A Novel Design for Cruiser Type Motorcycle Silencer Based on Micro-Perforated Elements

Abstract

...Micro perforated elements are innovative acoustic solutions,which silencing effect is based on the dissipation of the acoustic wave energy in a pattern of sub-millimeter apertures. Similarly to fibrous materials the micro-perforated materials have been proved to provide effective sound absorption in a wide frequency range. Additionally, the silencer is designed as a two-stage system that provides an optimal solution for a variety of exploitation conditions. In this paper a novel design for a cruiser type motorcycle silencer, based on micro-perforated elements, is presented. It has been demonstrated that the micro-perforated elements can successfully be used to achieve high attenuation of IC-engine noise in strictly limited circumstances. A technical description of the design and manufacturing of the prototype silencer is given and technological issues are discussed. The acoustical and fluid-dynamical performance of the silencer is characterized by transmission loss and pressure drop data. The influence of the two-stage system valve operation has been analyzed by studying the acoustics data and engine output characteristics. In addition to the experimental investigations, numerical 1-D models were developed for the optimization of the silencer geometry and the results are compared in a number of operating conditions. The studies have resulted in development of a silencer system for a small series cruiser type motorcycle. The first silencer prototypes have been tested on the motorcycle. While maintaining acceptable pressure drop characteristics, it has proven to comply with standard noise criteria without incorporating fibrous materials....

Full text

Preprint of the paper:
Kabral, R.; Rämmal, H.; Auriemma, F.; Luppin, J.; Koiv, R.; Tiikoja, H.; Lavrentjev, J. “A Novel Design for
Cruiser Type Motorcycle Silencer Based on Micro-Perforated Elements”. SAE Technical Paper 2012-32-
0109, 2012. https://doi.org/10.4271/2012-32-0109

Preprint of the paper: Kabral, R.; Rämmal, H.; Auriemma, F.; Luppin, J.; Koiv, R.; Tiikoja, H.; Lavrentjev, J. “A Novel Design for
Cruiser Type Motorcycle Silencer Based on Micro-Perforated Elements”.
SAE Technical Paper 2012-32-0109, 2012. https://doi.org/10.4271/2012-32-0109
Page 2 of 12
A Novel Design for Cruiser Type Motorcycle Silencer Based on
Micro-Perforated Elements
Kabral, R.; Rämmal, H.; Auriemma, F.; Luppin, J.; Koiv, R.; Tiikoja, H.; Lavrentjev, J.
Tallinn University of Technology, Tallinn, Estonia
ABSTRACT
Regulations stipulating the design of motorcycle silencers are strict, especially when the unit incorporates fibrous absorbing
materials. Therefore, innovative designs substituting such materials while still preserving acceptable level of characteristic sound
are currently of interest.
Micro perforated elements are innovative acoustic solutions, which silencing effect is based on the dissipation of the acoustic wave
energy in a pattern of sub-millimeter apertures. Similarly to fibrous materials the micro perforated materials have been proved to
provide effective sound absorption in a wide frequency range. Additionally, the silencer is designed as a two-stage system that
provides an optimal solution for a variety of exploitation conditions.
In this paper a novel design for a cruiser type motorcycle silencer, based on micro-perforated elements, is presented. It has been
demonstrated that the micro perforated elements can successfully be used to achieve high attenuation of IC-engine noise in strictly
limited circumstances. A technical description of the design and manufacturing of the prototype silencer is given and technological
issues are discussed. The acoustical and aerodynamical performance of the silencer is characterized by transmission loss and pressure
drop data. The influence of the two-stage system valve operation has been analyzed by studying the acoustics data and engine output
characteristics.
In addition to the experimental investigations, numerical 1-D models were developed for the optimization of the silencer geometry
and the results are compared in a number of operating conditions.
The studies have resulted in development of a silencer system for a small series cruiser type motorcycle. The first silencer prototypes
have been tested on the motorcycle. While maintaining acceptable pressure drop characteristics, it has proven to comply with
standard noise criteria without incorporating fibrous materials.
The radiated motorcycle sound, as one of the key features of successful design, has been evaluated. The sound design has been
recognized as well suitable for the product.
INTRODUCTION
A motorcycle design is a complex task integrating challenges of engineering analyzes with marketing goals while complying with
a number of regulations. The exhaust system of this type of motorcycle (see Fig. 1) has to provide not only adequate engine noise
cancellation but also a pleasant characteristic sound while preserving acceptable level of exhaust gas flow restriction.
The present work is focusing on the exhaust system design for exclusive small series motorcycle. Traditionally, this type of
motorcycle is equipped with a relatively large displacement internal combustion engine (see Table 1) designed for a moderate rpm
range.

