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Appendix 1 - Acoustic Fact Sheet
Attached to the e-mail MCU17/0524 - Information Response.
FACT SHEET
Acoustic Properties of Precast
Concrete
Acoustic Properties of Precast precast concrete is not the problem, but
Concrete rather, the solution when one is trying to
In today’s cities people are living closer limit noise intrusion.
and closer together primarily due to
urban consolidation. The need to utilise Noise Levels & Measures
services and infrastructure, to be close to Noise values quoted in reports and
work, and to be close to entertainment is technical datasheets most times will be
of high priority. a number approximately in the range
of 40 to 100 with the letters dB after
Medium density living in modern the number eg 70 dB. The dB stands
architect-designed glass and concrete for decibel (one tenth of a Bel) and is a
structures is very much in demand, and logarithmic scale (similar to the Richter
thus it becomes the task of the designer scale for earthquakes). Whilst noise
to ensure the building inhabitants have levels can be outside this range, those
a comfortable living standard. Precast values would tend to be in the extreme
concrete structural walling and flooring (eg values over 100 dB are extremely
systems are becoming an integral part loud and often short in duration).
of this construction process of multi-unit
buildings. Knowledge of the acoustic If however one sees values quoted
properties of such forms of construction in dB(A) then this scale relates to the
is crucial to ensure quality lifestyle living sound measured in particular with its
in these buildings. reference to the human ear. This scale
has been “weighted” to correspond to
Part of this process it to ensure that the way the human ear hears sounds at
noise from people working and living corresponding loudness levels in each
above, below and outside of a dwelling,
does not hinder one’s lifestyle. Building
designers need to understand the basic
science of acoustics and noise control.
The following information shows how
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FACT SHEET
frequency. Road traffic noise levels are (generally taken as internal)
usually quoted in terms of dB(A) and the
”difference” between outside and inside S is the area of the dividing wall
noise levels (due to barriers, windows,
walls etc) is quoted as dB(A) reduction. A is the equivalent absorption area of
the receiving room (determined from
Sound Reduction Ratings – Wall & reverberation measurements).
Floor Panels
Precast concrete floor and wall panels In recent times the TL rating system
(as well as other materials eg glass) are has been changed to conform to the
tested both under laboratory conditions International Standards Organisation
and on-site, to establish their acoustic system of Weighted Sound Reduction
ratings. This is basically to establish Index Rw. This, in essence is similar to
how efficient they are at reducing the the previously used Sound Transmission
transmission of noise from outside to Class system.
inside environments. For example,
measurements of noise levels taken near
Both the rating systems and Sound
a busy highway and on the outside of
Transmission Class were based on
a building wall may show that average
a speech related noise spectrum
noise levels are around 80 dB(A), yet
and in this regard are successful at
when measured on the inside of that
determining required performance
building only record 50 dB(A). The
for the sound insulation required by a
difference between the two levels is
dividing barrier. In the case of the Sound
30 dB(A), so an approximate noise
Transmission Class, it’s deficiency in
reduction rating for that wall would be
the low frequency area was identified
30 dB(A).
and there was an attempt to introduce
Music Transmission Class to take this
Sound transmission loss of an acoustic into account. In the case of the ISO Rw
barrier system was previously rated as system it was recognised that it did not
Sound Transmission Class, a system adequately insulate against external
derived from America. This system not traffic noise and the correction factor
only takes into account the difference Ctr was introduced to provide enhanced
between noise levels either side of a low frequency sound insulation.
wall (or floor, or ceiling) but also the Work done in the United Kingdom
absorption capacity of the room itself eg showed that using the Ctr factor also
a room lined with soft textured fabrics provided more appropriate sound
will record a lower noise level since insulation when dealing with modern
many of the sound waves entering that home entertainment systems. The Ctr
room get absorbed by the soft materials. correction factor has been introduced
The formula that is normally quoted in into the Acoustic Provisions of the
this application is: Building Code of Australia since 2004.
TL (dB) = L1 – L2 + 10 log 10 (S/A)
Where: L1 – L2 is the noise reduction
or difference either side of the barrier
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FACT SHEET
The Precast Concrete Handbook gives 150 precast concrete wall as complying
values for various common concrete with the BCA 2004 onwards acoustic
wall arrangements (see Tables 1 and provisions.
