tober corakci msc thesis

7/30/2019 Tober Corakci MSc Thesis

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Modeling of interior sound field in railway vehiclesSpecial focus on sound transmission between vestibules and saloons

Masters Thesis in the Masters programme in Sound and VibrationATA CAN CORAKCI

STEFAN TOBER

Department of Civil and Environmental Engineering

Division of Applied Acoustics

Vibroacoustics Group

CHALMERS UNIVERSITY OF TECHNOLOGY

Gteborg, Sweden 2009Masters Thesis 2009:11


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CHALMERS UNIVERSITY OF TECHNOLOGY

Gteborg, Sweden 2009

Modeling of interior sound field in

railway vehicles

Special focus on sound transmission between vestibules and saloons

ATA CAN CORAKCI

STEFAN TOBER

MASTER'S THESIS 2009:11


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Cover:

Photography of scale model built for sound field measurements.

Master's Thesis 2009:11




Chalmers University of TechnologySE-41296 Gteborg

Sweden

Tel. +46-(0)31 772 1000

Modeling of interior sound field in railway vehicles

Special focus on sound transmission between vestibules and saloons

Ata Can Corakci, Stefan Tober, 2009

Reproservice / Department of Civil and Environmental Engineering

Gteborg, Sweden 2009


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Acknowledgements

I would like to thank my colleague Stefan Tober for his great friendship and to

express my grateful to our supervisor Wolfgang Kropp, the head of the Department

of Applied Acoustics at Chalmers University, for his great help during our master

thesis work. Thanks to all staff and all my friends in the department for their interest

on our nice train model, and would like to thank everyone who helps us to assembly

the model.

And also want to thank everyone who has great help on our work during the time in

Bombardier Transportation in Vsters.

Can

I would first and foremost like to thank my colleague Can Corakci and our

supervisor Wolfgang Kropp for the fun we had while working on this project.

Wolfgang was a great help for this work in so many ways. He was always willing to

sacrifice some of his valuable time, not only during the day, but also at night and on

weekends. The many Skype-discussions we had, significantly contributed to this

work. Many thanks also to Brje Wijk for his endless help in technical belongings

and for letting me use his workshop. Thanks to all students and staff at applied

acoustics who contributed their part to the great atmosphere at the department.

I am also grateful for the help from the team at Bombardier Transportation, namely

Karl-Richard Fehse, Ulf Orrenius, Anders Frid and Thorsten Kohrs. I appreciated

their valuable inputs during our conference calls. Furthermore, I would like to thank

the rest of the Specialist Engineering team in Vsters for the nice time there. Last but

not least, I would like to express my gratitude to Bombardier Transportation for

giving me the opportunity to work on this project and especially for paying for my

new bike

Stefan


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Contents1. Introduction ...................................................................................... 92. Background ..................................................................................... 10

2.1. Sound distribution in closed rooms ..................................................... 102.2. Sound distribution in long, corridor shaped rooms .......................... 122.3. Brains Bombardier Railway Noise Software .................................... 12

3. Scale model ...................................................................................... 143.1. Scaled sound source ................................................................................ 163.2. Microphones ............................................................................................ 193.3. Microphone rack 1 .................................................................................. 223.4. Microphone rack 2 .................................................................................. 233.5. Measurement setup ................................................................................ 253.6. Data post processing ............................................................................... 26

4. Reverberation time measurements .............................................. 284.1. Motivation ................................................................................................ 284.2. Setup ......................................................................................................... 284.3. Reverberation time measurement using MATLAB ........................... 294.4. Results ....................................................................................................... 30

5. Sound field measurements ........................................................... 335.1. Influence of absorption on sound decay .............................................. 37

5.1.1 Measurement in empty scale model...................................................385.1.2 Measurement with absorption on ceiling ..........................................395.1.3 Measurement with absorption on seats and ceiling (as in Regina)405.1.4 Measurement with absorption on seats and ceiling and two toilet

rooms ......................................................................................................415.1.5 Summary influence of absorption on sound decay ......................42

5.2.

Influence of screen opening area on sound decay ............................. 43


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5.2.1 Measurement with small screen opening ..........................................445.2.2 Measurement with standard screen opening ....................................455.2.3 Measurement with large screen opening ..........................................465.2.4

Summary influence of screen opening area on sound decay ......47

6. Ray tracing method ........................................................................ 48

6.1. Theory ....................................................................................................... 486.2. Odeon ........................................................................................................ 486.3. Simulation ................................................................................................ 506.4. Influence of absorption on sound decay .............................................. 50

6.4.1 Simulation of empty model .................................................................516.4.2 Simulation with absorption on ceiling ...............................................526.4.3 Simulation with absorption on seats and ceiling as in Regina ....546.4.4 Simulation with absorption on seats and ceiling and two toilet

rooms ......................................................................................................556.4.5 Summary influence of absorption on sound decay ......................57

6.5. Influence of screen opening area on sound decay ............................. 586.5.1 Simulation with small screen opening ...............................................586.5.2 Simulation with standard screen opening .........................................596.5.3 Simulation with large screen opening ...............................................616.5.4 Summary influence of screen opening area on sound decay ......63

7. Statistical Energy Analysis (SEA) ................................................ 647.1. Theory ....................................................................................................... 647.2. Implementation Setup of SEA model for Regina ............................ 657.3. Influence of absorption on sound decay .............................................. 69

7.3.1 Simulation of empty scale model........................................................707.3.2 Simulation with absorption on ceiling ...............................................717.3.3 Simulation with absorption on seats and ceiling (as in Regina) ....727.3.4 Summary influence of absorption on sound decay ......................73

7.4. Influence of screen opening area on sound decay ............................. 747.4.1 Simulation with small screen opening ...............................................747.4.2 Simulation with standard screen opening .........................................757.4.3 Simulation with large screen opening ...............................................767.4.4 Summary influence of screen opening area on sound decay ......77

8. Comparison ..................................................................................... 78


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8.1. Influence of the absorption .................................................................... 788.2. Influence of the Screen opening ............................................................ 808.3. The Regina case ....................................................................................... 82

9. Conclusions: Possible implementation for sound fieldprediction in trains ......................................................................... 869.1. Estimation of sound decay from SEA results ..................................... 869.2. Introducing sound decay by an increased number of subsystems .. 93

Appendix A .......................................................................................... 95Detailed comparison ........................................................................................ 95

Empty .................................................................................................................95Absorption on ceiling ........................................................................................96Small opening .....................................................................................................97Standard opening ...............................................................................................98Large opening .....................................................................................................99Regina ...............................................................................................................100Regina two toilet rooms ...............................................................................101

Appendix B ......................................................................................... 103

Odeon Results ................................................................................................. 103Appendix C ........................................................................................ 109

MATLAB Code SEA model: ......................................................................... 109Appendix D ........................................................................................ 128

Reverberation time measurements using paper-level-recorder .............. 128Appendix E ......................................................................................... 129

Scale model photos ........................................................................................ 129Scale model drawings .................................................................................... 130

Sign convention ................................................................................. 134Bibliography ....................................................................................... 135


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1.Introduction

The sound field in vehicles, in trains as well as passenger cars has become an

important part of the design process over the past years. Knowledge about the sound

perceived by the passenger is not only interesting in terms of avoiding annoyance

from high noise levels; it also helps to build a quality image of a vehicle. In the same

way as an appealing sound in a car can give the passenger a feeling of quality and

maybe even an indication of the purchase price of the car, this can be applied to

trains. Therefore it is desirable for train designers to predict the interior sound field

of their train to be built as early in the design phase as possible. In an early state

sound design measures can be relatively simple and cheap to do where on the otherhand changes to an already built train are mostly very hard to implement and can be

very cost-intensive.

