Classroom Design for Good Hearing
Ewart A. Wetherill
simply because of the way
they are constructed and finished.
It is a shocking fault, for the need to hear well
is basic in education.
McQuade, Schoolhouse, 1958
In the summer of
2002, the American National Standards Institute published Standard 12.60,
a totally new standard that provides acoustical performance criteria,
design requirements and design guidelines for new classrooms and
renovation of existing classrooms. The goal is to ensure a high degree of
speech intelligibility in learning spaces. In order to achieve this, the
noise level in an empty classroom should be kept to less than 35 decibels,
and reverberation or echoes controlled.
While the impetus for the standard
began initially as an effort to improve schools for children with impaired
hearing or other learning disabilities, children with normal hearing will
also benefit greatly from these standards.
The good news for architects and
builders is that compliance with the acoustical standards need not be
costly if they are incorporated early into the planning and design
process, although remodeling existing facilities could be expensive
depending on the actual situation. The requirements for good hearing were
first presented formally to the American Institute of Architects in 1898
and have been successfully applied to many schools. However, in the
absence of enforceable standards far too many schools have been built with
little or no concern for good hearing. Since acoustical problems are
created by the design they can just as easily be avoided by the design.
EXISTING CONDITIONS IN U.S. SCHOOLS
Elementary and secondary education, the
nation's largest public enterprise, is conducted in more than 80,000
schools in about 15,000 districts. America's public schools serve more
than 42 million students. In February 1995, the U.S. Government Accounting
Office (GAO) presented a report to the U.S. Senate on the results of a
survey of school officials across the country on the physical condition of
their facilities. The report comprised hard facts concluding that more
than $100 billion would be needed to restore all of the schools to good
condition. The most frequently mentioned of all the "unsatisfactory
environmental conditions" was "acoustics for noise control."
One outstanding example of acoustical
inadequacy can be found in the standards set by the Los Angeles Unified
School District, one of the largest in the country. These allow the use of
classroom ventilation/air conditioning units that are up to 20 decibels
noisier than would be permitted by Swedish standards. The inevitable
conclusion is that school children cannot hear much of what is said, while
teachers must shout to be heard at all. A second example that should be
familiar to many was the disastrous trend in the late 1960s to open-plan
schools. These created a situation in which some school children could
hear the teacher of an adjacent class more clearly than their own teacher.
Thus, a combination of outdated
facilities and unfortunate design or construction decisions leave us with
an inheritance that will be a burden for decades to come. This legacy of
past policies will consume a very significant part of the limited funds
that many communities seem currently willing to allot to school
construction or renovation; so skillful planning and site selection will
be essential to attain the new goals.
CHILDREN AT RISK
In December 1997, representatives of
eleven national groups joined the Acoustical Society of America in a
workshop on Eliminating Acoustical Barriers to Learning in Classrooms.
From this workshop has developed a coalition that worked actively to
further improved hearing conditions in schools. Leaders in the field of
audiology and a wide range of disciplines related to design and
construction of educational facilities presented the results of surveys
and research on the prevalence of hearing disorders and substandard
facilities, and their effects on hearing. The truly alarming statistics
clearly show the disadvantage resulting from poor hearing conditions for
both normal and hearingimpaired school children.
Studies of speech recognition confirm
that an adult listener hearing words in the context of a sentence can fill
in words or syllables that are not heard clearly, depending on the size of
the listener's vocabulary. Since children have smaller vocabularies, they
are less able to fill in the words not heard clearly. Similarly, someone
using English as a second language or someone who suffers from an
attention deficit disorder are at a significant disadvantage in a noisy
classroom. In addition, many children with usually normal hearing have
temporary hearing losses from illness. Otitis Media, a bacterial
infection of the middle ear that is the most frequently-occurring
childhood medical complaint, has more than doubled in the last decade.
Compounding the learning disadvantages
that confront children in noisy classrooms or with impaired hearing are
the constant discouragement and frustration that can inhibit the
motivation of even the most talented to learn and to excel.
The importance of clearly hearing the
teacher seems self-evident, but this has not been a design criteria of
many schools in the past.
REQUIREMENTS FOR GOOD HEARING
Two basic criteria must be satisfied to
meet the requirements for good hearing:
1. A quiet background (e.g. noise from intruding traffic, adjacent
classes, ventilation systems etc.)
