Educational Experience: Meeting The Acoustical Standard for Schools
Extensive research over the last two decades indicates that the acoustical environments in schools interfere with education. Based on this research, a new American National Standard, titled “Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools,” was approved three years ago. The standard, ANSI S12.60, is the result of work by technical experts affiliated with the Acoustical Society of America.
Classrooms should be spaces where listening conditions are excellent so that students can learn. Three factors are important in achieving a good listening environment. The first is correct room acoustics, specifically avoiding the speech-blurring effects of reverberation. The second is good isolation of sounds from elsewhere, so as to avoid distraction from competing conversations in adjacent classrooms or interfering sound from street or air traffic. The third factor is adequately low levels of background sound from HVAC equipment. Especially for students farthest from the teacher, ventilation-system noise often masks the intelligibility of the spoken word. All three factors are addressed in good classroom designs.
Acoustical consultants regularly assist in school designs. That assistance routinely involves identifying logical acoustical criteria for educational settings. The new standard identifies criteria for all three factors that must be considered: reverberation time, sound-isolating constructions and HVAC noise levels.
Reverberation time is the time it takes sounds to die down in a room. Speech intelligibility is better in a room with a lower reverberation time. Reverberation is reduced through use of sound-absorbent finishes such as acoustical ceilings and wall finishes. Requirements for maximum reverberation times (500, 1000, and 2000 Hz octave band average) for unoccupied classrooms are .6 seconds for classrooms under 10,000 cubic feet and .7 seconds for classrooms from 10,000 to 20,000 cubic feet.
The Northeast typically has classrooms with floor areas of 750 to 850 square feet, which results in classrooms that are smaller than 10,000 cubic feet. A reverberation time calculation shows that if the entire ceiling is composed of a board achieving an Noise Reduction Coefficient (NRC)/Sound Absorption Average (SAA) of .70, the .6 second requirement will generally be met. (NRC and SAA are equivalent measures of sound absorption according to ASTM standard C423.) Pendant light fixtures suspended below the ceiling allow for the ceiling to be fully absorbent. If the older type of lay-in fixtures is used, supplemental surface treatments, such as acoustical wall panels, are required.
Sound Isolating Construction
The new standard provides minimum sound isolation requirements for constructions surrounding classrooms by adjacency. The requirements, given in terms of Sound Transmission Class (STC) values, are:
• Classroom: STC-50
• Outdoors: STC-50
• Toilet: STC-53
• Corridor: STC-45
• Music, mechanical, cafeteria, gym, pool: STC-60
STC is a speech frequency range metric.
For simple constructions, such as 8-inch concrete block walls, the STC-50 requirement between classrooms may be met with walls having surface weights exceeding about 50 psf. STC-50 can also be achieved with good gypsum board and steel stud construction. Some form of resiliency, such as resilient clips, and a cavity blanket (normal glass fiber or mineral wool insulation) are required. Besides the general improvement in performance a cavity blanket provides, the blanket also attenuates sound leaking through one face of a wall before it reaches leaks on the other face of the wall.
The one S12.60 criterion difficult to justify is the STC-50 requirement for exterior construction of classrooms. STC-50 frequently forces unnecessarily expensive glazing systems, for example 4-inch air space with two lites of laminated glass. Such constructions are sometimes needed to deal with loud transportation noise. However, universally requiring such performance puts an unnecessary financial burden on many schools.
The less stringent STC-45 for corridor walls is appropriate because corridors are generally inactive during class periods and typical classroom doors can be anticipated to provide no better than STC-30.
The STC-60 requirement for music room sound isolation is a minimum. Masonry STC-60 construction is effective for isolating music, but one needs to be careful with gypsum board construction because it typically provides poor performance at frequencies below the 125 Hz lower limit of the STC testing range.
It is prudent to place major music rehearsal and teaching spaces on grade so that slab breaks are feasible around the perimeter of these spaces. This is because with an unbroken floor slab, flanking of sound around walls through the slab will limit the effective STC value to about 55.
