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S. Floss, M. Kaltenbacher, G. Karlowatz:
"Application and simulation of Micro-Perforated Panels in HVAC Systems";
SAE International, 1 (2018), 2018015141 - 2018015148.

To reduce noise in a HVAC system for railway application
the usage of micro-perforated panels (MPP) is
proposed. MPPs offer some favorable characteristics,
like robustness and durability in harsh environments
and the possibility to optimize absorption in desired
frequency bands. The underlying acoustic mechanism can
be modelled via an equivalent fluid in accordance with the
Johnson-Champoux-Allard (JCA) approach, treating the
MPPs as a porous material with rigid frame. This allows
to conduct the necessary acoustic pre-evaluation in complex
HVAC application scenarios in order for the MPPs to
substitute the commonly used foam and fibrous
absorber materials.
Introduction
The reduction of noise emissions in HVAC systems is
still a demanding challenge. One has to deal with harsh
environments, the absorber must nevertheless last for
many maintenance cycles and satisfy demanding fire protection
standards such as the EN 45545-2. Broadband sound
reduction is desired and the pressure drops induced by the
absorber materials should be small. Usually foam and fibrous
materials are used to meet most of these tasks.
We propose the application of micro-perforated panels
to substitute the standard absorber materials, since they
exhibit characteristics being well appropriate to be used in
HVAC systems:(1) They can be made out of various robust
materials; (2) they are passively tunable to a special
frequency band and broadband absorption is a matter of
arrangement, which, however, must be examined beforehand,
since a single MPP is only narrow band absorbing;
(3) by applying the Finite Element method (FEM) in combination
with an equivalent f luid approach, the sound
absorbing properties of MPP configurations in complex
environments can be pre-evaluated.
The idea to use MPPs as acoustic absorbers goes back to
Maa et al. and the acoustic behavior of perforations in plates
has already been extensively studied by Ingard et al. (see [1,
2]). We demonstrate the applicability of these plates on the
example of a silencer section in a HVAC system for railway
application. Our results show that MPPs can substitute the
conventional absorbers up to a certain frequency band. For
pre-evaluating MPP arrangements with different number of
layers and layer distances, with the goal of broad band absorption,
targeting a certain frequency band and minimal overall
layer thickness, it is sufficient to use the transfer matrix method
(TMM) for conducting parameter studies. The method assumes
plane wave propagation and an incident angle perpendicular
to the absorber material. Depending on the dimensions of the
HVAC duct cross section, one can identify a cut-on frequency
from which higher order modes can propagate. These higher
order modes can be represented by inhomogenous waves, with
nonparallel equiphase and equiamplitude planes propagating
at different directions (plane wave decomposition) [3]. Unless
one partitions the MPP backing volume with a fine honeycomb-
like structure, one must assume extended reaction for
the combination of MPP and backing air volume in contrast
to the local reaction assumption for many porous materials
where the surface impedance is not dependent on the incident
angle [4].
MPPs may also be used as flow guiding structures. In this
scenario, several layers of MPPs and air volume in form of,
e.g., a flow guiding vane would be placed in section of a duct
where the flow changes direction, this is commonly known
as baffle silencer. In another application case one single MPP
could be placed where the duct cross section suddenly changes.
The change of cross section induces an impedance jump which
causes a reflection of sound wave. This expansion chamber
principle is used in automotive mufflers and produces sound
reduction where the attenuated frequency band depends on
the length of the expansion and the reduction peak depends
on the ratio of the cross sections. In the HVAC application
case the MPPs would guide the flow, hence reduce the occurring
pressure drop. At the same time the panel would increase
and by

http://dx.doi.org/10.4271/2018-01-1514

https://publik.tuwien.ac.at/files/publik_271551.pdf