2. Control of reverberation and self-noise
SPEECH TO NOISE RATIO
Speech in the classroom must be heard
over the prevailing background noise level, be it intruding noise from
traffic, adjacent classes, or a noisy ventilation system. A convenient and
easily measured descriptor is the Speech to Noise ratio (S/N). There is
general agreement that desired S/N ratios for speech recognition are:
Normal-hearing:
Adults: at least 6 decibels
Children: greater than for adults, at least 10 decibels
Hard-of-hearing listeners
Adults: at least 15 decibels,
Children: greater than for adults
By contrast, a survey of actual
classroom conditions taken between 1965 and 1968 indicated a Speech to
Noise ratio range from +5 decibels to -7 decibels. This information alone
adds support to the growing concern both for children's understanding and
for teachers' voice strain.
Reverberation (commonly known as an
echo) is defined as the persistence of sound in a room after the source
has stopped. In a reverberant space, successive syllables blend into a
continuous sound, through which it is necessary to distinguish the orderly
progression of speech. The level at which this sound persists is
determined by the size of the space, the speech level and the interior
finish materials. Reverberation time (the time it takes for a sound to die
off) is measured in seconds, with a low value-around 0.5 seconds or
less-being optimum for a classroom seating about 30 children.
Reverberation can be controlled by the use of readilyavailable
sound-absorbing wall and ceiling materials that comply with building code
requirements.
EFFECTS OF NOISE AND REVERBERATION ON
SPEECH RECOGNITION
Mean speech-recognition scores (the
percent of words correctly recognized) of adults with normal hearing for
various S/N ratios clearly demonstrate the connection between good
acoustics and effective hearing.
| S/N ratio | Word Recognition |
| +12 decibels (low-background noise)
+6 decibels 0 decibels (high-background noise) |
95.3%
80.7% 46.0% |
Mean speech-recognition scores (in
percent correct) of children for monosyllabic words with various
reverberation times (RT) show a similar correlation.
| RT - Seconds | Normal Hearing | Hearing Impaired |
| 0.0 (no echo)
0.4 1.2 (persistent echo) |
94.5 %
82.8% 76.5% |
87.5%
69.0% 61.8% |
The combined effects of poor Speech to
Noise and long reverberation time for children, which is the actual
situation encountered daily in many of the nation's schools, are
predictably a substantial handicap to entire classes. The following scores
are for monosyllabic words.
| Test Condition | Normal Hearing | Hearing Impaired |
| FOR REVERBERATION TIMES OF 0.0 SECONDS: | ||
| +12 decibels
0 decibels |
89.2%
60.2% |
70.0%
39.0% |
| FOR REVERBERATION TIMES OF 1.2 SECONDS: | ||
| + 12 decibels
0 decibels |
68.8%
29.7% |
41.2%
11.2% |
The following conclusions can be drawn
from these test results and from corroborating evidence compiled from
other test situations.
1. Understanding of children with normal hearing can be seriously
affected by a combination of excessive background noise and reverberation.
2. Hearing impaired children are always at a disadvantage compared to
those with normal hearing but the difference can be minimized by
acoustical controls.
3. Comprehension levels for multisyllabic and unfamiliar words can be
expected to be worse than indicated by monosyllabic testing.
4. Decrease in intelligibility with distance from the teacher can be
minimized by acoustical treatment and shaping of the space.
EFFECT ON TEACHERS
In addition to children's hearing
concerns, the effect of trying to compete with an acoustically-difficult
environment creates a problem of severe strain on the vocal chords for
many teachers. While not as well-known or studied as the listener's
ability to understand, voice strain is belatedly being recognized as a
serious and potentially incapacitating problem for teachers. However,
effective acoustical treatment of a classroom can create significant
benefits here also.
EXAMPLES OF EFFECTIVE CLASSROOM DESIGN
Designers and builders can improve
hearing conditions in schools by incorporating the basic principles of
acoustics into classroom design. For every new and remodeled school, the
control of unwanted sounds and enhancement of wanted sounds, without the
complications inherent in general amplification, should be placed high on
the list of design goals. For new classrooms accommodating from 30 to 40
children these requirements should not add anything to the cost of either
design or construction. However, correction of acoustical deficiencies in
existing facilities could be costly, depending on the particular
situation.
At least the following considerations
must be addressed (see appended sketches):
Control of unwanted sounds
- locate schools away from highways, rail tracks, and flight paths
- minimize noise intrusion from outdoors (figure D)
- minimize interference between classrooms
- design quiet ventilation system (figures E and F)
Enhancement of wanted sounds
- control excessive reverberation by sound absorption
- minimize echoes from distant surfaces (such as the back wall)
- use hard materials for useful sound reflections (such as on surfaces beside and above the teacher)
Figure G shows a suitable
acoustical treatment for a "traditional" classroom
configuration. For other desired class uses, redistribution of the
required sound absorption may be appropriate. The booklet Classroom
Acoustics, available at NPC's website, www.nonoise.org/quietnet/qc/,
and the ANSI Standard S12.60 are of help when working with teachers and
administrators.