Acoustics of Green Buildings with Open Plan Offices

by Basel Jurdy

The Green Building Rating System of the US Green Building Council advocates the use of daylight, natural ventilation, and passive cooling in buildings. Achieving these goals instills in owners and project teams a certain pride in the evolution to become environmentally sensitive.

For all of their admirable qualities, green buildings also tend to be noisier due to less reliance on mechanical systems, exposed walls and overhead concrete that reflect sound, open windows, and other factors. There's only so much acousticians can do without competing with the benefits and desirable functions of green buildings, especially those with open plan offices.

To maximize the benefits of a green building, it is crucial to establish acoustical expectations and requirements early in the programming phase, with the purpose of providing an interior acoustical character that addresses the users' needs.

Zone of Influence

In traditional buildings, speech privacy between open plan offices varies from poor to acceptable. Acceptable speech privacy implies that speech between cubicles may be partially or fully intelligible but could be ignored if the person was to focus on the task at hand.

The presence of sound absorptive acoustical tile ceilings, background masking noise from overhead mechanical systems, and the partition height that separates cubicles goes a long way toward achieving the desired acoustical privacy between cubicles. These factors also help limit the "zone of influence," which is how far an individual's sound intrudes on other occupants of the space, whether it's two or ten cubicles away.

Speech privacy deteriorates and the zone of influence dramatically increases when:

• Natural cooling and ventilation is used, which reduces the amount of background noise that would have been available to mask out voices.
  
  
• Radiant cooling is used whereby the concrete over the open plan office space is exposed, resulting in a "bouncing board" which can reflect up to 100 percent of noise to the adjacent cubicles. In a traditional building, an acoustical tile ceiling would have reflected 15 to 50 percent of this sound.
  
  
• Cubicle partitions are lowered to enhance the movement of natural air and the distribution of daylight among the work stations. Unfortunately, increasing the partition height is not an option in green buildings because there must be ample space for the air to circulate. Increasing the cubicle height from 48" to 60", for example, would impede the air from moving freely and cause occupant discomfort.

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Understand the Criteria

Acoustical criteria for buildings may be developed from either literature or experience. Specifications from past projects and commonly accepted guidelines are the basis of literature based criteria. The experience-based method is to evaluate user satisfaction of an existing space's acoustical performance and then set criteria based on this information.

The Articulation Index (AI) is a quantitative measure of speech intelligibility between open plan offices. While the building program may request the same AI for green buildings that is in standard buildings, attaining this value is simply not viable without the addition of electronic masking, covering the overhead concrete with sound absorptive materials, and/or raising the cubicle height, all of which might conflict with the goals of green buildings.

Therefore, it is incumbent upon owners and design teams to establish acoustical criteria before programming is written. This often means examining the acoustical performance of a space versus the AI, which may be a leap of faith for owners who have historically maintained a specific AI but don't really understand what that number actually represents.

To bridge the gap between literature and experience based criteria, acousticians and owners can walk through existing open plan offices that have an acceptable acoustical performance and measure the AI of those spaces, as well as conduct listening tests between cubicles. These are excellent opportunities to establish a relationship between the AI and the overall impression of a space. More often than not, owners are surprised that a satisfactory space has an AI of 0.37, for example, rather than a lower AI value.

Engaging an acoustical consultant early in the planning and programming stages of a green building is critical to integrate achievable criteria and avoid conflicting goals.

Staff education and behavior modification are also necessary as an organization makes the move to a green building. Teaching staff to control the level of their voice, for example, will make a green building much more effective and foster cooperation among colleagues.

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