Setting an acceptable sound level between floors

As a member of the Executive Committee of a multi-storey
unit block, we are getting a number of requests from owners to install timber
or tiled floors. As often commented in Flat Chat this can be a vexed problem
regarding noise with the minimum (and mandatory since 2004) Building Code of
Australia (BCA) performance criteria for floor impact sound insulation being
quite inadequate. It may be of interest that our solution has been to draft a
bylaw requiring a higher level of sound insulation to that of the BCA.  To minimize technical jargon, the measure of impact
sound insulation is reduced to a single number given by the parameter L_w;
the lower the number the better the sound insulation.

The Association of Australian Acoustical Consultants (AAAC) uses
this L_w parameter in their star rating system to set an acceptable
level for impact noise. This systems rates impact noise levels between units on
a 2-6 star rating; the more stars the better. The lowest rating of 2 stars,
where impact noise is clearly audible, is where the BCA figure of L_w
< 62 lies. (The use of ‘<’ means ‘less than’.) At 6 stars (where L_{w
< 40) is where impact noise is essentially inaudible and applies to carpeted
floors. As it is impossible for hard surface floors to achieve this level we
have to be satisfied with a 4-star rating (where Lw} < 50 and
heavier impact noise can still be audible) as the best that can be reasonably achieved
with appropriate floor impact treatment.

We had measurements taken in our block and carpeted floors
gave a low L_w = 33 while a timber floor just made a 4-star rating with
L_w = 50. Since a bare concrete slab of typically around 230mm thick,
has an L_w of 73, any floor treatment for a minimum 4-star rating must
reduce this figure to 50 or better. Thus the difference ΔL_w between
the bare slab and floor covering has to be 23 or better. To quantify this in
our bylaw for an AAAC  4-star or better rating,
we simply state floor treatment options are  1) carpet or 2) install a floor impact
treatment with ΔL_w better than 23.