December 1997 Workshop on Acoustics and Learning

Guidelines for Classroom Acoustics in New Construction

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Many of our children, and especially those with hearing or learning disabilities, are being deprived of a clear communication channel in educational environments because of inferior classroom acoustics. Poor classroom communication acts as a barrier to learning, stunting intellectual growth, lowering self esteem, and serving to diminish the potential for the child to grow into a productive citizen. The following classroom acoustics design guideline checklist was developed in response to the Request for Information of the Architectural and Transportation Barriers Compliance Board, established in response to the Americans with Disabilities Act (1991).

The problem of good acoustics is a very broad one. For the purposes of this brief statement on "Draft Guidelines for Classroom Acoustics in New Construction", we focus on the prototypical classroom of no more than 30 or so students. We shall leave out auditoria, cafeteria, gymnasiums, music rooms, offices, and the like. Each of these spaces must be addressed individually; however, the prototypical classroom is where most verbal interactions takes place.

In larger spaces where verbal communication is important (such as auditoria), and in the absence of a sound system, we rely on reflected sound to maintain the decibel level of the sound throughout the space. But in smaller spaces, the direct sound from teacher to student is at least as important, if not more important, than the indirect or reflected sound. In the intimate classroom environment, we can ask: What are the obstacles to verbal communication? There are two obvious culprits: Excess noise and excess reverberation.

The issue is complicated by the distinction between the design of new classroom spaces and the remediation of existing spaces. For newly designed spaces, it is our responsibility to specify an architect/contractor's worksheet and checklist of good classroom acoustics design practice. We should also give concrete examples of good classroom design. If these conservatively defined specifications and examples are followed and appropriately signed off on, then we should be confident that in a very large majority of the cases, the results will be successful or easily remedied.

For existing spaces requiring renovation, we need to specify performance standards that are both reasonably easy to specify and to measure. We should also generate a diagnostic checklist that suggests what remediation should take place, and we should be able to point to a data base of consultants and vendors who can supply services and/or equipment to bring the space into minimum standards for communication for all students.

Let us concern ourselves with the acoustical guidelines for new construction, since these will be the standards against which efforts at remediation will be compared. By understanding the environmental background noise, through an appropriate acoustical survey, architects and acousticians can work together to design school layout and classroom spaces 1) so that the external noise has minimal impact, 2) so that classrooms are sited away from noise generating spaces, and 3) so that wall and ceiling-floor partitions, windows, and doors are specified to achieve adequate acoustical isolation.

Even in the absence of external noise, we must realize that we are our own noise sources. The sound we made a moment ago should not be reverberating around a space to mask the information we are currently conveying. Students make noise not only vocally, but in their interaction with furniture and the like. Fortunately, excess reverberation can be easily dealt with through the judicious use of acoustical treatment. There is no mystery in calculating the approximate amount of absorbing material required for a room of a given volume. If the classroom has been well designed, the ceiling will not be too high and therefore the room will not have an excessively large volume. Absorbing material to be used in a room is ordinarily placed on the ceiling with an adequate air space to assure acoustical control over a broad frequency range. Sometimes some upper wall treatment is desirable to mitigate flutter echo effects associated with hard parallel walls. Neither of these tasks is particularly difficult or costly.

Reverberation control with absorbing material is more straightforward than controlling unwanted sound sources, such as heating, ventilating, and air conditioning sound, noisy fluorescent light fixtures, noises from sources external to the room, and self noise, as mentioned earlier. It is generally known that there are three ways to control unwanted sound: at the source, along the path between sound source and receiver, and at the receiving end. The last of these is not possible because it implies the use of earplugs, which only would be considered in particularly noise environments, such as a music practice space for bands and orchestras. The first option, controlling sound at the source, can be accomplished by purchasing equipment with low noise ratings and by installing it properly. Treating the path along which noise propagates may entail anything from lining the ventilation ducts between a fan unit and the room, to assuring that doors are heavy enough and are properly gasketed, with drop seals, if necessary, to prevent hall noise from leaking in (and out).

In the classroom, difficulties in hearing by a small group of children with special challenges are a warning that the classroom may not meet the minimum standards of classroom communication for all students. Although assisted listening devices and teacher amplification can improve one direction of the communication channel, learning requires the responses of students, and, therefore, some forms of electronic enhancements, which may be necessary in some instances, are not sufficient to guarantee clear and intelligible communication.


The following "Draft Guidelines for Classroom Acoustics in New Construction" and "Other Acoustical Issues" questions and answers are intended to assure that acoustical considerations are appropriately addressed in the design process and in construction.

