Winter 2005

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Classroom Acoustics

Does the classroom assist or impede the learning process?

Dr. John Bradley

Introduction
Even moderate levels of noise and poor room acoustics can impair children’s ability to understand clearly spoken words. In many everyday classroom situations, children will only understand a portion of the words that are spoken to them, even if they are clearly spoken simple words. The problem becomes more acute for younger children and others such as hearing impaired and second language listeners.

Most learning activities in classrooms are verbal and involve listening to speech. This is especially true for younger students whose reading and writing skills are still developing. Learning activities in classrooms involve verbal communication between teacher and students as well as among students. If students cannot readily understand all of the teacher’s words due to interfering noise or poor room acoustics, then the learning process is made more difficult. While some students may be able to cope with a situation where they can only understand a portion of the words that the teacher is speaking, many others cannot and they are more severely handicapped. When difficult listening conditions persist for some time, some children will ‘turn off’ and stop trying to understand the words.
It is therefore very important that the acoustical conditions in classrooms be specially designed to encourage and not impede the learning activities that take place in these rooms. This is best done with the assistance of a knowledgeable acoustical consultant. It is equally important that during critical teaching periods, where verbal communication is important, that student activity noise be controlled. An ideal classroom should not detract from or impair the learning process for any child. Classrooms should provide an environment that completely supports the needs of verbal communication in teaching activities.

Why is Noise a Problem?
From speech recognition tests, we know that the fraction of the words understood by listeners, (that is, the intelligibility of the speech), is most influenced by how loud the speech is relative to the level of ambient noise. For near complete understanding of speech, the speech must be at least 15 decibels louder than other interfering sounds. Teachers can talk louder to overcome noisy situations, but there is a limit to how loud teachers can comfortably talk for long periods. Therefore classrooms (and various meeting rooms) must be relatively quiet for maximum intelligibility of spoken words. The new Classroom Acoustics standard in the US recommends that ambient noise levels should not exceed 35 dBA. This is very quiet and not easily attainable without serious consideration of the complete acoustical design of the classroom. However, since the main purpose of a classroom is to support verbal communication for all students as part of the learning process, it is obviously important that efforts should be made to maximize the performance of the classroom.

The Consequences of Too Much Noise
If the classroom is not quiet enough, then the students may not understand all of the words spoken by the teacher, adding to their difficulties of learning new concepts and new information. New results from an ongoing Canadian Language and Literacy Research Network study show that in typical classroom conditions, where the students are attentively listening to a teacher, the average grade 1 student will not understand about 1 in 6 simple, clearly spoken words. About one quarter of grade 1 students will be less successful and will not understand 1 in 5 words spoken by the teacher. This is true for the average conditions found in apparently typical good classrooms. Of course many classroom designs are below average and in them students would understand less of the teacher’s speech. Although older students are better able to understand speech in noise, the typical level of noise found in classrooms clearly interferes with the learning process, making it more difficult for students to learn new information. This increased difficulty will be further exaggerated for certain groups of students such as: hearing impaired, second language listeners, students with colds or temporary ear infections etc. and students with various learning disabilities.

Excessive noise also creates problems for the teacher. Talkers naturally talk louder to overcome the masking effects of noise. (This is known as the Lombard effect). Many teachers talk significantly louder than the levels used in normal conversation to ensure that students can better understand them. The Canadian Language and Literacy Research Network classroom acoustics project found the average voice output of teachers to be almost always above a ‘raised’ voice level. Teachers are known to have a much larger incidence of voice impairment problems due to their prolonged use of higher voice levels. Moderate amounts of noise in the classroom, not only impede the learning process for students, but also increase the incidence of voice strain in teachers.

