Improved method for determining the absorption coefficient of high reflective surfaces
Invited paper
M+P
Wednesday 3 june, 2015, 09:40 - 10:00
0.2 Berlin (90)
Abstract:
The demands for the exterior noise test tracks defined in the ISO 10844:2014
are
more demanding than the demands in the ISO 10844:1994 version. Among others,
this certainly holds for the acoustical absorption of the road surface. This
may
not exceed 8% in each 1/3rd octave band between 315 and 1600 Hz, while in
the
former one the limit was 10% on an average value.
The mandatory test for acoustic absorption checking is the spot method
prescribed in ISO 13472-2 that applies a two microphone impedance tube
(standardized in ISO 10534-2). Straightforward application of this method
causes
complications since for each microphone position destructive interference
will
take place at certain frequencies. This introduces a significant error in
the
determination of the transfer function between the two positions and thus in
the
determination of the absorption coefficient.
We developed a procedure where signals are measured continuously at three
microphone locations. Between each of the three pairs of positions the
coherence
function is measured. Occurrence of destructive interference at one position
will result in loss of coherence for each pair that shares that position.
Then
we select each individual frequency that pair with the highest coherence,
and
use that for the calculation of the transfer function, leading to a much
better
estimate of the absorption coefficient. This has shown to be a major
improvement
that reduces errors over the total frequency range and allows the
determination
of absorption coefficients below 2%.