Preprint of the paper: Kabral, R.; Rämmal, H.; Auriemma, F.; Luppin, J.; Koiv, R.; Tiikoja, H.; Lavrentjev, J. “A Novel Design for
Cruiser Type Motorcycle Silencer Based on Micro-Perforated Elements”.
SAE Technical Paper 2012-32-0109, 2012. https://doi.org/10.4271/2012-32-0109
Page 3 of 12
Figure 1 – A side view of the motorcycle prototype [1]. The location of the silencer unit (orange rectangle) and the direction of
exhausting gas flow (dashed red arrow) are illustrated.
Table 1 – Characteristic parameters of the motorcycle engine [1].
layout
V2, 90°
total displacement
1326cm3
geom. compression ratio
10,8
bore
102mm
stroke
81,2mm
max. torque
134Nm (5600rpm)
max. power
123hp (7100rpm)
engine management
electronical (EFI, Euro4)
cooling system
water cooling
In order to introduce this type of vehicle for initial registration in European Union (EU) according to Directive 2002/24/EC [2] the
type approval evaluation has to be performed. Procedures for the evaluation of exhaust systems are stipulated in Directive 97/24/EC
[3]. Accordingly the maximum by-pass (vehicle in motion) noise level for this type of motorcycle is limited to 80dB(A).
Typically, a motorcycle silencer incorporates fibrous materials. To avoid negative side effects, e.g. deterioration of performance due
to relocation and blow out of the fibers, challenging demands have been stipulated in Directive 97/24/EC [3] on the implementation
of the fibrous materials. A recent investigation [4] on the use of micro-perforated (MP) elements in flow-duct silencers has proven
that the fibrous materials can be successfully substituted by the MP elements.
As a well-known issue the noise attenuation ability typically compromises the pressure drop of the silencer. As the pressure drop is
one of the key parameters affecting the engine performance and fuel consumption, innovative technical solutions (e.g. micro
perforations and meta-materials) providing satisfactory noise cancellation while preserving low pressure drop are of great interest.
The motorcycle exhaust system treated in this paper was designed taking into account the following technical constraints,
concentrated into Table 2. In the present work a technical description of the silencer is given together with experimentally and
numerically obtained characteristic parameters, relevant to successfully fulfill the technical constraints listed in Table 2.

Preprint of the paper: Kabral, R.; Rämmal, H.; Auriemma, F.; Luppin, J.; Koiv, R.; Tiikoja, H.; Lavrentjev, J. “A Novel Design for
Cruiser Type Motorcycle Silencer Based on Micro-Perforated Elements”.
SAE Technical Paper 2012-32-0109, 2012. https://doi.org/10.4271/2012-32-0109
Page 4 of 12
Table 2 – Technical constraints for the exhaust system design.
No.
Constraint
1
The geometry of the silencer is restricted by the
overall design of the motorcycle components (See
Fig. 1).
2
The motorcycle equipped with the silencer must
satisfy the stipulated noise limits [3].
3
The silencer design should provide an option for
less flow restrictive “straight flow” configuration.
4
The fibrous acoustic materials should be avoided
[3] inside the silencer.
5
The silencer should resist corrosive environments
6
The mass of the complete silencer system should
be minimal.
SILENCER DESIGN
OVERVIEW OF DESIGN PROCEDURES
The research and development of the silencer system including experimental testing and 1-D computer simulations were carried out
in cooperation with technical acoustics laboratory at Tallinn University of Technology. A dedicated hot flow test facility presented
in [4, 5 and 6] was implemented for the experimental acoustic characterization of the complete silencer and silencer components.
During the product development project the following design procedures were performed:
1. Analysis on the positioning of the silencer. Determination of the space available, the location of the mounting structures
and the inlet and outlet tubes;
2. Manufacturing and acoustic characterization of micro-perforated tubular elements (40 test samples);
3. Set up of acoustic simulation models in 1-D analysis software followed by preliminary analyses;
4. Manufacturing and acoustic characterization of the first geometrically simplified silencer prototype;
5. On-vehicle testing of the silencer prototype in a variety of operating conditions (stationary: idle, 3000rpm, 6000rpm and
by-pass tests [3]);
6. Development and road testing [3] of extra noise control guide valves;
7. Motorcycle testing on rolling road and analysis for the engine equipped with the 2-stage silencer prototype;
8. Acoustic 2-port testing and characterization of noise control guide valves;
9. Manufacturing of the complete silencer system for the motorcycle.
ACOUSTICAL DESIGN
Since the dominant noise radiation typically originates from the exhaust gas pressure pulsations related to the first harmonics of the
firing frequencies, the engine (see Table 1) tuned for medium crankshaft RPM range can be regarded as a low frequency source.
Hence, an effective noise cancellation for such engine is technically obtained by the maximization of the acoustic wave reflections.
In order to provide high reflections the cross section area ratio at the sudden exhaust system area discontinuities (e.g. in expansion
chambers) should be maximized. Therefore, it is natural to utilize all the limited volume available. A CAD model of the silencer
presented (Fig. 2) illustrates an effective use of the space available underneath the motorcycle (see Fig. 1).

