Classroom Acoustics

Classroom Acoustics

The classroom serves as the acoustic venue for primary and secondary education.

We have heard classical music aficionados tout the “glorious acoustics” of a particular concert hall, opera house, or theatre. These “acoustics” are the venue’s natural ability to support and enhance the musical or dramatic functions performed inside, without the need for an electronic sound system. Important parameters affecting room acoustics generally include geometric form, the appropriate quantity and location of acoustical materials, subdued noise from heating, air-conditioning and mechanical systems, and sound isolation from outdoors and adjacent spaces.

Acoustics are not only important to concert halls, however. The reason many (otherwise bashful) people break into song while in the shower is that the shower stall’s hard surface walls and geometric form provide intense resonance and reflective support for the human voice. The result is an impression that one’s voice is fuller, deeper and louder. Conversely, attempting to sing outdoors without a sound system usually is very difficult for untrained vocalists because of the lack of reflective support and high ambient noise levels.

The most commonly cited acoustical parameter for architectural spaces is Reverberation Time, or RT60. Reverberation refers to sound that is continuously reflected within a space, such that it continues for a period of time after the source has ceased. This is audible as a decaying “tail” following each syllable of speech, for example. We scientifically measure reverberation in terms of the length of time it takes this decaying sound to drop 60 decibels. Gothic cathedrals or other large hard-surfaced halls can have reverberation times greater than 3 seconds. The reverberation outdoors, without and buildings or sound-reflective surfaces nearby would be about 0 seconds. Reverberation can be beneficial to the performance of many different styles of music, but is often detrimental to clear speech intelligibility. It can lead to confused, hollow sounding, “echoing” speech.

It is vital to ensure a supportive acoustic environment for any critical listening space. The classroom serves as the acoustic venue for primary and secondary education. It must enhance and project the teacher’s voice, have a very subdued level of reverberation or late-arriving echoes, and prevent the intrusion of unwanted sound from building mechanical systems, adjacent spaces, and sources such as, children, lawnmowers, roadways, trains or airports.

Clear speech is essential to classroom function. About 60 percent of all classroom learning activities involve students listening to and participating in spoken communications with the teacher and other students, according to the Acoustical Society of America. Unfortunately, the presence of severe acoustical barriers is far too prevalent in American schools.

According to the United States General Accounting Office, millions of students attend schools with unsatisfactory acoustical conditions. 21,900 schools exhibit poor acoustics or noise control, affecting more than 11 million students. 28.1 percent of all schools reported unsatisfactory or very unsatisfactory environmental noise conditions. This was higher than ventilation (27.1 percent), physical security (24.2 percent), indoor air quality (19.2 percent), heating (18.9 percent), or lighting (15.6 percent).

The excessive noise levels and lack of support for speech in these classrooms have failed countless students and led to difficulties in learning and instruction. The impediment these classrooms pose to learning is often underappreciated by students, teachers and administrators. In some cases, problems caused by poor acoustic conditions may have been falsely attributed to other factors (teacher performance, socioeconomic factors, etc). This prevalence of poor listening conditions in classrooms results from a lack of understanding and awareness of the detrimental impact that noise and reverberation have on student learning.

A supportive and enhancing acoustic environment in the classroom will enable easier learning for students, with more retention, and less fatigue for teachers.

How Poor Acoustics Affect Students

The link between acoustical problems and academic achievement is demonstrated in dozens of studies from several scientific fields. Proper acoustical support is crucial to young children whose auditory and language faculties are still developing. According to the Acoustical Society of America:

“Young children are more susceptible than adults to the effects of background noise and reverberation on communication with spoken language. Because of this susceptibility, young children also require more favorable classroom signal-to-noise ratios and reverberation times to achieve the same level of speech intelligibility as adults do. Developmental status, linguistic and cognitive proficiency, temporary hearing impairments, and early receptive and expressive language disorders are all factors that affect the greater susceptibility of young children to background noise and reverberation.”

Adults and older children possess an ability to fill in missed or unintelligible words from a greater context. For example, consider the statement “Why ot so many old halls remain popular while shmu newer ewon have already undergone extensive renovation?” Our adult minds are easily able to fill in the meaning of unintelligible, but predictable words from the greater context and establish 100 percent of the meaning through 82 percent of the words. Younger children possess a less effective ability to discern these high predictability contexts than older children or teenagers. Additionally, missed words with low context predictability are disastrous for all students. Consider the following statement:

“An angle that has a measure less than a right angle (less than 90 degrees) is an sbtsu angle”

The presence of excessive noise or reverberation will lead to a greater percentage of missed words during classroom instruction. The affected words may be context predictable, or not.

