ACOUSTICS AND SCHOOLS
 "The school was established to promote learning, which is acquired largely by word of mouth and by listening. Therefore, acoustics is one of the most important physical properties that determine how well the school building can serve its primary function. Thus, the exclusion of noise and the reduction of reverberation are indispensable in adapting classrooms to the function of oral instruction." [i] ?
As an appreciation reemerges of the importance of the roll of acoustics in the classroom, it is interesting to note these words were not published recently, but 57 years ago in 1950.
Most learning occurs through correctly and clearly hearing oral presentations, particularly in the first through sixth grades. The Collaborative for High Performance Schools (CHPS) program acknowledges the importance of acoustics in creating better, more efficient facilities. The United States Access Board is a group that pushed for the adoption of a national standard for acoustic requirements in schools. The result of their efforts and many others is the "Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools", an American National Standard adopted in 2002 [ii]. This standard is expected to be adopted by California and parts of it are already in the CHPS program.
The goal of good classroom acoustics is to provide favorable speech intelligibility for all students, prevent speech interference, provide acceptable background sound levels from all sources not in the room and furnish sound decay times (reverberation time) that enhance speech, allow easy listening and reduce noisiness. These goals can easily be met with proper planning, but attention to details is required.
The plan and the work fall into 4 phases:
- Phase 1 - The first phase involves site selection, site layout and building layout.
- Phase 2 - Involves setting design goals for sound separation, background sound levels and reverberation times.
- Phase 3 - Consists of insuring that the design goals are implemented in the design effort, and then carried out during construction.
- Phase 4 - Testing or commissioning to insure that the design goals set were achieved or making modifications as necessary to meet the objectives.
PHASE 1 - Selecting an appropriate site is the first task. Because of the need to achieve specific interior sound levels, a site with prominent transportation or industrial sound sources increases the difficulty of achieving the design goal and the cost of the building. Numerous studies have shown that all other social-economic parameters being equal, children in schools near significant sound sources score lower on standardized tests. Keeping this in mind is a big first step [iii].
The second job is to orient the buildings and the most noise sensitive areas to reduce the influence of the dominant sound sources. Use natural shielding and distance to protect the most noise sensitive areas of the school.
The final task in the first phase is to layout the building with the desired adjacencies while considering the acoustical requirements for speech privacy, speech interference and background sound. Speech privacy occurs when a person in an adjacent space cannot hear enough words to have an understanding of what is being discussed. Speech interference occurs when sound from other sources prevents a person from understanding what is said by a person to whom they are listening. For example, placing the band room over the science classroom will lead to problems and increase the final costs of construction. Similarly, having the language room adjacent to a mechanical equipment room increases the cost of achieving your goals or reduces the effectiveness of the room to assist in imparting an education to the listeners. Background sound is all of the sound heard from extraneous sources that affects concentration, communication, privacy and productivity. Road traffic and mechanical equipment are examples of background sound sources.
PHASE 2 - The second phase of work is equally important for it is here that goals are set for speech privacy, speech interference, background sound levels and reverberation times in each space. Reverberation time is an echo or a measure of the time it takes sound to die out. Sound decay time is synonymous with reverberation time. These goals are converted to the respective elements of the design such as setting sound loss requirements through wall and floor/ceiling assemblies to achieve speech privacy and preventing speech interference. The ability of an assembly to prevent sound transmission between spaces is roughly estimated using single number indexes such as Sound Transmission Class (STC) or Noise Isolation Class (NIC) ratings. Specific background sound levels are set in each space for sources exterior to the building including the mechanical equipment or road traffic. Reverberation times are specified for each room type also. The goals are set based on the type of space. Classroom requirements are different from administrative offices, musical rooms, auditoriums, multi-purpose rooms, cafeterias, libraries, computer resource centers or counseling rooms. The type of activity occurring in each space and the number of people within the space are used to set goals for the various acoustic characteristics.