Preprint of the paper: Kabral, R.; Rämmal, H.; Auriemma, F.; Luppin, J.; Koiv, R.; Tiikoja, H.; Lavrentjev, J. “A Novel Design for
Cruiser Type Motorcycle Silencer Based on Micro-Perforated Elements”.
SAE Technical Paper 2012-32-0109, 2012. https://doi.org/10.4271/2012-32-0109
Page 5 of 12
Figure 2 – A CAD model of the geometry available for the exhaust system.
Acoustically the silencer (see Fig. 3) has been designed to incorporate a combined noise cancellation principle, where the exhausting
pressure pulsations are reflected backwards to the source by the three sequential reactive expansion chambers (a, b and c) and
attenuated by the dissipative micro-perforated tubes (1 and 2). Both the engine cylinders are designed to exhaust via autonomous
primary pipes and tailpipes, which are coupled inside the silencer housing through the micro-perforation. In order to satisfy the
described noise limits the perforated tubes inside the silencer have been equipped with guide valves (see Fig. 4), positioned in the
middle of the largest expansion chamber. The function of the guide valves is to direct the pulsating exhausted gas flow through the
micro-perforated elements before terminating from the tailpipe. The dissipative acoustic effect of the micro-perforated elements has
been found to be remarkably dependent on the viscous losses introduced inside the perforated apertures [6, 7 and 8] and the sound
dissipation typically improves in higher flow velocity conditions. The reduction of propagating sound pressure amplitudes can be
explained by the increase in acoustic losses originating from the flow induced vortex shedding in this region. Hereby, this technical
solution has acoustically and aerodynamically been proved to successfully enhance the noise cancellation while preserving
acceptable pressure drop (see results section).
Figure 3 – A geometrical layout of silencer unit revealing the three sequential expansion chambers: 1, 2 – micro-perforated
pipes and a, b, c – expansion chambers.

Preprint of the paper: Kabral, R.; Rämmal, H.; Auriemma, F.; Luppin, J.; Koiv, R.; Tiikoja, H.; Lavrentjev, J. “A Novel Design for
Cruiser Type Motorcycle Silencer Based on Micro-Perforated Elements”.
SAE Technical Paper 2012-32-0109, 2012. https://doi.org/10.4271/2012-32-0109
Page 6 of 12
TECHNICAL DESCRIPTION
The technical layout of the silencer incorporating three sequential expansion chambers and micro-perforated tubes is described in
Figs. 2-4 and the characteristic data of the micro-perforated tubular elements is presented in Table 3.
Figure 4 – A side view of the silencer unit exhibiting removable guide valve position (green line) and illustrating the gas flow
path along the ducts and through the micro-perforated walls (dashed red arrows).
Table 3 – Characteristic data of the micro-perforation used in the silencer.
Photo
Aperture
sketch
Porosity
10%
PA ratio
13.4
TECHNOLOGY AND MANUFACTURING
An overview of the materials and technological procedures used to manufacture the silencer system components is composed in
Table 2.
0.15
20
253
47
76
62
130

