Acoustic Design Performance in Green Buildings
Rendering of the Richmond Olympic Oal. The proprietary pre-fab “woodwave” panel system was designed, fabricated and installed by Structurecraft Builders Inc., and spans between the glulam arches.
by Doug Kennedy and Mike Noble .The increasing concern for energy efficiency and other green building strategies, as codified in the LEED Rating System [as well as other rating systems], considers many aspects of indoor environmental quality but does not explicitly provide credits for good acoustical design.
Acousticians, who are charged with providing an interior acoustical character which addresses the users’ needs, now find themselves faced with an acoustically challenging built environment as the building designers cope with innovative, energy efficient design strategies including day lighting, passive cooling and natural ventilation.
It is not too surprising then, that a recent survey of green buildings [mostly in the United States] indicated that the main drawbacks from the user’s perspective were poor acoustics [privacy and room acoustics deficiencies] and ventilation issues.
Energy efficient design strategies are usually built into the project brief and are central to any design solution. In contrast, many owners, architects and engineers perceive acoustic considerations as a [sometimes optional] extra to enhance the finished product if the budget permits, rather than an integral part of the design. Thus it is easy for project designers to discount acoustic design where it conflicts with the energy efficiency parameters.
Acoustic Design of Buildings
In general, we consider the acoustic performance of a space to be determined by the background noise in the space, the noise isolation from adjacent spaces and from the exterior, and the room acoustics [reverberation time and speech intelligibility]. We control these acoustic parameters by providing sufficient attenuation to the HVAC systems, by designing appropriate room boundaries, and by selecting suitable room finishes.
Green Buildings
The LEED rating system awards credits for strategies relating to sustainable sites, the efficient use of water, energy efficiency and emissions to atmosphere, the use of materials and resources, the indoor environmental quality, and innovation in the design process. These green goals often result in significant acoustic challenges related to:
- Natural ventilation - to promote better air quality, and lower energy consumption
- Radiant cooling - to reduce cooling costs and costs of moving air
- Use of green materials
Natural Ventilation
Natural ventilation often requires large openings in the building envelope which can let in noise from traffic, aircraft and other environmental noise. Natural ventilation may also result in more openness between interior spaces which can lead to a loss of acoustic privacy.
Naturally ventilated buildings can be better designed acoustically by designing air paths that are sound attenuated by labyrinths, lined elbows and silencers. However, these can reduce air-flow so much that traditional fans are required to overcome air resistance. This increases the cost and reduces overall building performance as it requires more energy.
Acoustically attenuated openings in the building envelope and acoustically attenuated passive stack systems have been successfully used in the design of naturally ventilated buildings. However, the buildings have to be designed around the requirements of both the ventilation system and the acoustic design. This includes site orientation and space planning. The ventilation systems can significantly affect both the internal and external appearance of the buildings.
Radiant Heating/Cooling
Passive heating/cooling uses concrete walls and floors to absorb heat during the day and to release it at night. Active systems provide greater control by incorporating cooling coils in the slabs to transfer heat to a central cooling system or from a central heating system. Airflow and its associated noise are consequently reduced.
Both passive and active systems require bare concrete or gypsum board surfaces for heating and cooling. Therefore, they cannot be covered with any substantial amount of sound absorbing insulation. Hence, the surfaces available for acoustic treatment are limited, and this can reduce the room acoustic quality. However, if such issues are identified early enough in the design process, appropriate trade-offs can sometimes be made.
For example, during the recent design of the Southeast False Creek Community Centre in Vancouver, a large Aerobics Studio was to utilize capillary radiant mats laid over top of a suspended gypsum board ceiling. Through discussions with the architect and mechanical consultant, it was determined that the ceiling could consist of a number of suspended gypsum board “clouds” with openings around and between the individual panels. While the suspended clouds/panels provided the required radiant area, the combined area of the openings was great enough to provide a significant amount of sound absorption.
Sound entering these openings was absorbed within the ceiling space by acoustic insulation applied to the underside of the structure. Additional acoustic treatment was still recommended, in the form of acoustic wall panels, but the recommended area of wall panels was considerably less than it would have been without the ceiling openings.
When radiant heating is used in open plan offices, the prevalence of hard, acoustically reflective surfaces can result in a lack of acoustic privacy since speech from one work station propagates very efficiently to other more distant stations by reflecting off the acoustically hard surfaces. Background noise from all sources [HVAC, office activity, exterior traffic] is also increased due to “reverberant amplification”. There are situations, however, where speech privacy may be inadequate even though sufficient acoustic treatment has been provided. This may occur, for example, in naturally ventilated spaces where exterior noise levels may be very low. In this case, electronic sound masking systems can improve privacy by increasing background noise to a more appropriate level [e.g. NC 40-45].
