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Problems: Poor Acoustics

There are two critical aspects involved in auditory learning: acoustics and the hearing ability of the child. Acoustics measure the impact that the physical environment will have on that most important piece of the learning process. The acoustic environment can dramatically decrease the effectiveness of the teaching. Students in today’s classrooms are unable to understand 25 to 30 percent of what their teacher said because of excessive noise and reverberation. ⁱ

It is important to have some understanding of the contributing factors that determine the success (or failure) of an acoustical environment. There are three major components of acoustics: ambient noise, reverberation, and the signal to noise ratio.

Ambient Noise

In a classroom, ambient noise is all the background noise going on in the classroom; externally generated and internally generated. It consists of any noise that is not produced by the primary instructional medium (typically the teacher.) External ambient noise is outside noise entering the classroom through open windows or poorly insulated windows. It consists of lawn mowers, auto traffic, birds, children on the nearby playground, air traffic, etc. Older school buildings, the majority of which exist, were not built with acoustic considerations in mind. The building materials used were not designed to be acoustic barriers. The locations are not necessarily acoustically desirable. Those without air-conditioning, require windows to be open allowing all these “foreign” sounds to invade the classroom and disrupt the teacher’s attention.

Ambient noise is also generated from within the school itself. Antiquated heating systems, the wall or ceiling fan, the hum of the lights, the noise from a computer or printer, hallway traffic, etc all create noise within the classroom. Ambient noise also needs to include the students themselves? Even the most well disciplined students create noise: chairs moving, feet shuffling, pencils dropping, supplies gathering, even the occasional whisper to the neighboring student. By themselves, these noises appear innocent enough, but multiply the shuffle of two feet or one pencil dropping by twenty-five students. These noises end up competing with the teacher’s voice and the teacher’s attention.

Reverberation

The second component of acoustics is reverberation. Think of walking into a large empty room. What happens to the sound in that room? What we think of as an echo is sound reverberating off the smooth surfaces of the room. Reverberation time is the phenomenon of the physical environment that measure how quickly the physical environment can absorb acoustic energy. The ability of the room to absorb the energy that is put into the room, determines how much interference the residual energy will interfere with the learning environment. Reverberation time (the echo time) increases in rooms with high ceilings and bare floors. The longer that it takes sound to reverberate, the harder it is to hear; particularly if you have multiple sounds (including that background noise we just talked about) reverberating. Now think again about the classroom. The older classrooms typically have the high ceilings and the bare floors. Even the newer classrooms are being denied carpeting by school officials due to bacterial concerns. These smooth surfaces will increase the reverberation time, degrading the child’s speech intelligibility.

Studies have found that in unamplified rooms with typical reverberance and typical background noise levels, children with and without hearing problems have difficulty understanding what was being said. The following table illustrates the results of a studyⁱⁱ of speech recognition scores in percent correct of children with normal hearing and with mild to moderate degrees of hearing loss under various controlled classroom listening conditions.

Signal to Noise Ratio (SNR)

The last component of acoustics is the most critical to understanding the acoustical environment and a child’s cognitive hearing ability. It is referred to as the Signal to Noise Ratio, or more commonly the SNR. The SNR is basically how much louder the teacher’s voice is, above the other noises in the room. For example, if the teacher’s voice is at 65 decibels and the background noises (students, heaters, computers, etc.) are at 55 decibels, we would say that the SNR is 10 decibels. Therefore, the other two components (ambient noise and reverberation time) affect the SNR. An increased background noise will lower the SNR. An increased reverberation time will lower the SNR. But what the SNR includes that the other two components do not, is the teacher’s voice. The teacher’s voice becomes the Signal.