Preprint of the paper: Kabral, R.; Rämmal, H.; Auriemma, F.; Luppin, J.; Koiv, R.; Tiikoja, H.; Lavrentjev, J. “A Novel Design for
Cruiser Type Motorcycle Silencer Based on Micro-Perforated Elements”.
SAE Technical Paper 2012-32-0109, 2012. https://doi.org/10.4271/2012-32-0109
Page 7 of 12
Table 4 – Material specifications and technological procedures used for the manufacturing of the silencer components.
Silencer
component
Quantity
Parts, material
Technological
procedures
Inlet ducts
2
Pipe with a bend, stainless
steel AISI316
(45x1.25-R100)
conical element, stainless
steel AISI316
(D45-D38x1.25-L44.1)
2 flanges, V-clamp
Pipe with a bend, stainless
steel AISI316
(38x1.25-R57)
Cut,
machined,
welded*
Perforated
duct
2
Pipe, stainless steel
AISI316 (38x1.25)
Laser
perforated
Outlet
ducts
2
Pipe, stainless steel
AISI316 (38x1.25)
Cut, welded*
Silencer
housing
1
Sheet metal, stainless steel
AISI304 (1.5mm)
Laser cut,
formed,
welded*
Baffles
3
Sheet metal, stainless steel
AISI304 (1.5mm)
Laser cut
Guide
valve
2
Rod, stainless steel
AISI316 (D40)
Machined
*TIG welding
EXPERIMENTS
In order to evaluate the overall performance of the silencer a variety of common engineering parameters were determined including
acoustic transmission loss spectra, the vehicle in motion noise emission level and the aerodynamic pressure loss (see the results
section). A symmetrical geometry of the silencer unit (see Fig. 3) is expected to offer equal TL and PD characteristics for both
exhaust passages (sides). To simplify the experimental procedures, according to the assumption, the characteristic data were
determined only for one side of the silencer.

Preprint of the paper: Kabral, R.; Rämmal, H.; Auriemma, F.; Luppin, J.; Koiv, R.; Tiikoja, H.; Lavrentjev, J. “A Novel Design for
Cruiser Type Motorcycle Silencer Based on Micro-Perforated Elements”.
SAE Technical Paper 2012-32-0109, 2012. https://doi.org/10.4271/2012-32-0109
Page 8 of 12
For the acoustic characterization the silencer unit was treated as an acoustic two-port [9]. The acoustic power was determined at the
inlet and outlet cross-section of the silencer by using the classical two-microphone wave decomposition method [10]. The TL as the
measure of the acoustic power reduction across the silencer was calculated [11]:



    
   
where W denotes the acoustic power, A is the cross sectional area of the flow-duct, ρ is the density of the flowing media, c is the
speed of sound, M is a Mach number and T is the transmission coefficient [11]. The subscripts denote the respective sides of the
two-port. The transmission coefficient T originates from the scattering matrix formulation [10] of the two-port and is defined as the
ratio between incident and transmitted complex acoustic wave amplitudes. In [5] a detailed description about the determination of
the transmission coefficient T has been treated by the authors together with the overview of the aero-acoustic hot flow facility used
for the experimental investigations in this paper.
The maximum by-pass (vehicle in motion) noise level was measured following the Directive 97/24/EC [3] and the aerodynamic PD
was obtained by measuring the static pressure difference across the inlet and outlet of the silencer.
MODELLING
The 1D analysis of the muffler has been performed by implementing commercial software packages Gamma Technologies GT-
PowerTM [12] and SIDLABTM [13]. GT-PowerTM is a widely employed tool for the analyses of internal combustion engines, and the
SIDLABTM is a specific tool for 1D acoustic simulations focusing on flow duct applications. In GT-PowerTM the 1D non-linear flow
equations of mass, momentum and energy conservation [14] are solved in the time domain for each component of the complete
engine (air box, intake, exhaust system, muffler, etc.).
In the specific case of a silencer, the chambers are schematized by sets of quasi-3D elements whose ports are oriented along the
three spatial directions. In this way the radial interactions between the perforated tubes and the chambers are accounted. Straight
tubes are represented by 1D elements. Eventually, perforated tubes are modeled by a set of quasi-3D elements connected to each
other along the direction of the tube and linked, through a proper number of orifices, to the quasi-3D elements constituting the
chambers (see Fig. 5).
The alternative software package SIDLAB uses a 1D linear frequency domain approach, which is very fast and especially suitable
for flow duct acoustic applications. The computations are based on the well-known acoustic plane wave equation [15]. In GT-
PowerTM the porosity is accounted considering a respective friction coefficient in the energy equation. Whereas in Sidlab the
perforated tubes are divided into a number of discrete lumped impedance two-port elements which are separated by hard segments
on both sides [16]. The two-microphone random-excitation technique proposed by Seybert and Ross [17] is used in GT-PowerTM in
order to calculate the transmission loss of the silencer.
Figure 5 – The two-microphone technique virtually implemented for the TL analysis in 1D GTPower simulations.

