How a Consensus Approach to Planning and Design Can Ensure Desired Acoustical Performance
Understanding basic acoustic space design criteria and strategies that building managers, users, planners, and designers can use to recognize and address the acoustic needs of laboratory spaces is deeply important for collaboration and efficiency in labs. This article suggests methods for minimizing the negative impact of HVAC systems and laboratory equipment often used in lab spaces.
The expectations for the acoustical performance of teaching and research labs have changed since many of the design standards were created. Older standards were based on assumptions about lab use and noise that may no longer be appropriate.
When planning for new or renovated lab space, it is important to clearly state the expected functions of the space and the required acoustical properties; this allows the HVAC design professionals to accommodate the acoustic needs.
The following four considerations are critical in laboratory noise control:
1. Understand the communication needs of the space.
2. Reference the appropriate acoustic metrics for the design and intended use.
3. Contain equipment and utility noise.
4. Integrate energy efficiency and acoustics in the HVAC design.
Each of these is discussed below.
Understand the communication needs of the space
Labs have a wide range of functional needs; however, human communication needs tend to be consistent across lab types. For example, teaching labs often function as both a full-service lab and a classroom. Since in-lab lectures and discussions are the norm, a teaching lab may need to exhibit acoustic properties similar to those found in a classroom to support all learning models. Instructional labs must also support individual work, teamwork, and a variety of presentation and discussion methods, including AV systems with or without amplified sound.
Research teams need spaces where people can easily communicate detailed information that may affect their safety and the accuracy of the research data. Scientists and lab technicians on a single team commonly speak a variety of native languages, with English as a second language. They must be able to communicate in the lab environment without confusion or fatigue. Collaboration is one of the keys to research advances, and noise should not prevent easy communication in a lab. If scientists have to move to another space to have an effective conversation, the opportunities for conversations that move the science forward are reduced.
In research labs, the current level of collaboration, team communication, and cell phone conversation requires a reevaluation of the acoustic requirements. In a typical biology, chemistry, physical science, or interdisciplinary lab, teamwork is common and often requires a setting more like an open office, in which normal conversation is an important part of the daily function.
Reference the appropriate acoustic metrics for the design and intended use
The most common measures of background noise are dBA (decibel A-weighting)—a measure of sound that de-emphasizes the very low and very high frequency components of the sound in a manner similar to the frequency response of the human ear—and NC (noise criteria)—an index of sound pressure levels based on equal loudness curves. The dBA measurement is approximately equivalent to an NC rating of +5, so, for example, a dBA measurement of 35 would roughly correspond to an NC rating of 30. The NC ratings are used herein. This article does not seek to establish the exact NC ratings required for lab spaces; rather, it suggests that readers consider lab acoustical design criteria based on a more current understanding of the verbal communication and collaboration that occurs in most labs.
For more information, the ASHRAE Handbook provides design criteria for minimizing background noise in a variety of spaces, and the ANSI/ASA S12.60-2010/Part 1 Acoustical Performance Criteria, Design Requirements, and Guidelines includes more specific criteria for learning spaces (see Additional Resources below).
During the programming phase for a new or renovated facility, the acoustic design criteria are established with reference to the dBA or NC standards noted above to provide a space that works well for its intended use. Mechanical engineers, architects, and acoustical consultants then work together to provide systems that meet the established criteria.
The established acoustic design criterion for a classroom is typically NC 25 to 35, and the criterion for a laboratory is NC 45 to 55, yet the functions are not very different. Ultimately, the acoustic needs for a teaching lab may be no different than those for a classroom.
It is important that the word “laboratory” not be used as a blanket term when programming acoustical metrics in a given space. When “laboratory” is used in a programming document, it is commonly misconstrued and interpreted as a space that includes fume hoods and/or processes that are noisy and assume a default lab NC of 55. The traditional standard of NC 55 was based on the assumption that one could not justify the cost of reducing the noise level further due to the high air-change rates and/or hood use—not on a good understanding of the communication needs in a lab. A space designed for NC 55 is near the threshold at which fatigue and stress are the natural results of trying to speak or hear normal speech over the background noise.
Using the term “laboratory” in a planning document may be unwise if the space is actually a “dry” computational lab, such as that used for bioinformatics or computing research. A typical research lab, wet or dry, could be considered acoustically similar to an open office with an NC rating of 40 or less. A computing-based lab that requires constant collaboration and teamwork throughout the day may be more appropriately treated as a conference room, which has an acoustic NC rating of 25 to 35.
