Reverberation is the measure of the time (in seconds) that it takes a given sound to decay by 60 decibels. Long reverberation times are not desirable because late-arriving sounds blur speech clarity and increase background noise. However, early sound reflections in rooms can actually reinforce the speech signal and improve SNR if they arrive at the listener's ear within 50 milliseconds. By placing materials to reflect early sound and absorb late-arriving noise, it is possible to optimize the reverberant characteristics of a given room.

A recent paper by Rebecca Reich and John Bradley of the Canadian National Research Council reports on their investigation of classroom reverberation through computer modeling. Using the ODEON room acoustics ray tracing program (version 2.6 for DOS), researchers were able to identify optimum conditions for speech as a reverberation time of 0.5 seconds (the research also showed that speech intelligibility varied only one-half of one percent between reverberations of 0.3 and 0.6 seconds). Nine different placements of material, each with the same total of sound absorption, were tested. When the source position was located at the head of the room, in traditional classroom style, speech clarity was found to be optimal when the absorptive material was located on the upper portions of classroom side and rear walls.

Interference

Interference from lighting ballasts, radio frequency sources, HVAC controls, and other electrical, electronic, microwave and even infrared sources can compromise the effectiveness of assistive technologies and has become an increasing problem for many people who are hard of hearing. Young children with hearing loss may not be able to identify and call attention to malfunctioning devices. In extreme cases, such as schools located in the path of transmission towers or equipment, it may be necessary to install shielding in exterior wall and roof assemblies.

Accessibility Recommendations

In 1995, the American Speech-Language-Hearing Association (ASHA) published a Position Statement on Acoustics in Educational Settings that called for 'appropriate acoustical environments in all educational settings, to include classrooms, assembly areas, and communications-related treatment rooms'. ASHA's Acoustical Guidelines recommend that:

a.. unoccupied classroom noise levels should not exceed 30 dB(A) or a Noise Criteria (NC)-20 curve [FN 2]

[FN 2] NC curves weight sound pressure levels across 8 standard frequencies to approximate human perception of sound, which is greater in the high frequencies. To meet NC-20, sound pressure level at the lowest standard frequency (63 Hz) can be as much as 50 dB, while at the highest frequency (8000 Hz) it can be no more than 16 dB).

b.. reverberation times should not exceed 0.4 seconds, and

c.. the SNR at a student's ear should exceed a minimum of +15.

Industry Recommendations and Standards

Industry coverage of acoustical issues rarely includes discussion of the characteristics of good listening conditions for people who are hard of hearing, although specialists in the design of facilities for people who are elderly have begun to recognize this as a significant issue. Acoustical design for children's environments is not typically distinguished from practices suitable for adults.

Criteria for classroom listening conditions at three levels of quality were recently outlined in "Goals and Criteria for Acoustical Planning", a presentation by R. Kring Herbert, FASA, at the 1999 conference "Eliminating Acoustical Barriers to Learning in Classrooms" in New York City, organized by the coalition formed to submit comment to the Board's RFI:

Textbooks on acoustical design typically contain guidelines for maximum background noise in different occupancies. Recommendations in current publications show a range of 25 dB(A) to 35 dB(A) maximum for the interior sound level in unoccupied classrooms. Most texts do not distinguish between classrooms for children and classrooms for adults. Only Egan, of those consulted in the Board's analysis, considered hard-of-hearing users. Egan recommends a 5 dB reduction in background noise for facilities serving people who have hearing loss. Reverberation times between 0.5 and 0.8 seconds have been recommended for classroom uses.

The American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) in its 1995 Handbook suggests a Room Criteria maximum of RC-40N for small classrooms (<750 SF) and RC-35N for larger classrooms. This is considerably higher than most acoustical textbooks recommend, and recognizes no adjustment for classrooms for children or for people who have hearing loss.

The American National Standards Institute (ANSI) in S12.2-1995, 'Criteria for Evaluating Room Noise' suggests RC-25-30 for lecture halls and classrooms and RC-35-40 for open plan facilities (where it is significantly more difficult to control background noise). Again, no adjustment is suggested for younger listeners or those who have hearing impairments.

Acoustical Modeling and Measurement

Computer modeling is a useful way to project the effects of various design decisions and materials selections on the speech intelligibility of a classroom. Professional engineering software for acoustics analysis has been used for many years in the design of performance halls. New user-friendly software packages are now becoming available to assist non-specialists to determine reverberation time and specify proper locations and areas of absorbency.