The purpose of this work is to analyze the physics behind the sound distribution in

train interiors to provide a solid base for the implementation of the acquired

knowledge in future prediction tools. This is not particularly easy as the well known

simple theories for sound distribution in regularly shaped rooms can only be used if

a diffuse and evenly distributed sound field can be assumed. For the special case of

rail vehicle interiors, the problem is more complex due to the corridor like shape of

the room. This leads to an uneven distribution of energy and sound decay along thelength of the corridor. For train interiors the sometimes present screens separating

the entrance vestibules from the seating area can have an important effect on the

sound field as well. To describe this in a proper way, a special theory is required to

predict sound distribution inside railway vehicles.

In order to understand the distribution of sound in trains better, a number of

different approaches were used. A scale model of a complete car of Regina, a

regional train built by Bombardier Transportation, has been built for the purpose of

extensive sound decay measurements (360 microphone positions in the train car).Additionally numerical simulation methods, particularly a ray tracing method, and

an analytical method, statistical energy analysis (SEA) have been used to describe the

sound field in the train.

Combining the results of all these different approaches gave a good understanding of

how sound distributes in railway vehicles and led to a suggestion of a simple model

based on statistical energy analysis for future implementation in sophisticated

prediction tools for the sound field in railway vehicles.


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2.Background2.1. Sound distribution in closed rooms

In a regularly shaped room with big enough dimensions to have a sufficiently high

modal density in the frequency range of interest; it is common to describe the sound

field in the room by a statistical model. This model is known as the diffuse field

model. A diffuse sound field is one in which there is an equal energy density at all

points in the room with an equal probability that sound will arrive from any

direction [11].

As the number of modes plays a key role in this definition, it has been calculated for

the saloons of Bombardiers Regina train.

For a room the modal density can be calculated to [10]

3

2

2

4

28 c

Vf

Vfc

Sf

c

Ln i

i

iii

+

+

=

. ( 2.1)

With Libeing the total edge length, Sithe surface area and Vithe volume of the room.

As ni represents number of modes per Hz, this quantity can be multiplied by the 1/3

octave band width to get the number of modes per 1/3 octave band.

Figure 2.1: Number of modes per 1/3 octave band 3 different saloons in Bombardiers Regina

(Saloon 1 and 3 have same size and therefore same modal density)


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As seen in figure 2.1 there is a high number of modes present in the saloons of

Bombardiers Regina train. The number of modes per 1/3rd octave band exceeds 30 for

frequencies greater than 200 Hz.

The eigenfrequencies of a rectangular room can be calculated to2/1

222

2

+

+

=

z

z

y

y

x

xqxqyqz

l

q

l

q

l

qcf . ( 2.2)

0 100 200 300 400 500 600 700 800 900 1000

nx 0 0

0 ny 0

0 0 nz

nx ny 0

nx 0 nz

0 nx ny

nx ny nz

Figure 2.2: Eigenfrequencies 1st class saloon Bombardier Regina

Figure 2.2 shows the eigenfrequencies calculated for the first saloon of Bombardiers

Regina.

Eigenfrequencies denoted nx, ny and nz refer to modes in lateral direction,

longitudinal direction and vertical direction respectively.

The Schrder frequency defined as,

V

Tfs

602000 ( 2.3)

is commonly used to estimate above which frequency the diffuse field model can be

considered valid. It calculates to =sf 180 Hz for the first saloon of Regina as an

example. (T60 = 0.3 s, V = 37m, see section 4)

This and the high number of modes would lead to the conclusion that the diffuse

field model can be considered valid above about 180 Hz for a saloon of Regina, and

that energy should be distributed evenly above this frequency.


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2.2. Sound distribution in long, corridor shaped rooms

Everything said above is only valid for regularly shaped rooms with evendistribution of absorption. When one room dimension is significantly different from

the others, as it usually is the case for trains, the diffuse field model and the

assumption of an even distribution of energy is not valid any more. In such cases

energy decay along the length of the room (the one dimension that is much greater

than the others) can be observed. To describe the sound field in such rooms,

alternative models are required that are able to include the uneven distribution of

energy. Such models have been developed by several authors. Hodgson [20] has

published a review of several of these models. He concluded that some models,

namely Kuttruffs [10] and his own model were predicting the sound field accurately,

whereas others were inaccurate. Redmore [14] published an experimental model

describing the sound decay in corridors already in 1982 and Franzoni [18] presented

an experimental and an analytical model in 1999.

Unfortunately, none of these models considers large internal barriers or

constructions can be found in train interiors. In the case of Bombardiers Regina train,

there are screens separating the entry area from the seating area. These screens cover

the whole cross section of the train except for a 0.75 m wide door opening. It is

believed, that these screens will have a strong influence on the sound field in the

interior of the train. This might be the reason why sound decay models usually used

for empty corridors, like the one from Redmore, are not able to represent the

sound field in train interiors properly.

Therefore a new model describing the sound field in the special case of trains would

be required. This work should help to develop such a model for the use in future

tools for prediction of the sound field in train interiors.

2.3. Brains Bombardier Railway Noise Software

Bombardier Transportation has developed their own in house prediction tool to

describe the sound field in train interiors. This software is an SEA compatible

framework to facilitate an efficient handling of energy input to different cavities

along the corridor based on Redmores corridor model [15]. In the present state of

Brains, the model treats the train interior as a corridor with no internal barriers. It

includes the effect of vestibule screens in a simplified manner in that the coupling

between cavities is reduced in proportional to the reduced open cross section. This is

believed to be the reason why the model does not give satisfying results for trains as


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Bombardiers Regina series, which have screens separating the entry area from the

seating area [1].


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3.Scale modelFor a detailed analysis of the sound field and the spatial sound decay in train

interiors and the evaluation of BRAINS as a prediction tool for spatial sound decay,

extensive measurements were necessary. These measurements should ideally be

performed in a number of different trains with different interior configurations to get

an idea of the influence of variations in geometry and absorption. To achieve this, we

would need access to a number of train cars for a significant amount of time. It

would also be nice to be able to change the interior of a given train (e.g. seating,

screens) or to evaluate the effect of absorption (e.g. the foam layer on the seats) or

screens on the spatial sound decay. All this is not really an option in a real train.

Train down time is usually very expensive and it is obviously not possible to alter

the interior of a train in normal revenue service just for the purpose of sound

measurements.

Therefore we decided to build a scale model of a train car for our measurements. The

model could be placed in the laboratory of Applied Acoustics, so it would be easily

accessible at any time and the interior could be designed flexibly to allow changes

with reasonable effort.

The first thing that needed to be decided was the scaling factor of the model. For the

measurement of air borne sound only, as in our case, a scaling of the geometry by a

certain factor requires a scaling of the wave length by the same factor. If wave

lengths are scaled down, frequencies will be scaled up. So the scaling factor is mainly

determined by the desired frequency range for the measurements. It has been agreed

with Bombardier that 125 Hz to 4000 Hz for full scale measurements would need to

be replicated in the scale model measurements. In order to perform measurements

efficiently, a multi channel measurement system would be desirable. The Hewlett

Packard VXI stations available at Applied Acoustics have a frequency range of 20

kHz, which gives the upper frequency limit for the scale model measurements. This

results in a maximal scaling factor of SF = 20 kHz/ 4 kHz = 5. The lower frequency

limit is determined by the ability of the used noise source. For a scaling factor of 5 the

source needs to produce sufficient power from 5 * 125 Hz = 625 Hz and above.

The model has been built as a 1:5 scale model of a Bombardier Regina, commonly

used in Sweden. With this scaling factor the desired frequency range was achievable

and the model had a length of 4.6m, which is quite long, but still manageable in the

laboratory.


