October 9, 2019
by Gary Madaras, PhD
Whenever the topic of school acoustics comes up, most often the discussion centers on the ability, or inability, of students to understand their teachers inside classrooms. While this is arguably the most important acoustical aspect inside education facilities, it is not the only one. Schools serve as an example of facilities having a variety of room types and desired acoustic experiences. In addition to classrooms, schools contain gyms, auditoriums, cafeterias, media centers, offices, recording and broadcast studios, industrial shops, natatoriums, and band rehearsal rooms—the list is long and diverse. These rooms play their part in the education process. To focus just on the acoustics of the classrooms, or to treat every room and space as if it were a classroom, limits the overall acoustical and academic experiences in these facilities.
A designer or specifier should not be overwhelmed by the variety of room types in schools. The first step in the acoustic planning process is to categorize each room by its core function as one for speech, focus, activity, or music. The core acoustics function of a black box theater or special education classroom is speech intelligibility. Other spaces in the facility including administrative offices, libraries, and lounges could be meant for relaxation, individual focus, or privacy. A natatorium or weight-training gymnasium is for neither speech nor music, but the acoustics are still important. The people in these potentially noisy rooms require overall acoustic control, while keeping the activity energized. An individual music practice room and choir rehearsal room are primarily for music listening.
The core acoustic function of any room or space tends to affect its natural shape and size. It would be challenging to achieve speech intelligibility in a 8-m (25-ft) tall classroom. It would be equally challenging to achieve ensemble playing in a circular orchestral rehearsal room. Beyond shape and size, the finishes also affect a room’s acoustics. Glass, metal, stone, wood, and concrete are sound-reflective and can be used in a lecture hall to passively amplify the speaker’s voice. If those are used too liberally though, it can result in excessive reverberance and make speech difficult to understand.
Due to this intimate relationship between room size, shape, finishes, acoustics, and core function, an advisable approach is to use the acoustical purpose of the room in question to drive its aesthetic design and the specification of its surface materials. The most important factor, and the focus of this article, is the specification of sound-absorbing finishes or treatments in relation to the size of the room.
Rooms for speech
Rooms for speech inside educational facilities can vary in size and function from a 10-seat meeting space and a 100-seat lecture hall to a 1000-seat theater. The primary acoustics goal in each is high speech intelligibility. In classrooms, this means students have longer attention spans and are less likely to drift away from the lesson mentally, even when seated in the rear of the room. A well-designed speech room can hold listeners’ attention even as the speaker moves, turns away, or is blocked from direct line of sight and sound.
Making speech intelligible inside rooms requires a loud sound source and low noise levels—in other words, a high signal-to-noise ratio.
As a speech room increases in size, it becomes more difficult to maintain speech intelligibility and more critical to optimize the acoustics of the space. It is necessary to view the area as a passive amplification system. Larger speech rooms naturally have lower signal-to-noise ratios because the average listener distance is greater and the speech signal has to travel farther. Therefore, speech rooms should be as small as practical for the intended capacity. The goal for the shape of the space is to minimize the average speaker-to-listener distance. This is why Greek amphitheaters are semi-circular. The ceiling should be as low as possible to limit the volume of the room and decrease the amount of acoustic treatments needed to control reverberance. This is why a number of good speech rooms are fan-shaped with very low ceilings.
Noise inside a speech room can potentially come from many different sources including those located on the exterior of the building, in other interior spaces, or the building systems themselves. The building envelope should be designed to attenuate the level of environmental noise specific to the site. Interior partitions around speech rooms should always be full height, extending from floor to floor or the roof above. Stopping the partitions at ceiling level and leaving an open plenum above adjacent rooms is not permitted by school design standards. Also it is neither recommended nor endorsed by professional acoustics or hearing organizations. The building’s mechanical, electrical, and plumbing (MEP) systems should operate quietly by locating noisy equipment remotely from speech rooms and routing ducts and piping around, and not over speech rooms.
The most important and often overlooked source of noise interfering with speech intelligibility is reverberation, or how long sound reflects around amongst the ceiling, walls, and floor surfaces. It is the most disturbing source of noise because, unlike a truck or airplane pass-by or someone laughing and talking as they walk down the hallway, reverberation is always present, interfering with every spoken word all day long. High-performing sound absorption should be positioned overhead and in taller rooms on the upper walls as well.