Academic Performance: A Progressive Deficit

Since the early 1980s, study after study has proven the detrimental affects of poor acoustic conditions on student learning and academic performance.

In the early 1980s, S. Cohen, Evans, Krants, and Stokols investigated elementary schools situated near a busy Los Angeles airport flight path. Children attending noisy schools scored lower on simple and difficult puzzle-solving tasks, even when tested in quiet conditions. They also were more likely to quit tasks than students tested from quiet schools. Since all possible socioeconomic factors were controlled, this study points to elevated transportation noise in classrooms as a real impediment to learning. Cohen, Evans, Krantz, Stokols, and Kelly also found that third-grade children who spent the year in acoustically treated classrooms had improved math scores compared to children in untreated rooms near the same airport.”

Impairment to language acquisition because of excessive noise during classroom instruction also can lead to deficits in reading skills according to a study by Evans, G. W. and Maxwell, L. First- and second-grade students exposed to chronic noise scored lower on standardized reading tests taken in quiet conditions. This evidence suggests that the detrimental effects of noise are related to a cumulative long-term effect and will leave students behind even after conditions improve.

Poor acoustical conditions in the classroom impede the teaching-learning process and lead to a cumulative and progressive deficit, according to Gary W. Evans, "Environmental Stress." Elevated noise levels, for example will shorten curricula by forcing teachers to repeat content. Other possible explanations include noise-produced influence on children’s information-processing strategies, on their feelings of personal control, and on level of arousal.

Special Needs

Students with hearing, language, speech, attention deficit or learning disabilities are especially vulnerable to poor acoustic classroom conditions. Children less than 15 years of age and those for whom English is a second language (ESL) also have greater difficulty adapting to a less-than-ideal listening environment.

Beyond simply reducing unwanted noise, classrooms that architecturally support and enhance desirable sounds (lessons from teachers, responses from students) are highly beneficial to students with mild hearing impairments. Permanent and temporary hearing impairment actually is more common than once thought. Two to 3 out of every 1,000 children in the United States are born deaf or hard-of-hearing, according to the National Institute on Deafness and Other Communication Disorders. Temporary hearing loss due to ear infections or head colds can affect anyone. Data submitted by parents to a 1988 National Health Interview Survey showed that 3.5 percent of children ages birth to 17 years in the United States had ever had “deafness or trouble hearing” in one or both ears.

Students with difficulties in attention or focus are very susceptible to intruding noises that have impulsive, rhythmic and/or tonal qualities. Sounds that carry information, such as student voices from an adjacent classroom, are especially intrusive, and can interfere with daily classroom instruction to all students.

Another alarming trend is the prevalence of temporary hearing loss because of unprotected exposure to extremely loud sound sources such as portable music players, music concerts, machinery, transportation noise, etc. Temporary Noise Induced Threshold Shift (NITS) has been measured among U.S. children, with an overall prevalence in 1 or both ears of 12.5%. Boys (14.8 percent) had a significantly higher prevalence estimate of NITS than girls (10.1 percent), according to an article in Pediatrics. Children aged 12 to 19 years (15.5 percent) had a significantly higher prevalence estimate of NITS than 6- to 11-year-olds (8.5 percent). These findings suggest that children are exposed to excessive and hazardous noise levels during personal activities, and that their hearing is vulnerable to these exposures. Over time this exposure can lead to permanent Noise Induced Hearing Loss. Although it is not within the power of school administrators to eliminate this hearing loss because of extreme sound exposure at home, providing an optimum acoustic environment will reduce the detrimental affect on student learning and academic performance that result.

How Poor Acoustics Affect Teachers

In addition to the cumulative negative impact of poor acoustics on pupil learning, excessive noise or reverberation can lead to additional stress and physiological harm for teachers.

When communicating in the presence of excess noise or reverberation, a speaker naturally will increase the intensity of his or her voice. This involuntary reflex is known as the Lombard effect and can include changes to loudness, pitch, rate, and duration of sound syllables in an attempt to increase the signal-to-noise ratio of his/her words. When continued for long periods of time, such as during a lengthy classroom workday, this will lead to increased strain on the vocal chords. Given the prevalence of poor acoustic conditions in American schools, it is little wonder that occupational voice disorders are so common among teachers.

