By Gary Madaras, PhD, acoustic specialist at Rockfon
Whenever the topic of school acoustics comes up, the discussion most frequently 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 certainly not the only one. Schools serve as a perfect example of facilities that have a wide 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, band rehearsal rooms – the list is long and varied. These rooms all 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 educational experience 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, comfort or music. The core acoustics function of a black box theatre or special education classroom is speech intelligibility. A natatorium or weight training gymnasium are for neither speech nor music, but the acoustics are still important. The people in these potentially noisy rooms require overall acoustic comfort. Other spaces in the facility could be meant for relaxation, individual concentration or privacy. An individual music practice room and choir rehearsal room are primarily for music.
Whether a room is for speech, comfort or music affects its natural shape and size. It would be challenging to achieve speech intelligibility in a classroom that is 25 feet tall. It would be equally challenging to achieve ensemble playing in an orchestral rehearsal room that is circular. Beyond, shape and size, the finishes also affect whether a room is appropriate for speech, comfort or music. Glass, metal, stone, wood and concrete are typically sound-reflective and can be used beneficially in a lecture hall to passively amplify the speaker’s voice. If they are used too liberally though, they result in excessive reverberance and can make speech difficult to understand.
Because of this intimate relationship between room size, shape, finishes, acoustics and core function, an advisable approach is to use the acoustical purpose of the room to drive its aesthetic design. The alternative is to ignore these relationships and then struggle to address the acoustics without negatively impacting the visual design intent. The former approach tends to result in a more fluid and linear design process, and ultimately, highly successful experiences inside the rooms. The latter tends to result in multiple design iterations, sacrifices and compromised acoustics.
Rooms for Speech
Rooms for speech inside educational facilities can vary in size and function from a 10-seat meeting room to a 100-seat lecture hall to a 1,000-seat theatre. The primary acoustics goal in each is high speech intelligibility. This means that listeners can hear the speaker and understand what he or she is saying. It also relates to feeling close to and connected with the speaker. This is called acoustic intimacy. In the classroom, it means students have longer attention spans and are less likely to drift away mentally from the lesson, 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. The designer controls a lot that can affect both the strength of the speech signal and quietness of the noise level.
As a speech room increases in size, it becomes more difficult to maintain speech intelligibility and more critical to optimize the acoustics of the room. It is necessary to view the room 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 farther to travel. Therefore, speech rooms should be as small as possible for the intended capacity. The goal for the shape of the room 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 come potentially from many different sources including those located on the exterior of the building, in other interior spaces or the building systems themselves. Reverberation inside the room is also considered noise when it persists too long and interferes with the intelligibility of the spoken word. To minimize the noise component of the signal-to-noise ratio, the building shell and room enclosure should attenuate noises from outside the room, the building systems should operate quietly and absorptive finishes should be used inside the room to limit the reverberation time.
For a detailed set of guidelines, refer to the American National Standards Institute’s and the Acoustical Society of America’s ANSI/ASA S12.60 Acoustical Performance Criteria, Design Requirements and Guidelines for Schools. The criteria in this standard also form the basis for the acoustical requirements in the U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) v4 and the Collaborative for High Performance Schools (CHPS) Criteria for New Construction and Renovation. These all provide designers with acoustical performance criteria and noise isolation design requirements and guidelines for schools. Sections include limiting background sound from building systems, limiting noise intrusion from outside the room and using sound absorption inside the room to limit reverberation time. Even if a school is not officially going to be a LEED or CHPS building, these standards can still serve as a basis of good acoustic design for any educational facility.
What is not covered adequately by the existing standards is guidance on the nuances of adding absorption to a speech room to optimize the speech intelligibility. These standards only list a maximum permissible reverberation time of 0.60 seconds, but do not warn what can happen if too much absorption is added or if it is added on the wrong surfaces. One of the most common mistakes designers make is to place too much sound absorption all over speech rooms thinking more is better. Unfortunately, this is why electronic audio reinforcement systems, including those geared specifically for standard classrooms, are so common. When a speech room is overly absorptive, the only sound reaching the listeners is that traveling directly from the speaker’s mouth to the listeners’ ears. This is known as the direct sound. All reflections off the architecture are absorbed. This creates an acoustically dead space and increases the perceived acoustic distance between the speaker and listeners, making them feel remote. To compensate, the speaker must raise his or her voice significantly, which over time, can lead to vocal fatigue and temporary or permanent injury.
Instead, the speaker’s voice can and should be amplified passively by the room with proper surface materials and design. Strong, early-arriving sound reflections off hard room surfaces can combine with the direct sound and amplify it significantly. If these reflections reach the listener too late in time, they can become problematic echoes or general reverberance, interfering with speech intelligibility. Therefore, any hard, sound-reflecting surfaces need to be close to the person speaking or to the listener. Surfaces that are farther away from the speaker and will not provide useful, early-arriving, reflections should be sound-absorptive to control reverberance.
