Appendix 1 - Acoustic Fact Sheet

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
                                                                         making precast easy