Classroom Acoustics: A Review
The year 2001 is predicted to be the peak year for school construction and renovation in the United States with an estimated dollar amount of $27 billion. And while Rome burns, the American National Standards Institute technical committee on classroom acoustics fiddles with its pending standard. In the meantime, in an effort to introduce more school facility planners, architects and contractors to the virtues of good classroom acoustics, the Acoustical Society of America has recently published a 12-page pamphlet titled “Classroom Acoustics,” a resource for creating learning environments with desirable listening conditions.
Written by five college seniors of the Architectural Engineering program at the University of Kansas, the introduction, as well as the rest of the report, is peppered with anecdotal statements designed to impress if not to be entirely accurate. One such introductory statement is that many classrooms have speech intelligibility ratings of 75 percent or less. It is pointed ___ that this is _________ to reading a ____ in which every ______ word has been ________. Possibly an easy task for expert Mad Libs players but not so easy for “young children with limited vocabularies, students with learning disabilities, those with auditory processing problems, and people for whom English is a second language.” (Missing words: out, equivalent, book, fourth, deleted.)
The whole report is best likened to a set of Cliffs Notes, those yellow-and-black-cover study guides popular with students trying to cram for a test on anything from “Alice in Wonderland” to “Wuthering Heights.” Unfortunately, as any student who has taken a test by reading the summary but not the actual book (guilty) knows, a lot of important details are left out of the summary. As no student should expect to pass an exam with only the shallow knowledge gained by a summary, no one should expect to be able to properly design a classroom for good acoustics after reading a 12-page pamphlet. The publication seems to be confused on this point as it makes several conflicting statements of purpose.
In the beginning was the wordIn the frontispiece, it clearly states that it “is not intended to replace the services of a professional consultant.” However, in several places it tacitly implies that the reader will be able to design a successful classroom but that the “design of spaces with special acoustical requirements, such as theaters or music rooms, or any spaces with complex noise problems, are best handled by a professional acoustical consultant.”
To complete the confusion, the writers give detailed tables, recommendations, design equations and example problems to use in the design of the optimal classroom, cafeteria, gymnasium, auditorium and music rehearsal room. The publication gives the reader just enough information to get oneself into trouble. The danger of all this confusion is the increased likelihood that some architect, pulling an all-nighter to finish a school design, will rely on this summary as his or her sole acoustic design resource.
The publication does do a good job of explaining the basic concepts of sound propagation and how it is affected by architectural elements. It adequately explains the basic measurements of reverberation time, noise reduction, signal-to-noise ratio and speech intelligibility that are used to judge the acoustics of a space and then attempts to give guidelines for reverberation time of classrooms. In the appendix, the writers give the 100-year-old Sabine equation and tables of absorption coefficients for calculating the reverberation time of a room. Sadly, the use of reverberation time and, especially, the Sabine equation is questionable if not totally incorrect for small classrooms. To put it simply, the concept and calculation of reverberation time assumes that the sound in the room is “well mixed.”
To the contrary, sound in a small room, such as a school classroom—especially in the mid to low frequencies—is not mixed at all. Although sound at the higher frequencies will be sufficiently mixed throughout the room, sound waves at frequencies below 500 Hz typically will not even fully “fit” in the small space of a classroom. The difference between the total mixing of small, high-frequency sound in the room and the impossibility of mixing for large, low-frequency sound is analogous to the difference between a glass filled with peas and another filled with golf balls. The glass of peas is a mixture while the other is just a glass with some golf balls in it.
The real problem with using the Sabine equation in the design of a classroom is that it will, when followed blindly, predict large reverberation times in rooms too small to produce reverberation and cause the designer to introduce sound absorbers where none are needed. Good practice in large rooms with well-developed reverberation fields is to spread out the applied sound absorbers throughout the room. This is not good advice for the treatment of small classrooms. Since no reverberation field exists in small classrooms, when sound absorbers are used they are best placed only in specific areas to control specific energy returns in the room. The hapless user of the Sabine equation has no means to determine where acoustic treatments should go.
Dated informationThe sections on speech intelligibility testing are also dated. Originally, the speech intelligibility testing of a room was determined by reading a list of words to a group of listeners seated throughout the room and then determining what percentage of words were heard correctly. This is the basic explanation given in the text and appendix of the pamphlet. Times have changed and manual reading of word lists have long been replaced with computerized analyses in which a computer produces a set of signals that mimic human speech components, plays them through a loudspeaker in the room, listens to the signal and noise of the room picked up by a microphone, and determines the quality of the resulting signal. The test is much cheaper, faster and more repeatable than the old word list test.
The publication does point out many useful design features for classrooms. The use of diffusion to eliminate specular reflections is touched upon. It is shown how strategic placement of reflecting panels in a suspended ceiling grid will reinforce the transmission of speech to students. A section on mechanical equipment noise gives common mistakes and rules of thumb to prevent excessive mechanical system noise from disturbing the class. Examples of gypsum-board wall construction are given with a comparison of sound isolation properties. Positioning of doors in adjacent rooms and location of high-noise rooms relative to critical listening spaces is briefly discussed. The recommendation section ends with a curt discussion of exterior noise sources and the use of electronic sound reinforcement systems.
The report concludes with examples of good and bad classroom designs and a case study of the remodeling of an older classroom having poor acoustics. The good and bad examples give only anecdotal and rule-of-thumb-type direction on the use of a limited number of acoustic treatments to improve classroom acoustics. The examples focus on the use of acoustic lay-in tiles, fiberglass absorbers and thin carpet—a solution common in many architectural texts of the 1960s.
The case study cited in the report, far from being a showcase of a modern retrofit, is simply a study of how suspended fiberglass panels reduced the calculated reverberation time in an old university classroom (no doubt, a senior project at the university).
All things considered, the Acoustical Society of America’s Classroom Acoustics brochure is too little, too late. It is a bit surprising that the ASA would use what amounts to a mediocre senior architectural project as a position paper on such an important subject as classroom acoustics. Certainly, it was never meant to take the place of a true standard for classroom acoustics, but, as sure as students will use Cliffs Notes as a main source of knowledge, many architects will use this report as such.