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Figure 3.1: Bombardier Regina

All dimensions for the scale model have been taken from the drawing shown in

Figure 3.1. The paper drawing has been scaled to 1:50 with the help of a photo copier

and all dimensions have been measured from there. (probably not the most accurate

method, but the only one available) With these dimensions a simplified full scale

CAD model has been drawn. The CAD model was then scaled to 1:5 and all wall

thicknesses where adapted to commercially available material dimensions. The

model was built from medium dense fiber board (MDF) and Plexiglas. (Makrolon)

Figure 3.2: Scale model drawing


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Figure 3.3: Photo of scale model (more photos in appendix)

Figure 3.4: Notation for different sections of Regina interior

All sections in the train are referred to as indicated in Figure 3.4 throughout this

report.

3.1. Scaled sound source

It would have been desirable for our measurements to have a noise source with

monopole characteristics. A breathing sphere would have such a radiation

characteristic. One way to approximate this for a certain frequency range is the use of

a so called dodecahedron source. It consists of 12 loudspeaker cones mounted on the

surface of a dodecahedron. This approximates a breathing sphere for frequencies

where the individual loudspeakers radiate with low directivity. Commercially


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available dodecahedron sources intended for room acoustic measurements usually

have a diameter of about half a meter. For our scale model, we would obviously need

something much smaller. As the market did not fulfill our need, we had to build a

source ourselves. With the above mentioned dodecahedron source in mind we triedto utilize the same principle but shrink it as much as possible. This led to a cubical

source of 55mm edge length with 6 pieces 20mm dome tweeters mounted on the

surface.

Figure 3.5: B&K source Type 4292 and homemade Regina source Type 0001

The perfect noise source for our scale model measurements would have no

directivity and a flat power spectrum for frequencies from 630 Hz to 20 kHz. Toevaluate the performance of our real source, we measured its sound pressure at

different angles from 0 to 180 with increments of 15 at a distance of 200mm. The

source was placed on a rotating pole in a semi absorbent environment. For

comparison, also a single tweeter was measured under the same conditions. All

measurements on the source were done with ARTA (commercial software for

acoustical measurements) and a Creative E-MU sound card on a standard laptop

computer. The measurements are not calibrated but the same setup has been used for

all measurements. So they are comparable to each other. The desired frequency

response is indicated in the graphs below (dashed line). A perfect source wouldshow this frequency response for all angles.


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103

104

-70

-65

-60

-55

-50

-45

-40Directivity of the Tweeter

Magnitude[dB]

Frequency

Figure 3.6: Single tweeter SPL at different angles

Figure 3.6 above shows the results for the single tweeter. The frequency responsemeasured for different angles shows strong deviations (~4dB@5kHz and 30) and the

radiated sound power is low for frequencies below 2 kHz. This would not be a

desired characteristic for our measurements.

103

104

-65

-60

-55

-50

-45

-40

-35Directivity of Regina Source

Magnitude[dB]

Frequency

Figure 3.7: Regina source SPL at different angles

The measured response of the Regina source shown in Figure 3.7 is much more

similar to the desired response. It shows negligible directivity up to approximately 12

kHz. This proves that the cubical source is much closer to the ideal breathing sphere

as the single dome tweeter. The power output for low frequencies is much higher as

for only one tweeter. (~20dB at 2 kHz) This is due to the improved radiation

impedance when using more tweeters and a passive network that boosts the

electrical input power at about 600 Hz. The electrical network is a 12dB high pass

consisting of 82F and 1mH that has been developed using BOXSIM [19]. The thick

red line in Figure 3.7 shows the average sound pressure over all directions.


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Figure 3.8 shows the input voltage in dBV at the Regina source due to the electrical

network, relative to a case without any electrical network (0dBV).

Figure 3.8: Regina source amplification characteristics of the electrical network

3.2. Microphones

The microphones for our measurements should be scaled down in the same way as

the source. We used very small commercially available electret condensermicrophones from Panasonic. (Type WM60) The microphones have an outer

diameter of 6mm which corresponds to 30mm in full scale and is therefore slightly

bigger than a common 1 microphone. They should be sufficiently small for our

purpose. (d=30mm => k*a=1 at 3600 Hz)

Figure 3.9: Panasonic ECM Type WM60

All microphones have been individually calibrated in an anechoic environment using

the Regina source and a high quality reference microphone (Larson&Davis). Each

microphone was powered by its individual power supply and amplified by the


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matching channel of the later used microphone pre-amplifier (MIC AMP 8.0).

Therefore possible deviations in these components have been calibrated as well. The

microphone in test has been put as close as possible on the reference microphone and

sound from the Regina source has been recorded by an Agilent VXI station. Distancefrom source to both microphones was 250mm. The power spectrum of each test

microphone has been calculated relative to the reference microphone and saved as a

correction vector for individual calibration. This calibration vector, individual for

each microphone, has then been used to correct all scale model measurements.

Figure 3.10 shows the sound pressure level (1/3 octave band) of the Regina sound

source measured with calibrated microphones in an anechoic environment. As seen

in the graph, the microphones measure virtually the same power spectrum for

frequencies up to 10 kHz but show little deviation in the 12.5 and 16 kHz band. This

has later been identified as caused by placing the different microphones not exactly

in the same position while measuring in the anechoic room.

103

104

-10

-5

0

5

10

15

20Comparison of Calibrated Microphones

1/3-Octave Band Center Frequency [Hz]

RefMic

Mic1

Mic2

Mic3

Mic4

Mic5

Mic6

Mic7

Figure 3.10: Comparison of calibrated microphones (anechoic)

To evaluate this closer, all microphones have been measured at the exact same

position in the scale model. Figure 3.11 below shows the result of that measurement.

It shows the sound pressure level measured with different microphones at exactly

the same position and the average of all these measurements. The high frequency

deviation is present in the scale model as well.


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103

104

-20

-15

-10

-5Different microphones in identical position relative to RefMic


SoundPressure

Level

Mic7

Mic6

Mic5

Mic4

Mic3

Mic2

Mic1

AVG

Figure 3.11: Different microphones in identical position in scale model

103

104

-5

-4

-3

-2

-1

0

1

2

3

4

5Correction to average


S

oundPressureLevel

Mic7

Mic6

Mic5

Mic4

Mic3

Mic2

Mic1

Figure 3.12: individual microphone correction

103

104

-25

-20

-15

-10

-5Different microphones in identical position relative to RefMic - average corrected


SoundPressureLevel

Mic7

Mic6

Mic5

Mic4Mic3

Mic2

Mic1

Figure 3.13: Corrected microphones show identical SPL reading


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To correct this deviation, the average of all measurements has been calculated. Then

the difference from each microphone (figure 3.12) to this average has been subtracted

from each microphone. After this procedure every microphone gives exactly thesame result for the measured position. (Figure 3.13) Of course the calibrated relation

from the real value of sound pressure to the one measured by the corrected

microphones is not valid any more after this procedure. But the differences between

the individual microphones have vanished. For our measurements, the differences

between microphones are much more important than the absolute value so we

decided to focus on getting the difference as accurate as possible.

3.3. Microphone rack 1

For measuring the spatial sound decay with all seats in the model a set of six

microphones, hanging from the ceiling, needed to be moved through the train model.

The individual microphones have been mounted to a rack made of 2 mm steel wire.

The rack was hung from a string tied to both end walls of the model. In this way the

rack of microphones could be moved through the train model on a straight line and

with constant distance between the microphones. The microphones hung 1.2m above

ground and had a constant distance of 1m to each other. (Full scale dimensions)

Figure 3.14: Sketch of microphone rack 1 in scale model

(longitudinal section view, scaled dimensions in mm)


(cross section view, scaled dimensions in mm)


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Figure 3.16: Microphones on rack 1 in scale model (view through ceiling)

3.4. Microphone rack 2

For measuring the spatial sound decay of the empty scale model (without seats and

toilet room) another microphone rack has been built. With this rack, the microphones

were spread in a 4x3 matrix in the cross section of the train. As we could only use

seven microphones at a time, the train has been measured in two passes. Six

microphones recorded one half of the 4x3 matrix and one was used for comparison

on a mirrored position. For the second pass, the rack was flipped and now measuredthe other six matrix positions plus the one for comparison. Figure 3.17 shows the

microphone rack 2 for both passes. The microphones were placed at 0.5m, 1.0m and

1.5m above ground with a distance of 0.75m to each other. The rack has been used at

30 longitudinal positions with 0.5m and 1.0m between them. (Full scale dimensions)


(cross section view, scaled dimensions in mm)


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Figure 3.18: Microphone rack 2 in scale model (position for pass 2)

As the measurements were done in two passes, it might be worth while looking at

the difference between each pass. For that purpose seven and not only 12/2 = 6

microphones have been recorded in each position.

