Many aspects of our busy modern times have deteriorated the public schools classroom acoustical environment, such that teaching in them is more challenging than ever (ask any public school teacher). In past times one could create a quiet and pleasant classroom with natural ventilation to a quiet outdoors, or convection heating. Absorbent ceilings (often of asbestos) were installed in the early to mid 20th century. But changes occured. Mechanically driven air conditioning became the norm. Asbestos has had to be removed, and that was not always replaced by an equivalent sound absorber, thus introducing more reverberation. Noise in the classroom became the norm. 
     Around 1995, the acoustical attributes of classrooms became a serious matter of attention to members of the Acoustical Society of America, including Mike Nixon, Dave Lubman, Lou Sutherland, and later myself. It was clear by then through studies by John Bradley of NRC Canada that speech intelligibility in rooms suffered when any amount of noise was introduced. More than a decade previous, in 1983 and 1987,  I had participated in a study on the acoustical design of building interiors, bringing to my attention the results of John Bradley of NRC Canada. He had concluded that only a reverberation time of 1/2 second or less and a noise level as low as NC-25 provided optimimum speech intelligibility in a room, especially for young listeners and in the United Sates and Canada for listeners for whom English was a second language (ESL) . A decade later, it was evident that these conclusions should become the goal for good classroom acoustical design.
    These members of the Acoustical Society then formed a Working Group under the American National Standards Institute. They first produced a 'white paper'  on classroom acoustics;

     http://home.att.net/%7Ea.campanella/ClasRox5.pdf

    Later, they produced ANSI 12.60, a standard that contains the recommended classroom acoustical design methods and objectives. It is largely a compendium of facts, data and architectural design recommendations that may be adopted in whole or in part by municipalities and school administrations to ensure that their renovated and new schools will be optimum places of learning.

    The general recommendation is that classrooms should be designed to have a background noise level of less than 35 dBA (less than NC-30) and a reverberation time of less than 1/2 second. In detail, they considered the following factors:  

1. HVAC has advanced to the point where air conditioning has been designed into and retrofitted to many old classrooms. It is now clear that  that the ideal background noise level for the teaching environment is a maximum RC/NC/NCB-25. Both address the problem of speech intelligibility across a room.
   
2. Reverberation time (RT) must be reduced to 1/2 second by installing sound absorption materiel on the walls and ceiling. In previous years, sound absorption material often contained asbestos. That has all been removed or passivated. Carpet has a lesser effect on reverberation time. Its main function is to minimize surface  generated noise (foot and chair scuffing). Carpeting may be targeted for removal, due to its propensity to harbor molds, . 
3. Urban noise has increased due to transportation evolutionary developments. The noise isolation of windows and doors to the exterior may not have been proportionately upgraded.
4. Student behavior is perturbed by social and family changes.

     Survey classrooms and teachers to identify noisy environments. Inspect these rooms for the presence of sound absorbers. An acoustical tile ceiling, or walls covered with loose fabrics or bookshelves will suffice. Determine the reverberation time if possible. If you have access to a sound level meter, determine whether the ambient noise (vacant room, normal air conditioning fan on) is not more than 30dBA. With a companion, evaluate the room-to-room sound isolation. You should not be able to carry on a conversation between rooms using your raised voices. "Open-Plan" spaces might have been inadvertently designed according to 'open-office' parameters that are NOT recommended for classroom learning spaces. Clearly, demising walls that do not extend to the ceiling will leak distracting sound from one room to another. When acoustical tile is lightweight (e.g. fiberglass), sound will purvey through them to the adjacent room despite that wall. It is preferred that the wall partition be extended to the underside of the floor or roof above. If good room-to-room sound isolation is not found, make a careful record of these facts (notebook entries). Then bring this matter up at the next PTO, PTA or School Board meeting.

     The HVAC and background noise limit of any room is determined by the intended use of that room, e.g. a private residence, bedroom, classroom, auditorium, etc. In the case of classrooms of any size, reduction down to and including RC/NC/NCB-25 is productive. The offending noise source is most commonly a ventilation fan associated with air conditioning or room air recirculation.

      Sound attenuation devices for built-in fans, and alternative air fans need to be required. The addition of acoustical tile and/or sound absorber panels for reverberation control - to achieve the 1/2 second reverberation time - will reduce background noise in any room.  Construction suppliers or manufacturers can be surveyed for these practical noise control materials. Sound absorption data (e.g. NRC) and transmission loss (TL) data are often available from manufacturers on a frequency band basis. (TL applies to windows and walls, usually of concern only for noisy outdoor environments such as near freeways and airports.

     Design and test for quiet HVAC and reverberation control often requires the services of an experienced acoustical consultant. Our successful projects include numerous school and higher education projects in Ohio, for instance,

     If you have acoustical or noise control design problems you would like to discuss, feel free to call or FAX us any time.