Room Acoustics
Voice levels in rooms are naturally enhanced by reflected speech sounds that arrive at the listeners within a small fraction of a second after the sound travelling directly from the talker to the listener. Consequently sound levels at the rear of a typical classroom are not so much lower than at positions closer to the teacher. These early-arriving reflections of speech sounds are not heard separately by the listener, but enhance the apparent loudness of the speech sounds. This makes the speech effectively louder relative to noise levels and increases the intelligibility of the speech. When the teacher turns and talks away from a particular group of students, they can still usually hear well because of the early arriving reflections of the speech sounds. It is important to design rooms to preserve and strengthen such useful reflections of speech sounds. They are essential to ensuring that speech is loud enough relative to various interfering noises.

In some rooms, speech sounds can be heard reverberating around the room for more than a second after the talker has stopped. A gymnasium is an extreme example of a room with too much reverberant sound that may persist for more than a second after the speech has stopped. Such reverberant sounds cause one word to blur into the next and decrease the intelligibility of speech. The reverberant character of a room can be assessed by its reverberation time. (This is the time it takes for a loud sound to fade away to inaudibility after the source has been stopped). For typical classrooms, intended for 20 to 35 students, the reverberation time should be about 0.6 s. Although it is not important to get exactly this value, you can have too much or too little reverberation. Too much sound absorbing material in a classroom reduces reverberation time (which is good), but at the same time will reduce useful early-reflected speech sounds and hence will reduce effective speech levels (which is bad). It is very important that the room enhances the effective levels of the speech sounds for all listeners by providing useful early reflections of the speech sounds.

Although this seems a little complicated, in practice conventional Canadian construction tends to lead to classrooms with acceptable room acoustics. Cost concerns usually dictate that the classrooms are not too large (less than about 250 m3) and conventional (inexpensive) construction usually leads to a suspended acoustically absorbing ceiling. With the added absorption of students, teacher, and furnishings, most classrooms tend to have close to ideal reverberation times. Problems occur when more extravagant architecture is used with larger room volumes.

Excessive noise is a much more serious and widespread problem than poor room acoustics. It is quite common for noise energy to be 10 times greater than optimum; it is almost impossible for reverberant sound to be 10 times too high.

Student Activity Noise
Noise from student activity is also an important concern. Even well behaved students attentively listening to the teacher make noise, and occupied classrooms are always noisier than the unoccupied room. While it may be quite acceptable to have higher levels of student activity noise during many classroom activities, it is important that teachers understand the importance of controlling student activity noise when critical teaching activities are happening. To ignore this problem is to place an additional burden on children trying to learn new concepts and ideas.

It may not be obvious to the teacher that the students are having increased difficulty, because adults can understand speech in noise much better than younger children. Although grade 6 students can discriminate speech in moderate noise almost as well as adults, grade 1 students require, on average, conditions that are 7 decibels quieter to understand the same percentage of clearly spoken simple words as would the grade 6 students. Thus many teachers probably do not fully appreciate the problems that the younger students experience in moderately noisy conditions because they as adults do not have the same difficulty.

Open Concept Schools and Other Bigger Issues
The new results referred to in this article were obtained from enclosed elementary school classrooms of quite conventional designs. Not all schools are like this and some have much larger problems. One school included in our study had been originally built as an open concept school with no walls between groups of 3 classrooms. Over the years there had been various attempts to create some form of partitions between the classrooms. However, these attempts were not successful because the various temporary partitions were not effective barriers to sound and because large areas had to be left open to allow adequate ventilation to the spaces. The result was a disturbing cacophony of sound. Sometimes the voice of the teacher from the adjacent space was louder than that of the teacher situated in the space. Students had great difficulty deciphering the confusing mix of messages, and teachers found the experience very frustrating. Open concept architecture for schools is completely incompatible with providing environments where children can easily understand speech. Since verbal communication is a significant part of the educational process, one must conclude that these schools are a major impediment to academic achievement for many students.

