Classroom Acoustics

Building classrooms with inferior acoustics is a long-standing American tradition. I can remember re-learning the Pledge of Allegiance when “under God” was added, having not yet understood who Richard Stans was, or why the republic was for him. Later, it became clear that the phrase was “for which it stands,” not an unknown person.

While small class size may be ideal for individual attention to the student, the size, shape, and structure of the classroom is equally important. College lectures are often in large auditoria, but the professor can still be heard and understood.

The recent trend to larger church sanctuaries does not prevent the message from reaching an expanded congregation attending the service, if acoustics have been addressed and the sound system is adequate. The same principles hold true for any classroom, if the students are well-behaved and attentive.

Classrooms often have a speech intelligibility rating of 75 or less. This is the equivalent of missing every fourth word. This impossible and stressful situation faces many young students everyday. Acoustics have most often been improved for the hearing impaired.

However, bad acoustics deprive those with normal hearing but who may have learning disabilities or problems in processing sound. It is an increasing problem for those where English is a second language. Young children are less able to “fill in the blanks” due to their limited vocabulary. How many have sung “Bringing in the Sheep” not knowing what “sheaves” were?

Acoustical requirements vary with room size, shape and purpose. Where a traditional musical performance can sound much better with extra reverb (everyone sounds good singing in the shower), speech intelligibility generally requires that reverberation be significantly less than one second. 

The good news, and the bad news, is that correcting acoustics is a relatively low cost even before taking into account the toll taken by poor acoustics on learning. The bad news is how few know this.

Sound radiates in waves from a point source until it encounters obstacles. Walls and ceilings reflect sound back into the room and block sound from passing through.

Attention to wall detail during construction will prevent the distraction of outside noise (analogous to watching outside activity through a window) and is inexpensive compared to fixing after the fact. Materials that are heavy, dense, and massive block sound, but require that air be trapped to prevent them from acting like a drum, thereby defeating their purpose.

The material to avoid the “drum effect” is the same as that needed to reduce reflections, echo, and reverberation within the room: absorption. Absorption traps sound and is typically a porous material with many internal pathways. Sound must travel through the labyrinth and loses energy in the process, literally wearing itself out.

A common material for absorption in classrooms is acoustical wall covering. Although it has a lower sound absorption coefficient than the common acoustical wall panel, effectiveness is made up by having more surfaces covered. Acoustical wall coverings are often called “wall carpet,” but floor carpet, although having acoustical properties, will not always meet fire codes for vertical installations.

Completing the acoustical picture, there are two other common components to sound control: diffusion and vibration isolation.

Unlike absorption, sound diffusion reduces the intensity of sound by redirecting, spreading, and scattering it throughout the acoustical space. Diffusion devices have limited application to typical classrooms and might only find use in school band and choral rooms where ambiance is an enhancement.

Lastly, vibration isolation for HVAC and other equipment will prevent machinery vibration from traveling through the building structure and using the classroom as a “speaker” to put noise in the air.

As mentioned earlier, good student behavior and attention in class are necessary to learning. Bad acoustics make both harder. When the brain must sort the important information from background noise, and try to guess missing words, it creates stress.

Stress can be manifested as bad behavior. When someone gives up trying to understand, it may be seen as lack of attention. Removing the barriers to learning will clarify cause and effect in the classroom experience.

Installing an acoustical ceiling and adding carpet can greatly improve room acoustics. Care should be taken to ensure that ceiling tiles have a high rating. Price usually follows acoustic effectiveness, with cosmetics having some influence, also.

Acoustical wall covering wears well, is easy to install, and provides a uniform reduction in sound throughout the classroom. Acoustical wall panels require less wall surface to achieve the same acoustical effect and can be made tackable to double as bulletin boards. However, covering the panel with excessive amounts of art can defeat its acoustical purpose.

In summary, the classroom is most effective as a teaching environment when it is free of interfering sound reflections (echo and reverberation), outside noise, and internal vibration. When speech is clearly heard and understood, learning can begin. 

Nick Colleran is past-president of SPARS (Society of Professional Audio Recording Services), past president of the VPSA (Virginia Productions Services Association), a former recording artist and recording engineer. He is a principal of Acoustics First Corporation, which designs, manufactures, and distributes products to control sound and eliminate noise for commercial, residential, and industrial uses, www.acousticsfirst.com.