It is important to recognize, however, that masking noise will not resolve speech privacy problems that are caused by inadequate sound absorption within the room. Inadequate absorption often results in elevated levels of background noise, a problem that would be further exacerbated if sound masking noise is added. There are also potential energy considerations when specifying sound masking systems, thus this technology must be considered as just one aspect of an integrated approach to acoustical design.
Green Materials
Green materials are those that do not present any potential health hazards, and which may contain a proportion of recycled material, or at least can be recycled at the end of their useful lifetime.
Many acoustic finishes contain glass-fibre. Although there is little evidence to suggest that glass fibre acoustic insulation is hazardous to health and a large percentage of glass-fibre insulation is made from recycled material, all fibrous materials now tend to be viewed with suspicion, and some health care and education authorities require that no glass-fibre based materials be used in new construction projects.
This presents a difficult challenge since glass-fibre based products are routinely recommended for duct-lining to control HVAC noise, and cross-talk noise transmission via ducts. Glass-fibre ceiling tiles and acoustic panels are generally proposed to control reverberation and encourage good speech intelligibility.
It is possible to use alternative materials such as packless silencers in HVAC systems which contain no insulation, or silencers such as Vibro-Acoustics “MoldBlock Media” silencers that utilize all natural fibres.
However, “packless” silencers provide considerably less sound attenuation than conventional silencers and, as a result, they tend to be used primarily where corrosive fumes are being exhausted. The use of natural fibres such as wool or cotton as an acoustic medium in silencers is relatively new. According to one manufacturer, their natural fibre silencers are acoustically equivalent to silencers utilizing glass fibre media and they can contribute to earning credits in several of the LEED Ratings program criteria. They are, however, about 15% more expensive than conventional silencers.
Many other acoustic products utilize recycled material and have other attributes which may contribute to LEED credits under the Materials and Resources and Indoor Environmental Quality categories. Some manufacturers have specific information relating to LEED credits on their websites [e.g. Tectum roof and wall panels] but most do not and, therefore, it is not always easy to determine whether any given product might contribute to LEED credit.
Green Roofs
Green roofs are acoustically beneficial to buildings. They increase the mass of the roof, thereby improving sound isolation through the roof. They also increase the damping of the roof and thereby reduce panel resonances. Therefore, green roofs are particularly beneficial for buildings close to airports or other elevated noise sources and for acoustically critical spaces such as theatres and conference rooms that typically have no exterior windows, which are usually the “weak link” for intrusion of exterior noise. For example, the green roof on the new Vancouver Convention Centre will help to attenuate noise from float planes taking off and landing nearby.
Acceptable room acoustics is largely dependent upon achieving an appropriate reverberation time for the room use and room size. Relatively short reverberation times are desirable for good speech intelligibility whereas longer reverberation times are desirable for music.
Large rooms inevitably have longer reverberation times than small rooms but for moderate sized rooms, reverberation times should generally be 1 second or less where speech intelligibility is important and 1.5 to 2 seconds where music is important. The reverberation time is determined both by the room volume and the total amount of sound absorption within the room.
Although some sound absorption is provided by typical room finishes, it is often necessary to provide additional sound absorption through the use of acoustic ceilings, acoustic wall panels and/or carpet on the floor. Any such treatments must be compatible with the architectural design, they must be sufficiently durable and they must not interfere with the performance of HVAC and lighting systems. Material costs may also be a factor.
Inevitably, some trade-offs and compromises are necessary to satisfy all of the above requirements and it is difficult to generalize on how these sometimes conflicting requirements can best be satisfied. In this regard, a team approach is essential, preferably from the earliest stages of design.
A recent example of such an approach is the innovative roof assembly for the Richmond Olympic Skating Oval, which is currently under construction. The simplest treatment, acoustically, would have been to use fully perforated acoustic steel roof deck but to achieve the desired LEEDS credits and to promote BC wood products, there was a strong desire to use pine beetle damaged wood.
Considering the very large room volume of the Oval and the relatively short reverberation time that would be required for intelligible speech, the entire roof area would have to be sound absorptive. Since wood provides minimal sound absorption, this created some serious concerns, at least initially. However, with input from the architect, building envelope consultant and acousticians, the structural engineers developed an innovative roof design which met all of the structural, aesthetic, thermal and acoustic requirements. The design incorporated staggered 2×4 wood members laminated together in such a way that there were sufficient openings to allow reverberant sound to pass through to acoustic insulation above.
Conclusion
In conclusion, the implementation of new strategies for green buildings under LEED and other rating systems has created significant challenges for acousticians. These have not always been addressed in an effective manner especially where acoustical needs are at odds with other green building concerns explicitly identified in the rating system.
As design professionals, we must all be more proactive in promoting interior acoustics, and must also lobby for the establishment of credits for achieving a good acoustic environment.
Doug Kennedy, P.Eng and Mike Noble, M.Sc. are acoustical consultants with BKL Consultants Ltd. in Vancouver which provides specialized advice in the field of architectural acoustics. .