Figure 3.19 shows the difference between the two passes for a number of measured

positions. It has been calculated as the difference of two microphones that end up in

the same position when the rack is flipped. Microphone 7 pass 1 has been subtracted

from microphone 6 pass 2 and microphone 6 pass 1 from microphone 7 pass 2

respectively. The difference is lower than 1 dB and takes into account the

repeatability in positioning of the rack and any change that might be caused by

opening and closing the model to turn the rack around. The differences are

reasonably small, so the method seems to be appropriate.


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103

104

-3

-2

-1

0

1

2

3i i i


Powerspectrum

Figure 3.19: Difference between the two passes

3.5. Measurement setup

Figure 3.20: Block diagram of measurement setup

Figure 3.20 shows a block diagram of the scale model measurement setup. The

signal of seven microphones on a moveable rack and the signal of a static reference


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microphone was amplified by 6dB and then fed to an Agilent VXI station. (VXI

E8408A Mainframe) There the time signal of 9 channels was recorded and handed

over to MATLAB for real time processing. The MATLAB code Trigger Happy Real

Time Version 4.0 written by Patrik Andersson from Applied Acoustics was used tocalculate and plot auto spectra, cross spectra and coherence for each channel. The

excitation signal was random noise produced by the VXI station, amplified by a

NAD 310 series and radiated by the Regina Source Type 0001 described earlier. The

output noise was looped back to input CH1 as a reference.

3.6. Data post processing

103

104

-90

-80

-70

-60

-50

-40

-30

-20

-10

0Raw data

Frequency [Hz]

Voltage(dB)

Mic1

Mic2Mic3

Mic4

Mic5

Mic6

Mic7

Mic8

Figure 3.21: Raw data- transfer functions from Trigger Happy

Figure 3.21 shows the raw data as it was saved by Trigger Happy. It shows the

transfer functions from the electrical output signal (CH1 in Figure 3.20 ) to the input

signal from each microphone. (CH2 to CH9 in Figure 3.20 ) These transfer functions

have been filtered to 1/3 octave bands in the frequency domain. For each of these 1/3

octave bands the power spectrum has been calculated to

xx

f

f

xy SHH *~

22

1

= ( 3.1)

With Hxy being the H1 estimate of the frequency response function and Sxx and Sxy the

averaged auto and cross spectra, respectively.

xx

xy

xyS

SH = ( 3.2)

Then the individual calibration correction was applied to the resulting power

spectrum of each microphone and the data was presented relative to a reference


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microphone located in a corner of vestibule 1. This procedure results in a frequency

response for each microphone as seen in Figure 3.22.

103

104

-30

-25

-20

-15

-10

-5

0Calibrated Microphones rel. to RefMic


Powerspectrum

Mic1

Mic2

Mic3

Mic4

Mic5

Mic6

Mic7

Figure 3.22: 1/3 octave power spectrum of each microphone relative to a static

reference microphone


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4.Reverberation time measurements4.1. Motivation

In order to make scale model and full scale Regina comparable, not only the

dimensions must be scaled properly, also the absorption respectively, the

reverberation time has to be adjusted in a proper way. This means that absorption

has to be introduced to the scale model in a way, which is similar to the spatial

distribution of absorption in full scale Regina. Of course the absorption should also

have similar frequency characteristics. In order to achieve this, different materials

have to be tested in a scale model reverberation chamber.

To measure the absorption coefficients of materials with Kundts tube was not

realistic due to the frequency limit depending on the tube diameter. The desired

frequency range (up to 20 kHz) was not suitable for the Kundts tube that was

available at the department of applied acoustics.

Therefore the scale model reverberant chamber was considered as best choice.

However, it required to divide the saloon 1 with an MDF board from the rest of the

train. With this separation of saloon 1 from the first vestibule we obtained a room

with dimensions of (5.8 x 2.9 x 2.1m, full scale). The room was much smaller than itshould be according to ISO 354 [6], but the room shape and reverberation time of the

empty room are according to the standard

4.2. Setup

After the scale model reverberation room was built, microphones and the

loudspeaker were placed according to ISO standard (ISO 354-1985) [6]. A 3D plot of

the interior setup can be seen in figure 4.1. Eventual leaks were covered with tape

and the room was rearranged with the absorption material under test for each case.

We used the same microphone rack during all the measurements.


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Figure 4.1: Setup of reverberation time measurementsred - first microphone positions, blue - second microphone positions

yellow - first source position, green - second source position

No commercial product was found to do the measurements up to our desired

frequency range, which was 20 kHz. So the same VXI station which was used for all

other scale model measurements in this work was used for the reverberation time

measurements as well. Based on measured frequency response functions, impulse

response functions were calculated and the Schroeder backwards integration [10]

was used to calculate the energy decay. The raw data was processed with the help of

MATLAB.

4.3. Reverberation time measurement using MATLAB

Before we started to do our measurements, we wanted to be sure that the signal to

noise ratio was good to do our measurements properly. This ratio can be seen in the

following figure. The achieved signal to noise ratio of more than 50dB is sufficient for

the measurements.

102

103

-10

0

10

20

30

40

50

60

70

80Signal to noise ratio - RT measurements


Powerspectrum

Source on

Source off

SNR

Figure 4.2: Signal to noise ratio for reverberation time measurements


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For each interior configuration, time signals were recorded with the VXI station from

five microphones mounted on microphone rack 1. Impulse responses were calculated

from the recorded time signal for each microphone position. The energy decay was

obtained from calculated impulse responses by Schroeder backwards integrationmethod. 45 different combinations of source and microphone positions were

measured and averaged to give one reverberation time curve. To get the

reverberation time, the -10dB drop in energy decay was found and extrapolated to

get RT60 for the scale model.

The disadvantage of this method is the limited signal to noise ratio which only

allowed for evaluating the 10 dB drop time. This certainly brings up the question of

how accurate the extrapolated results are. Therefore we have used an old Brel &

Kjaer paper-level-recorder to verify the reverberation time measurements done with

the VXI station and Schroeder backwards integration. The paper-level-recorder

basically plots the sound decay inside the room over time after a source is switched

off. The following table shows a comparison of both methods. The differences are

reasonably small to consider the measurements as correct. Paper plots can be found

in appendix D.

f Paper-level-recorder VXI+MATLAB

125 Hz 1,8 1,8

160 Hz 4,5 4,9

200 Hz 4,0 [4,5-5,5]

Because we wanted to compare this reverberation time with the full scale model, the

time axis needed to be scaled to get RT60 for full size Regina. A reverberation time,

measured in the scale model, of 1 second results to 5 seconds after scaling it back to

full scale. (Scale model 1:5)

4.4. Results

A wide range of materials were measured but the following plots are only showing

those configurations/materials, which at the end were selected to adjust thereverberation time to the one in the Regina train. The following figure shows the

reverberation time of saloon 1 scaled to full scale. The red, dash-dot curve shows the

reverberation time as function of frequency in the Regina train for saloon 1, as it has

been measured by Bombardier [5]. The blue curve called mineral wool 114x39 15mm

seats is the final reverberation time for our scale model (transferred to full scale).

This was the best fit to the reverberation time measured in full scale Regina, which

we could achieve with reasonable effort. 114x39x2cm mineral wool on the ceiling

(570x195x10cm full scale) and 1.5cm thick foam on all seats (7.5cm full scale) was

used to achieve this result.