Preprint of the paper: Kabral, R.; Rämmal, H.; Auriemma, F.; Luppin, J.; Koiv, R.; Tiikoja, H.; Lavrentjev, J. “A Novel Design for
Cruiser Type Motorcycle Silencer Based on Micro-Perforated Elements”.
SAE Technical Paper 2012-32-0109, 2012. https://doi.org/10.4271/2012-32-0109
Page 9 of 12
RESULTS
EXPERIMENTAL RESULTS
Experimentally determined results for the acoustic transmission loss spectra with and without the guide valves, measured in the
presence of 40m/s room temperature mean flow, are presented in Fig. 7. The aerodynamic pressure loss characteristics measured in
0-40m/s flow velocity range for both the silencer settings are shown in Fig. 8. The respective output torque curves measured in
rolling road test facility for the motorcycle engine equipped with the silencer are exhibited in Fig. 9.
Figure 6 – Transmission loss spectrums of the silencer measured in the presence of 40m/s mean flow velocity and presented for
straight flow (grey dashed line) and high attenuation (green solid line) setup.
Figure 7 – Experimentally determined pressure drop of the silencer presented for straight flow (grey dashed line) and high
attenuation (green solid line) setup.

Preprint of the paper: Kabral, R.; Rämmal, H.; Auriemma, F.; Luppin, J.; Koiv, R.; Tiikoja, H.; Lavrentjev, J. “A Novel Design for
Cruiser Type Motorcycle Silencer Based on Micro-Perforated Elements”.
SAE Technical Paper 2012-32-0109, 2012. https://doi.org/10.4271/2012-32-0109
Page 10 of 12
Figure 8 – The output torque curves measured in rolling road test facility for the motorcycle engine equipped with the silencer
in straight flow (grey dashed line) and high attenuation (green solid line) setup.
MODELLING RESULTS
In Fig. 9 the 1-D modeling results by using GT-PowerTM and SIDLABTM for the acoustic TL of the silencer in straight flow
configuration (in the absence of the guide valves) are compared to the experimental ones in no-flow conditions.
It is clearly noticeable that the entire plane wave region shown is well represented by the 1D simulation. The result demonstrates
that the use of the quasi 3-D elements for the chamber modeling allows to provide reliable results almost up to 1000 Hz.
Figure 9 – The acoustic TL of the silencer in straight flow configuration and in the absence of mean flow: experimental results
(red solid line), 1D simulation results with GT-PowerTM (blue dashed line) and SIDLAB (black dots).
A comparison of the TL results for 40m/s mean flow velocity through the silencer in straight flow configuration (Fig. 10) and in
case of the extra attenuation guide valves (Fig. 11) is presented. Due to the viscous effects generated by the flow passing the
apertures, both the experimental and the numerical results obtained by GT-PowerTM demonstrate an enhancement of the attenuation
performance of the silencer at low frequencies. This effect is even more pronounced when the guide valves are equipped (see Fig.
11).

Preprint of the paper: Kabral, R.; Rämmal, H.; Auriemma, F.; Luppin, J.; Koiv, R.; Tiikoja, H.; Lavrentjev, J. “A Novel Design for
Cruiser Type Motorcycle Silencer Based on Micro-Perforated Elements”.
SAE Technical Paper 2012-32-0109, 2012. https://doi.org/10.4271/2012-32-0109
Page 11 of 12
Figure 10 – The acoustic TL of the silencer in straight flow configuration and in the 40m/s mean flow condition:
experimental results (magenta solid line), 1D simulation results with GT-PowerTM (orange dashed line).
Figure 11 – The acoustic TL of the silencer equipped with the guide valves and in the 40m/s mean flow condition:
experimental results (magenta solid line), 1D simulation results with GT-PowerTM (orange dashed line).
The PD calculated by GT-PowerTM simulations for two silencer configurations are 31mbar and 135 mbar. The simulated PD values
agree well with the experimentally determined ones: 35mbar and 118mbar respectively (see Fig. 7). The relatively small difference
between the simulated and measured PD of the silencer indicates well-captured viscous losses that occur in the presence of mean
flow.
CONCLUSIONS
A novel design for a cruiser type motorcycle silencer incorporating custom micro-perforated elements has been presented in this
paper. The silencer represents an effective engine noise cancellation solution in constricted conditions without the implementation
of the traditional fibrous materials.
The performance of the silencer as well as the engine output characteristics are presented for two different silencer configurations.
The numerical 1-D simulation models developed for the optimization procedures have exhibited a good agreement with the
experimentally determined data.
While maintaining acceptable pressure drop characteristics and providing pleasant engine sound, the silencer has proven to comply
with standard noise criteria.

