Tuning up Classroom acoustics is an exciting adventure for the acoustical society.
Who is at Risk
One of the important facts is that normal hearing children, normal hearing older adults, normal hearing individuals with speech and learning disorders as well as hearing-impaired persons of all ages do not understand speech as well as normal hearing young adults when the listening conditions are less than maximally favorable. This should be taken into account in designing the acoustics of environments where these population groups study, work, and play.
Since available information about speech perception has been primarily been for young adults, there has been little appropriate information regarding children’s perception of speech to assist architectural acousticians designing classrooms for special population groups. Certain groups, including normal children and certain handicapped persons with presumably “normal” hearing, require enhanced acoustic signals in order to understand speech that must be considered in designing the acoustics of classrooms.[i][1]
Children have significantly poorer listening comprehension than adults.
There are several populations of children with normal hearing sensitivity that experience greater speech perception difficulties in degraded listening conditions. (Crandell, C.)
· Young children (less than 13 – 15 years old)
· Conductive hearing losses
· Articulation Disorders
· Learning Disabled
· Non-native English
· Central auditory processing disorders
· Minimal sensorineural hearing losses (SNHL)
· Unilateral hearing losses
Children with SNHL require signal to noise (S/N) ratios greater than +15 dB and reverberation times no longer than 0.4 seconds for maximum communicative efficiency.
Traditional listeners for speech perception have used the proverbial college student with normal hearing as the listening subject. More recent investigations have also employed children, hearing impaired listeners, older persons and certain handicapped persons. Information about the effects of noise and reverberance on perception of speech by children and certain handicapped individuals from some of these studies will help the consultants re-examine the importance of establishing realistic acoustical criteria in classrooms.
Intensities required for normal hearing 3, 5, and 10 year
old children and adults to achieve essentially 100% correct performance using
the NU-CHIPS testing procedure.[ii][2]
|
|
Age |
Intensity (dB
SPL) |
|
|
|
|
|
|
|
|
3 |
38 |
|
|
|
5 |
25 |
|
|
|
10 |
18 |
|
|
|
Adult |
11 |
|
In this study, the task of the child or adult is to point at the proper picture on one of the NU-CHIPS 2 that represents the word presented.
Construction of new K-12 school building has recently re-commenced after three decades of relatively little activity. Present design guidelines are drawn from the last major construction period important bodies of scientific knowledge gained during the ensuing thirty years. For example: no current architectural guidelines exist to promote good communication conditions between teachers and students. However, significant scientific research has investigated the acoustical parameters needed to foster good speech intelligibility conditions in classrooms.
This recent commencement of school construction is occurring and school districts, here in Washington State and elsewhere across the United States, are finding that they do not have contemporary design guidelines to dictate how school buildings should be laid out and outfitted. Consequently, school districts are, for the most part, relying on guidelines that were used during the last construction push and are updating these guidelines only when design and construction practices have markedly changed since the 1960’s.
But there are a number of topics for which changes in design guidelines have not been instituted, largely because the design and construction professions are unaware of new bodies of knowledge.
One such topic concerns speech communication needs in classrooms. There are no established architectural/acoustical guidelines for designing k-12 classrooms which specifically foster good communications between teachers and students. For good speech conditions to exist in classrooms, it is necessary that children be able to clearly hear and understand what teacher are saying. But surveys of K-12 classroom through the United States – examining schools both from the last major construction period and before – have shown serious deficiencies in speech communication in classrooms. Studies show that k-12 communication conditions perform less well than the norm on topics like speech recognition and reading comprehension[3], [4], [5]. These studies indicate that students experiencing poor speech communication conditions fall increasingly further behind the performance of students schooled in setting with better acoustics. There is evidence also that students who habitually experience difficulty in speech communications in schools are more likely to display disruptive behaviors and considerably higher rates of truancy.
Numbers of studies made by research audiologists have investigated classroom attributes needed to foster good speech intelligibility. The major parameters which have been indentified as causing inferior speech communication between reverberation [6], [7], [8], too great distances separating teachers from students [9], and inadequate teachers’ voice levels. Additionally - and related to the fourth parameter - is that many teachers’ voice levels have been reported to decrease over the course of the teaching day as voice fatigue sets in.