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It turned out that the absorbing mineral wool on the ceiling was essential to gain

sufficient low frequency absorption but it added too much additional absorption at

higher frequencies. Therefore the mineral wool has been covered with a thin foil to

reduce its absorption for high frequencies. The foil is effective above approximately 1kHz.

102

103

0

1

2

3

4

5

6Reverberation Time


RT10[sec

]

Mineral wool 114x39cm

Mineral wool 114x39cm 15mm seats

Empty Train

Bombardier Regina Measurement

Figure 4.3: Results of scale model reverberation time measurements and reverberation time

measured in full scale Regina

Figure 4.4: Absorber setup in scale model 1.5cm foam and 2cm mineral wool covered with foil

Figure 4.4 shows the absorber setup in our scale model.

The reverberation time curves were used to calculate the absorption coefficients

using Sabines formula. These values were mainly used in our ray tracing model,


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described in section 6, to assign the materials to correspondent surfaces. The

absorption coefficients can be seen in figure 4.5.

102

103

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

absorption coefficient - full scale


absorptioncoefficient

empty

15mm 10seats

mineral 114x39

Figure 4.5: Absorption coefficient of different materials used in ray tracing simulation

black solid MDF walls, blue dash - seat foam, red dash dot mineral wool

Figure 4.6 shows the mean absorption in saloon 1 for different cases. The mean

absorption was used in a statistical energy analysis in section 7 and is also used in

BRAINS.(Mean absorption is the absorption coefficient which the total boundary surface of

the room would need to have to replicate the measured reverberation time)

102

103

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1Average Absorption - full scale


averag

eabsorption

Scale model - full abs

Scale model - ceiling abs

Scale model - empty

full scale Regina

Figure 4.6: Average absorption in saloon 1 for different absorber setups


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5.Sound field measurementsDetailed scale model measurements have been performed to investigate parameters

affecting the sound field in train interiors. The sound absorption in the highly

damped saloons and the sound insulation due to vestibule screens is believed to

have a major effect on the sound field. Therefore these parameters have been studied

by measurements. The purpose of the measurements was not only to understand the

sound field in trains better but also to give a solid baseline for judging the quality of

computer models which will be designed and studied in the following sections of

this work.

Before starting with the scale model measurements it was important to know if the

model represents reality appropriately. For this we compared the sound decay

measured in full scale Regina [5] with the decay measured in our scale model of

Regina. The two measurements were unfortunately not directly comparable as the

setup was a bit different. In full scale Regina, the source was placed in Vestibule 2

and in our measurements it was placed in Vestibule 1. As the interior layout is

almost symmetrical this should not result in big deviations. For comparison, the full

scale data is presented similar to our scale model measurements, so the source was

assumed to be in Vestibule 1 for this comparison. The dashed lines in figure 5.1 show

the decay for full scale Regina and the solid lines represent the decay in our scale

model (configuration of the scale model can be seen in section 4.4). The decay rates

are similar. Even though the two measurements are not directly comparable, this

indicates that the scale model represents full scale Regina appropriately and can be

used for further investigations.

0 5 10 15 20-35

-30

-25

-20

-15

-10

-5

0

5Decy comparison - full scale vs. scale model 1:5

length

SPL1/3oct.

scale 250Hz

scale 500Hz

scale 1000Hz

scale 2000Hz

scale 4000Hzscreen

full 250Hz

full 500Hz

full 1000Hz

full 2000Hz

full 4000Hz

Figure 5.1: Comparison sound decay full scale Regina vs. scale model Regina


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For all measurements in the scale model with seats removed, it was possible to use a

floor standing microphone rack as described in section 3.4. With this rack we

measured 12 microphone positions in the cross section at 30 longitudinal positions.

This gives the sound pressure level for a total of 360 different positions in the train asa result. (Figure 5.2a, one layer visible)

For the case with seats in the model the floor standing microphone rack could not be

used. In all these cases, a hanging microphone rack as described in section 3.3 has

been used. With this rack, only positions in one longitudinal line have been

measured as the vestibule screens and the seats interfere with the rack or the

supporting string in other positions. (See figure 3.15 and 5.2b)

a)

b)

Figure 5.2: Microphone positions for different interior setup

a) cases with no seats b) cases with seats

All frequencies shown in the following plots are transformed to full scale. So 1 kHz

in the plots equals 1 kHz in reality. As all measurements have been done in the scale

model, the measured frequencies were actually five times higher than this.The results from all microphones in one horizontal layer (120 of 360 positions) are

shown in figures 5.3 to 5.6. The figures show the sound pressure level in each

microphone position relative to a static reference microphone in Vestibule 1.

(Absorption on ceiling only) The positions of the vestibule screens are indicated by 4

walls in the plots.

Table 1: Microphone positions (full scale) rack 2 no seats

Saloon 1 Vestibule 1 Saloon 2 Vestibule 2 Saloon 3

micPos.(m) mic

Pos.(m) mic

Pos.(m) mic

Pos.(m) mic

Pos.(m)

1 0,28 7 6,44 9 7,79 23 15,75 25 17,24

2 1,20 8 6,68 10 8,25 24 16,25 26 18,22

3 2,20 11 8,74 27 19,23

4 3,21 12 9,24 28 20,23

5 4,20 13 9,74 29 21,23

6 5,20 14 10,25 30 22,23

15 10,75

16 11,24

17 11,75

18 12,25

19 12,75

20 13,25

21 13,7422 14,25


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Table 1 shows the longitudinal positions for all microphones when rack 2 was used.

In cases with seats in the model, where microphone rack 1 had to be used, the first

microphone was placed at 0.5m and all others were spread with a distance of 1.0m

between them. (Full scale dimensions)

05

1 01 5

2 0

0

0 .5

1

1 . 5

2

2 .5

3

-3 0

-2 5

-2 0

-1 5

-1 0

-5

w i d t h

l e n g t h

l o w l a y e r 1 2 5 H z

SPL

1/3oct.

Figure 5.3: Measured SPL for 120 microphone positions 125 Hz 1/3 octave band

(relative to reference microphone)

0 5 10 15 20

0

0.5

1

1.5

2

2.5

3

-30

-25

-20

-15

-10

-5

wi

length

low layer 250Hz

SPL1/3oct.




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0 510 15 20

0

0. 5

1

1. 5

2

2. 5

3

-30

-25

-20

-15

-10

-5

widt

length

low layer 500Hz

SPL1/3oct.



0 5 10 15 20

0

0.5

1

1.5

2

2.5

-30

-25

-20

-15

-10

-5

length

low layer 1000Hz

SPL1/3oct.



The figures above give a fairly good impression of the sound field in the train

interior, but are somewhat hard to compare for the different cases we measured.

Therefore it has been decided to present the data in 2D plots instead.

We calculated the average sound pressure level for each cross section as the mean

value of the 12 microphones in this cross section. This results in 30 data points along

the longitudinal axis for every 1/3 octave band between 125 Hz and 4000 Hz. For

frequencies as low as 125 Hz where single modes can dominate the sound field in

lateral and vertical direction, this averaging can result in inconsistent values. Figure


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5.3 gives an indication of the lateral sound distribution. This problem can be seen in

some of the following plots. Therefore any conclusions drawn from the plots for

frequencies below about 200 Hz should be treated with care.

The location of the vestibule screens is indicated by vertical lines in each plot. Allshown curves in the plots are shifted in a way to have their maximum values in

vestibule 1, where the sound source was placed. This way, the attenuation along the

longitudinal axes can be easily compared.