Preprint of the paper: Kabral, R.; Rämmal, H.; Auriemma, F.; Luppin, J.; Koiv, R.; Tiikoja, H.; Lavrentjev, J. “A Novel Design for
Cruiser Type Motorcycle Silencer Based on Micro-Perforated Elements”.
SAE Technical Paper 2012-32-0109, 2012. https://doi.org/10.4271/2012-32-0109
Page 12 of 12
REFERENCES
1. http://www.renardmotorcycles.com.
2. Directive 2002/24/EC of the European Parliament and of the Council of 18 March 2002 relating to the type-approval of two or
three-wheel motor vehicles and repealing Council Directive 92/61/EEC, Official Journal of the European Communities L124
(2002).
3. Directive 97/24/EC of the European Parliament and of the Council of 17 June 1997 on certain components and characteristics
of two or three-wheel motor vehicles, Official Journal of the European Communities (1997).
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2011-24-0219 (2011).
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20119514 (2011).
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2012320030 (2012).
7. D.Y. Maa, Theory and design of Microperforated-panel sound-absorbing construction. Sci Sin, XVIII (1975), pp. 55–71.
8. S. Allam, M. Abom, A New Type of Muffler Based on Microperforated Tubes, J. Vib. Acoust., (2011), 133 (3), 8 pp.
9. H. Bodén, M. Åbom, Modeling of fluid machines as sources of sound in duct and pipe systems, Acta Acustica 3 (1995) 549-
560.
10. M. Åbom, Measurement of the scattering-matrix of acoustical two-ports, Mechanical Systems and Signal Processing 5 (1991).
11. M. Åbom, An Introduction to Flow Acoustics, 164 (2004).
12. Gamma Technologies, GT-SUITE Flow Theory Manual Version 7.0, the GT-SUITE Interactive Simulation Environment
(2009).
13. SIDLAB Acoustics User Manual, version 2.6.
14. D.E. Winterbone, R.J. Pearson, Theory of engine manifold design, Wave action methods for IC engines, Professional
engineering publishing (2000).
15. H.P. Wallin, U. Carlsson, M. Abom, H. Boden, Sound and vibration, KTH MWL, ISBN 978-91-7415-553-2.
16. T. Elnady, M. Abom, S. Allam, Modelling perforates in muffler using two ports, Journal of Vibration and Acoustics, December
Vol. 132 / 061010-3.
17. A.F. Seybert and D.F. Ross, Experimental determination of acoustic properties using a two-microphone random-excitation
technique, Journal of the Acoustical Society of America, 61, 1362-1370, 1977.
CONTACT INFORMATION
Raimo Kabral: The Marcus Wallenberg Laboratory, The Royal Institute of Technology, Teknikringen 8, Stockholm, SE-10044,
Sweden, kabral@kth.se, phone: +372 50 24 992;
Dr. Hans Rämmal: Department of Automotive Engineering, Tallinn University of Technology, Ehitajate tee 5, Tallinn, 19086,
Estonia, hansra@kth.se, phone: +372 56 465 738;
ACKNOWLEDGMENTS
The authors would like to acknowledge technical consultancy companies Lettore and Triple Seven for successful co-operation
including financial, technical and technological support. In addition, the support from Estonian Science Foundation (Grant No.
7913), Marie Curie European doctoral student program and “DoRa” program of Archimedes Foundation are acknowledged.
DEFINITIONS/ABBREVIATIONS
MP
micro-perforated
TL
transmission loss
PA Ratio
perimeter to area ratio
PD
pressure drop
RPM
rotations per minute