Unacceptable background noise levels are produced by:
The background noise in a classroom is generated by several types of sources. If the school is located adjacent to a heavily traveled highway or near an air traffic route, the noise levels that intrude can be extremely disruptive to understanding speech. However noise that intrudes into the classroom from gymnasium, band room, adjacent classroom or the corridors can also interfere with a child’s ability to hear and understand speech. Uncarpeted floors amplify foot scuffling and chair bottoms squeaks. Any teacher will agree that students become fidgety and tend to generate more noise when the mechanical noise in the classroom increases. This can happen when the fan in the air conditioning system cycles on and off. Children with attention deficits have significantly greater difficulty controlling impulsiveness and attention in the presence of auditory distractions.
Children who have fluctuating or permanent hearing loss also have attention and speech understanding problems that stems from trying to understand fragments of speech they perceive with their defective hearing. Their hearing disability is further debilitating when they are trying to listen in spaces with low levels of reverberant background noise. Low frequency noise creates “upward masking” that tends to cover up soft consonant, high pitch word endings and brief words. Reverberation tends to acoustically smear the speech fragments that a child does hear.
One excellent method of improving the signal to noise ratio in classrooms is to use equipment similar to a public address system. Probably the classroom FM system is one of the most popular. The teacher wears a portable microphone which transmits a radio signal to an amplifier that is connected to various configurations of loudspeakers in the classroom. This system can be adjusted to raise the speech level in the classroom significantly above the background noise that is produced by the environmental and ventilating noise systems and is therefore able to permit the teachers voice to reach every student without having to unnecessarily project her voice level.
Teaching styles have changed in the last few years and to overcome the noise of more interactive classrooms, teachers usually spend more of their teaching day trying to project their voices over the noisy classrooms. Approximately 80% of teachers report vocal fatigue and there are a growing number of complaints to workman’s compensation for vocal injury due to the need to speak loudly enough to hopefully reach the students in the rear rows. One study found that teachers take an average of sick leave of 2 days per year for voice related problems.
Reverberation is the subtle prolongation of sound that persists after the source has ceased to emit the sound. This prolongation occurs because some of the sound waves reflect from one surface to another until they become inaudible. The sound arrives at the listener by the direct path and also with innumerable reflected paths. The rate of decay of the sound will depend on its volume and the distributed absorption of the surfaces. The time of the reverberation is defined as the time it take s the sound pressure level to day to one millionth (60dB) of its former value.
Although the reverberation time is an important subjective quality of the room, for people with hearing disabilities, other factors such as the ratio between the direct and reverberant sound are probably more important since the listener hears a mixture of the direct sound from the source and the reverberant sound from the room. Direct sound allows a listener to localize on the source whereas the reverberant sound is random and comes from every direction. Moving away from the source the sound becomes weaker and at some point which is called the critical distance, the sound from the two paths will be equal. The critical distance is surprisingly small for most rooms but normal hearing people that have binaural hearing and are reasonable close to the critical distance can still localize on the source. Unfortunately people with hearing disabilities find it difficult or impossible to make this distinction. Increasing distance from the source obscures the direct path and the listener only hears the reflected sound.
Objectively measured reverberation time may not be necessarily observed by the listener if the reverberant sound may be at a higher or lower level when it is compared to the direct sound. For good speech intelligibility, too much reverberance makes speech recognition difficult for normal hearing listeners and is extremely detrimental for listeners with hearing disabilities.
Mean speech-recognition scores, in percent correct of children with normal hearing and hearing impairment for monosyllabic words across various signal to noise ratios and reverberation times. (Crandell, C.)
|
Test Condition |
|
Normal Hearing |
|
Hearing Impaired |
|
RT=0.00 second |
|
|
|
|
|
Quiet |
|
94.5 |
|
83.0 |
|
+12 dB |
|
89.2 |
|
70.0 |
|
+6 dB |
|
79.7 |
|
59.5 |
|
0 dB |
|
60.2 |
|
39.0 |
|
RT=0.4 seconds |
|
|
|
|
|
Quiet |
|
92.5 |
|
74.0 |
|
+12 dB |
|
82.8 |
|
60.2 |
|
+6 dB |
|
71.3 |
|
52.2 |
|
0 dB |
|
47.7 |
|
27.8 |
|
RT=1.2 seconds |
|
|
|
|
|
Quiet |
|
76.5 |
|
45.0 |
|
+12 dB |
|
68.8 |
|
41.2 |
|
+6 dB |
|
54.2 |
|
27.0 |
|
0 dB |
|
29.7 |
|
11.2 |
These Studies show that for un-amplified rooms with increasing reverberance, both the normal hearing student and the hearing impaired students have difficulty understanding what was being said.