5.1. Influence of absorption on sound decay

The spatial decay in a corridor shaped room is mainly caused by absorption of

acoustical energy at its boundaries. In order to investigate the influence of absorption

on the spatial sound decay, different cases have been measured in the scale model ofBombardiers Regina. The first case measured was the almost empty scale model. All

seats and the toilet room have been removed so that only the vestibule screens and

the luggage shelves remained. For the second case, absorption was added to the

ceiling only. Mineral wool was used as an absorber. The mineral wool was 2cm thick

and 39cm (10cm x 195cm in full scale) wide and covered the entire length of each

saloon. (See section 4 for more details) It was covered with plastic foil to reduce the

else present increase of absorption for high frequencies (effective from about 1 kHz).

Having the absorption evenly spread along the longitudinal direction and on the

ceiling only, should not introduce any unwanted effects as diffraction or shielding.This could happen when absorption is introduced by seats with absorbing foam on

them. This is why we have included the seats not until case three where we tried to

replicate the absorption as it is in full scale Regina as close as possible.

No additional absorption material has been added to the vestibules.

Figure 5.7 shows the reverberation time for the different cases as it was measured in

Saloon 1. (See section 4)

102

103

0

1

2

3

4

5

6Reverberation Time - full scale


RT10-fullscale

empty

full abs ceiling+seats

mineral wool ceiling

Figure 5.7: Measured reverberation time with different amounts of absorption (same as Figure

4.3)


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5.1.1 Measurement in empty scale model

0 5 10 15 20-35

-30

-25

-20

-15

-10

-5

0

5Energy in Cross Section - empty (rel to ref mic)

length

averageS

PL1/3oct.

125Hz

250Hz

500Hz

1000Hz

2000Hz

4000Hz

Figure 5.8: Measured result for empty scale model

The measurement in the empty scale model shows strong influence of the vestibule

screens. For low frequencies, there is no sound decay visible in the saloons and for

high frequencies a slight decay can be seen in the second saloon. This can be

explained by the strong variation in absorption which can be seen in the

reverberation time shown in figure 5.7 as well.

The sound decay from vestibule 1 to saloon 3 is measured to about 13dB at 1 kHz.

Please keep in mind that the plot shows the average SPL in the cross section

calculated from 12 microphone positions. For the 125 Hz 1/3 octave band, this

average might not be a good representation due to the possibility of dominant single

modes which were only picked up by some of the 12 microphones in the cross

section. See also figure 5.3 for an impression of the sound field at low frequencies.


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5.1.2 Measurement with absorption on ceiling

0 5 10 15 20-35

-30

-25

-20

-15

-10

-5

0

5Energy in Cross Sect ion - with Absorption (rel to ref mic)

length

averageSPL1/3oct.

125Hz

250Hz

500Hz

1000Hz

2000Hz

4000Hz

Figure 5.9: Measured result for scale model with absorption on ceiling

In this case, only the mineral wool absorber described earlier in this chapter has been

used. The resulting reverberation time in saloon 1 is shown in figure 5.7 mineral

wool ceiling. ( sec7.060 T )

When absorption is introduced to the saloons, a slight sound decay can be observed

in the measurements. Nevertheless, the influence of the screens is still dominant over

the decay due to absorption.

With increased absorption the sound decay from vestibule 1 to saloon 3 increases to

about 19dB at 1 kHz.


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5.1.3 Measurement with absorption on seats and ceiling (as in Regina)

0 5 10 15 20-35

-30

-25

-20

-15

-10

-5

0

5SPL along longitudinal line - Regina (rel to ref mic)

length

SPL1/3oct.

125Hz

250Hz

500Hz

1000Hz

2000Hz

4000Hz

Figure 5.10: Measured result for scale model with absorption as in Regina

When more absorption is added by adding foam covered seats, the sound decay in

the saloon increases significantly. The reverberation time with added seats is roughly

half of the one with mineral wool on the ceiling only. (See figure 5.7, sec3.060 T )

The screens still seem to have a strong effect but it is harder to distinguish between

sound insulation due to screens and the sound decay due to absorption.

With absorption increased further to a level as in real Regina, the sound decay from

vestibule 1 to saloon 3 increases dramatically to about 31dB at 1 kHz.

Please keep in mind that microphone rack 1 was used in this case. Only a single

position has been measured in each cross-section. For low frequencies, single modes

can dominate the sound field and therefore the measured sound pressure can vary

strongly for different positions. The microphone could measure a modal peak in one

position and a dip in another. This might be the reason why the curves for 125 Hz

and 250 Hz are not as smooth as the ones for higher frequencies.


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5.1.4 Measurement with absorption on seats and ceiling and two toilet

rooms

0 5 10 15 20-35

-30

-25

-20

-15

-10

-5

0

5SPL along longitudinal line - Regina - 2 toilets (rel to ref mic)

length

SPL1/3oct.

125Hz

250Hz

500Hz

1000Hz

2000Hz

4000Hz

Figure 5.11: Measured result for scale model with absorption as in Regina

and a second toilet room (as in Regina variant)

This case was measured as a replica of a Regina variant where there will be two toilet

rooms opposite to each other. The measurement has been performed to investigate if

there is any influence from the resulting narrow corridor.

The results are very similar to the above case with only one toilet room. So the

influence of this configuration on the sound decay can be neglected.


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5.1.5 Summary influence of absorption on sound decay

0 5 10 15 20

-30

-20

-10

0

Measurements - 125 Hz

length

averageSPL1/3oct.

0 5 10 15 20

-30

-20

-10

0


length

averageSPL1/3oct.

0 5 10 15 20

-30

-20

-10

0


length

averageSPL1/3oct.

0 5 10 15 20

-30

-20

-10

0


length

averageSPL1/3oct.

0 5 10 15 20

-30

-20

-10

0


length

averageSPL

1/3oct.

0 5 10 15 20

-30

-20

-10

0


length

averageSPL

1/3oct.

Figure 5.12: Comparison influence of absorption

red solid - empty, blue dashed abs on ceiling, green dash dot abs as Regina

Figure 5.12 shows a comparison for different absorption in the scale model. Theinfluence of absorption on the sound decay can be clearly seen. It can also be noticed,

that the difference in decay between the empty case and the case with absorption on

the ceiling is decreasing with frequency. This can be explained by looking at the

reverberation time measurements for these two cases in figure 5.7. For high

frequencies the reverberation times are similar (diff. ~ 0.5 sec), whereas for low

frequencies the difference is fairly big (diff. ~ 4 sec). Therefore also the decay rate is

similar for high frequencies. The sound insulating effect of the vestibule screens can

be clearly seen as well. This effect will be studied further in the following section.


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5.2. Influence of screen opening area on sound decay

The influence of the screen opening area has been measured in three differentconditions:

1. with wider screens leaving only a gap of 0.31m (full scale),

2. with the Plexi glass screens removed leaving a 1.8m wide gap formed by the

remaining MDF boards and

3. with the standard opening of 0.75m for comparison.

The three conditions are indicated in figure 5.13. Only the screen between Vestibule 1

and Saloon 2 has been changed. All other screens have been left untouched.

Therefore measurements were only done in Vestibule 1 and Saloon 2 as the sound

pressure in other cavities should hardly be affected by these changes. Hence, the

length coordinate in the plots is shown only from 5m to 15m.

The sound pressure level is again shifted to have its maximum value in Vestibule 1 to

ensure ease of comparison. For all the following measurements the scale model was

empty, except for the screens and luggage shelves.

Figure 5.13: Screen opening for three different cases (only right screen has been changed)


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5.2.1 Measurement with small screen opening

5 6 7 8 9 10 11 12 13 14 15-20

-15

-10

-5

0

5Energy in Cross Section - small opening (rel to ref mic)

length

average

SPL

1/3oc

t.

125Hz

250Hz

500Hz

1000Hz

2000Hz

4000Hz

Figure 5.14: Measured result for empty model with small opening

Figure 5.14 shows the sound pressure measured with small screen opening. The

sound insulation from the screens is with about 10dB at 1 kHz significantly greater

than in the standard case. As the saloons are empty, there is hardly any sound decay

visible.


