Classroom Acoustics

More than three-quarters of the teachers in a recent poll gave the acoustics in their classrooms a failing grade. The problem is most commonly a “signal to noise” ratio issue, which results in poor speech intelligibility. In general terms – the room is too noisy and it takes a long time for sound to decay within the room. Studies by Maxwell and Evans (1997) have shown that students in schools with high background noise levels tested lower than students in schools with lower background noise levels.

Recently, the American National Standards Institute (ANSI) approved a new set of recommended acoustical specifications for schools (ANSI S12.60-2002). “The criteria, requirements, and guidelines of this Standard are keyed to the acoustical qualities needed to achieve a high degree of speech intelligibility in learning spaces,” according to Paul Schomer, standards director for the Acoustical Society of America (ASA), which publishes the “Classroom Acoustics Design Guide” (2000).

“If followed, this Standard removes acoustical barriers to learning,” he says. “It provides equal access to education for a sizable minority of school children in the United States who may have mild-to-moderate hearing, learning, or attention deficits, suffer frequent ear infections, have limited English language skills, or may otherwise suffer from a substandard acoustical environment. For teachers, working in a classroom that conforms to this Standard can reduce or eliminate voice strain and reduce stress.”

The new ANSI Standard covers:

– Background noise from both inside and outside the building

– Sound transmission of walls, floors, ceilings, and doors

– Impact insulation of floor-ceiling assemblies above core learning spaces

– Reverberation times or the length of time for sound to decay.

The quieter a room is, the lower its background noise level, enhancing speech intelligibility and raising the room’s “signal to noise” ratio. In the case of a teacher in a classroom, “signal” means speech and “noise” is the base noise level in the classroom without the teacher speaking. Noise is measured in Room Criteria (RC) or Noise Criteria (NC) values, which are commonly used to describe the noise from HVAC systems. The primary difference between the NC and RC curves is that the RC curves are more stringent in the low (rumbling) and high (hissy) frequencies.

When documenting the noise level in a room from the mechanical systems, all sources must be considered: How much noise is being transmitted though the wall or roof from the air conditioner unit itself? How much noise travels from the unit down the ductwork to the diffusers? Do the air diffusers create their own noise due to the airflow/air turbulence as air passes thorough the grill? And finally, how is the unit mounted to the structure and is it properly vibration isolated?

Mounting units over or in hallways, and then ducting the air into the classrooms, can resolve many of these acoustical issues. This helps to attenuate the noise from the unit. Silencers or mufflers can be installed in the duct run and acoustical lining can be added to further reduce the noise. If air quality is a concern, one may wish to install AP/Armaflex (manufactured by Armacell), instead of traditional acoustical duct lining.

Unlike noise coming from within a building, exterior noise sources are measured by the A and C weighted decibel (dBA/dBC) system. The A-weighted decibel is filtered to respond to the way we hear, while Cweighting is closer to a true flat response. According to the Standard, the building shell should control outside noise sources to levels of 35 dBA. If the exterior C-weighted sound level is more than 45 decibels above the A-weighted sound levels, the Standard requires additional acoustical mitigations, for example at schools near airports, major rail lines, or roads with a high volume of truck traffic.

Sound Isolation or Noise Reduction between classrooms is an issue that must also be addressed. In general we have found that walls should have a minimum Sound Transmission Class (STC) of 50 as has been supported by the ANSI Classroom Acoustics Standard. When addressing sound isolation between rooms it is very important to look at all of the pathways for sound to enter the room: doors, windows, penetrations of the walls by ducts, pipes and conduits, and holes made in walls by recessed electrical panels and fire extinguisher cabinets. These can all significantly reduce the Noise Reduction properties of the partition.

Another section of the Standard for Acoustical Design of Classrooms covers the build-up of noise in the room itself. This is typically measured in Reverberation Time (RT60), or the length of time it takes for sound to decay 60 decibels or to one millionth of its initial level. Reverberation can be reduced by installing sound absorbing panels or an acoustical tile ceiling system. A major design challenge in
classrooms concerns the need for wall-mounted writing surfaces to be abuse resistant while also non-reflective of sound.

The suggested Reverberation Times for classrooms are based on classroom size:
Less than 10,000 cu. ft. – 0.6 seconds

10,000 to 20,000 cu. ft. – 0.7 seconds

More than 20,000 cu. ft. – consult an expert

In addition to these specifications, the Reverberation Time for distance learning rooms should be less than 0.5 seconds, with no discrete echoes.

As school districts begin to adopt this standard, architects and engineers will need to address these issues just as they do for egress and heating and cooling building codes. While primarily directed at K-12 schools in the United States, the new Standard also could be applied internationally and at the college level. ANSI S12.60-2002 (Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools Standard) is 50 pages long. The entire document can be downloaded for $35 from the ANSI web store

This is an abridged version of the “Classroom Acoustics ANSI Standard” article for Archi-Tech magazine in their July/August 2003 issue.