According to a study published in the Journal of Speech, Language, and Hearing Research, teachers were significantly more likely than other professionals to experience voice symptoms such as hoarseness, discomfort and increased effort while using their voice. Teachers also reported tired or reduced voice quality, difficulty with vocal projecting, trouble speaking or singing, and a loss of vocal range.

The paper continues:

“Teachers consistently attributed these voice symptoms to their occupation and were significantly more likely to indicate that their voice limited their ability to perform certain tasks at work, and had reduced activities or interactions as a result. Teachers, as compared with non-teachers, had missed more workdays over the preceding year because of voice problems and were more likely to consider changing occupations because of their voice. These findings strongly suggest that occupationally related voice dysfunction in teachers can have significant adverse effects on job performance, attendance, and future career choices.”

Improving natural acoustic support for the spoken word as well as reducing ambient noise levels in the classroom will allow teachers to communicate effectively with little vocal effort and less repetition.

Important causes of poor acoustics

The term “reverberation” refers to sound that is continuously reflected within a large space for an extended period of time. Reverberation time greater than 0.60 seconds typically is too long for a classroom environment, and will lead to muddled syllables and unclear speech. This problem is common in large rooms with tall ceiling heights (greater than 10 feet), and many hard surfaces on walls, floors and ceilings. Another negative effect of excess reverberation is that it amplifies unwanted noise from intrusive sources. This is why underground concrete subway stations are so much louder than elevated outdoor stops for light-rail systems.

Elevated ambient noise levels can mask speech and degrade intelligibility even if significantly lower in amplitude than the teacher’s voice. Consider the white- noise masking systems frequently employed in medical and human resource offices. These systems output steady broadband noise at relatively low volume to mask consonant sounds and increase speech privacy. Unfortunately, many classroom heating, ventilation and air-conditioning systems emit a similar noise spectrum, where speech “privacy” is the exact opposite of what is desired. Students seated far from the teacher, at the rear of the classroom, will be most susceptible to these effects.

Common sources of noise with speech masking properties include:

  • HVAC systems, interior-mounted air conditioners and heating units.
  • Transmission of hallway activity and noise from other classrooms.
  • Cooling fans from computers, projectors, etc.
  • Office equipment operation: phones, fax, printers, etc.
  • Plumbing noise from adjacent bathrooms or kitchens.
  • Buzz & hum from fluorescent lighting ballasts.

Another frequent problem is noise transmitting from outdoors into the classroom. Walls, windows or doors with a poor sound insulation rating (Sound Transmission Class, or STC), will enable noise generated from loud, distant sources to penetrate into the classroom. This noise can often have an impulsive or dynamic character that brings further distraction and annoyance into the classroom. Open windows during warmer months will compound this problem and allow wind, tree rustling, and other natural sounds to interfere with classroom instruction.

Common sources of noise penetration from exterior sources include:

  • Automobile traffic and highways.
  • Trains or light-rail systems.
  • Aircraft noise.
  • Playgrounds or athletic fields.
  • Lawn maintenance equipment.
  • Transmission of hallway activity and noise from other classrooms.

Open plan classroom designs, popular in the U.S. during the 1960s and early 1970s, present serious challenges to classroom noise control. If fully open, the students have very little sound isolation from adjacent classes. Even with operable partitions, the noise transmission through weak points and small openings can provide significant distraction and challenges to speech clarity. The acoustic problems posed by open plan classrooms can outweigh the intended educational benefits.

Proper Planning and Abatement

The most cost-effective way to introduce noise-abatement measures into classroom design is during the initial construction process. Changes made after the fact can be somewhat more intrusive and time-intensive. Unfortunately, thousands of schools have already been constructed with little or no concern for student hearing. This has continued despite decades of scientific research into the detrimental effects of noise on learning. These outdated and unfortunate designs leave us with a challenging duty to improve conditions for today’s students.

For this reason, the Acoustical Society of America has published the comprehensive American National Standard ANSI S12.60-2002. Acoustical Performance Criteria, Design Requirements, and Guidelines For Schools. This document is available as a free download.

Important classroom criteria presented in ANSI S12.60-2002 include:

-Maximum acceptable ambient noise levels in the classroom: One hour average A-weighted steady background noise level: 35dB.

