acoustic performance

Acoustic performance in open-concept offices

New challenges underscore roles of ceiling and walls in absorbing, blocking sound
Monday, October 31, 2016
By Gary Madaras

As walls have come down in offices to create open-concept spaces, the trend has brought welcome change, such as encouraging collaboration, but it also means that the old ways of managing noise no longer work. Distraction and a lack of privacy are real concerns that must be addressed in the acoustic design of today’s buildings.

With the popularity of open-concept spaces and use of sound-reflective and sustainable materials, facility managers and designers are looking to the ceiling for noise control. However, the notion that dual-purpose or multi-functional panels can both absorb and block sound effectively has raised confusion about the primary role of ceilings.

In reality, acoustic ceilings are good at sound absorption because they are porous, and in open spaces, the general rule is: the higher the absorption rating, the better. Modular acoustic ceilings do not have enough mass to block sound between private rooms, and penetrations for lights and air devices let sound leak through. Walls are more effective at blocking noise transfer between rooms because of their mass and solidity.

Controlling noise in today’s open building designs begins with effective noise absorption. Acoustic ceiling panels hold the potential to absorb the lion’s share of noise generated in spaces, preventing widespread disturbances and lack of productivity. This explains why the WELL Building Standard 2016 requires the ceiling in open offices to have a noise reduction coefficient (NRC) of 0.90 or higher.

Enclosed spaces, such as meeting rooms, demand noise absorption too. The maximum permissible reverberation times are decreasing in building standards and guidelines, demanding high-performing ceilings. Where low-performing ceilings are installed, the walls and floor may require additional absorption, which adds avoidable costs.

In the past, for example, a conference room with carpeting and acoustic wall panels may have only required an NRC of 0.60 for the ceiling. If the carpeting and wall panels are removed, the ceiling now needs an NRC of 0.90 to maintain the same reverberation time. To achieve successful acoustic experiences, focus on the high absorption ratings of ceiling panels.

Dual-purpose or multi-functional ceiling panels that attempt to both absorb and block sound sacrifice absorption performance for higher blocking performance. Increasing density and mass for improved sound blocking capacity decreases the porosity and open pore structure needed to absorb sound. Yet the higher blocking performance that these panels can provide is not high enough to be effective because the standards are based on the blocking levels of much heavier walls. So, if a product is able to absorb sound effectively, it cannot also block an adequate amount of noise to meet the requirements in the standards.

Lights and air distribution devices can also cause serious noise flanking paths or leaks that make the blocking properties of ceiling systems even less effective. This reduction in noise control is enough to make speech from adjoining spaces intelligible — meaning privacy is lost.

Consequently, in commercial spaces, a preferred acoustic design approach is to use the ceilings for noise absorption and the walls or other architectural components for isolation or blocking performance. One of the easiest and most cost-effective ways to comply with minimum reverberation times and keep noise from spreading through open spaces and down corridors is to use a highly sound-absorptive ceiling. When privacy between rooms is required, full-height walls or a lightweight plenum barrier will help meet the isolation, or blocking, requirements for minimum sound transmission class (STC) ratings in the acoustic standards, guidelines and building rating systems.

Start by selecting the correct sound absorption level for the ceiling panel. Then, if sound blocking is still needed, develop the right system approach to the room design. This can encompass full-height walls, lightweight plenum barriers and other noise control measures that work in conjunction with the ceiling panels to effectively block sound.

Consider:

  • What are the expectations for speech intelligibility, privacy and noise control for users of the space (i.e. sensitivity to noise)?
  • How is the room being used (i.e. How much noise can be expected inside the room and from adjacent rooms)?
  • Are there any applicable standards or regulations that need to be met?

After defining the requirements of the space, select the levels of absorption and blocking that are required — low, moderate or high. As examples:

A waiting room would have a low sensitivity to noise and a low to moderate amount of noise inside the room. As such, the waiting room would require an absorption level of NRC 0.70.

By way of comparison, an emergency room would have a high sensitivity to noise and a high amount of noise inside the room. It would require an absorption level of NRC 0.90.

An office next door to another office would have a low to medium sensitivity to noise and a low to medium amount of noise in adjacent rooms. As such, the office would require a blocking level of STC 40 to 45.

By way of comparison, a classroom next to another classroom would have a high sensitivity to noise and a medium to high amount of noise in adjacent rooms. The classroom would require a blocking level of STC 50.

Today’s approach to building acoustics is not a one-size-fits-all solution. To get the best acoustic experience, independently identify the optimal levels of absorption and blocking for a specific space to get exactly what is needed and not pay for any properties that are not.

Gary Madaras, PhD, Assoc. AIA, leads Optimized Acoustics™ at ROCKFON. He helps facility managers and designers select the appropriate acoustical ceiling products and apply them effectively.

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