Acoustics in School Buildings – Guidance to Achieving Better Acoustic Performance

Aim

School buildings need to meet the standards set out in “Building Bulletin 93 – Acoustic Design for Schools”. Poor acoustic standards frequently provoke complaints, necessitating costly and disruptive remedial works. Areas where complaints are most common include classrooms, music rooms, assembly halls and gymnasia.

This guidance noteis based on experience from a number of school building projects in Cornwall and the South West. It contains solutions that would have avoided the problems encountered and reminds designers and contractors of established solutions which are nevertheless frequently overlooked.

Scope

The guide covers four key areas of acoustics:

  • Internal noise levels – noise from building services or off-site activities (e.g. traffic, industry).
  • Reverberation – noise bouncing off of surfaces in a room due to inadequate sound absorption.
  • Airborne sound insulation– can noise in one room be heard in another?
  • Impact sound insulation –can footsteps be heard in the room below?

Hints for Compliance

A brief summary of common solutions is listed below.

Internal noise levels

  • Noise from outside:
  • Site layout: noise-sensitive rooms (e.g. classrooms) need to face away from busy roads.
  • Ventilation: where natural ventilation is specified:
  • External noise is not usually a problem when external ambient noise levels are
    below 45dB(A).
  • Designing for cross-ventilation (openings on more than one façade, including rooflights) reduces the area of opening needed to supply the ventilation rates that must be considered when demonstrating acoustic compliance, and normally means that external noise levels up to 50dB(A) will not be problematic.
  • There is a trade-off between the provision of attenuators in ducted passive ventilation, and the larger ducts required due to the impeded airflow.
  • Rain impact noise: On lightweight roof systems, this is best controlled using a system that suppresses resonance of the outer sheet with compressed mineral wool insulation. Rain impact noise can be further reduced with plasterboard underlining or sound-attenuating ceiling tiles.

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Source: Building Bulletin 93 /
Source: Rockwool
Plan site to shield noise-sensitive rooms from main noise sources / Cross-ventilation is more effective and will result in reduced noise ingress / Control rain impact noise using certified products or by underlining lightweight roofs
  • Noise from building services:
  • Mechanical ventilation: noise attenuation is essential to reduce noise from air handling units and to control cross-talk between rooms. Fans located within teaching spaces are unlikely to comply.
  • Noise from boiler rooms: these tend only to be a problem where walls/floors are poorly specified and detailed, or when plant generates high levels of noise and vibration.
  • Other plant locations: the effect of noisy plant located in ceiling voids or outside of the building also needs consideration.
  • Fan-cooled server cabinets:need to be located in un-occupied spaces.

Ensure that noise from ventilation systems and other services is adequately controlled / Although often not problematic, check whether plant requires noise control or vibration isolation / Fan-cooled server cabinets should not be installed in teaching spaces

Reverberation and sound absorption

  • Classrooms: sound absorbing ceiling tiles will normally provide sufficient absorption of sound to keep reverberation time within prescribed limits. Carpeted, furnished classrooms with low ceilings may not need additional sound absorptionbut this should always be checked.
  • Corridors and stairwells: sound absorbing ceiling tiles are needed to give adequate sound absorption.
  • Large spaces (school assembly halls, drama studios, sports halls): all require sound absorption, ideally placed at low level as well as high level (there may be practical limitations for this - particularly in sports halls).

Ceiling tiles with an appropriate sound-absorptive specification are effective at reducing reverberation in classrooms and corridors / Large spaces will require more detailed design to ensure an appropriate reverberation time

Internal airborne sound insulation

  • Sealing
  • Service penetrations and sealing joints:sound will find the path of least resistance. It is vital to specify details to seal around service penetrations in walls and floors(e.g. rammed mineral wool, sealed with flexible mastic or plasterboard sealed in place). This includes holes above suspended ceilings (partitions should be carried up full height and the junction with the roof or floor slab above sealed), unless the ceiling system is specified to provide adequate control of noise transfer, smoke and fire.
  • Junctions between partitions, floor and roof: where partitions meet profiled floor or roof constructions particular attention is required to fully stop and seal the joint. NOTE: perforated roof liners must not be carried across the top of a partition if flanking transmission of sound is to be prevented.

Seal service penetrations and joints, even above suspended ceilings. / Sealing the junction between a partition and a profiled roof deck requires careful detailing. / Perforated roof liners can compromise sound insulation between rooms.
  • Walls
  • Partition types: avoid numerous different specifications that could get confused or ignored on site. Try to use one or two standard solutions wherever possible.
  • Lightweight partitions: resilient bars can significantly improve sound insulation performance (if flanking paths are adequately controlled). Sound-absorptive quilt in wall cavities will help to reduce the effect of small defects or service penetrations. As far as possible light switches and socket outlets should not be installed in partitions where sound transmission is critical.
  • Flanking walls: avoid lightweight blockwork or plasterboard wall leaves that are continuous between rooms.


