THINK ACOUSTICS FIRST – NOT LAST
Classroom acoustics are an important, often neglected, aspect of the learning environment. Up to 60% of classroom activities involve speech between teachers and students or between students, indicating the importance of environments that support clear communication.
However, classrooms that have been constructed in the last 20 to 30 years to better engage students in hands-on activities or discussions have often resulted in active, noisy environments. Additionally, HVAC systems have created distracting background noise in classrooms. Inappropriate levels of background noise, reverberation, and signal-to noise ratios can also inhibit reading and spelling ability, behaviour, attention, concentration, and academic performance.
Furthermore, children who develop language skills in poor acoustic environments may develop long-term speech comprehension problems.
Good classroom acoustics are a basic classroom need, not an accessory, to give all students access to spoken instruction and discussion. Acoustic problems persist in classrooms because of a lack of acoustics education for architects and engineers, the prohibitive expenses of acoustic refurbishment, and because adult listeners often do not consider the limitations of children’s hearing abilities.
WHO BENEFITS FROM IMPROVED ACOUSTICS?
Put simply, the answer is both students and teachers.
Children, especially those younger than 13 years of age, have an undeveloped sense of hearing, making the impacts of background noise on hearing, comprehending, and learning more pronounced for children than adults. Students with learning, attention, or reading deficits are more adversely affected by poor acoustic conditions than the average student. Students speaking English as a second language (ESL) require significantly better acoustic conditions to hear the teacher than others. Hearing-impaired students require a significantly better acoustic environment to adequately hear than the average student. Loud or reverberant classrooms may cause teachers to raise their voices, leading to increased teacher stress and fatigue.
CONSIDER KEY LEARNING ENVIRONMENT CRITERIA
Acoustical barriers to learning may exist even if teachers and/or students are unaware that they exist. Adults’ perceptions of speech intelligibility are often better than children’s perceptions, indicating adults should not rely on their own subjective assessments of listening conditions inside of a classroom.
Speech intelligibility decreases when background noise increases or with long reverberation times. When both background noise and long reverberation times are present, they have a combined effect on both people with and without normal speech, hearing, and language.
Designers should focus on controlling background noise levels, reverberation times, and signal-to-noise ratios to improve the acoustic environment of schools.
BACKGROUND NOISE
Excessive noise in schools has a negative impact on student learning and performance While a 1 decibel (dB(A)) change in sound level is barely noticeable, background noises are perceived as doubling in loudness every 10 dB(A). Background noise in unoccupied classrooms should not exceed 30-35 dB(A). Major sources of background noise include: • HVAC noise (vents, ductwork, A/C unit) • Outdoor noise (automobiles, airplanes) • Reflected speech sounds (echo) • Noise from adjacent spaces
REVERBERATION
Reverberations occur when sound waves strike surfaces (e.g., floors, walls, ceilings) in a room and are reflected back into the space. Reverberation will continue until all the sound waves have been absorbed or have dissipated. Reverberations are affected by the quantity and effectiveness of sound-absorbing surfaces in a room. Sound reflective surfaces are typically hard and smooth. They provide little friction to absorb sound energy, prolonging sound reverberation. Sound-absorbing surfaces are typically fibrous or porous, significantly reducing sound energy through friction between the air and material fibres. Sound-absorbing surfaces can help reduce sound reflection and reverberation, but they do not reduce the intensity of the sound’s source itself.
Reverberation times (RT) should not exceed 0.4 seconds in classrooms primarily used by hearing disabled students or 0.6 seconds in general classrooms. Reducing the RT to acceptable limits will help with speech intelligibility, and the added absorption will reduce the overall sound level within the room without adversely affecting the signal-to-noise ratio.
SIGNAL-TO-NOISE RATIO
Signal-to-noise ratios (SNRs) generally become less favourable for hearing as the distance between the speaker and the student increases, suggesting that different locations within an individual classroom may have different SNRs SNRs are typically lowest at the back of classrooms or near a noise source (e.g., air conditioning unit). Students seated in the rear of a classroom may not understand a teacher’s speech to the same extent as one seated at the front of the classroom. Children with hearing disabilities generally require significantly higher SNRs than children with normal hearing. Environments with SNRs of +20 dB to +30 dB provide optimal speech comprehension for children with hearing disabilities. SNRs should meet or exceed +15 dB in allocations of a classroom.
