Building Bulletin 93 is a pretty daunting assignment for anyone involved in the design and specification of school buildings. It sets tough new targets for acoustic performance and offers guidance on what construction details should be used to meet the requirements. The cane-snapping Department for Education and Skills says BB93 represents a "significant tightening of the regulations concerning the acoustic design of schools".
And it can't simply be ignored like a supply teacher. BB93 has clout because from July 1 2003 school buildings in England and Wales will have to comply with the acoustic Building Regulation Part E. This means that new school buildings will be subject to detailed design checks and on-site inspections by Building Control.
No, the new bulletin is more like final exams than first grade. If architects' and specifiers' completed buildings don't meet the strict standards for noise levels, sound insulation and room acoustics expensive remedial work will have to be carried out at their expense. This was not the case with BB93's predecessor BB87, which was not referred to in Part E, as schools had crown exemption from the Building Regulations. Consequently many designers ignored the standards, which led to a lot of schools being built with poor acoustics (see "Changes at a glance: Key differences between BB87 and BB93", below).
One of the main drivers behind the government's desire to improve acoustic design is its belief that poor performance adversely affects pupils and teachers. Recent research by South Bank University indicated that noise levels affected the performance of schoolchildren taking SAT tests. BB93 also draws attention to a survey of voice clinics that suggests that bad classroom acoustics increases the strain on teachers' voices by forcing them to shout.
The new bulletin has been updated to reflect current acoustic research and the relevant requirements of the Disability Discrimination Act. The DDA places a duty on all schools and local education authorities to integrate children with special needs into mainstream schools, which means designers will have to incorporate acoustic improvements and aids for hearing impaired pupils in all school buildings. The new document carries guidance on providing for children with hearing difficulties.
The most significant change in targets in BB93 is the reduction of classroom ambient noise from 40 dB LAeq,1hour (including activity noise from neighbouring spaces in the school) to 35 dB LAeq,30min, (excluding school activity noise) measured over a period of time. This means that designers will have to incorporate more acoustic insulation in external classroom walls and windows. Designers will be able to use less insulation if they position classrooms away from loud noise sources such as busy roads.
The demand for more acoustic protection on the external wall means that designers have to be careful how they provide ventilation, as it could allow unacceptable levels of ambient noise into rooms. BB93 states that natural ventilation rather than mechanical ventilation should be used wherever possible, and the guidance offers four choices of natural ventilation (see "Possible types of natural ventilation to minimise acoustic noise in a classroom", page 4). A case study in Section 7 of BB93 also includes examples of ground floor and first floor vents details that would meet requirements (see "Ground floor air vents", below).
The lower ambient noise target in classrooms means that the performance standards of separating walls have also risen. The amount of sound insulation required depends on the amount of noise generated from the adjoining room – defined in BB93 as low, average or high. The sound insulation targets between rooms are generally more onerous in order to allow for the needs of hearing impaired children in mainstream schools and to meet the general improvements.
To ensure good insulation between rooms and corridors the new document requires that doorsets, ceilings and walls meet minimum performance standards of 40 Rw dB for walls and 30 Rw dB in classrooms. Because there is no in-situ measurement standard for corridors, compliance is through calculation so designers should simply be able to specify doorsets and wall types that meet the target figures.
BB93 provides a form of student counselling for the dunces of the class – guidance to help designers ensure that the airborne sound insulation of the walls and ceilings is not compromised by flanking sound (see "Eliminating flanking transmission via floating screed", below).
For the first time there is a requirement for performance standards for impact sound insulation (see "Floor impact sound insulation", below). Section 1.1.4 states that impact sound insulation should be designed and measured for floors without a soft covering.
The document has also introduced a requirement for additional sound absorption in corridors, entrance halls and stairwells to reduce reverberation. The amount of absorption required is calculated according to two calculation methods in Section 7 of Part E.
One area in which BB93 particularly cracks the whip is the control of noise in open-plan teaching spaces. Initially the DfES was encouraged by its acoustic advisors to bar open-plan spaces as it considers cellular classroom design to be the most effective way of controlling noise between classes. Under pressure from architects and educationalists, however, it is allowing open-plan spaces on condition that designers use a computer prediction model to predict the Speech Transmission Index (STI) and thus the speech intelligibility of the open space. BB93 requires that the STI be between 0.6 and 1. A rating of 0.6 is enough for speech to be easily heard.
Because of the complexity of achieving a good STI rating, the document says that designers should establish the expected open-plan layout and activity plan with the client at an early stage.
