The 35 dBA Limit in ANSI/ASA S12.60
ANSI/ASA S12.60 sets the greatest allowable background noise as a one-hour average, measured in a furnished but unoccupied room:
- Core learning spaces up to 20,000 cubic feet (566 cubic meters), which covers a typical classroom: 35 dBA, or 55 dBC.
- Core learning spaces larger than 20,000 cubic feet: 40 dBA, or 60 dBC.
- Ancillary learning spaces of any size: 40 dBA, or 60 dBC.
Background noise here means the building and its surroundings: traffic and schoolyard noise from outside, the classrooms, corridors, restrooms, and mechanical rooms next door, and heating, ventilation, plumbing, and lighting. Because it is measured in the empty room, the noise the class makes is not part of it.
What WHO Recommends
The WHO Guidelines for Community Noise set out the same level for schools:
- Classrooms: background sound no higher than 35 dB LAeq during teaching sessions. Children with hearing loss may need it lower still.
- Reverberation time in the classroom: about 0.6 seconds, and preferably lower for children with hearing loss.
- Assembly halls and school cafeterias: reverberation time under 1 second.
- Outdoor playgrounds: noise from outside sources no higher than 55 dB LAeq, the same as for outdoor living areas in the daytime.
Why 35 dB: The 15 dB Speech Margin
WHO says that to follow complicated messages, such as a lesson or a foreign language, speech should be at least 15 dB louder than the background. A casual speaking voice is about 50 dBA at 1 meter, so the background should not go above 35 dBA.
The Acoustical Society of America's booklet makes the same sum for a classroom. With useful reflections from the ceiling and walls, a teacher's voice reaches about 50 dB at the farthest seats. 50 dB of speech minus 35 dB of background leaves a 15 dB margin.
To see what dBA and dBC mean, read what dBA is.
Reverberation Time
Reverberation time is how long a sharp sound, such as a loud clap, takes to fade by 60 dB. A shorter time keeps speech sounds from blurring into each other.
ANSI/ASA S12.60 sets a maximum of 0.6 seconds for a core learning space up to 10,000 cubic feet and 0.7 seconds for one up to 20,000 cubic feet, measured in the unoccupied, furnished room at 500, 1,000, and 2,000 Hz. A classroom 40 feet long, 25 feet wide, and 9 feet high is 9,000 cubic feet, so its limit is 0.6 seconds.
Soft surfaces bring the time down: acoustic ceiling tiles, wall panels, and carpet. Carpet, or neoprene tips on chair legs, also cuts the scraping of chairs and feet.
How to Check Your Classroom
- Measure the empty room with the ventilation, lights, and projector running as they do in a lesson. The standard is about the room, not the class.
- The decibel meter shows the average level (Leq) since you started, so leave it running for a few minutes and read the average.
- Compare the reading with 35 dBA. A level well above it points to a source to track down: a fan, a projector, a corridor, or traffic.
- During lessons, the classroom noise meter shows a green, amber, and red traffic light for the noise the class makes.
A phone or laptop microphone gives an estimate, not a certified reading. See how accurate a phone decibel meter is, and the types of sound level meters used for formal measurements. The booklet recommends bringing in an acoustical consultant early when a school is designed.