Page 30 - Fall 2005

THE ASA CLASSROOM ACOUSTICS EFFORT
Peggy B. Nelson
Department of Speech-Language-Hearing Sciences University of Minnesota Minneapolis, MN 55455
One of the most rewarding recent efforts of the ASA has been our involvement in the development of the classroom acoustics standard (ANSI, 2002). The accomplishment was a true joint venture, involving contri- butions from experts in speech perception, psychoa- coustics, architectural acoustics, engineering acoustics, physical acoustics, noise control, and others. The stan- dard’s development was the epitome of a team effort, with contributions from academicians and researchers as well as industry representatives and product manufacturers. Particular thanks are due to the co-chairs of the working group, Lou Sutherland and David Lubman. Additional thanks go to the staff of the ASA Standards organization, in particular Daniel Johnson, Susan Blaeser, and Paul Schomer. The beneficiaries of this joint effort will include children learning in quieter spaces around the country.
Hard-working ASA members of the standard’s working group are listed in the table. The working group also includ- ed representatives from industry, consumer advocacy groups, government agencies, architects, teachers’ unions, and other interested associations. The group met periodically for at least two years. Throughout the meetings the panel exten- sively discussed the literature, design principles, costs, and proposed performance criteria. Discussions were lively and heated, and it should be no surprise that we occasionally moved backwards before moving forward again. Ideas were included and later discarded as the focus of the proposed standard became clearer. In December 2001 the task force agreed upon a final version of the standard that was soon approved by the American National Standards Institute. The new standard immediately met with
controversy, receiving a last-minute
challenge from certain industries
(modular classrooms, Air-conditioning
and Refrigeration Institute).
Nevertheless, the standard was adopted
and is now embraced by some of those
early “opponents.”
In this brief article, I hope to explain the basic principles of the standard, summarize the rationale and need, and show examples of how it is working around the country. This information should provide ASA members with the information they need to be advocates for the standard in their own communities and schools. In the paragraphs below I will address questions about the stan- dard that I frequently hear from con- stituent groups.
Did we really need a new standard for acoustics in classrooms?
As the working group started to meet, it quickly became apparent to the members that the current status of acoustics in schools was unsatisfactory for children’s learning. Surveys of noise and reverberation in existing unoccupied classrooms commonly reported background noise levels of 50 – 65 dBA (e.g., Knecht et al., 2002) in both newer and older schools in urban, suburban, and rural school districts. Primary sources of unoccupied classroom noise included classroom appli- ances (especially heating, ventilation, and air-conditioning (HVAC) systems), traffic noise entering through the build- ing’s exterior, and school noise emanating from a classroom’s adjacent hallways and spaces. Average reverberation times (RT60 for .5, 1, and 2k Hz) varied from 0.4 s to as long as 1.5 s. Although many classrooms had acceptable RTs (0.6 s and below), high ceilings in older classrooms resulted in long RTs and significant temporal smearing of speech signals (see Nabelek and Nabelek, 1985 or Nelson, Soli, and Seltz, 2003, for a description of temporal smearing due to reverberation.)
With background noise levels such as those measured, it could be expected that target voices, such as the teacher’s voice, frequently arrive at students’ ears around the room at signal-to-noise ratios (SNRs) as low as 0 dB. Basic estima- tions suggest that if a teacher’s voice starts at an expected rms of 65 dBA, and is then reduced by the distance between the source and the student, the voice level and the level of the background noise may be approximately equal at locations farthest from the source. In fact, direct measurement of SNR in classrooms suggests just that, that the SNR for students
Fig. 1. Speech recogni- tion thresholds in noise for children and adults (from Soli and Sullivan, 1997). Using the Hearing in Noise Test (HINT), one can see a system- atic improvement in speech recogni- tion thresholds as listeners develop into adolescence and adulthood.
28 Acoustics Today, October 2005