SNR is the most critical component because it determines speech intelligibility; i.e. the ability to understand what you hear. This is where the difference between a child’s ability to hear and an adult’s ability to hear becomes most evident. Children, NORMAL hearing children, require an SNR of +15 decibels. Therefore, a child must have the teacher speaking at least 15 decibels louder than the background noise in the room, in order to fully understand what he is hearing. In comparison, an adult with normal hearing requires an SNR between +4 and +6 decibels. So conservatively speaking, children (with normal hearing) need a teacher to speak at least 9 decibels louder than adults need the teacher to speak, in order to fully comprehend the speech. This basic understanding allows us to realize how detrimental poor acoustics can be to auditory learning, which again, makes up a significant portion (up to 75%) of a child’s day in school.

Comparison of Desired Acoustics to Actual Acoustics:
There are two major authorities that have set the standard on acceptable acoustics. The first is the American Speech Language Hearing Association, commonly referred to as ASHA. According to ASHA, ambient noise should be no louder than 30-35 decibels in an empty room. Reverberation time should not exceed .4 seconds and the SNR should be no lower than +15 decibels (which is what a child requires for intelligible comprehension).

The second authority on acoustic standards is the American National Standards Institute, or ANSI. ANSI adopted in 2002, standards that were developed by the Acoustical Society of America (ASA) regarding acoustical performance criteria and design requirements for classrooms and other learning spaces. These standards are referred to as ANSI S12.60-2002, “Acoustical Performance Criteria, Design Requirements and Guidelines for Schools.” The general guidelines are based on just two of the three acoustic performance factors: background noise and reverberation. ANSI specifies that average sized core learning spaces have a background noise level not exceeding 35 decibels, with a maximum reverberation time not to exceed .6 seconds for small rooms and .7 seconds for large rooms. Unlike ASHA, ANSI fails to address the Signal to Noise Ratio, the most critical component.

While ASHA and ANSI both provide recommended acoustical standards, their intent is different. ANSI standards were adopted to provide school districts with guidelines for new construction and renovation that would improve the acoustical environment in the schools. For example, the architect or designers are responsible for specifying HVAC, electrical, lighting, plumbing systems and installation methods that will control the background noise levels down to the specified standards. Similarly, ANSI specifies certain sound absorbing materials be used to control the reverberation time. However, these standards are still voluntary and relate mainly to new school construction. They were not adopted into the International Code Council’s 2003 Building Code because of concerns about associated costs and compliance issues.

Additionally, ANSI does not address the Signal to Noise Ratio, the fundamental determinant to a child’s ability to hear in the classroom. There are intentional reasons why this component was ignored. First, the background noise level specified (35 dBA) ignores the reality of the occupied classroom. It fails to consider what happens to background noise levels once children occupy the room along with the use of instructional equipment, such as a computer or overhead projector. As you will see in a 2005 study done in Los Angeles, background noise increases dramatically when a classroom is occupied, making the 35 decibel requirement meaningless. The importance of this cannot be emphasized enough. If the actual background noise is significantly high, the Signal to Noise Ratio will be low, too low for children to hear.

Actual paragraphs taken from the ANSI standards:

• 1.1.2 acoustical performance criteria are specified in this standard by limits on maximum one-hour a-weighted and c-weighted background noise levels and limits on maximum reverberation times. An objective of these performance criteria is to achieve a level of speech that is sufficiently high relative to the background noise level for listeners throughout the classroom or learning space. However, a requirement for the relative difference between speech levels and levels of background noise, usually referred to as the signal-to-noise ratio, is not within the scope of this standard.
  
• 1.1.4 this standard does not apply to noise generated within a classroom by its occupants. Occupant- generated noise sources include voices and the sounds of classroom activities such as the moving of chairs. Furthermore, this standard does not apply to the noise from portable or permanent built-in equipment used during the course of instruction, such as audiovisual equipment and computers. However, the background noise generated by occupants and instructional equipment can seriously degrade communication or speech intelligibility in learning spaces.