For a detailed set of performance criteria and guidelines, it is advisable to refer to the American National Standards Institute/Acoustical Society of America (ANSI/ASA) S12.60, Acoustical Performance Criteria, Design Requirements and Guidelines for Schools. The criteria in this standard also form the basis of the acoustical requirements in the U.S. Green Building Council’s (USGBC’s) Leadership in Energy and Environmental Design (LEED) v4 and the Collaborative for High Performance Schools (CHPS), Criteria for New Construction and Renovation. As a minimum, these standards require an acoustic ceiling with a noise reduction coefficient (NRC) of 0.70 to 0.75 or a reverberation time (RT) no longer than 0.60 seconds.
Even if the applicable school district does not require compliance with one or more of these standards, they can still serve as a basis of good acoustic design for any educational facility. Professional acoustics and hearing organizations, such as the ASA, the American Academy of Audiology (AAA), and American Speech Language Hearing Association (ASHA), endorse and recommend that all schools comply with the requirements in the standards mentioned above.
Rooms for focus
Many rooms in educational facilities are not used by groups for understanding speech or appreciating music. Instead, occupants must work independently, concentrate, focus, or relax. Examples include computer labs, administrative offices, study lounges, and media centers. In these focus rooms, speech needs to be attenuated, not amplified. When these spaces are excessively loud and reverberant, they are very stressful. Speech carries, and distracting noises negatively affect concentration, productivity, or decompression. The acoustic goals in these focus rooms can vary from quiet relaxation or contemplation in an area of respite to individual concentration in a niche of a library or computer lab.
The basic design approach for focus rooms is to provide a lot of sound-absorptive finishes and a high level of designed background sound. Unlike rooms for speech, rooms for focus can vary in size from small and intimate to enormous relative to the function and occupant capacity of the room and still be acoustically successful. Designers and specifiers can base the size and shape of the room on non-acoustic drivers. For example, when a room is oversized, the sound sources are just simply not loud enough to energize the whole room and it begins to simulate being outdoors without any architectural enclosure.
The amount of absorption in focus rooms should be maximized in extent and performance rating (NRC). Reflections off architectural surfaces are not needed to amplify speech for intelligibility. Overhead sound absorption in the form of acoustic ceilings, islands, or baffles should have a minimum NRC of 0.90 and cover 75 to 100 percent of the room. Walls must be absorptive as well with a minimum NRC of 0.70 for 50 percent of the wall area or more. Ideally, the floor would be carpeted. While carpet is not optimal in many types of rooms in educational facilities, it is sometimes acceptable in spaces meant for focus.
In addition to implementing high-performing sound absorption, focus rooms benefit from designed background sound. It is as important as the size, shape, and absorptive treatments. Designed background sounds mask or cover up transient sounds that can disturb, distract, or annoy. They provide speech privacy by making words more difficult to hear and understand outside close proximity. Options include background music, nature sounds, water features, and electronic sound masking. Each serves a different purpose and results in a different acoustic experience. Music rejuvenates and energizes. Nature sounds sooth and relax. Electronic sound masking is benign and is best at contributing to speech privacy, for example, in an administrative area or healthcare center.
The designed background sound level should be significantly louder than permissible in speech and music rooms. Levels of 35 to 50 A-weighted decibels (dBA) or more are required compared to levels of just 20 to 25 dBA in large speech and music rooms. The unit of sound loudness is called decibel, and the A-weighting process adjusts the loudness at different frequencies to better represent how humans perceive sound. The approach is to create a low signal-to-noise ratio by absorbing any reflections off the architecture so the signal is diminished and by increasing the noise level. In this case though, the noise is beneficial background sound and not annoying noises.
Rooms for activity
Activity rooms in educational facilities can include cafeterias, natatoriums, gymnasiums, weight-training centers, and dance studios. Like focus rooms, they are not intended for groups to assemble for understanding speech or appreciating music. Unlike focus areas, activity rooms are not meant for individual concentration or relaxation either. They can, and perhaps should be, louder and more energetic than any other room type in educational facilities.
Other rooms and areas such as corridors, stairways, lobbies, and atriums also belong in the activity rooms’ category because people are generally gathering in or circulating through these spaces without need to focus or listen carefully.
The size and shape of activity rooms are determined by the functions within them. A pool or soccer field has specific size and shape requirements. The acoustic purpose does not drive them. Instead, the designer and specifier should determine how much sound absorption is required based on the room size in order to prevent excessive loudness and make announcements or sports commentary intelligible over an audio system. While more sound-absorbing materials are often required for these rooms due to their large sizes, it is less in relation to the size in speech or focus areas. Additionally, the performance level for the ceiling can be decreased to NRC 0.70 since less overall control is required. Ceiling panels with abuse-resistant faces for activity areas, at times, have lower NRC performance anyway.