Both background noise and reverberation time can also be calculated from relatively simple equations contained (and explained) in most acoustics texts. Editions of M. David Egan's text "Concepts in Architectural Acoustics" has been a standard reference work for students of architecture since 1972. Tables of material and assembly characteristics needed for acoustics computations, including values for absorbency, sound transmission, impact isolation and other factors, are published in many textbooks; 'Part IX Acoustics', in "Mechanical and Electrical Equipment for Buildings", by Stein, Reynolds, and McGuinness, has been an assigned text for architecture and engineering students through eight editions. Many manufacturers of acoustical finishes and products also provide details on wall, partition, slab, ceiling, and roof design in catalogs and product data sheets. "Architectural Graphic Standards" and "Timesavers Standards", key resources for design professionals, both contain basic information on architectural acoustics and noise control, including design and construction details and noise reduction values.

Background noise in existing facilities can be metered on several scales, including the A scale, which is adjusted for human hearing. Simple inexpensive devices may be adequate to determine the existence of an acoustical problem, but more sophisticated and costly devices are necessary to perform an acoustical analysis. Reverberation meters also exist, although they do not seem to be much used by consultants.

Standard-setting and Regulation of the Acoustical Environment

Acoustical standards are of two general types: performance standards, usually combined with a testing protocol, as with ANSI and ASTM standards, or design and construction standards that require a specified sound absorbency or sound transmission or resistance value in building elements -- ceilings, walls, windows -- known through prior testing to achieve certain results.

Because design, construction, and use all affect the acoustics of a space, design professionals are understandably wary of single-number requirements for reverberation and background noise. A 5 dB difference in room performance could be due to meter quality, changes or omissions in construction, lack of equipment maintenance, teacher fatigue, or even a new flight pattern at a nearby airport.

Sweden, Portugal, Germany, and Italy all have acoustical standards for educational facilities. The Swedish standard is based upon room area and absorbency values for ceiling tiles (the higher the absorbency rating of the material, the less area is required) and on the sound transmission class of wall, floor, and roof/ceiling assemblies. Italy's standard prohibits school construction where environmental noise exceeds certain levels (as, for example, near airports, rail lines, and highways). Research is underway in Great Britain to establish classroom standards for children who are hard-of-hearing.

In the United States, the New York State Department of Education published a manual for classroom design and construction that sets 35 dB(A) as a background noise 'objective' for State school construction. Washington State Department of Health regulations also limit background sound to 35 dB(A) in classrooms. The Los Angeles Unified School District has attempted to limit noise from through-the-wall and rooftop HVAC units through their purchasing program, specifying a 35 dB maximum for equipment noise. The Access Board understands that the School District has not been able to identify a manufacturer of complying units. The District hopes that purchasing volume may encourage manufacturers to develop quieter models.

The model codes (BOCA, UBC, SBC), several state departments of education or health, and the Department of Housing and Urban Development have already adopted acoustical standards for multifamily residential occupancies that establish minimum values for Sound Transmission Class (STC) and Impact Isolation Class (IIC) of wall and slab/roof assemblies. Multifamily housing in California is subject to design and construction standards for acoustical performance. Environmental (exterior) noise is also limited by regulation in many jurisdictions, and others require construction that will provide an interior noise level of no more than 45-55 dB.

Resources

There are many other resources available for parents, schools, audiologists, advocates, and design professionals who wish to improve their understanding of issues in classroom acoustics. A coalition of organizations assembled in 1998 to respond to the Access Board's Request for Information (RFI) maintains a lively listserv and archive at classroomacoustics@onelist.com and contains links to other sites of interest. Professional members include the Acoustical Society of America, Alexander Graham Bell Association for the Deaf and Hard of Hearing (AG Bell), the American Academy of Audiology (AAA), the American Speech-Language-Hearing Association (ASHA), the Educational Audiology Association (EAA), the National Council of Acoustical Consultants (NCAC), Self Help for Hard of Hearing People (SHHH), and the Council of Educational Facility Planners, International (CEFPI). The U.S. Department of Education maintains a National Clearinghouse on Educational Facilities. Its website on classroom facility design (http://www.edfacilities.org) includes references to research and publications on classroom acoustics.

Additional reading and reference material, including electronic links to other websites of interest, will be posted on the Access Board's website (www.access-board.gov/rules/acoustic3.htm).