There are a number of studies in the scientific literature that also report much larger problems than found in the Eastern Ontario schools that we tested. In warmer climates very high noise levels have been measured in schools in high-density urban areas. For example, an Italian study reported high noise levels due to the combination of a school with normally open windows located close to very busy road traffic. While such extremes may not be common in Canada, there are probably many schools in urban areas exposed to significant levels of outdoor noise. In these situations, the building facade and especially the widows must be designed to be effective barriers to the unwanted outdoor noise. This would typically require that the windows not be opened and hence would involve at least a very quiet ventilation system. In most parts of Canada, keeping the windows completely closed in warmer seasons would also need a quiet air conditioning system. For new construction, a better approach would be to locate the school away from high levels of outdoor noise.
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The Problem - If children are unable to hear and understand the words spoken by the teacher, they cannot learn the new concepts and ideas being taught. If only some of the words are understood, learning becomes more difficult depending on the child’s own abilities to cope. Classrooms should fully support the learning process and not impede it.

Speech Recognition Tests - Speech recognition tests can measure children’s ability to recognize clearly spoken, simple words in various classroom environments. From these tests we know that noise and any other clearly audible sounds will reduce children’s ability to understand spoken words. They also show that less than ideal room acoustics can further degrade speech communication in rooms.

Noise - For accurate speech communication, the speech sounds must be noticeably louder than noises and other interfering sounds. The recommended ideal for noise to not exceed 35 dBA in a classroom is very quiet and indicates that most clearly audible sounds interfere with speech communication.

Sources of Noise
Heating and ventilation system noises. Solution: use quieter ducted central ventilation systems and avoid units located in the classroom.

Student activity noise from adjacent spaces or from outdoors. Solution: Move or reschedule the activity causing the noise; improve the sound insulation of walls and windows.

Other outdoor noises. Solution: Better sound insulation of exterior walls and windows, and locate new construction away from noisy roads and other noise sources.

Student activity noise in the classroom. Solution: When a critical teaching activity is in progress, the teacher must ensure that students are quiet and attentive.

Room Acoustics - Some reflecting surfaces are essential to enhance speech sounds at positions far from the talker and also when the talker turns his/her head away from a listener. If a room is too large (>250 m3) speech sounds may reverberate around for more than a second and degrade intelligibility as occurs in many gymnasia. Solution: Classrooms should not be too large and should include some sound absorbing surfaces.
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What to do?
The new results from current investigations will be used to more precisely define ideal criteria for classrooms according to the age of the students. However, initial results suggest that the conclusions will not be very different than the recommendations of the new US Classroom Acoustics standard. That is, for ideal conditions for accurate speech communication, noise such as that from heating and ventilating systems or that intruding from adjacent spaces, should not exceed 35 dBA. Although not ideal, a little higher level (40 dBA) may often suffice for older students. This will normally require ducted ventilation systems with the fans (and any other noise producing components) located outside of the classroom.

For normal sized classrooms (less than 250 m3), the inclusion of a sound-absorbing ceiling along with typical classroom furnishings will probably provide an acceptable combination of beneficial early-arriving reflections and reduced reverberant speech sound. Where calculations of expected conditions are to be made, a reverberation time goal of about 0.6 s should be the design goal. Surfaces that can provide strong early-arriving reflections to many listeners such as the central portion of the ceiling are to be encouraged and large room volumes should be avoided.

Measurements of noise levels in classrooms show, that the noise levels of attentive students during a teaching activity are 5 to 10 decibels louder than the noise levels found in the unoccupied room. It is therefore very important to control such student activity noise during periods when accurate speech communication is critical. For example, if a teacher wishes to explain a new concept to the entire class, he/she will be much more effective if the students are as quiet as possible.

Conclusions
There are many factors that can interfere with a child’s academic development. Many of these may be difficult to control. However, acoustical conditions in classrooms are completely under the control of teachers and school authorities and can be optimized to maximally support accurate speech communication in the classroom. Being able to understand the words of the teacher’s explanation must be an essential first step towards understanding the message too.

Dr. John Bradley, Canadian Language and Literacy Research Network, Project Leader is based at the University of Ottawa and National Research Council Canada.

The Canadian Language and Literacy Research Network supported this research project. For a full listing of the research team and for more information on this research paper’s references please visit The Network’s Web site located at http://www.cllrnet.ca . Click on the Resources Focus Area and download the complete paper titled Does the Classroom Assist or Impede the Learning Process?