For example, in a classroom, hard, sound-reflective surfaces should surround where the teacher normally stands. A reflective ceiling over this zone will project the teacher’s voice at a higher sound level out into the student seating zone. The strong reflections off the ceiling, combined with the direct sound, create a loud and clear signal. The rest of the ceiling can be specified as sound-absorptive to attenuate sound that would otherwise reverberate in the room and interfere with the original speech signal. If the ceiling of the room is relatively low (i.e., 10 feet or less), it is easy to achieve the maximum permissible reverberation time of 0.60 seconds prescribed in the standards by using moderately absorptive acoustic ceilings or treatments that have a Noise Reduction Coefficient of NRC 0.75 to 0.85. Higher NRC ratings might be needed only if the ceiling is significantly higher, but otherwise, could be too absorptive for most classrooms. This passive amplification of the teacher’s voice will increase speech intelligibility, increase attention span by creating acoustic intimacy and decrease teacher voice fatigue. This same approach can be used in medium- and large-sized speech rooms as well. If designed correctly, an audio system should not be necessary for speech amplification in rooms sizes up to 100 seats even with untrained speakers. Much larger speech rooms over 500 seats can be successful acoustically without speech amplification if the speakers are well-trained, the room is shaped correctly and the noise levels are low.
Rooms for Comfort
Many rooms in educational facilities are not used for understanding speech or appreciating music, but are still occupied by people that need to be acoustically comfortable for the room to be successful. Excessively loud and reverberant spaces are stressful and negatively affect concentration and productivity. The acoustic goals in these comfort rooms can vary from quiet relaxation or contemplation in an area of respite to individual concentration in a niche of a library. Rooms or spaces for acoustic comfort in educational facilities also include common areas, corridors, computer laboratories, cafeterias, natatoriums, gymnasiums, media centers, offices and many other types of rooms.
Acoustic comfort does not equate to silence. In fact, silence is quite uncomfortable for people and is not the goal even if it could be easily achieved. The basic design approach for comfort rooms is to provide a lot of sound-absorptive finishes and a high level of designed background sound, perhaps with the option to add foreground sound at the room occupants’ discretion.
Unlike rooms for speech, rooms for acoustic comfort 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 can base the size and shape of the room on non-acoustic drivers. For example, when a room is greatly oversized, the sound sources simply are not loud enough to energize the whole room, and it begins to simulate being outdoors with no architectural enclosure.
The amount of absorption in rooms for comfort should be maximized in extent and performance rating (NRC). Reflections off architectural surfaces are not needed to reinforce the direct sound as in speech rooms, nor to promote quality reverberance as in music rooms. 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% of the room. Walls should be absorptive as well with a minimum NRC of 0.70 for 50% of the wall area or more. Ideally, the floor also would be carpeted, but in many rooms of educational facilities, such as gymnasiums, that is not possible.
After ensuring that the building envelope and room enclosure have been designed to isolate the room from intruding noise and that the building systems are not generating annoying noise, thought should be given to designing the background sound and perhaps offering options for adding foreground sound. What will room occupants hear while using the room or space? Silence is not possible, nor desired, and noise is unacceptable. That means designed background sound is
required. It is as important to room design as the size, shape and visual aesthetics. 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 energizes and invigorates. Nature sounds soothe and relax. Sound masking is benign. An outdoor respite area may be designed with tall grasses that rustle gently in the breezes and a wind chime. A food court may be designed with a tall water feature, so nearby tables cannot eavesdrop on conversations. An administrative office area may be designed with electronic sound masking to decrease auditory distractions and improve productivity.
The designed background sound level should be significantly louder than that in speech and music rooms. Levels of 35-50+ A-weighted decibels (dBA) are required compared to levels of 20-25 dBA in large speech and music rooms. 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 noise.
Some comfort rooms in educational facilities should also be designed with options 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. The main common area at the entrance of the school may have a stage platform with nearby storage and changing rooms for planned or spontaneous performances.
Rooms for Music
Rooms for music instruction, practice, rehearsal and performance are the most critical and challenging from an acoustics perspective inside educational facilities. 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 is usually only one or two main music performance spaces, and even those are typically multi-purpose 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 diverse functions with different acoustic goals.
Due to the critical nature of music rooms, it is highly advised that designers seek out an experienced acoustics consultant, such as a member of the National Council of Acoustics Consultants (NCAC). The NCAC is an international organization committed to supporting the acoustical profession through recognizing expert acoustical consultants and engineers, promoting opportunities for peer interaction, providing a reference tool for the public to learn more about the profession and to find a consultant matched to their needs. Those seeking the design advice of a qualified and experienced acoustics consultant can search the NCAC’s membership directory in several different ways on the organization’s website (www.ncac.com).
