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9.1.3 Materials /absorption data
Wrong or imprecise absorption data are probably one of the most common sources of error in
room acoustic simulations. This may be due to lack of precession in the measurements (or
limitations to the measurement method itself) of the absorption data or because the material
construction assumed in the simulations are really based on guesswork – in any case it is a
good idea to remember this and to estimate the size of error on the material data as well as
the impact on the simulated results. It has been seen that absorption coefficients outside the
range 0.05 to 0.9 should be used with care (Christensen, Nielsen, & Rindel, 2008). In the
Material list in Odeon there is a button which will limit the range of absorption coefficients
assigned to surfaces in a room to a selected range.
Solution if materials data are uncertain:
There is really not much to do about the uncertainty of material data if the room does not
exist except taking the uncertainty of the materials into account in the design phase. If the
room does indeed exist and is being modelled in order to evaluate different possible changes it
may be a good idea to tweak (adjust) uncertain materials until the simulated room acoustical
parameters fits the measured ones as good as possible.
Absorption properties in a material library are often, by users, assumed to be without errors.
This is far from being the truth. For high absorption coefficients and high frequencies the
values are probably quite reliable. However, low frequency absorption data and absorption
data for hard materials will often have a lack of precision.
Low frequency absorption
At low frequencies the absorption coefficients measured in a reverberation chamber are with
limited precision because:
There are very few modes available in a reverberation chamber at lowest frequency
bands.
Low frequency absorption occurs partly due to the construction itself rather than its
visible surface structure. Often it may not be possible to reconstruct a complete
building construction in a reverberation chamber and if reconstructing only a fraction of
the wall in the reverberation chamber it will have different absorption properties,
because it becomes more or less stiff.
Hard materials
Hard materials such as concrete are often listed as being 1 % or 2 % absorbing. It may sound
like a difference of 0.5 % or 1 % is not a significant difference. However if a room is
dominated by this material (or if one of the dimensions of the room is) a change from 1 to 2
% is a relative change of 100 %!
9.1.4 Materials /scattering coefficients
The knowledge on scattering coefficients is currently rather limited. Hopefully in the future,
the scattering coefficients will be available for some materials. Meanwhile the best that can be
done is to make some good guesses on the size of the scattering coefficients and to do some
estimates on the effect of uncertainty.
9.1.5 Measurements
Eventually the reference data, which you may compare with simulated room acoustical
parameters, are not perfect. We must accept some tolerances on the precision of the
measured parameters.
9.1.6 Receiver position(s)
Common errors are:
to base the room acoustic design on simulations in one or only few receiver positions
to place the receiver close to a surface.
to use too short source-receiver distance
9.1.7 Source-Receiver distance
Point response calculations made in ODEON are to be compared with point response
measurements and as such the (ISO 3382-1, 2009) standard should be followed:
9-92