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5.2.2 Measurement with standard screen opening

5 6 7 8 9 10 11 12 13 14 15-20

-15

-10

-5

0

5Energy in Cross Section - s tandard opening (rel to ref mic)

length

average

SPL1

/3oc

t.

125Hz

250Hz

500Hz

1000Hz

2000Hz

4000Hz

Figure 5.15: Measured result for empty model with standard opening

The measurement with the standard opening is shown for comparison. The sound

insulation is about 7dB at 1 kHz for this case and therefore less then with the small

opening.


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5.2.3 Measurement with large screen opening

5 6 7 8 9 10 11 12 13 14 15-20

-15

-10

-5

0

5Energy in Cross Section - large opening (rel to ref mic)

length

average

SPL1

/3oc

t.

125Hz

250Hz

500Hz

1000Hz

2000Hz

4000Hz

Figure 5.16: Measured result for empty model with large opening

With the large screen opening the achieved sound insulation is down to almost 4dB

at 1 kHz.


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5.2.4 Summary influence of screen opening area on sound decay

5 10 15-20

-15

-10

-5

0

5

length

averageSPL1/3oct.


5 10 15-20

-15

-10

-5

0

5

length

averageSPL1/3oct.


5 10 15-20

-15

-10

-5

0

5

length

averageSPL1/3oct.


5 10 15-20

-15

-10

-5

0

5

length

averageSPL1/3oct.


5 10 15-20

-15

-10

-5

0

5

length

averageSPL1/3oct

.


5 10 15-20

-15

-10

-5

0

5

length

averageSPL1/3oct

.


Figure 5.17: Comparison influence of screen opening

red solid small, blue dashed standard, green dash dot large opening

Figure 5.17 shows a summary of the influence of screen opening area on the sound

field. The results for all cases are quite consistent. There seems to be a strong

dependence of the sound insulation on the screen opening area. The insulation even

for the standard case of a 0.75m wide opening is significant. So the screens should

not be neglected in any model used for predicting the sound field in train interiors

with screens.


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6.Ray tracing method6.1. Theory

In physics, ray tracing is a method for calculating the path of waves or particles

through a system with regions of varying properties of the medium and the

boundaries i.e. absorption, surface impedance. Under these circumstances, wave rays

may bend, change direction or reflect from the surface they hit while propagating.

This aspect is used to understand the sound propagation in space with different

medium characteristics.

It can be explained in the following way; a sound source in space emits sound rays in

all directions. The strength variation of the source gives the directivity pattern of the

source itself. All sound rays travel in a straight line, and when they hit an obstacle,

the rays are reflected, diffracted or scattered according to the geometry of the

obstacle.

When a ray hits an object some of the energy is absorbed by the object in proportion

to the absorption coefficient of the object surface. The energy absorbed is usually

calculated in a way where the incoming energy can be reduced by a factor of 1-,

where is the absorption coefficient of the object.The path of the ray is followed until the total energy of this ray is lower than a

certain level. Each time only one ray can be calculated. When the ray reaches one of

the receivers a result can be calculated with the energy it has at that time at that point

[13].

6.2. Odeon

During our thesis work ODEON was used for the ray tracing method. Before we

started to use the software, 3-D drawings of each model were created with

commercial 3-D software. Then these models were imported to ODEON. Full scale

of the models was investigated with different interior configurations. The interior

details were adjusted for each case to get the same geometry effect as in the scale

model for sound field measurements.

Six different models were simulated and presented in the report for different

frequencies.


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1. Empty train

2. Empty train with absorption on the ceiling

3. Regina train

4.

Regina train with two toilet rooms5. Empty train with small screen opening

6. Empty train with large screen opening

A point source was defined and placed exactly at the same position where the real

source was placed during the measurements.

The overall gain of the point source was set to 90 dB; this gain gave 99dB total power.

Then the materials were assigned to the surfaces and the absorption coefficient and

the scattering coefficient of each material was entered as an input to the software.

The absorption coefficients of the materials were calculated from the scale model

measurements. More details can be found in reverberation time section (section 4).

After the materials were selected and placed to their place, with the help of the

software the estimated reverberation time can be seen, the longest reverberation time

of the room has to be noted and 2/3 of the time should be used as an impulse

response length to simulate it correctly. This is an important parameter, if it is shorter

than 60% of the reverberation time in the room, the T60 cannot be calculated because

the dynamic range of the decay curve is less than 35dB. 33204 rays were used for

each model to have even distribution in the saloons.

Before running the program, the receiver grid has to be defined if a grid response is

wanted. In our work; a pre-defined grid was used which can be created by the

software by just entering the step size and the height of the grid, and a manually

entered 120 receiver positions (multi receiver point) were created to replicate one

layer of the microphone positions used during the measurements (120 of 360

positions). Both responses can be seen in the following part. The positions of the

multi receiver points, grid and the source can be seen on the following figure.

Figure 6.1: Position of the receiver points (blue dots), source (red dot). Height is 1.2m for all

receivers. Distance between the receivers in the first saloon is 1.0 m, 0.5m in the

second saloon and again 1m in the third saloon.


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Figure 6.2: View of the grid which is defined by the software.

Area of each square is 0.5m and at 1,2m height

6.3. Simulation

In the following part the results from the 6 cases are presented in two different plots.

The energy in the cross section for each case with a view of the correspondent train

model can be seen in the first figures. These curves were calculated from the

response of the manually defined receiver points (Figure 6.1). The average soundpressure level was calculated as the mean value of 4 microphones for each

longitudinal position. All curves have been shifted to have their maximum level in

vestibule 1 to allow an easy comparison of the sound decay curves.

The second plot is the response in all grid positions of the whole train. The resolution

of the grid is (0.25m*0.25m) and the height is 1.2m from the floor (Figure 6.2). More

detailed plots of the grid response of the whole train can be seen in Appendix B.

6.4. Influence of absorption on sound decayFirst of all, the influence of the absorption material was investigated. For that reason

the empty train model has been simulated to see the energy decay in the cross

section. In the second case the same size of mineral wool with the same absorption

coefficient as in the measurements was used in the ray tracing model. Lastly we

simulated the Regina train itself with the same interior configuration as in the real

train, and with an extra toilet room.


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6.4.1 Simulation of empty model

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averageSPL1/3oct.

Odeon - Energy in Cross Section empty train (rel to ref mic)

125Hz

250Hz

500Hz

1000Hz

2000Hz

4000Hz

Figure 6.3: Odeon result - Energy in Cross Section Empty (without any absorption material, seefigure 4.3)

Figure 6.3 shows the average sound pressure over the length axis of the train. All SPL

curves are normalized to the SPL in the vestibule where the source is situated for

ease of comparison. The vertical lines in the plot show the positions of the vestibule

screens in each figure.

The sound reduction for the empty train is mainly due to the screens. For low

frequencies, there is no visible sound decay in the saloons and for high frequencies a

slight decay can be seen in the second saloon. This can be explained by the strong

variation in absorption. The sound decay from vestibule to the third saloon is about16dB for 1 kHz. Figure 6.4 shows the response in all grid positions of the whole train

for 1 kHz. The difference in grey scale represents the sound decay in the train. It gets

lighter when we go further from the first vestibule where the source is situated.

Screens ScreensPoint source Shelf


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Figure 6.4: Odeon result - Grid response SPL of the empty train at 1000Hz (Not normalized to

the SPL in the vestibule)

6.4.2 Simulation with absorption on ceiling

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averageSPL1/3oct.