2 – which are reproductions of Table 9.8
& Table 9.9 from the Precast Concrete This would be fine if people lived in
Handbook-Sept 2002). Added to the acoustic laboratories, but they do not,
Rw values are the spectrum adaptation they live in real apartments. There are
terms Ctr. some walls that perform well in an
acoustic laboratory but do not perform
When dealing with external noise well when installed. The acoustic
intrusion, particularly traffic noise, there laboratory has precise openings built to
is a rule of thumb used in a number of fine tolerances. This does not exist in the
Australian Standards that the effective field.
dB(A) reduction equals the Rw rating
minus 5. If the Ctr spectrum adaptation There are other walls however that
term is used the dB(A) reduction is likely better accommodate site tolerances and
to be closer to the Rw factored down by experience has clearly demonstrated
the Ctr rating. that precast panels lose very little
performance between that tested in
In the case of external noise intrusion a laboratory and that measured in
the external wall is usually a mixture of the field, assuming that the rest of
solid wall and windows and/or doors. the structure is up to the appropriate
The concrete panel has a typical Ctr acoustic standard. What is more
value of -5 which is very good. If double important is the field performance
glazed windows are used the Ctr value (designated as Dntw). The requirement
could be -10 for that component. It is of the current BCA is that the installed
therefore necessary to examine the performance including Ctr should
composite sound transmission loss of exceed 45 dB. In this regard the precast
the solid wall area together with all the element easily achieves this value with
components that make up that wall, in a field result in the range Dntw + Ctr
order to determine the Ctr value for the equals 47 to 48 dB.
composite construction.
The acoustic laboratory measurement
Performance of 150mm precast is essential and is the first starting point
panels in acoustic design, however it is the
A 150 mm thick precast concrete panel installed result in the finished apartment
is an interesting situation in respect or town house that is vital and will
to compliance with the Building Code become the dominant factor in the
of Australia. The typical laboratory future. As previously said, people do not
performance of a 150 thick precast live in acoustic laboratories, they live
wall is Rw 54 and the Ctr is -5 giving an in apartments and it is there where the
overall rating of Rw + Ctr equalling 49 acoustic performance must be delivered.
dB. Technically this does not comply
with the BCA, which requires 50 dB
and there has been experience with
some certifiers that will not approve a
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FACT SHEET
Effect of Fixing Plasterboard
Whilst a 150 thick precast concrete panel
has a Ctr value of -5, this value is made
more negative if a plasterboard wall on
furring channels is fixed to the precast
wall, giving a less favourable result
overall. The thinner the furring channel
and the lighter the plasterboard, the
more the Ctr factor is made negative.
This should be watched carefully. When
dealing with internal noise and furring
channels are applied to both sides of
the precast wall together with the use
of 10mm plasterboard the effect of the
two cladding systems is compounded
in respect to Ctr. If it is necessary to
provide cladding with airspace for the
running of services then it is important
that the plasterboard is either 16mm
or 13mm sound rated, that the furring
channels are no less than 28mm deep
and that effective insulation is applied to
both cavities.
It can be seen from Table 1 that a 150
mm precast concrete panel (which
weighs 360 kg/m2) would provide at
least 50 dB reduction of sound levels
from outside to inside when exposed
to road or rail traffic (or factory noise –
refer to the Handbook for more detailed
information).
Another simple rule of thumb that can
be applied to materials to calculate their
approximate transmission loss is the
‘mass law’ equation:
TL = 20 log 10 (mf) – B
Where: TL = transmission loss (dB)
m = surface mass (kg/m2)
f = frequency (Hz)
B = 48 dB (on average but can range
from 45-53).
