Source: Consulting-Specifying Engineer
By: Timothy Cape, CTS-D, JBA Consulting Engineers, Atlanta; and Michael Schwob, PE
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The acoustical environment in a building—good or bad—is the result of design. For optimal occupant comfort and facility functionality, attention to noise and vibration issues should be included early and throughout the building design and construction process.
- Classify the various systems that may cause noise or vibration in a building.
- Explain how to measure noise as it relates to building occupants.
- Apply noise or vibration mitigation as needed.
Building occupants have always known (sometimes after the fact, unfortunately) that the acoustical environment can be a key comfort factor, and just as important as temperature and light. Acoustics also can go beyond comfort into the realm of artistic quality—like in performance spaces, music practice environments, and recording studios.
Noise and vibration in a building are either caused by the building elements directly, as in the case of mechanical system noise, or allowed to affect the building from outdoors, as in the case of automobile or air traffic noise. Reverberation in enclosed spaces is also an important acoustical parameter determined by room shape, finish selections, the room size, and the number of people in the space.
The responsibility for the acoustical environment falls squarely on the building design team, requiring attention to noise sources, such as mechanical, electrical, and plumbing (MEP) systems, as well as architectural elements that both enclose spaces and isolate noise.
Acoustical aspects of a building are being formally recognized as a key factor in indoor environmental quality as a part of certifications such as U.S. Green Building Council LEED, BEAM Plus, and other design guidelines, and the building design process must incorporate appropriate attention to these acoustical criteria.
The vocabulary of acoustics
Just as in architecture, MEP, and other building trades, there is a large body of jargon that may not be understood by those not in the particular trade. This is also true of acoustics. We won’t define all of the acronyms and specialty terms here, but it is important to have a good understanding of what the broad common terms mean in the context of acoustical design in a building.
Four key terms are discussed: noise, vibration, reverberation, and speech intelligibility. These are the basic acoustical human-comfort parameters, just as temperature, humidity, and air movement are basic human-comfort factors for mechanical system design.
Noise—The simple definition of noise is unwanted sound. Chillers, fans, powered air terminal units, pumps, and generators are sounds that are not welcome in excess at any time by building occupants. Even human speech can be noise. Desirable speech includes that between people inside of offices, conference rooms, and training rooms, for example. Unwanted speech comes from others outside of offices, conference rooms, and training rooms—which makes it noise.
Noise also can come from an outdoor sources, such as cooling towers and electrical generators, as well as from transportation or industrial sources like nearby airports, roads, rails, or outdoor operations, such as mining or steel processing.
Figure 1 relates some HVAC noise sources to the subjective descriptions of the noise as well as the general frequency ranges associated with these types of sources and descriptive terms.
Vibration—Vibration from mechanical equipment propagates through the building structure and then radiates from building elements; it is called structure-borne noise. It consists of structural movement that either disturbs building occupants directly (vibration of floor or table surfaces, for example) or indirectly as airborne noise radiating from a wall, floor, or ceiling. Vibration in buildings is most often caused by machines (i.e., mechanical system fans, pumps, chillers, cooling towers) and electrical system elements (i.e., generators and transformers).
Other vibration sources can be human, such as footfall noise on hard floors or moving tables and chairs in a ballroom above an office area. Vibration also can be caused by external sources, such as nearby trains or other heavy equipment outside the building.
Perceptually, vibration may be lower in frequency as compared with audible sounds, usually being below about 20 Hz (cycles per second). Vibration also can be in the audible frequency range of roughly 20 to 20,000 Hz and radiate from the structure as airborne noise.
Besides the human-comfort issues, vibration can be a functional problem in a building, ranging from disruption of electron microscopes to wobbling ceiling-mounted video projectors.
Reverberation and speech intelligibility—Reverberation is heard as sound dies away in an enclosed space. Spaces with hard surfaces will be more reverberant that ones with a large total area of soft surfaces. The accepted and standard method to measure the level of reverberation in a space is by measuring the time required for sound to decay by 60 dB after the source sound has stopped, defined as the reverberation time, and denoted as RT60.
Noise and reverberation affect speech intelligibility, which is a measure of how well words are heard between a talker and a listener in a given environment. We increase speech intelligibility when we reduce reverberation and noise.
Reverberation is reduced when a space has more absorptive surfaces, such as acoustical tile ceilings, carpeted floors, and fabric-wrapped fiberglass acoustical panels. The role of reverberation in the acoustical environment is complex; this article will focus more on noise and vibration control.
Identifying the scope of work for acoustics
In any building or space design, the scope of work for noise and vibration as well as the related architectural elements should be determined early in the design process. Just like waiting to start the mechanical system design during the contract-documents phase after the building design is almost done would be a disaster, waiting to address acoustical issues too late in the process can have similar consequences in terms of both added cost and decreased occupant comfort.
Identifying the acoustical scope of work should be done at the beginning of the project when the architectural, MEP, and other scopes of work are determined. Almost any project can benefit from attention to acoustics during design, but some project types should include acoustical design as a matter of course. Typical facilities, such as office buildings, hospitality developments, educational facilities, conference centers, medical facilities, and houses of worship, would normally include an acoustical scope of work. Museums, theaters, recording studios, secure government facilities, and other specialty facilities also require special acoustical consideration.
The acoustical scope of work includes a range of fundamental tasks that apply to almost every project. This can generally be categorized into three basic acoustic goals:
- Noise and vibration control: The reduction of noise and vibration in the building caused by either internal or external sources
- Sound isolation: Reducing the transmission of unwanted sound from one space to another
- Reverberation control: Controlling the amount of reverberation in spaces to both reduce noise and increase the intelligibility of speech for occupants in spaces.
There are, of course, other special goals that are variations or combinations of these three goals. One example is creating functional open-office spaces, where the increasing background noise and reducing the intelligibility of speech can be the goal. Even these specialty goals involve parameters associated with the basic three noted.
All of these goals then need to be related to the type of facility, the size of the facility, and the specific spaces or areas of the building that need to be addressed to create the acoustical scope of work for the project.
Designing for noise and vibration control
With goals established and scope of work defined, the design work begins. At the beginning of a project, noise- and vibration-control goals can play an important role in determining space adjacencies. As the project design progresses, more detail is developed that requires integration into the deliverables for each building trade, particularly architecture, interior design, and the MEP trades.
In the early stages of a project, noise and vibration criteria should be set so that the goals are quantifiable and provide a basis for design. The most common criteria used for noise control is the noise criterion (NC) curve, which is commonly used for rating of air diffusers and other HVAC components. An enhanced rating system called room criterion (RC) also is recommended by ASHRAE, but it is still not as common as NC. Both are defined in detail in the ASHRAE Handbook—HVAC Applications.
ASHRAE has long-standing and evolving recommendations for NC curves to be used as design goals for various common spaces. These have been refined over the last 50 years or so, and the ASHRAE list covers a wide range of room types. A few of the common space recommendations are shown in Table 1.
Vibration criteria is tied to NC, in that structure-borne noise radiated from building elements should not produce audible sound beyond the airborne NC for any given space. Vibration isolation also is intended to reduce vibration that can be felt but is below the audible spectrum. To that end, ASHRAE includes a definitive and long-standing set of recommendations for selecting the type and performance of vibration isolators based on the equipment being isolated and the structure on which it is supported.