Another consideration for activity rooms is the option for foreground music. A fitness center may have a permanent audio system with various source inputs, such as Bluetooth connection to students’ mobile devices. This positive auditory distraction can help long workouts seem shorter.
Rooms for music
From an acoustics perspective, rooms for music instruction, practice, rehearsal, and performance are the most critical and challenging. The primary acoustics goal is to make the music clear, full, loud, enveloping, and enjoyable. Most of the music rooms in an educational facility are for individual or small group instruction, or for practice and ensemble rehearsals. Unless the facility is a music college, there are usually only one or two main music performance spaces, and even those are typically multipurpose in nature, not dedicated music performance venues. This adds a level of complexity to the acoustic design because the rooms need to be appropriate for the different functions with varied acoustic goals.
Due to the critical nature of music rooms, it is important for designers and specifiers to seek out an experienced acoustics consultant, such as a member of the National Council of Acoustics Consultants (NCAC). NCAC is an international organization committed to supporting the acoustical profession through recognizing expert acoustical consultants and engineers, promoting opportunities for peer interaction, and providing a reference tool for the public to learn more about the profession and to find a consultant matched to their needs.
Music rooms are larger in volume relative to their occupancy than speech areas are. The increased size is required to ensure sound reverberates in the room, thereby making the music fuller, louder, and enveloping. As room volume increases, RT lengthens.
The proportions and shape of a music room’s volume are also very important. Only certain configurations will develop a quality reverberant field. While low, squatty rooms may have the needed volume, the shape of the room prevents a reverberant sound field from developing and instead, the room is plagued by echoes off the greatly spaced walls. Acoustics consultants would help the design and specification team program the music rooms during the conceptual design phase, and provide overall massing, shape, and size that will work appropriately as the design progresses. Often, because the music rooms are large and dominate the building massing, their sizes and locations anchor the overall site and building design.
When a dedicated music performance space is sized and shaped correctly, very little, if any, permanent sound absorption is required. Typically, the furniture and patrons in the seats provide adequate absorption in music rooms. If a lot of permanent sound absorption is required, either the space is oversized and one is trying to decrease the maximum RT, or the room is incorrectly shaped and the attempt is to attenuate echoes. Both can be avoided with good acoustic design. Conversely, multipurpose auditoriums for speech and music typically require more sound absorption. While it is beyond the scope of this article, music rooms also require a different type of surface known as sound diffusive. These surfaces are hard and sound reflecting, but are sculpted or shaped to disperse sound energy in specific directions. This is another reason why experienced acoustics consultants should help guide the design and material specifications.
Rooms for music practice and rehearsal require more sound absorption at higher NRC ratings above 0.80 than properly designed performance spaces. This is true especially in early education because young musicians have not yet mastered volume control. A marching band rehearsal room in a middle school requires most surfaces to be covered with NRC 0.90 or higher absorption to help control the power of the band. Another reason for installing more sound absorption in music practice and rehearsal rooms is so the music students and their teachers or directors can accurately hear subtleties of intonation and embrasure.
The rooms and spaces inside an educational facility can be categorized by their core acoustic function:
Speech rooms need to be as small as possible with low ceilings and small volumes, and include the correct amount of sound-absorptive surfaces (NRC 0.75+) that decrease RT, but still allow early reflections to make speech intelligible and intimate. Focus rooms could be sized and shaped per non-acoustic drivers, but require a large extent of high-performing sound absorption overhead (NRC 0.90), on the walls (NRC 0.70), and on the floor. The size and shape
of activity rooms are determined by the functions occurring within them. Their heights should be minimized. Moderate sound absorption (NRC 0.70) is needed overhead and on the upper walls.
Music room design is complex and challenging. The size and proportions are critical and must be determined early in the conceptual design phase. They require not only sound-reflection, but also sound-diffusive surfaces in the correct locations. Rooms for younger musicians and individual practice or group rehearsal require more sound absorption from their ceilings (NRC 0.80+) to help with volume control and the hearing subtleties of playing.
Designers and specifiers can create wonderful sounding schools, making the acoustics a positive factor in student and teacher success, by remembering most spaces fall into one of four core acoustic functions, each having different size, shape, and absorption requirements.
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