The ANSI 12.60 (PDF - 43KB) standard provides performance criteria for each goal. For example, for core learning spaces with volumes less than 10,000 cu.ft., the reverberation times is to be less than 0.6 seconds. A sound decay time of 0.7 seconds is allowed for volumes greater than 10,000 cu.ft. but less than or equal to 20,000 cu.ft. Background sound levels averaged over an hour are not to exceed 35 dB(A) when measured in rooms with a volume of less than or equal to 20,000 cu.ft. Walls separating core learning areas are to have an STC rating of 50, the same value required by Uniform Building Code for walls between apartments. The wall between the core learning area and the corridor is to have a minimum STC rating of 45 while the door is to have an STC rating of 30 or better. The STC ratings for walls between several other adjacencies are specified also. Each requirement is higher than standard for most school construction, so cost implications exist.
PHASE 3 - Involves insuring that the design criteria are put onto paper correctly and accurately. Only a small fraction of the possible wall and floor ceiling assemblies have been tested for their sound loss capability and some have been tested more than once. Only a trained eye will know when the STC rating quoted for an assembly is not reasonable. The California Catalog of STC and IIC Ratings [iv] is one resource that can be used to learn whether proposed walls meet the design goals, though results are not available for any tests done since about 1980. Confirmation of the exterior acoustical environment should be made before completing the design by completing field sound tests. These tests are used to identify sound sources and to show that measured conditions can be predicted based on knowledge of the sources and their location. Then predictions can be made accurately of the acoustic environment for at least 20 years in the future so that the design will meet both existing and future needs. The heating, ventilating and air-conditioning (HVAC) systems require careful acoustic and vibration analysis to insure that sound and vibration levels in all core learning spaces and other sensitive spaces will meet the background sound goals. Selection of acoustical treatment, i.e., sound absorbing material, within the rooms to meet sound decay goals requires more than a simple evaluation because the typical space has treatment only on the floor (carpet and people) and on the ceiling (acoustical tiles). This treatment scheme renders the normal evaluation methods inaccurate because sound is not evenly distributed in the room. The remedy is to include some wall treatments to meet the design goal or to use more complicated analysis tools.
Attention to detail is required during construction as penetrations can greatly diminish the acoustical value of a partition. One (1) square inch of opening in a wall significantly reduces the acoustical value of the wall, lowering the STC rating by 10 to 20 points. Thus, pipe, duct, conduit, fire sprinkler penetrations, electrical and communications boxes and gaps around doors can degrade a wall to where its acoustical value is lost and the expenditure for the wall is wasted.
PHASE 4 - Finally, the acoustical properties must be tested to insure compliance with the design goals. This is to be sure that what was requested and paid for was received. A random selection of rooms of a given type can be tested for background sound levels, sound loss through wall and floor/ceiling assemblies and sound decay times. Done correctly, changes will not be required and the learning environment will be greatly enhanced.
Because the cost of the designing and building a school typically makes up less than 10 percent of a school system's budget [v], the cost of providing good acoustical conditions is very small. These are capital costs, one-time expenditures for a facility with a lifetime of thirty or more years during which time the school will serve thousands of students with a wide range of needs. Most of the materials required to meet the acoustical needs of schools do not require special maintenance and do not have higher life-cycle costs. Just selecting the right materials and designs in the beginning is important.
Learning is about listening. Little has changed from the words written by Knudsen and Harris 57 years ago but the realization that this is true and a concerted effort is needed to achieve even the minimum conditions for good listening. Many resources are available to learn more about design goals, application and materials. Links to some of these follow the text of this article.
Resources
http://www.edfacilities.org/rl/acoustics.cfm
http://www.nonoise.org/
http://www.quietclassrooms.org/
http://www.access-board.gov
References
- V.O. Knudsen & C.M. Harris, Acoustical Designing in Architecture, Copyright 1950, John Wiley & Sons, Inc., New York, copyright 1978 Acoustical Society of America, New York.
- American National Standard, Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools, American National Standards Institute, ANSI S12.60-2002, approved 26 June 2002, available from Acoustical Society of America, Melville, NY.
- Anon., Noise: A Health Problem, United States Environmental Protection Agency Office of Noise Abatement and Control, Washington, DC, August 1978.
- R. Dupree, Catalog of STC and IIC Ratings for Wall and Floor/Ceiling Assemblies, Office of Noise Control, California Department of Health Services, Berkeley, CA.
- Anon., "Listening for Learning 3: Counting the Costs of Noisy vs. Quiet Classrooms", from United States Access Board, a federal agency committed to accessible design, Washington, DC.
- Steve Pettyjohn
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