The second reason that the Signal to Noise Ratio was ignored has to do with the way sound travels. Unlike background noise which remains relatively constant around a room, the teacher’s voice (signal) varies greatly depending on where that teacher is located and where the SNR is being measured. This is because sound decreases over distance; specifically it drops 6 decibels for every doubling of distance. For example, a teacher speaking at an 60 decibels 3 feet out into the front of the classroom, will be heard at 54 decibels 6 feet into the room; 48 decibels 12 feet into the room and so on. Since the background noise levels remains essentially the same, this decline in the teacher’s voice, means that the Signal to Noise Ratio (difference between the Teacher’s voice and background noise) will be declining over the distance of the room. At some point, that SNR will drop below +15 decibels, precluding some students from hearing.

The following depicts this Inverse Square Law:

Click a link below for a brief one minute video that further explains the Inverse Square Law and provides a simulated classroom example.

Media Player Format    QuickTime Format

An investigation (Crandell C., Bess, F., "Speech Recognition of Children in a 'Typcial' Classroom Setting." ASHA, Vol. 29, 1986, pp.87) examined the speech recognition ability of young children, ages 5 to 7, with normal hearing in a "typical" classroom - teacher's speech 6 dB louder than background noise; reverberation time of .45 seconds. Single syllable words were presented at speaker listener distances commonly encountered in the classroom (6, 12, 24 feet). Results indicated a systematic decrease in speech recognition ability as speaker-listener distance increased. Specifically, mean recognition scores were 89%at 6 feet, 55% at 12 feet and 36% at 24 feet. Overall, these results suggest that normal hearing children, seated in the middle to rear of a typical classroom, are at a disadvantage. Students not seated near the teacher have greater difficulty understanding speech due to the decline in the teacher's voice over distance; i.e. a decline in the SNR.

The following table summarizes the standards, the reality, and the huge discrepancy:

Believe or not, up until a study in 2005, all references and recommendations relating to background noise had to do with an unoccupied classroom, ignoring the reality of student generated noise. In 2005, a pioneering study was done to measure the classroom sound levels in a student occupied classroom, shedding new light on the noise level in the classroom and discounting the ANSI background noise standard of 35 decibels.

This study, referred to as the Los Angeles Unified School District (LAUSD) study by Dr. Paul McCarty and Jack Rollow, investigated the Los Angeles student occupied 4th grade classroom, through a two-day recording and statistical analysis of the recorded sound data. Classroom activities such as silent reading, working together, talking, and out of class activities such as lunch and recess, were recorded by the teacher so that classroom activities could be correlated with varying sound levels. Actual sound levels were recorded at 10 second intervals in a class size of 30 children. The school was new and designed with good acoustical qualities. The classroom was not considered a noisy room, even though the noise of traffic and air-conditioning could be measured.

Up until now, efforts to reduce classroom noise have been focused on reducing the noise generated by air conditioning and traffic. Because there was no information on the noise generated by the occupants, their role in the problem was dismissed as an issue of classroom management. The LAUSD study concluded that background noise levels, ranging from 43 to 52 decibels, exceeded the ANSI requirement of 35 decibels by 8 to 17 decibels. Given these results, how are the ANSI standards expected to make any improvements at all in the student occupied classroom?

Noise generated from the HVAC, previously thought to be the culprit of background noise, had little to no effect on background noise. Rather it was noise generated from the children themselves that had the greatest impact. “Working together/talking” activities were measured at 67 to 72 decibels, more than double that of the quiet activities. “Silent Reading” activities were measured at 45 decibels, only one to two decibels louder than the unoccupied room. Unoccupied levels during lunch, before and after school were about 43 to 45 decibels.

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This study raised the question, if the ANSI background noise levels are not achievable, how does a teacher generate the +15 decibel signal to noise ratio in ALL areas of the classroom, without screaming at 80 decibels?

Looking at the results of the LAUSD study combined with information about acoustics leads to any easy conclusion that the classroom is a poor environment for auditory learning to take place. It is inconceivable that a teacher can project his or her voice to the back of the room all day, in order to achieve the signal to noise ratio of +15 decibels in ALL areas of the classroom, for ALL students, ALL day.