Although BB93 has been written for designers, the DfES recommends that an acoustic consultant be appointed for all but the simplest of projects. The role of the acoustic consultant ranges from checking architect's plans for problems, to checking on-site construction and carrying out acoustic tests. BB93 includes a checklist to help architects judge when they need to use an acoustic consultant (see "Acoustic planning: do your homework", below).
BB93 doesn't make post-construction testing compulsory but it does recommend that the client include a requirement for acoustic testing in the building contract. There is extensive guidance as to how testing should be carried out. Building Control may require tests on completed school buildings if it believes that the acoustic design is suspect or compromised by poor workmanship.
The preferred method of demonstrating compliance to Building Control is by submitting plans, construction details, material specifications and calculations when seeking planning approval. The DfES is designing a standard form available on its website, which should simplify the task of submitting the plan and supporting information.
BB93 is a crucial document and it will have a major impact on the way schools are designed. Until now, pressure on finances has meant that acoustics have been low on the list of design priorities. After 1 July, acoustics will have a much higher priority and architects working in the lucrative schools sector will have to get to grips with BB93 fast to ensure they don't get pulled up before the headmaster. That means you – so keep your head down and get back to work!
Acoustic planning: Do your homeworkYou’ll need a structured approach to acoustic design at each stage of the planning and design process. So do your homework and follow the bulletin’s advice. Feasibility study and sketch design
- Selection of the site
- Noise survey to establish external noise levels
- Orientation of buildings
- Massing and form of the buildings
- Consideration of need for external noise barriers using the buildings, fences and screens and landscape features
- Preliminary calculation of sound insulation provided by building envelope including the effect of ventilation openings, for example using SIBE spreadsheet, which is available from the DfES website.
- Determine appropriate noise levels and reverberation times for the various activities and room types
- Consider the special educational needs of the pupils
- Consider the design of music, drama and other specialist spaces separately from that of normal classrooms as the design criteria are very different
- Provide the necessary facade sound insulation while providing adequate ventilation
- Architectural/acoustic zoning: plan the disposition of quiet and noisy spaces, separating them wherever possible by distance, external areas or neutral buffer spaces such as store rooms or corridors
- Consider sound insulation separately from other aspects of room acoustics, using walls, floors and partitions to provide adequate sound insulation
- Design the acoustics of the rooms by considering their volume and shape, and the acoustic properties of their surfaces
- Specify the acoustic performance of doors, windows and ventilation openings
- Specify any amplification systems.
- Submit plans, including specific details of the acoustic design for approval by Building Control.
BB93 Terminology: a non-technical guideNoise
LA – A sound level measurement in which the frequency content of the sound is compensated to imitate the ear’s varying sensitivity to sound of different frequencies. All sound levels in BB93 are in terms of A-weighted sound pressure level. LAeq,T – the A-weighted noise level averaged over time T. LA1,T – the A-weighted noise level exceeded for the noisiest 1 % of the time during a measurement period T. Indicative of the highest noise levels measured during that period. Room acoustics
T – the reverberation time of a room. The number of seconds taken for a noise (eg a gunshot or a balloon popping) to die away. Tmf – T varies with frequency. Tmf is an average of the reverberation times measured at 500 Hz, 1000 kHz and 2000 kHz. BB93 sets maximum permissible reverberation times for each type of room. Sound insulation
Dw – D is the sound level difference between two rooms, used in measuring airborne sound insulation. D varies with frequency, but for convenience the result is expressed as a single figure rather than as a table of values at each frequency. The method used for weighting the results at different frequencies is Dw – the weighted level difference. DnT (Tmf, max) – The sound level difference D described above is also a function of the reverberation time of the room in which the sound is measured. DnT (Tmf, max) is simply the measured sound level difference D, corrected to the maximum reverberation time permitted for that room.
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The new Building Bulletin 93 The Acoustic Design of Schools produced by DfES and published by the Stationery Office, ISBN 0 11 271105 7 will be available in print in June and is already available for download from the internet at www.teachernet.gov.uk/acoustics. Section 1 of BB93 is quoted as the normal means of compliance with Part E of the Building Regulations and contains the DfES constructional standards on acoustics for new school buildings. Other sections contain: guidance on noise control; typical construction details; designs for speech and music; acoustic design and equipment for pupils with special hearing requirements; and 10 case studies. The appendix includes a glossary of technical terms, calculation methods and a standard form for submitting plans and information for approval by a Building Control body. Adrian James Acoustics firstname.lastname@example.org