MCU19-0132 Applicant's Response - Att 3 Acoustic

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
                                                                             making precast easy