BY NIKLAS MOELLER
Sustainability is one of the driving forces in the field of construction today, a road that has largely been paved by the Leadership in Energy & Environmental Design (LEED®) rating system developed by the United States Green Building Council (USGBC).
However, for much of its history, LEED paid little attention to acoustics, drawing criticism from building professionals and weakening the overall performance of many green buildings. In fact, surveys performed by the Center for the Built Environment show that their occupants are generally more dissatisfied with noise control and speech privacy than those in traditional structures.
USGBC is attempting to address this weakness through LEED Building Design and Construction (BD+C) v4 and Interior Design and Construction (ID+C) v4, which broaden consideration of acoustic performance (EQc9) to include, for example, new construction and commercial interiors. The Occupant Comfort Survey credit within Building Operations and Maintenance (O+M) also requires an acoustic evaluation.
While credits draw much-needed attention to this aspect of the work environment, it is essential to have a good understanding of the basic elements needed to achieve acoustic comfort, as well as the reasons why some of the current sustainable design strategies tend to undermine them.
The ABC Rule provides a solid framework for this discussion. This acronym stands for the principal methods used by acoustic professionals to achieve effective acoustics: absorb, block and cover.
Absorptive materials reduce the volume of noises reflected back into a space, the length of time they last and the distance over which they travel. The amount of absorption in a room is generally indicated by the Reverberation Time (RT) measured within the space. Attaining a low RT is essential to reducing the echo or ‘liveliness’ of the room, which can otherwise irritate and tire out its occupants.
Because the ceiling is usually the largest uninterrupted surface in a facility, using a good absorptive tile is important. Select a ceiling tile with at least a 0.75 Noise Reduction Coefficient (NRC) for open plans. In closed space, use tiles with a high Ceiling Attenuation Class (CAC) because they are better at containing noise. Ideally, tile coverage should be uninterrupted.
However, many green buildings have open ceilings. It is important to research whether this decision will have the desired heating/cooling or cost benefits and, if an exposed structure is still desired, treat an appropriate percentage of the deck with an absorptive material sufficient to provide the RT deemed acceptable for the type of space (i.e., open or closed). Generally speaking, this strategy has an impact, as do vertical baffles. Depending on the building construction, another option is to use a perforated corrugated metal deck with an absorptive material placed behind the perforations before the concrete is poured.
Workstation panels should also be absorptive, particularly if there is no acoustical tile. If the space is narrow in order to promote natural light penetration, use absorptive panels on select walls in order to prevent noise from ricocheting between the exterior wall and the core. Use soft flooring to reduce footfall noise, at least in high traffic areas.
In open plan spaces, workstation partitions above seated head height (150 to 165 centimeters; 60 to 65 inches) are essential to attenuate the noises passing to an occupant’s nearest neighbors. If they are shorter, they will do little more than hold up the desks.
Where daylighting is a concern, use absorptive panels up to a 120-centimeter height (48 inches) and top them with 30 centimeters (12 inches) of glass or another transparent material. The top 12 inches introduces a reflective surface, but the reduction in absorption relative to the increase in blocking is an acceptable compromise. Also, ensure the panels have a high Sound Transmission Class (STC) rating and are well sealed along any joints, with no significant openings between or below them.
If there is no ceiling, build walls to the deck. If there is a suspended ceiling, walls can stop at the ceiling. Walls should have a high Sound Transmission Class (STC) rating to prevent airborne noise transmission. Do not locate penetrations such as outlets back-to-back on opposite sides of a wall. Ensure the STC rating of doors and interior windows at least meets the wall standard.
Many of these requirements also apply to demountable wall systems, which are often used to enclose spaces in green buildings. These systems may have lower STC ratings than conventional walls and the joints between the panels may provide conduits for noise. Address any gaps along the ceiling, exterior walls and floor during installation or they will easily transmit noise.
HVAC systems must also meet several criteria to avoid compromising acoustic isolation. For example, supply ducts should not connect adjoining closed rooms prior to connection to the main supply duct. Air return grills should not be placed straddling walls between closed spaces.
The ambient level in the majority of traditional offices is already too low. The use of high-efficiency heating and cooling systems means that it is generally even lower in green buildings. In these types of environments, conversations and noises can easily be heard, even from a distance, and are very disruptive to occupants.
Sound masking is the only acoustic treatment that can replenish and maintain the ambient level. This technology consists of a series of loudspeakers installed in a grid-like pattern in or above the ceiling, as well as a method of controlling their output. Though the sound the loudspeakers distribute is often compared to that of softly blowing air, it is specially engineered. It either completely covers up conversations and noises or reduces their disruptive impact by decreasing the amount of change between the baseline volume and any peaks in the space.
The generally recommended sound masking volume is between 43 and 48 dBA in open plans and from 40 to 45 dBA in private offices. In order to ensure uniform coverage – maximizing performance and occupant comfort across the entire space – the system should be designed to provide small zones (one to three loudspeakers) that are individually controllable for both volume and frequency. The system should also provide a suitable sound masking spectrum, such as the one developed by the National Research Council of Canada (NRC).
Using a sound masking system can help support other sustainable efforts, especially when included in the project’s design stage. For instance, masking increases noise isolation in open plans. Natural ventilation can be employed without affecting speech privacy and the amount of disruptions occupants experience. It can also pave the way for using demountable wall systems, contributing to the space’s flexibility and reducing waste following future renovations.
The project team should establish performance targets (e.g., for speech privacy) for their facility, particularly if pursuing LEED credit. These goals are best set together with an acoustic professional, who can also assist in product selection, make measurements and confirm compliance. However, for those who lack the budget for a consultant, this article provides a clear list of the required steps and technologies, which is far preferable to ignoring acoustics altogether.
Niklas Moeller is the vice-president of K.R. Moeller Associates Ltd., manufacturer of the LogiSon Acoustic Network sound masking system (www.logison.com). He also writes an acoustics blog at soundmaskingblog.com.