once installed, the sound masking system plays a neutral (not white or pink) background noise through speakers in the ceiling. The sound spectrum (range of frequencies emitted) and sound level are controlled to effectively mask most speech sounds without becoming a distraction.

A professional must install a sound masking system because the spectrum and sound level requirements are unique for every situation. If the spectrum is incorrect, these systems can create hiss, roar, or rumble and be distracting. Also, if the masking noise is too loud, occupants will speak louder and defeat the masking effect. It is not possible to use ventilation systems for sound masking because noise levels vary as ventilation requirements vary.

A good sound masking system balances the need for high frequencies to mask sound with the annoyance factor of excessive high frequencies. The sound level should be adjusted after the masking spectrum is found. Masking noise should have a maximum level of 48 dB(A). Generally, 45 dB(A) was most widely acceptable in COPE experiments ^[7].

Once a sound masking system is installed, it should not be turned off during occupation or the silence/noise contrast will make the office with sound masking seem very noisy. It is also advisable to broadcast sound masking noise in corridors and other areas so that the masking noise in the office does not seem noisy and irritating.

Note: An acoustical professional should install the system after the office is complete to create appropriate frequency content, effective sound level, and even distribution.

Office Etiquette
Office etiquette is another way to reduce excess noise. If employees are encouraged to keep conversations, music, and other noises down, the office will be quieter. Private conference rooms and lunchrooms can be used for large meetings and social conversation, which keeps the work area quiet for concentration. Encouraging respect will also improve the social atmosphere and respond to employees' need for recognition.

Acoustical Evaluation
Hiring an acoustical consultant is the best way to create a good acoustical environment. He or she can recommend strategies specific to the space and budget of the organization. In the case of renovations, a consultant will make sure that any changes are effective and worthwhile.

Consultants test workstations for specific SII ratings and recommend solutions for any problems. To determine speech privacy, they measure the ambient noise level of the office and sound propagation between workstations. The SII rating will predict the expected occupant satisfaction rate: SII 0.2 is acceptable for normal privacy, and SII 0.1 is necessary confidential privacy.

COPE-CALC Software
Users interested in acoustical open-plan office design or in evaluating a proposed design can model sample cubicles with the COPE-CALC software. The cubicles will be evaluated according to most of the principles listed above. The user can listen to the acoustical environment, read suggestions and make changes to and compare their designs. Acoustic models can be very effective at predicting the noise and privacy levels in a given situation.

To download the software click here.

References:

1: Bradley, J.S. (2003). "The Acoustical design of conventional open plan offices." NRCC-46274. Ottawa, Canada: Institute for Research in Construction/National Research Council (IRC/NRC).

2: Bradley, J.S. & Wang C. (2001). Measurements of Sound Propagation between Mock-Up Workstations. COPE project report. Ottawa, Canada: Institute for Research in Construction/National Research Council (IRC/NRC).

3: Hodgson, M. & Warnock, A.C.C. (1992). "Noise in Rooms." Noise and Vibration Control Engineering Principles and Applications. Chapter 7. ed. LL. Beranek and I. Ver. New York: John Wiley and Sons, Inc. pp 175-201.

4: Hegvold, L.W. (1971). "CBD-139. Acoustical Design of Open-Planned Offices." Canadian Building Digest.

5: Warnock A.C.C. (1974). "CBD-164. Acoustical Effects of Screens in Landscaped Offices." Canadian Building Digest.

6: American Society of Testing and Materials (ASTM). (1998). Standard Guide for Open Office Acoustics and Applicable ASTM Standards (ASTM E1374). West Conshohocken, PA: ASTM.

7: Veitch, J.A.; Bradley, J.S.; Legault, L.M.; Norcross, S.G.; Svec, J.M. Masking Speech in Open-Plan Offices with Simulated Ventilation Noise: Noise-Level and Spectral Composition Effects on Acoustic Satisfaction, Internal Report, Institute for Research in Construction, National Research Council Canada, 846, pp. 53, Apr, 2002