Acoustics: The Sounds of Sustainable Design
As the field of sustainable design grows and matures, more practitioners are turning away from a field that is based only on clear-cut metrics and realizing that the fuzzier areas of human comfort and productivity are very important as well. Consequently, space acoustics are garnering growing attention in sustainable projects. Acoustics are an aspect of design that is hardly ever discussed when the design works well, but they can be a major cause of annoyance or worse when there are problems.
Different space types have different specific concerns, but acoustics have been shown to play an important role across space typology. For example, a study titled “Productivity: How Acoustics Affect Worker’s Performance in Offices and Open Areas” by David M. Sykes, Ph.D., has indicated that proper acoustic design in office environments has been shown to improve focus by 48 percent, decrease error rates by 10 percent and decrease physical symptoms of stress by 27 percent.
Similarly, American National Standards Institute (ANSI) Standard S12.60 on classroom acoustics notes that learning environments with good acoustical characteristics have been shown to make learning deeper, more sustained and less fatiguing.
In general, solutions to acoustical issues fall into three categories: absorption, blocking and covering. Absorption is used to address the intelligibility and understanding of sounds that occur within a room by controlling reverberation time and echo. Reverberation time is the period of time it takes for a sound event to become no longer audible.
The primary method of controlling reverberation times is through the introduction of absorptive and reflective surfaces and materials into the space. The more absorptive materials a space contains, the lower the space’s reverberation time will be. Designers can understand a specific material’s absorptive qualities by obtaining the material’s noise reduction coefficient (NRC). NRC uses a scale of 0 to 1, with 0 indicating perfect reflection and 1 indicating perfect absorption.
Since sound occurs at many different frequencies, the NRC value for a material takes into account the material’s performance at multiple sound frequency ranges.
Blocking is a strategy used to combat the noise transfer between spaces. There are two primary metrics or ratings for an assembly’s ability to block sound transmission. Sound transmission class (STC) is a measure of how well an assembly blocks airborne sound. STC must be established for the entire building assembly, not for individual components and then added together. STC is typically established through laboratory tests that document the decibel loss of a sound that is emitted in one space as it passes through the proposed assembly to the adjacent space.
Similarly to NRC, the STC calculation also takes into account multiple frequencies of sound. Assemblies with greater mass, more air space, resilient materials or fibrous materials like sound batt insulation will have higher STC ratings, meaning they attenuate, or block, sound better.
The other measure of sound blocking is Impact Insulation Class (IIC). As its name suggests, IIC is focused on an assembly’s ability to attenuate impact sounds such as walking or moving furniture and is especially important in multiple story buildings. IIC is calculated very similarly to how STC is calculated; it must be calculated for an entire assembly and it also takes into account multiple frequency bands.
While mass and air space impact an assembly’s IIC to some extent, IIC is primarily increased by adding fibrous and/or resilient materials. Often these materials are added as a floor finish such as carpeting, rubber or cork flooring, but resilient channels, insulation and fiber board underlayments also can be used to increase IIC. Similar to STC, an IIC of 50 or higher is considered ideal for blocking impact sounds.
Covering is the final strategy of the acoustical ABCs, and it is also often called sound masking. Sound masking raises the background noise level of a space to mask or cover unwanted sounds like a neighbor’s conversation in an open office. Sound masking can often be cheaply provided in a space through the sound produced by a constant airflow HVAC system. The trick with this strategy is to remember that a background noise level of over 45 decibels becomes a distraction, so background noise levels produced by HVAC systems should be calculated to ensure that they fall at 45 decibels or below.
A more advanced and targeted way to provide sound masking is through a specially design sound masking system that uses small speakers to admit “white noise” through a space. White noise is a random sound signal emitted at a wide range of sound frequencies.
The strategies described above can be used separately or in conjunction to address the primary acoustical concerns of a space which often vary by space type. The following chart summarizes what are likely to be the most common acoustical concerns for several common project types.
Many sustainable design certification systems now contain credits related to acoustic performance. Below is a summary of how acoustics can help you satisfy prerequisite and credit requirements in several versions of the LEED Rating Systems. Green Globes and the Collaborative for High Performance Schools (CHPS) rating systems also contain acoustical requirements and suggestions for some space types.
LEED 2009 for Schools
This version of the LEED Rating System has an acoustics prerequisite that has limits for HVAC background noise and reverberation time. There is also a credit that lowers the acceptable HVAC background noise levels and adds STC requirements for interior and exterior walls.
LEED 2009 for new construction OR LEED 2009 for commercial interiors
The base rating system does not have any acoustics-related prerequisites or credits. However, there are “pilot credits” for exterior noise control (new construction only) as well as one that has reverberation time, STC requirements and requirements for sound reinforcement and sound masking in certain situations.
LEED v4 for schools
The acoustics prerequisite requirements in this version of the rating system are very similar to those contained in LEED 2009 for Schools, but it adds an exterior noise control requirement and lowers the allowable HVAC background noise decibel level. Similarly, the related credit also lowers the allowable HVAC background noise level.
LEED v4 for new construction or LEED v4 for commercial interiors
There is a new credit for acoustic performance that limits HVAC background noise, sets STC requirements and reverberation time requirements and requires sound reinforcement and sound masking systems in certain situations.
Allison McKenzie, AIA, LEED AP, USGBC faculty is an architect and director of sustainability for SHP Leading Design in Cincinnati, Ohio.
(Caption: Primary noise concerns for elementary school classrooms are interior noise transmission, exterior noise transmission, background noise and reverberation time.)