4.1 General
The designer should hold discussions with the Aircrete manufacturer
from the outset.
Full details of the project should be provided to the Aircrete manufacturer,
including timings and quantities, to enable the best advice and quotations
to be given.
The designer should provide full plans of the relevant parts of the
project as soon as they are available to enable the manufacturer to
provide all the necessary skills including IT and CAD resources to
deliver the project.
The Aircrete manufacturer should furnish the designer with recommendations
for the use of specific products in the most appropriate locations
with suitable mortar mixes, finishes, ancillary components and fixings
as required.
Changes to the design should be avoided after construction has commenced.
A constant dialogue should be maintained between all involved in the
supply chain. This means expensive late changes can be minimised and
any cost implications fully taken into account at the time.
Attention should be given to all types of interfaces i.e. both between
Aircrete blockwork and other parts of the building and also between
the designer and the Aircrete manufacturer and their Technical Advisory
Service.
4.2 Acoustic
The effect on acoustic performance of the building layout, construction
form, details of connections with other components and site workmanship
should all be
taken into account in design.
Designs which minimise common areas of separating walls between
dwellings
e.g. designs that include steps and staggers between dwellings will
improve sound insulation between the dwellings. Similarly split
foundations will be beneficial by reducing acoustic coupling.
Where timber floor joists are supported on separating walls, they
should be supported on joist hangers. Timber joists should not be
built into separating walls.
Any reduction in the thickness of Aircrete blockwork in separating
or flanking walls should be avoided, but where this is unavoidable,
e.g. to accommodate electrical sockets, positions on opposite sides
of the separating wall should be staggered.
Movement joints should not be built into separating walls.
If walls are finished with plasterboard, joints should be staggered
to avoid direct air paths. If plasterboard is not used, wet plaster
should be specified to seal air paths in masonry walling.
Mechanical equipment should not be mounted on separating walls.
If this cannot be avoided, the equipment should be mounted on acoustic
mountings.
Concrete floor slabs including beam and block floors should be discontinuous
across cavity separating walls as illustrated in Figure 1 which
is based on details given in the BRE Quiet Homes document.
Beam and Aircrete block floors should be adopted to reduce noise
transmission within dwellings between ground and first floors.
Successful acoustic design also involves careful onsideration of
often conflicting thermal and structural requirements.
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4.2.1 Sound Absorption
Sound absorption depends largely on the surface texture characteristics of a surface and is a highly specialised subject.
Sound absorption can be enhanced by recessing mortar joints and forming
grooves in the surface of the Aircrete.
Where significant levels of sound absorption are required however,
specialist acoustic finishes should be adopted.
4.2.2 Sound insulation
Airborne sound can be transmitted from one part of a structure to
another.
The degree to which a wall may reduce the passage of airborne sound
depends on its ‘mass law’. The Aircrete blockwork mass
law curve derived from laboratory
tests and supported by field tests may be used to calculate the sound
reduction index of single leaf internal walls and partitions. A close
relationship exists with field tests.
Sound Reduction Index
R= 22.9 log (m) – 4.2 dBaverage for the range 100-3150Hz
Where m = superficial density in kilograms per square metre.
The superficial density is the mass per unit area of the wall and
its finishes and may be calculated from the formula: = MB+ ρmTd (L+H-d) + the weight of the finishes kg/m
LH
Where, MB is block mass (kg)
ρm is mortar density (kg/m )
T is block thickness (m)
d is mortar joint thickness (m)
L is co-ordinating length (m)
H is co-ordinating height (m)
4.2.3 Airtightness
Airtightness of all parts of the construction likely to affect sound
insulation performance should be a design requirement for all types
of masonry. All mortar joints should be filled and sealed with mortar.
Air paths should be avoided.
A small hole in a wall or floor can significantly degrade the performance.
Poorly sealed pipe penetrations or electrical sockets are a common
cause of poor sound insulation. It is essential that any openings
for services are detailed and built correctly to maintain a seal.
All joints in blockwork and sheet materials must be correctly specified
and built.
4.2.4 Resistance to the passage of sound
Part E of the Building Regulations for England and Wales is concerned
with external noise, noise between dwellings and noise within dwellings
and rooms for residential purposes but generally does not apply to
prisons and hospitals. A revised version of AD
E came into effect from 1st July 2003 and introduced the concept of
pre completion testing (PCT) of dwellings. However, on 1st July 2004 a
new concept of ‘Robust Details’ was introduced, giving constructions which are not
required to be tested, but that have been shown by extensive testing to provide
an average test result of at least 5dB above the minimum values given in Table 2.
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