HOW TO CONSIDER FOR HEARING IMPAIRED STUDENTS
Hearing-impaired students or students using amplification devices (e.g., cochlear implants, personal FM sound field systems) require lower reverberation times and less background noise than an average student to hear adequately. Typical classrooms do not meet the acoustical needs of hearing-impaired students.
The following design recommendations pertain specifically to classrooms occupied by hearing impaired students:
For cost-effectiveness, ensure materials are easy for school maintenance or custodial staff to install. Reverberation times in classrooms with hearing-impaired students or students using hearing assistive technologies (e.g., cochlear implants) should not exceed 0.4 seconds. Assign support personnel e.g., speech/language pathologist, teacher’s aide to assist hearing-impaired students in classrooms.
CONSIDER THE TEACHER
The acoustical characteristics of unoccupied classrooms may vary significantly from occupied classrooms. Even the sound quality in classrooms designed for excellent acoustics can decline when the classroom is occupied. Working with teachers to anticipate the activities that will take place in the classroom is an important aspect of creating a satisfactory acoustic environment.
Following are important points for consideration:
SUBJECT SPECIFIC LEARNING AREAS
Different rooms in schools (e.g., study, lecture halls, music halls, nurseries, auditoriums) require different acoustic performance standards based on the room’s purpose. Large rooms (larger than 20 square metres; e.g., auditoriums) require different acoustic design requirements than general classrooms, as they often differ in size, shape, and function. The teacher student configuration within spaces is often fixed; however, the shape of the room can vary greatly. Additionally, HVAC systems often have greater capacities and speech reinforcement systems and other audio-visual aids are typically present in these rooms. The following are design considerations for some of these principal spaces commonly found in schools.
MUSIC ROOMS
LECTURE HALLS AND AUDITORIUMS
LIBRARIES
GYMNASIUMS
ACOUSTICS AND AIR QUALITY
Using sound-absorbing materials in some classrooms has led to concerns about indoor air quality (IAQ) and multiple chemical sensitivity. However, good acoustical design and IAQ can be achieved when proper materials are specified and maintained over time. Review the following factors to provide satisfactory acoustics and IAQ simultaneously: Consider how material composition, potential off-gassing, and operation and maintenance strategies may affect IAQ when specifying materials for acoustic purposes.
Be aware that needs of other building systems (e.g., lighting, thermal, ventilation) may interfere with optimal acoustic designs; however, special architectural elements (e.g., acoustic doors, acoustic windows) are available that decrease the negative effects on acoustics. Consider acoustical ceiling tiles and wall panels in elementary classrooms over soft furnishings and cloth wall-hangings to improve the acoustic environment while minimizing fire and health hazards.
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ACOUSTICS GLOSSARY
Absorption Coefficient (alpha)
The dimensionless ration of absorbed to incident sound energy from a single interaction between a sound wave and a partition. Values range from 0 to 1.
Absorption
The product of absorption coefficient and surface area of a material, in units of sabins, used to designate the amount of sound absorbed by that material. The properties of a material composition to convert sound energy into heat thereby reducing the amount of energy that can be reflected.
Attenuation
The reduction of sound energy as a function of distance traveled.
Decibel (dB)
A unit of sound level implying 10 multiplied by a logarithmic ration of power or some quantity proportional to power. The logarithm is to the base 10. Sound intensity is described in decibels. For example: breathing, 5 dB; office activity 50 dB; jet aircraft during take-off at 300′ distance, 130 dB. Frequency (f) The number of oscillations or cycles per unit of time. Acoustical frequency is usually expressed in units of Hertz (Hz) where one Hz is equal to one cycle per second. Interpreted subjectively as pitch. Humans can hear sounds having frequencies between 20 and 20,000 Hz.
Hertz (Hz)
Frequency of sound expressed by cycles per second.
Noise Reduction Coefficient (NRC)
A single number rating system for absorption coefficients over the speech frequency range. The NRC of an acoustical material is the mathematical average to the absorption coefficients at 250, 500, 1000, 2000 Hz.
Pitch
The perceived auditory sensation of sounds expressed in terms of high or low frequency stimulus of the sound.
Sound Absorption Coefficient (SAC)
The fraction of energy striking a material or object that is not reflected. For instance, if a material reflects 70% of the sound energy incident upon its surface, then its Sound Absorption Coefficient is 0.30. SAC = absorption / area in sabins per sq. m.
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