Our challenge is to develop convincing public relations that
will be necessary to convince all of the different diversities that they should
be cooperating with each other to solve the problem of ensuring that every
child should not only hear the teacher but understand what was being said.
Acoustically inadequate classrooms are one of the most insidious factors that are contributing to the countless numbers of students who have graduated from high school without the skill of being able to read and write.
The audiologists have paved the way in discovering these acoustical shortcomings for the last 20 years but have not been interfacing with experts in the design field to convince them to make the necessary changes to correct the problem. They have already suggested recommendations for limiting the reverberation time and the background noise from the ventilation system and providing sound amplification systems to amplify the teacher’s voice. The importance of these critical requirements has to be recognized and acknowledged by parents, educators, school boards, architects, acousticians, the owners that build the schools if we want our children to hear and participate in the classroom activities.
Architectural acoustical designers have the skill to offer cost effective recommendations for designing classrooms that will meet these standards and provide the appropriate reverberation time and to ensure that the noise levels generated by the mechanical system and from adjacent spaces and the outdoor environment will be acceptable.
Many school districts have a goal to mainstream children with varying degrees of hearing loss into regular classes with their peers.
If we are to succeed in correcting classroom acoustical deficiencies the creative scientists must join forces and direct their efforts into explaining the reasons why children are not understanding what the teacher is saying and making positive suggestions for appropriate designs to correct these problems:
1. To parents
2. Students
3. Teachers
4. Administrative Educators
5. School Districts
6. Facilities Departments
7. Architects
8. Acousticians
9. Audiologists
Their understanding of the importance and consequences of improving speech intelligibility will help them focus on making this a high priority in the construction of acoustically superior classrooms.
Summary
Fine Tuning the acoustical environment in classrooms is a multifaceted process. Recognizing that the importance of correcting each of the many deficiencies that have an interrelation on performance, such as lowering the reverberation time, reducing the environmental and internal noise levels, and raising the signal to noise levels. Improving each of these will help to optimize the performance but they may not be cost effective. We need to prioritize changes that will offer substantial improvements and have the greatest opportunity to meet our goals. We should consider the following:
[1] Elliot, L.L. (1982) “Effect of Noise and Perception of Speech by Children and Certain Handicapped Individuals”, Sound and Vibration, Dec 1982, 10 – 14.
[2] Elliot, L.L., Katz, D. (1980), The Northwestern University-Children’s Perception of Speech Test (NU-CHIPS),, Auditec, S. Louis.
[3] Davis, J.M., J. Elfin, R. Schum, and R.A. Bender, “Effects of mild and moderate hearing impairments on language educational, and psychological behavior of children,” Journal of Speech and Hearing Disorders, 51 (1), 1986 ppg 53-62.
[4] Blair, J.J., Person, and S. Vierwig, “The effect of mild hearing loss on academic performance of young school-age children,” Volta Review, 87,(2), 1985, ppg 87-93
[5] Zinkus, P.W. and I.M. Gottleib, “Patterns of auditory processing and articulation deficient juvenile delinquents,” Journal of Speech and Hearing Disorders, 48(1), 1983, ppg 36-40
[6] Nabelek, A.K. and P.K. Robinson, “Monaural and binaural speech perception in reverberation for listeners at various ages,” Journal of Acoustical Society of America, 71(5), 1982, ppg 1242-1248
[7] Neuman, A.C. and I. Hochbert, “Children’s perception of speech in reverberation,” 73(6), 1983, ppg 2145-2149
[8] Helfer, J.S., “Binaural cues and consonant perception in reverberation and noise,” Journal of Speech and Hearing Research, 37(4), 1994, ppg 429-438
[9] Peutz, V.M.A., “Articulation loss of consonant as a criterion for speech transmission in a room,” Journal in Audio Engineering Society, 19(11), 1971, ppg 915-922
[10] Lipscomb, David Environmental Noise is Growing – Is it Damaging our Hearing? Clinical Pediatrics, Vol 11, No 7, July 1972
[11] Lipscomb, David Environmental Noise is Growing – Is it Damaging our Hearing? Clinical Pediatrics, Vol 11, No 7, July 1972
[12] Lipscomb, David Environmental Noise is Growing – Is it Damaging our Hearing? Clinical Pediatrics, Vol 11, No 7, July 1972