National standards or guidelines for classroom acoustics are a needed response to President Clinton's Call to Action for American Education in the 21st Century. This initiative for classroom acoustic standards is timely because of the national thrust to modernize school buildings and undertake new school construction.
Poor classroom acoustics may be one reason why Johnny can't read and why Janie can't get her homework assignments down correctly. Acoustical conditions in many classrooms are unsuitable for such tasks as learning to read, to listen or to understand unfamiliar material. High noise levels and excessive reverberation frustrate and discourage students and teachers alike. Indeed, teacher surveys consistently rank noisy classrooms high on their list of frustrations. Students, in contrast, may not be fully aware that poor acoustics are contributing to their learning difficulties.
Students are not equally at risk. Normal students may function adequately in an acoustically marginal classroom. This is explained by the redundancy of language and the remarkable ability of normal ears and brains to process sounds under adverse conditions. Those with mild-to-severe hearing disabilities or learning disabilities such as ADD (Attention Deficit Disorder) are at much higher risk. So are non-native English speaking students, who represent a large and growing fraction of the American population. Very young students need better listening conditions because much of the material and vocabulary is new to them. Older students need better acoustics because they are more likely to have hearing disabilities. Each of these categories of students have a high stake in good classroom acoustics.
All students will at some time suffer from colds and other disabilities that temporarily render them somewhat hearing disabled. An acoustically good classroom design will include enough margin to compensate for transient hearing problems of normal students. If not, a student may be able to recover from a short period of hearing disability by extra study. Not so the student with a permanent hearing, learning or language disability. Even worse are the effects of poor acoustics on very young students just learning to read. The deficit in reading and language skills lost through poor classroom acoustics are cumulative and often devastating to their educational development. Many of these children become underachieving adults because of inattention to their listening needs in early years. The money that our society may have saved by tolerating poor classroom acoustics it loses in lost revenue over the victim's occupational lifetime or in higher costs for welfare, law enforcement or incarceration.
As noted above, teachers also suffer from poor classroom acoustics. Teachers are less likely to talk with students or will talk with them for shorter periods when noise levels are high. A teacher must raise her/his voice level to be heard over noise. After even a few minutes of speaking in an elevated voice to overcome noise, a teacher may have voice fatigue. When this happens day after day it may contribute to discouragement and burnout. Many teachers have limited voice power and some are hearing handicapped. Poor classroom acoustics magnify these otherwise slight limitations. Students will sense if the teacher does not understand them and may drop out.
There appears to be a wide consensus that the most satisfactory spoken communication occurs when the speech-to-noise ratio exceeds 15 dB (the speech signal exceeds the background noise by 15 decibels). Since the strength of the unaided human voice is limited, ideal listening conditions are achieved by reducing the background environmental noise levels.
According to a Swedish standard, an acoustically satisfactory classroom should have an unoccupied environmental noise level of not more than 30 dBA, and a dining room or gymnasium not more than 40 dBA (decibels). (This can be measured with a reliable sound level meter set to the "A"-weighting scale, a simplified method of rating noise that is often used.). Many American classrooms have unacceptably high noise levels of 50 dBA or more, and many American school cafeteria and gymnasia have unoccupied levels exceeding 60 dBA. Occupied levels are still higher as children's voices rise to overcome the noise of other children in a classic example of what acousticians have termed the "cocktail party effect." This is worse in cafeterias, gymnasia, and other spaces that have long "reverberation times" because their walls, floors and ceilings lack sound-absorbing materials needed to reduce reverberation. Bare walls and hard ceilings are common features of today's cash-strapped schools. (Hard, sound-reflecting walls are also common in tropical areas because of the rapid deterioration of fibrous material in humid environments.)
The Swedish standard specifies the amount of sound absorption needed to control reverberation for each type of space. This does not translate easily into terms that can be checked in American classrooms. But according to hearing experts of ASHA (American Speech and Hearing Association), an acoustically satisfactory classroom should have a reverberation time (RT) not exceeding 0.4 seconds. Many classrooms exceed that by a factor of two or three or more. Cafeterias and gymnasia are often much worse! Reverberation does not only increase room noise levels. It reduces speech intelligibility by "smearing" the speech signal over time. Reverberation reduces speech intelligibility even more for hearing disabled listeners than for normal listeners. (Reverberation time can be accurately measured by an acoustical consultant but one may get some idea of its value by clapping ones hands in the space and estimating the time it takes for the reverberation to become inaudible.)
What are the physical sources of classroom noise? Interior noise sources such as HVAC (Heating, Ventilating and Air Conditioning) are usually the main source of steady noise. Central heating and cooling are usually the quietest followed by rooftop installations. Quiet systems may be avoided because of their relatively high cost coupled with the absence of a clearly stated benefit of their quiet operation. Interior appliances such as the fans in overhead projectors and computers are another important source of interior noise. Quiet appliances will become available if school administrators learn to use their large purchasing power in the marketplace to demand quiet equipment. Exterior noise sources such as aircraft and highway traffic are also common and troublesome. A recent study at Cornell University has connected frequent aircraft flyover noise to student's reading deficiencies, poor language acquisition and worse--the learned habit of inattention to sounds. Transportation noise can be reduced by proper school and highway siting, by use of sound walls and earth berms for noise shielding, and by special design of the building shell to make it resistant to noise.
Why do so many American classrooms have poor acoustics? Unlike Sweden and
some other countries which have taken the lead in this area, America has no
acoustical standards or nationally recognized acoustical guidelines for
classrooms. Without specific guidelines, economic considerations or
ignorance may tempt architects and school building officials to use
standard commercial practices for classrooms. These standards are
inappropriate for a large and increasing fraction of classroom inhabitants.
A subcommittee of the Acoustical Society of America's Technical Committee
on Architectural Acoustics has been formed in recognition of this need.
This session is one step toward meeting this need. It is hoped that the
papers given by authors in this session and in the workshop that follows it
will give momentum toward the goal of national standards and guidelines for
classroom acoustics.