-Maximum acceptable reverberation time: 0.6 seconds. Acoustical ceiling tile is an acceptable and effective sound- absorptive surface used to soak up excess reverberation, however select areas of sound reflective material above the teacher and classroom center can help support and project his/her voice to students.

-Classrooms with ceiling heights less than 10 feet typically will have acceptable reverberation time with acoustical ceiling tile throughout.

-Ceiling heights 12 to 14 feet typically will require some additional absorptive material on the side walls in addition to acoustical ceiling tile.
-Ceiling heights 15 feet and over will require significant quantities of sound absorption treatments such as 1-inch fabric-wrapped fiberglass wall panels or equivalent.

-All interior and exterior classroom walls must meet or exceed Sound Transmission Class Rating STC-50. Sound Transmission Class is the officially tested sound insulation rating for a given wall, floor, ceiling, door, window assembly.

-Partition walls between classrooms and music rooms, mechanical rooms, cafeterias, gymnasiums, indoor swimming pools, or exterior machinery must meet STC-60.

-All classroom partitions must extend and seal to true structural ceiling and not end at false drop ceiling.

-Classroom entry-door systems must meet STC-30 with fully sealed perimeter and threshold.

-Exterior windows must be evaluated based on existing outdoor ambient noise levels and total glazed area.

-Carpeting on classroom floors can help control reverberation, reduce footfall noise to classrooms below, and minimize noise from moving chairs and feet. However, carpeting does not effectively absorb low frequency sound and may require an additional quantity of acoustical ceiling tile or wall panels. Neoprene or rubber chair leg tips can reduce chair-shuffling noise on hard floor surfaces.

-Classrooms must be free of discreet echoes or “slap-back” caused be large, flat wall areas, especially opposite the teacher. These echoes can be eliminated through careful sound absorbing material placement, or “broken-up” with strategically situated 3-dimensional features such as, large bookshelves, furniture, coat racks or sculpted wall hangings.

HVAC Noise Control
Heating, ventilation and air-conditioning systems must be designed/treated to suppress noise. Measured ambient noise due to HVAC system equipment should not exceed 35dB averaged over a one hour period of operation. The dB output of specific HVAC systems is a function of the cumulative effect and relationship of all system components and elements. Necessary treatments to achieve an acceptable ambient noise level may include sound absorbing duct liners, alternative fan types, low air velocity and noise attenuators. These treatments may increase the system’s initial cost and slightly reduce energy efficiency. These tradeoffs are necessary for any critical listening space, including classrooms. HVAC designers typically will not consider acoustical or noise control requirements unless specifically asked to do so.

Noise from HVAC systems is a very common problem that is often under-appreciated or ignored by school administrators and faculty. It is possible to achieve quiet system operation in any existing classroom, if noise- control treatments are installedproperly .

The 2007 ASHRAE Handbook – HVAC Applications: Chapter 47 Sound and Vibration Control is an excellent source for HVAC engineers or building technicians seeking to design quiet systems.

Amplification systems (microphones and loudspeakers) in the classroom are a common, but misguided, attempt to solve acoustical problems. The official position of the Acoustical Society of America is that sound amplification should NOT be routinely employed in typical small mainstream classrooms.

Sound-reinforcement systems will not solve classroom reverberation problems, and if calibratedimproperly , actually can increase syllable confusion. Sound- reinforcement systems will increase overall sound levels in the classroom, sometimes in excess of comfortable listening levels. Further, if partitions have poor sound insulation qualities, amplified sound may become audible and disruptive to students in adjacent classrooms.

Amplification systems also create an additional level of complication for instructors. They must worry about microphone placement and equipment operation, at the expense of classroom engagement. A desirable natural acoustical environment will effectively distribute and enhance spoken communication without any user training, maintenance expense or equipment upgrades.

For students with permanent or temporary hearing impairment, personal headset listening systems are available that transmit via FM radio-frequency. These systems are much more effective than amplified loudspeakers for students with special needs and exhibit none of the above stated disadvantages.

Learn More

The process of testing, evaluating, and treating classrooms acoustical issues can be daunting, especially to today’s overburdened school administrators. A qualified, independent acoustical consultant can help facilitate this process. Ensure that your consultant has experience with classrooms, is familiar with current industry testing and design standards is not a representative or salesperson for particular products or materials.

Wolfram is the lead acoustical engineer and noise control consultant for Riedel & Associates, Milwaukee, an acoustical consulting firm. He can be reached at [email protected].

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