HVAC Noise Control
The maximum background sound levels for classrooms is 35 dBA for classrooms smaller than 20,000 cubic feet and 40 dBA for classrooms larger than 20,000 cubic feet. For corridors, the level is 45 dBA.
ANSI S12.60 sets 35 dBA as the maximum background sound level for classrooms. Based on the traditional systems that have been used in the Northeast to ventilate classrooms, achieving 35 dBA is a challenge. Many schools use under-window unit ventilators, which typically have fans that are acoustically exposed to the classroom. Such units cannot meet the 35 dBA requirement. If unit ventilators are to be used in the future, these will have to be of a radically different design.
For fully air-conditioned classrooms, schemes with potential to meet S12.60 and provide adequate cooling include central constant air volume, variable air volume and fan coil units.
Central constant air volume systems provide more opportunities for noise control but can be a space challenge because of the need for large distribution ducts in corridors. This problem is exacerbated by codes that continue to increase the amount of fresh air required in classrooms.
Because variable air volume boxes radiate noise, a box placed above a classroom ceiling is likely to require an acoustical enclosure. Such enclosures are undesirable because the boxes need to be accessible for service. Conventional VAV boxes should be located over corridors, not classrooms. This requires careful planning to accommodate the boxes and ductwork. Analyses of classroom VAV systems generally show that duct silencers or acoustical lining are needed downstream of VAV boxes serving classrooms.
New fan-powered VAV boxes are becoming available that include acoustical treatment of the return air path. It appears that 35 dBA can be achieved if the boxes are selected for flows of less than 1,200 cfm, are above a mineral tile ceiling, and have duct silencers or acoustically lined ductwork on the discharge.
Fan coil units are another option. Such units may be housed in a manufactured steel cabinet or supplied as an unhoused component to be installed in an architectural enclosure. Careful design is needed to control inlet and discharge noise as well as case-radiated noise and vibration. Frequently, fan-coil systems have ducted discharge, which provides some attenuation, but air is returned almost directly into the unit and this path is unlikely to allow 35 dBA to be met without further treatment.
A look at a pair of school aircraft-noise-abatement projects shows strategies for meeting ASNI requirements. (See boxes below.) These schools originally were served by unit ventilators. Often the optimum solution is to use fan-coil units with ducted central fresh air supplies, which allows the original fresh air openings in the walls to be closed. Alternatively, fresh air can be ducted from facade openings to new fan coils.
Planning to meet ANSI Criteria
Meeting the ANSI S12.60 criteria requires evaluation of several aspects of typical classrooms. These include:
- A reverberation analysis in frequency bands to determine if the proposed finishes will meet the reverberation time criteria.
- Selection and detailing of walls for adequate sound isolation.
- An analysis of the HVAC design for classrooms to determine compliance with the 35 dBA requirement. This analysis, which is done in frequency bands, begins with the sound rating of the air-moving devices and takes into account the acoustical influence of duct system elements.
Acoustical engineering expertise and a knowledge of the ANSI standard are needed to carry out these analyses. This is the domain of experienced acoustical consultants who are typically members of the National Council of Acoustical Consultants.
Classroom Acoustical Solutions: Example 1
In this New York City school space was limited to house fan coils. The decision was made to remove a few lockers in each classroom and provide a custom enclosure for a fan coil equal in depth to the lockers. Analysis indicated that this specific solution will meet 35 dBA. Acoustical design features for meeting 35 dBA include:
- Acoustically lined sheet metal plenums on both supply and return sides
- Low air speeds for supply and return openings to classrooms
- Acoustically designed access doors to closet
R. Kring Herbert is a principal of Ostergaard Acoustical Associates, West Orange, N.J. He is a fellow of the Acoustical Society of America and an INCE board certified noise control engineer.
Classroom Acoustical Solutions: Example 2
An aircraft noise abatement project was undertaken for a 1923 Georgian brick elementary school in northern New Jersey. The decision was made to bring in outside air through the original unit ventilator openings and duct to new fan-coil units located in closets. Features include acoustical duct lining, low air speeds and a balanced design for over-ceiling supply ductwork without the need for dampers.