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FACT SHEET
Example: If one is interested in the TL Thermal Advantages of Concrete’
value of a glass panel (density ~ 2700kg/ by Paul Uno) shows the reduction in
m3) with thickness 6 mm at a frequency sound level efficiency when openings
of 500 Hz (ie this frequency corresponds of different sizes are present in panels.
to the C above middle C on a piano) For example, if a panel has a sound
then the m value would be 0.006 x 2700 transmission rating of 50 dB (when
= 16.2 kg/m2, the mf value would be appropriately sealed) and an opening
16.2 x 500 = 8100 kg/m2.Hz and thus the in that panel which only constitutes 0.01
TL would be 30 dB. % of the area of that panel (eg a 20mm
diameter hole in a 2.4m x 1.2m panel),
Weak Links then the panel will now only have a 40
Whilst the information above gives the dB rating – well below most accepted
designer a good reference point to start ratings for building design.
with when trying to account for noise
levels in buildings, much of this noise Figure 1
reduction can be forfeited if the so-
called ‘weak links’ are not understood Insualtion vs % Openings
and accounted for when designing a
structure.
(a) Flanking
Noise entering a structure does not
always follow a direct path from an
outside noise source through the wall in
question, to the person inside hearing
the noise. Often the sound waves
can enter from indirect paths such as
through ceilings, service penetrations
and joints. This being the case, a
designer needs to reduce the noise
levels from these flanking paths (eg
providing separation materials between
walls and/or ceilings; caulking perimeter
walls –again refer to the Handbook for
more detail). (c) Windows
Glazing is obviously a key part of any
(b) Openings, Holes & Cracks building envelope and as such the
Any concrete element will have its designer has to take into account the
acoustic rating downgraded if openings, limitations of glass with respect to
holes or cracks are left in the element acoustic efficiency. The transmission loss
after installation and during the life of of standard residential glass is around 25
the element. These openings provide an dB and for 6mm commercial thickness
avenue for sound waves to penetrate
directly through the panel. Figure I
below (reproduced from an ABIC 1992
conference paper titled ‘Acoustic &
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FACT SHEET
glass it is around 30 dB. If one puts Chart A - Composite Walls Sound
glazing into a precast concrete panel Insulation
where the panel thickness is 150mm (TL
= 50 dB) and the glazing is a single 6mm
sheet (TL = 30 dB) then one cannot
simply just average the two acoustic
ratings as both values are logarithmic
units. Whilst the mathematics is
complicated for non-mathematicians,
Chart A quantifies this ‘composite
grading’ process.
From Chart A it can be seen that if
the concrete panel is 6.0m x 2.4m and
the glazing is 1.2 x .2m, then the ratio
of the small panel to the large panel
is (1.2 x 1.2 / 6.0 x 2.4 = 0.10 or 1: 10).
The acoustic rating difference between
panels is 20 dB so the ‘loss of insulation’
from the higher value is 9 dB which
means the composite panel TL value is
41 dB.
If however, the panel contained a louvre Summary
panel for air-intake (which has no sound In conclusion, when one is concerned
insulation) rather than the glass panel, about acoustic performance in building
then the sound level difference between construction, precast concrete is seldom
the two panels would be 50 – 0 = 50 a concern as it has excellent sound
dB, and thus the composite partition transmission loss properties.
TL value would be 10 dB (i.e. 50 dB
minus 40 dB where the latter value As well as performing well acoustically,
corresponds to the x axis value from aspects such as quality control, curing
the chart for 50 dB difference and 1:10 and crack elimination are issues which
ratio). have been addressed during the
manufacturing process.
This is sometimes not the case
with concrete which is not factory-
manufactured. For example, lack of
vibration on site may result in incorrect
element density, thus the mass of the
panel is reduced leading to small TL (Rw)
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FACT SHEET
values. Cracks may occur due to lack of
quality control on site (eg lack of curing,
or incorrect protection procedures on
site against adverse weather conditions),
thus providing acoustic leakage paths
and thus downgrading the overall sound
rating of the building envelope.
Precast elements have very high sound
transmission loss values (i.e sound
reduction index values) and as such the
designer needs to address the TL (Rw)
values of other elements incorporated
into the panel or building envelope to
ensure the whole façade
or envelope is not compromised by
other elements with lower TL (Rw)values.
References:
(1) Precast Concrete Handbook, Sept
2002
(2) “Acoustic & Thermal Advantages
of Concrete” by Paul Uno, ABIC
Conference 1992 p67-69
(3) “Concrete in Energy Efficient Design
& Noise Control” by Paul Uno, TN62,
CCAA, 1993
(4) “Acoustics Noise & Buildings”,
Humphreys & Cowell
(5) Peter Knowland & Associates
(Acoustic Consulting Engineers)
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