Odeon - Energy in Cross Secti on with Absorption (rel to ref mic)

125Hz

250Hz

500Hz

1000Hz

2000Hz

4000Hz

Figure 6.5: Odeon result - Energy in Cross Section with Absorption on the ceiling

Absorption material on the ceiling


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Figure 6.5 shows the case with the absorption material on the ceiling. The calculated

absorption coefficients of the mineral wool from the scale model measurements are

used. As more energy is dissipated in the saloons, less energy reaches to the second

vestibule. The sound decay from vestibule 1 to saloon 3 is about 21dB at 1 kHz whichis bigger than in the empty case. The effect of the absorption material on the ceiling

can be seen in figure 6.6 as well. Presence of the mineral wool on ceiling absorbs

more energy in comparison to the empty case. This can be seen by observing the

difference between the second saloon and the third saloon.

Figure 6.6: Odeon result - Grid response SPL of the train with absorber on the ceiling at 1000Hz

(Not normalized to the SPL in the vestibule)


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6.4.3 Simulation with absorption on seats and ceiling as in Regina

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averageSPL1/3oct.

Odeon - Energy in Cross Section with Absorption on seats and ceiling (rel to ref mic)

125Hz

250Hz

500Hz

1000Hz

2000Hz

4000Hz

Figure 6.7: Odeon result - Energy in Cross Section with Absorption on the seats and ceiling

(Regina Case)

When more absorption is added to the saloons in form of seats with absorption

material, the sound decay increases further in comparison to the first two cases. The

interior of the train is close to be as similar as possible to Regina train. The measured

absorption coefficient of the 15 mm open cell foam is used for the seats and the same

coefficient for the mineral wool which has been already used in the previous case.

With this setup the reverberation time represents the conditions in full scale Regina

(see section 4.4). The sound decay from vestibule 1 to the end saloon 3 is about 33dB.

Again the figure 6.6 shows the grid response of the whole train. The big difference

can be seen clearly from the vestibule to the end of the train.


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Figure 6.8: Odeon result - Grid response SPL of Regina train with absorber on the seats and the

ceiling at 1000Hz (Not normalized to the SPL in the vestibule)

6.4.4 Simulation with absorption on seats and ceiling and two toilet

rooms

In this case an extra toilet room is placed in the Regina train, the rest stays the same.

The two toilet rooms form a narrow corridor right after the vestibule screens as seen

in figure 6.10. From figure 6.9, an increase in the sound decay can be observed when

it is compared to the case where there is only one toilet room. But then there is a

sudden drop after the toilet rooms (more than the previous case with one toilet

room). This sudden drop has an effect on the sound decay at the end of the train. The

sound decay from vestibule 1 to saloon 3 is 30 dB at 1 kHz for this time. The corridor

effect of adding an extra toilet room can be also seen in figure 6.10.


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averageSPL1/3oct.

Odeon - Energy in Cross Section with Absorption on seats and ceiling and 2 t oilets (rel to ref mic)

125Hz

250Hz

500Hz

1000Hz

2000Hz

4000Hz

Figure 6.9: Odeon result - Energy in Cross Section with Absorption on seats and ceiling

and two toilet rooms

Figure 6.10: Odeon result - Grid response SPL of empty train with absorber on the seat and the

ceiling (2 toilet rooms) at 1000Hz (Not normalized to the SPL in the vestibule)


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6.4.5 Summary influence of absorption on sound decay

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averageSPLoct.

ODEON - 125 Hz

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averageSPLoct.

ODEON - 250 Hz

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averageSPLoct.

ODEON - 500 Hz

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averageSPLoct.

ODEON - 1000 Hz

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averageSPLoct.

ODEON - 2000 Hz

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averageSPLoct.

ODEON - 4000 Hz

Figure 6.11: Comparison influence of absorption

red solid empty, blue dashed abs on ceiling, green dash dot abs as Regina

Figure 6.11 shows a comparison of different absorption in the scale model. The

influence of absorption on the sound decay can be clearly seen. The differences on

the sound decay can be explained due to the different reverberation times for each

case. Also the decay due to the screens can be seen from the above plots. This effect is

going to be studied further in the following section.


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6.5. Influence of screen opening area on sound decay

As shown in section 6.4 even though the sound decay in the saloons is due to the

absorption in the train, the screens also have a strong effect on the sound field. Tounderstand the effects of the screens different sizes of the opening between vestibule

1 and saloon 2 have been investigated. All other openings have not been changed.

The width was chosen to be 0.31m for the small opening, 0.75m for the standard

opening as it is found in Regina, and lastly 1.8 m for the large opening. All

simulations have been done using an empty train model as it is in section 6.4.1.

6.5.1 Simulation with small screen opening

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averageSPL1/3oc

t.

Odeon - Energy in Cross Section empty with s mall opening (rel to ref mic)

125Hz

250Hz

500Hz

1000Hz

2000Hz

4000Hz

Figure 6.12: Odeon result - Energy in Cross Section empty with small opening

As we are expecting, the screens have substantial sound insulation depending on the

width of the opening. The reduction is almost 10dB from the first vestibule to the

second saloon when we have a small opening. As the saloons are empty, there is no

clear sound decay due to absorption. The sound insulation due to the vestibule

screen between the saloon 1 and saloon 2 can be clearly seen in Figure 6.13.


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Figure 6.13: Odeon result-Grid response SPL of the empty train with small opening at 1000Hz


6.5.2 Simulation with standard screen opening

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averageSPL

1/3oct.

Odeon - Energy in Cross Sect ion empty with s tandard opening (rel to ref mic)

125Hz

250Hz

500Hz

1000Hz

2000Hz

4000Hz

Figure 6.14: Odeon result - Energy in Cross Section empty with standard opening


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In this section standard Regina opening has been investigated. The SPLs in saloon 1

and saloon 2 are almost equal as the same opening area is used on both sides of the

vestibule resulting in similar energy flow from vestibule 1 to both saloons. There is

almost 5 dB drop in SPL from the vestibule to the following saloons. These equal

energy flows can be seen in the figure 6.15. The energy on the saloon 1 and saloon 2

is almost at the same level.

Figure 6.15: Odeon result - Grid response SPL of the empty train with standard opening at

1000Hz (Not normalized to the SPL in the vestibule)


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6.5.3 Simulation with large screen opening

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averageSPL1/3oct.

Odeon - Energy in Cross S ection empty with large opening (rel to ref mic)

125Hz

250Hz

500Hz

1000Hz

2000Hz

4000Hz

Figure 6.16: Odeon result - Energy in Cross Section empty with large opening

In this section a large opening at the vestibule has been investigated. The energy flowto saloon 2 is higher than to saloon 1. This result agrees with the previous variations

of screen opening size. There is 3dB sound reduction from vestibule 1 to saloon 2,

whereas we have 6dB to saloon 1.The results seem to be consistent. A decrease in

opening area by a factor of 2.4 results in an increase in sound decay by 3dB. From the

following figure the effect of the screens can be clearly seen for 1 kHz.


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Figure 6.17: Odeon result - Grid response SPL of the empty train with large opening at 1000 Hz



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6.5.4 Summary influence of screen opening area on sound decay

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ODEON - 125 Hz

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SPLoc

t.

ODEON - 250 Hz

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SPLoc

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ODEON - 500 Hz

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average

SPLoc

t.

ODEON - 1000 Hz

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SP

Loc

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ODEON - 2000 Hz

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SP

Loc

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ODEON - 4000 Hz

Figure 6.18: Comparison influence of screen opening

red solid small, blue dashed standard, green dash dot large opening

Figure 6.18 shows a summary of the influence of screen opening area on the sound

field. It seems that the screens have strong effect on the sound decay and due to that

they cant be neglected. The sound decay difference is proportional to the screen

opening area.


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7.Statistical Energy Analysis (SEA)

7.1. Theory

In this section a very brief introduction to SEA will be given. Thus mathematical

developments were left out for the benefit of a quick overview. Readers interested in

a deeper insight into SEA are referred to Statistical energy analysis, an overview [7].

The main idea in statistical energy analysis is dividing the structure under study intosubsystems and analyzing their stored and exchanged energies. Let us consider first

a system consisting of only one subsystem. Any excitation of this subsystem can be

described by its p

tober corakci msc thesis

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