Other acoustical issues [see Guidelines Checklist]:

1) Under what circumstances should external windows be operable?

In the closed configuration, the windows must satisfy the criteria outlined under School Siting. Open windows should not be for ventilation (which is the task of the HVAC system). Teachers must be informed that external noise can compromise their classroom communication, but there may be instances (e.g. study hall, project work, etc.) where open windows are reasonable.

1) When is carpet appropriate?

Carpet is especially appropriate for young students who may sit on the floor in groups. Low pile carpet will not be comparable to a hung acoustic ceiling for reverberation control, and therefore should not substitute for ceiling treatment. In some instances, carpet may aid in achieving a reverberation time 0.4 seconds or less, which is optimal for students with hearing and/or listening disabilities. The use of electronic amplification may be required in order to achieve sufficient teacher speech level.

2) What shape should rooms have?

There are no strict rules for room shaping, except that hard, concave walls can focus sound rather than evenly distributing it. Length-to-width proportions are not strictly fixed, but avoiding even multiples of the width is desirable. Side-walls that are not strictly parallel can aid in avoiding flutter echoes. Usually it is sufficient to have a very gentle cant to the side-wall (e.g. 10%). Parallel walls are acceptable if furnishings (e.g. closets, bookshelves, etc.) are placed so as to break up the direct path across the room.

3) Where does one add extra absorbing material, if needed?

Generally, if the room is less than 9 feet tall and if a hung acoustic ceiling with a NRC of 0.75 is employed, the reverberation in the room will be controlled. In taller rooms, the best place for absorbing material is high on the walls, so as not to overly damp speech-reinforcing reflections from the side-walls. Also, soft material that is out of the reach of children is more easily maintained.

4) What are the ideal materials for partitions?

There are no absolute rules for this? There are examples of both filled cinder block and sheet rock on staggered studs that can be found in noise control and architectural design handbooks. These should have STC ratings of 50 or higher. Cost may be a factor that influences the choice of materials.



Perform New School Site Environmental Acoustic Survey

School design layout places classrooms away external noise sources and away from (including vertically) internal noise-producing rooms (e.g. equipment rooms, gymnasia, cafeteria, music rooms, et al.)

External windows--thermal pane with STC rating sufficient to reduce environmental noise levels to 30 dBA or below. Example: 65 dBA outside requires window system with STC of >35.

All internal walls, floor-ceiling construction, STC> 50

Doors with drop-seals, or gaskets with effective STC rating of tbd


1) No through-window ventilation

2) Rating of compressors, tbd

3) Installation detailing:

(a) Duct work not in common with noisy rooms

(b) Resilient caulking for all penetrations

(c) Vibration isolation to industry standard


Hung acoustic ceiling with NRC rating of >= 0.75

Height of acoustical ceiling < 9 ft.

If not, then addition of acoustical treatment by the following formula:

Extra square feet of acoustical material with NRC >= 0.75 for each foot above 9 ft is: 0.11 x ceiling area.

Furnishing recommendation to end-user: Closets, shelves/fixtures used in room to break up strictly parallel side and front/back walls.

In (acoustic) ceiling fluorescents light fixtures with noise ratings less than tbd.

All fixtures (ducts, wires, piping, radiators, etc.) going through structural walls, floors, or ceilings are resiliently caulked.

Sheet rock seams fully taped and caulked to floor (avoiding seams/joints just covered with facings, etc.)

Light from surrounding rooms cannot be seen from darkened subject room.

Post-construction, in-use dBA level <40/35; dBC < 65/60.

Bibliographical References Related to Classroom Acoustics

Apfel, RE (1998). Deaf Architects and Blind Acousticians? A Guide to the Principles of Sound Design, New Haven: Apple Enterprises Press.

Bergman, M (1980). Aging and the Perception of Speech. Baltimore: University Park Press.

Bronzaft, AL, and McCarthy, DP (1975). “The effects of elevated train noise on reading ability,” Environment and Behavior 7(4), 517-527.

Courts Design Guide (1997). Guide Acoustic Design of U.S Federal Court Facilities.

Crandell, CC (1993) "Speech recognition in noise by minimal degrees of sensorineural hearing loss," Ear and Hearing, 14 (3), 210-216.

Crandell, CC, and Smaldino, JJ (1996). “Speech perception in noise by children for whom English is a second language,” American Journal of Audiology 5, 47-51.

Finitzo-Hieber, T. and Tillman, T. (1978) Room acoustics effects on monosyllabic word discrimination ability for normal and hearing-impaired children, Journal of Speech and Hearing Research 2, 440-458.

