Classroom Acoustics

Have you ever wondered if acoustics in learning spaces are impacting how a student hears and retains information? Communication between teachers and students is a key component of learning. Many teachers have different communication styles and each student also processes information differently, but if the student cannot hear the information then that is another problem all together. The good news is that it is a correctable problem. This is why high performance facilities are addressing the impact acoustics have on communication between teachers and students in lecture and group activity settings.

In 2002, the American National Standards Institute created ANSI S12.60, the Acoustical Performance Criteria, Design Requirements and Guidelines for Schools. It was last updated in 2009. It provides a standard that sets specific criteria for maximum background noise and reverberation time for unoccupied classrooms.
Currently, ANSI S12.60-2002 is voluntary unless adopted by state or local jurisdiction. It has been adopted by several northeastern and some western school districts as a design standard. There are also European countries that have embraced it in addition to LEED for Schools and CHPS.

In 2010, The US Access Board, a federal agency advancing the ADA began the process of developing regulations for classrooms based on ANSI S12.60 to apply to all new classrooms nationwide in the future.

One of the key ingredients to good acoustics in a classroom that is less than 10,000 cubic feet is keeping the Reverberation Time between (0.4 – 0.6) seconds at 500 Hz. The time is increased to 0.7 seconds and greater for larger classrooms over 20,000 cubic feet) depending on the use. The specified reverberation times allow for optimum acoustic performance of a direct sound to reach the listener before the reflected sound becomes inaudible.

The Sabine Equation is used to calculate the Reverberation Time RT60

A typical classroom of 25x30 with 9 foot ceilings = 6,750 cubic feet

The Sabine Equation takes into account qualities of a typical classroom; such as the square footage areas for floor, walls, ceilings, marker boards, doors, windows, light fixtures and typical furniture; including the sound absorption coefficients of typical finishes used in a classroom. The results vary depending on the absorption qualities of the materials.

ANSI S12.60-2002, recommends that the most benefit of controlling the RT in a classroom under 10,000 cubic feet with 9 foot ceilings; is to place all the sound absorbing material on the ceiling. Part of the direct sound spoken from the teacher is being bounced (reflected) off the floor, ceiling and then the walls. At a volume less than 10 feet the critical location to absorb the sound is at the ceiling. Carpet on the floor provides very minimal absorption and mainly benefits reduction of foot traffic. In order to reduce the Reverberation Time to between (0.4 – 0.6) seconds at 500 Hz the ceiling material needs to have an NRC of 0.70 or higher. The Noise Reduction Coefficient (NRC) is an average measure of how much sound a material absorbs at 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz rounded to the nearest 0.05 when tested in accordance with ASTM C 423.

An (NRC) lower than 0.70 provides too long of an RT at 500 Hz. When the sound is reflected for too long, it is mixing earlier words with later words thus leaving the listener confused. Adults are good at using context clues to figure out what they thought they heard, but children; especially young students and those with learning disabilities or those taking a foreign language are academically affected by the missing words.

The Armstrong website offers a great alternative to calculating the Sabine Equation. It has an Interactive Reverberation Tool that automatically calculates the RT of a space after inputting the square footage and surface materials of a room. It provides numerical and audible before and after data, so you can hear the difference.

To access the tool, visit:
http://www.armstrong.com/commceilingsna/article21088.html

The Ceiling Attenuation Class (CAC) is another part of the puzzle, it blocks sound from escaping through the ceiling and being transmitting to the plenum space and over to the neighboring room, as well as mechanical noise. Air handlers are recommended to be located above spaces that are typically noisy such as cafeterias and corridors. It is preferred not to have units above classrooms, offices or libraries. A CAC of 35 or higher meets the ANSI S12.60.

Overhearing noise from a neighboring classroom seems to be a common problem for teachers. The Sound Transmission Class (STC) is a rating that measures the effectiveness of a wall (such as between classrooms, exterior and corridor walls) to block sound from escaping, just like the CAC for ceilings. The higher the STC the better the wall is at blocking sound transfer. For example, loud speech can be understood fairly well through an STC 30 wall, but should not be audible through an STC 60 wall. According to ANSI S12.60 the drywall classroom partition shall have a minimum STC of 50. This rating is achieved through the series of materials that are assembled. An STC of 50 is made up of two layers of gypsum board on each side of a metal stud with batt insulation. A cmu wall with insulation would also be acceptable. In addition, sealant is necessary to seal any penetration, air-gap, or “flanking” path that can degrade the isolation quality of a wall. Special consideration needs to be given to spaces where the noise transfer concern is other than from speech, such as mechanical equipment or music.

Due to the size of Lecture Hall spaces, these rooms benefit from a longer RT. It is recommended to have a 10’ ceiling or higher and install Gypsum board or a low NRC tile from the teaching wall into the middle of the classroom. Then provide an Acoustical Absorptive NRC = 0.75 or greater along three sides of the perimeter. In addition, acoustical wall panels on three sides of the room shall be added above 9’ to reduce echoes. This helps the direct sound reflect off the ceiling to the listener in the back of the room and then the perimeter tile and wall panels absorb it to NRC of 0.75 on the ceiling then the direct sound would not make it to the back of the room for the listener. The combination and location of the higher NRC at the perimeter creates the necessary balance.

The ceiling is the first line of defense in achieving good acoustics. Investigating how well acoustics are performing in your classrooms can provide valuable information to improving the learning environment for both teachers and students.

For the resources on this topic visit:

The School Noise/Quiet Classrooms www.classroomacoutics.org
ANSI Classroom Acoustic Standard ANSI S12.60 http://asastore.aip.org/
Armstrong www.armstrong.com/schools.