Source: British Gypsum / /

Specify resilient bars or staggered studs and sound-absorptive quilt where necessary to obtain sufficient acoustic performance from stud partitions. / Consider the effect of placement of electrical fittings on acoustic performance. / Avoid lightweight wall leaves that are continuous between rooms.
  • Floors
  • Concrete floors (cast or plank): a floating floor and underlining will be required to achieve 50dB or more sound insulation horizontally between rooms. The problem becomes more severe with lighter floor slabs.
  • Windows
  • Open windows: adjacent opening windows provide an unwelcome sound path. Increase window spacing and arrange direction of opening to minimise. Other means such as balconies or barriers are possible but inevitably increase capital cost.
  • Vision panels: as well as ensuring an adequate glazing specification, avoid lightweight metal frames.
  • Doors
  • Doors to corridors: these form a weak point in the wall. Lobbies with two doors will work best where sound reduction is critical (e.g. music suites). Solid doors with acoustic perimeter and threshold seals will be required to meet “Building Bulletin 93”. Maximise spacing between doors.
  • Doors to adjacent classrooms: should be avoided where possible as they will inevitably compromise the performance of the partition wall.
  • Folding partitions: ensure that detailing of walls, floors and ceilings allows the potential of the partition to be achieved.

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Adapted from Building Research Establishment /
Floating floors or an independent underlining may be required to provide sufficient insulation of airborne and impact noise. / Arrange spacing and opening of windows to reduce flanking transmission between rooms. / Specify acoustically rated doorsets where necessary.

Internal impact sound insulation

  • To attenuate impact sound from upper floors, a soft floor covering will assist (but may only be taken into consideration on concrete floors), otherwise a floating floor, an independent sound-attenuating ceiling, or a combination of these will be required.

Recommendations

  1. That this note forms part of the project design brief.
  2. Designers, contractors and clerk of works agree a stringent schedule of inspection for critical areas of detailing that could compromise acoustics and quite probably fire integrity too.

Glossary

This note is intended to inform those that arenot construction professionals as well as to communicate some basic points on design to designers and contractors and so every effort has been made to avoid technical jargon. However for brevity some terminology has remained:

dB:short hand for decibel - a measurement of sound. In this note it either describes the loudness of sound produced by a source (e.g. traffic) in dB(A), orthe effectiveness of a partition in reducing sound transmitted from one space to another.

Flanking transmission: acoustic separation would be easy to achieve if there was no requirement for doors, windows, services, roofs or ventilation! Flanking transmission is sound transmission via indirect paths, e.g. horizontally between rooms via the floor slab, roof construction, external walls or window openings.

Internal ambient noise levels:levels of background noise within a space expressed in dB(A). Typically this will be from services or off-site noise (e.g. traffic or industry). Acceptable levels are defined in “Building Bulletin 93” and vary depending on the specific use of a space.

Noise:sound that has the potential to annoy. This is highly subjective and individuals can respond very differently to noise nuisance. Sometimes acoustic tests will reveal that design criteria have been met although a nuisance is felt to exist.

Resilient: in sound terms a feature that reduces the passage of sound by damping vibrations. Corrugated metal straps in metal stud partitions might be a typical feature.

Reverberation: sound bouncing off hard surfaces (in essence creating an echo); made worse by parallel hard surfaces.

Sound absorption: a feature that ‘soaks up’ sound both to reduce sound levels and to damp down the effects of reverberation. Good sound absorbers include very heavy curtains, mineral wool in a partition or appropriately specified ceiling tiles or panelling.

Sound attenuation: a means of reducing the nuisance from a noise source, whether mechanicalor natural in origin.

Further Information

This guidance was produced for Cornwall Council by the Centre for Energy and the Environment at the University of Exeter. Further design guidance and a full acoustic consultancy service may be obtained from the Centre:

Centre for Energy and the Environment

Physics Building

University of Exeter

Stocker Road

Exeter

Devon

EX4 4QL

Telephone 01392 264143

E-Mail

Internet

The following resources are also available:-

  1. Inspection for Acoustic Performance – a leaflet for clerks of works and site managers.
  2. Building Bulletin 93 Summary Poster – summarises the criteria.
  3. Reverb software – to predict reverberation times.
  4. Transmit software – to predict noise ingress and sound insulation.