General Accounting Office (1995). “School facilities, condition of America’s schools,” Report to the US Senate, GAO/HEHS-95-61, Feb. 1, 1995

Gengel, R (1971). “Acceptable signal-to-noise ratios for aided speech discrimination by the hearing impaired,” Journal of Auditory Research 11, 219-222.

Hawkins, D (1986). “Options in classroom amplification systems,” in Hearing Impairment in Children, Bess, Ed., York Press, 253-265.

Kerlf, Leif (1996). Acoustic Guide, Selection of Acoustic Quality in Buildings, Swedish Council for Building Research.

Nabalek, A, and Pickett, J (1974). “Monaural and binaural speech perception through hearing aids under noise and reverberation with normal and hearing-impaired listeners,” Journal of Speech and Hearing Research 17, 724-739.

Nabalek, AK (1988). “Identification of vowels in quiet, noise and reverberation: Relationships with age and hearing loss,” Journal of the Acoustical Society of America 80, 741-748.

Nabalek, AK (1993). “Communication in noisy and reverberant environments,” in Acoustical Factors Affecting Hearing Aid Performance 2nd Edition, Studebaker & Hochberg, Eds., Allyn & Bacon, 15-28.

Nabalek, AK, and Donahue, AM (1984). “Perception of consonants in reverberation by native and non-native listeners, Journal of the Acoustical Society of America 79, 2078-2082.

Nilsson, M, Soli, SD, and Sullivan, J (1994). "Development of the Hearing In Noise Test for the measurement of speech reception thresholds in quiet and in noise," Journal of the Acoustical Society of America 95, 1085-1099.

Olsen, W. (1986) “Classroom acoustics for hearing-impaired children,” in Hearing Impairment in Children, Bess, Ed., York Press, 266 – 277.

Pearsons, KS, Bennett, RL, and Fidell, S (1977). “Speech levels in various noise environments,” US Environmental Protection Agency Report EPA-600/1-77-025.

Ross, M (1990) ."Definitions and descriptions," in Davis, J. Our Forgotten Children: Hard-of-hearing Pupils in the Schools, U.S. Department of Education, Washington DC, 3-17.

Stelmachowicz, P (1998). “Effects of advanced signal processing on speech: Implications for fitting hearing aids to young children,” Paper presented at the Issues in Advanced Hearing Aid Research Conference, Lake Arrowhead, California.

Stollman, MH, Kapteyn, TS, and Sleeswijk, BW (1994). “Effect of time-scale modification of speech on the speech recognition threshold in noise for hearing-impaired and language-impaired children,” Scandinavian Audiology 23(1), 39-46

Egan, D (1988). Architectural Acoustics, McGraw Hill.

Rosenberg , C 1996). “Sound Control,” in AIA Architectural Graphic Standards, Eighth Addition, John Ray Hoke, Jr. Ed. AIA (EIC).

Apfel, R (1998). Deaf Architects and Blind Acousticians, A Guide to the Principles of Sound Design,.

Irvine, LK, and Richards, RL (1998). Acoustics and Noise Control Handbook for Architects and Builders, Kreiger Publishing Co.

Salter, CM 1998). Acoustics: Architecture, Engineering the Environment,” William Stout, San Francisco.

Members of the Acoustical Society of America Who Have Contributed Input to the Request for Information on Acoustics

Robert E. Apfel, Ph.D., Fellow, Faculty of Engineering, Yale University, Technical Committee on Architectural Acoustics

Angelo J. Campanella, Campanella Associates, Technical Committee on Architectural Acoustics

T. James DuBois, Consultant in Acoustics, Technical Committee on Noise

David Lubman, Fellow, Consultant in Acoustics, Technical Committee on Architectural Acoustics

Peggy B. Nelson, Ph.D., Department of Otolaryngology, University of Maryland Medical School, Technical Committee on Psychological and Physiological Acoustics

Michael T. Nixon, Enviro-Acoustics Company, Inc., Technical Committee on Architectural Acoustics

Brad S. Rackerd, Ph.D., Department of Audiology, Technical Committee on Psychological and Physiological Acoustics

Mark E. Schaffer, McKay Conant Brook Inc., Technical Committee on Noise

Sigfrid D. Soli, Ph.D., Fellow, Department of Human Communication Sciences and Devices, House Ear Institute, Technical Committee on Speech Communication

Norel D. Stewart, Stewart Acoustical Consultants, Technical Committee on Noise

Louis C. Sutherland, Fellow, Consultant in Acoustics, Technical Committee on Noise