School Acoustics
Whether it is the introduction of acoustic treatment to control high levels of sound reverberation in a
gymnasium or drama hall, sound absorbers to improve speech intelligibility within the classroom or
a custom designed practice room or recording studio for the music department, Kestrel Acoustics
have a wealth of experience dealing with the control of sound within the educational environment.
All of our recommendations are aimed at ensuring compliance with the requirements of Building Bulletin
93, Acoustic Design of Schools.
There are two potential problems with acoustics in schools. The first is reflected sound and the need to
increase the amount that is absorbed. The second relates to sound transmission and the structural ability
of a room or space to either contain or prevent the ingress of sound intrusion. Let's look at each of these
in a bit more detail.
Sound Absorption
Classrooms, assembly halls and gymnasiums are, of necessity, constructed with hard, robust surfaces to
withstand the rigours of school life. This may afford a degree of decorative and structural longevity, but it
provides very poor acoustics. This is due to sound impacting upon the hard surfaces and being reflected
back into the room in a new direction, causing a build up of reverberant sound that mixes with direct sound.
The result is an acoustic muddle that makes communication and listening difficult.
So what's the solution?
The first step is to assess the existing reverberation time (RT) for the space. This is the time taken in
seconds for the average sound energy created within the space, to decrease to one millionth of it's original
steady level, after the source has ceased ie. the time taken for a 60dB decrease to take place. The RT can be
measured directly by charting the decay of sound following a gun shot or balloon burst, or it can be
estimated from the following: R = 0.16V ÷ S`a where R is the reverberation time in seconds, V is the room
volume in m³, S is the surface area of the space in m² and`a is the average room absorption
coefficient. Our approach is to create a simple acoustic model, based on the latter, that will allow us to
assess both the existing situation and predict the improvement.
What is the absorption coefficient?
This is the amount of sound energy that can be absorbed by a surface or material. This is expressed in
values between 0 and 1. The former absorbs none of the sound incident upon it and the latter will absorb all
of the sound. Our computer models consider the full sound frequency spectrum for surfaces and materials as
the absorption coefficient will vary over the sound spectrum. Sound absorption is generally poor at low
frequency but usually much better at high frequency.
How do we determine the correct RT?
Building Bulletin 93 makes this very easy as recommended times are tabulated in Section 1, Table 1.5. For
example classrooms should generally be <0.8s, assembly halls 0.8 - 1.2s, sports halls <1.5s and music
classrooms <1.0s.
With this information we can readily determine how much sound absorption needs to be introduced to
comply with BB93. Careful consideration must be given at this point to the selection and positioning of
acoustic products within the space, to provide a satisfactory acoustic solution that is also practical and
aesthetically pleasing.
Solutions will vary dependent upon many factors and we are always pleased to offer our advice. The
products usually proposed are sound absorbers or acoustic curtains, both of which can be found under the
'Products' tab.
Sound Transmission
Problems relating to the unwanted transmission of sound can be attributed to any number of potential areas,
such as lightweight wall structures, hollow core doors, single glazed windows or 'flanking paths' via
suspended ceilings or timber floors.
How do we fix this?
This needs careful site investigation and the solution will depend on several factors. The assessment will
need to consider the sound source and volume, the relative position of the areas where sound is intruding,
the construction of the rooms/building and any practical constraints both physically and financially. These
will all have to be considered alongside the recommendations in BB93, which advises performance standards
for airborne sound insulation between spaces. See Section 1, Table 1.2 with reference also to Table 1.1.
Every situation is different and solutions have to be tailored accordingly, although the principles of acoustic
control remain much the same.
Acoustically weak walls can be improved with the introduction of a secondary wall, this could involve just the
treatment to a party wall or all the walls within a room. Poor door performance can be rectified by replacement
with an acoustic door or, if the budget is tight and the door is fairly solid, it might be improved with the addition
of some acoustic seals. Internal windows, particularly between rooms in a music department, can be improved
with double or triple glazed arrangements. Sound flanking transmission via ceilings can be improved, either with
dense acoustic material laid over the top of an existing suspended ceiling, or with the introduction of an MF
ceiling structure (metal frame with several layers of Soundbloc plasterboard) suspended below the existing
substrate. An acoustic floor overlay can be used to mitigate the passage of noise via floors, both in terms of
airborne transmission and footfall. Where higher levels of sound reduction are required, such as a music
practice room or recording studio, a separate acoustically isolated modular room structure may be required. This
approach is often termed as a 'box within a box'. Most of the above products can be found under the
'Products' tab.
Building Bulletin 93
The acoustic design of schools is dealt with through the Building Regulations under Building Bulletin 93. With
its introduction in July 2003, BB93 has brought about many changes in the design and construction of schools.
These changes are far reaching, the old Building Bulletin 87, which BB93 replaces, was a guidance document,
whereas the requirements of BB93 are now mandatory.
Building Bulletin 93 aims to:
- Provide a regulatory framework for the acoustic design of schools in support of the Building Regulations.
- Give supporting advice and recommendations for planning and design of schools.
- Provide a comprehensive guide for architects, building control officers, building services engineers, clients
and others involved in the design of new school buildings.
Section 1 of the document describes the 'Specification of Acoustic Performance'. This section gives the
performance targets for compliance with the Requirement E4 from Part E of Schedule 1 of the Building
Regulations 2000. 'Each room or other space in a school building shall have the acoustic conditions and the
sound insulation against disturbance by noise appropriate to its normal use'. All school buildings must conform
to BB93 and are subject to detailed design checks and on site inspections by building control officers.
The full overall scope and content of BB93 is far too comprehensive to summarise here. The complete document
can, however, be found at http://www.teachernet.gov.uk/.