Music rooms are generally much larger in volume relative to their occupancy than are speech rooms. The increased size is typically required, so the sound reverberates in the room making the music fuller, louder and enveloping. As room volume increases, reverberation time lengthens. Some of the most oversized music rooms are organ recital halls, which require very long reverberation times over three seconds. Due to the long reverberation, these large music rooms are not able to serve multiple functions. As a benchmark, standards require speech rooms in educational facilities to have reverberation times of 0.60-0.70 or less.
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 will help the design team and academic facility’s (owner’s) representative program the music rooms during the conceptual design phase, and provide overall massing shape and size that will work appropriately as the design progresses. Oftentimes, because the music rooms are so large and dominate in the building massing, their sizes and locations anchor the overall site and building design.
Unlike rooms for speech and comfort, when a music room is sized and shaped correctly, very little, if any, permanent sound absorption is required. If a lot of permanent sound absorption is required, either the music room is oversized and one is trying to decrease the maximum reverberation time, or the room is incorrectly shaped and one is trying to attenuate echoes. Both can be avoided with good acoustic design.
Music rooms require a different type of surface beyond sound-reflective and -absorptive. They require sound diffusive surfaces. Just as light and air distribution require uniformity across rooms, so does sound. Diffusers for sound are hard, sound-reflecting, three-dimensional wall and ceiling surface undulations ranging in depth and height and width from several inches to several feet. They can be commonly recognized as pyramidal or hemispherical solids on the ceiling of an orchestral rehearsal room or as rough textured masonry on the walls of a band room. A diffusive surface prevents all the sound energy in a wave from being redirected only in an equal and opposite direction of the wave’s incidence angle. Instead, the sound energy is more evenly distributed in space and even smeared a bit in time – both beneficial attributes. Diffusive surfaces promote a quality reverberant sound field, maintain music strength, prevent echoes, help ensemble playing and give the music an enveloping feeling.
Music strength and clarity are also important in music rooms. It is not all about quality reverberation of the appropriate duration. Clarity and strength are related to the surfaces inside the music room, for example, suspended overhead reflectors, parterre walls, stage enclosures and balcony facias. These hard, sound-reflecting or sound-diffusing surfaces are positioned close to the musicians and audience, which provide early-arriving sound reflections much like those discussed for speech rooms. These early reflections combine with the direct sound of the instruments to increase strength and clarity. These surfaces are positioned inside the music room and do not cut off room volume, which is needed to maintain reverberance.
Music rooms in educational facilities are typically multi-functional, so the varying acoustic needs must be addressed. Even a dedicated choral rehearsal room must be versatile acoustically. At times, the choir director may want the rehearsal room very controlled with a short reverberation time to focus on individual or section intonation. That is difficult to accomplish under very reverberant conditions. At other times, the director may want the room to sound more like a cathedral with a very long reverberation time while preparing the ensemble to perform at an upcoming Sunday Mass.
The main multi-purpose venue could be used for films, community lectures or rock concerts, thus requiring short reverberation times, or be used by the school or community orchestras for concerts that require much longer reverberation times. This functional and acoustical variety means that variable acoustics are oftentimes needed in music rooms in education facilities. This is achieved by sizing and shaping the room for the most reverberant conditions, and then by designing devices – such as motorized, thick, cotton velour, acoustic curtains that track into the room from storage cabinets – when a shorter reverberation time is required. Up to 75% of a music room’s wall surfaces could require variability between reflective and absorptive. Swings in reverberation time of one to two seconds or more can be achieved with this approach. There is a limit, however, to the acoustic variability that can be achieved practically. If both organ recitals and amplified rock concerts are desired, they will need to occur in different rooms.
With advances in technology, the acoustic variability can be achieved completely with electronics. The prime example is premanufactured, modular, music practice rooms. These rooms are small, typically only for one or two people, and acoustically dead with all surfaces being highly sound-absorptive. The student can select on an electronic control panel in which type of room he or she will be practicing from practically anechoic, to a jazz club or even a grand cathedral. Similar systems exist for full-size auditoriums and are sometimes used at the college and university levels of education, but rarely at the primary or secondary school levels.
The rooms and spaces inside an educational facility can be categorized by their core acoustic function: speech, comfort or music. Speech rooms need to be as small as possible with low ceilings and small volumes and include the correct amount of sound-absorptive surfaces that decrease reverberation time, but still allow early reflections that combine with the direct sound to make speech intelligible and intimate. Comfort rooms can be sized and shaped per non-acoustic drivers, but require a large extent of high-performing sound absorption overhead, on the walls and on the floor. Music room design is complex and challenging. The size and proportions are critical and typically must be determined early in the conceptual design phase. They require not only sound-reflective surfaces, but also sound-diffusive surfaces in the correct locations. Because of their diverse functionality, they require variable acoustics, whether physical or electronic or both. It is therefore advisable to include an expert in music room acoustic design on the team.