Thus, more clearly defining the acoustical function of the space in addition to the NC criteria during a lab’s design phase is recommended to avoid misunderstandings that could impact the lab’s acoustical properties.
Contain equipment and utility noise
Sources of noise that must be managed include the supply air system, supply valves, and general and fume hood exhaust ducts. Equipment such as vacuum pumps, chillers, and other noisy process equipment should be housed either in a lab cabinet lined with acoustical material or in another room away from the open work environment.
Noise-producing refrigerators, freezers, incubators, centrifuges, and environmental chambers should be kept in equipment rooms or alcoves when possible. Computer server racks and their associated cooling systems should also be in enclosed spaces.
High-bay labs, robotics labs, and various types of engineering labs are often designed without ceilings to afford greater overhead clearance and flexibility to accommodate unique structure-mounted apparatuses. In spaces without acoustical ceilings, more attention must be given to additional sound reduction. Placing supply terminal units in acoustical containers or outside the lab above a corridor ceiling helps. Supply air valves for hoods will need additional sound attenuators in a ceilingfree room.
Duct noise due to excessive supply air velocity is another culprit. Maintaining a design airflow rate of 300 to 500 feet per minute (fpm) at a terminal unit is good practice to minimize unnecessary noise due to air velocity. Individual terminal units for private offices and multiple smaller terminal units in larger spaces will help keep the airflow velocity within the appropriate range for an acoustically comfortable work environment. When possible, low-flow/highperformance hoods with lower-containment airflow velocities of 60 to 80 fpm can help reduce noise when the sash is open. Radiused and/or tapered exhaust transitions and multiple exhaust outlets for hoods more than six feet wide will also help reduce noise generated by the hoods.
Air handlers can be an additional source of unwanted noise. Fan array technology that includes multiple smaller fans inside the air handler unit instead of a single, large fan helps reduce low-frequency noise and provides greater flexibility to serve changing/dynamic loads.
Supply and exhaust ductwork for labs cannot include internal acoustical lining. However, insulated, nonmetallic flexible duct often can reduce noise at the final run to a ceiling diffuser or can be used for the first few feet of a small general exhaust duct. Axial-type exhaust fans characteristically generate the least low-frequency noise of any of the fan types, given good aerodynamic flow conditions.
Integrate energy efficiency and acoustics in the HVAC design
Chilled beams and Venturi wedges are becoming more prevalent in labs with equipment-driven cooling loads and lower air-change rates. When radiant ceilings or chilled beams are used in an open office environment, they temper noise levels in the space with much less airflow than a typical variable air volume (VAV) system. Because of the higher chilled water temperature and elimination of the reheat typical for a VAV system, the chilled beam system is much more energy efficient. It is not unusual for a radiant ceiling or chilled beam system to have a background noise level of NC 20. Some believe this is a problem insofar as it limits speech privacy. Others suggest it contributes to a sense of community and collaboration.
The trend toward lower air-change rates in labs helps to reduce energy use and improve overall acoustical quality in lab spaces. Systems that monitor lab air quality and deliver fresh air on an as-needed basis also help to reduce air noise and energy use.
Perhaps one day it will be common to consider the acoustical properties of a lab as part of the typical building commissioning process. Until then, designers, planners, and building managers need to bear in mind the four considerations discussed herein when planning laboratories of all types. A consensus approach to reducing lab noise will lead to greater assurance that the desired acoustical performance will be achieved.
Note: There are very few publications on laboratory acoustics. Most of the references below are for learning spaces, but the concepts described in the documents are transferable across space types to some degree.
• ASHRAE Handbook, HVAC Applications Volume, American Society of Heating, Refrigerating and Air-Conditioning Engineers, ASHRAE 2011, Chapter 48, Table 1 http://www.ashrae.org/publications/page/158
• ANSI/ASA S12.60-2010 Part 1 Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools, Part 1 Permanent Schools. http://asastore.aip.org/shop.do?pID=594
• Acoustical Barriers to Learning/Classroom Acoustics ll, Acoustical Society of America, 2002 http://www.centerforgreenschools.org/docs/acoustical-barriers-to-learning. pdf
• Acoustic Comfort, Whole Building Design Guide, National Institute of Building Sciences www.nibs.org/
• Classroom Acoustics, Acoustical Society of America, 2000 http://acousticalsociety. org/about_acoustics/acoustics_of_classrooms