The paper is a part of investigation near one of the busiest Lithuanian roads with big percentage of heavy traffic where the spectrum of traffic noise and efficiency of existing noise screens also enhancement of them have been studied. The paper presents use of finite element method (FEM) for traffic noise prediction at low (16-200 Hz) frequencies also for modelling of noise barriers and enhancement of them by adding tops (heads). Acoustic field transformation simulation model is prepared within COMSOL software. Model is verified according investigation in situ (measurements) and deals with divergence loss, diffraction from obstacles, absorption and reflection from surfaces of cylindrical sound waves solving Helmholtz equation. Numerical prediction showed that investigated additional tops can improve effectiveness of noise barrier either at low frequencies (Leq(16-200 Hz)) up to ~2 dB and up to ~7 dB at discrete frequencies

Darius Saliunas

Vitalijus Volkovas

English

Vibroengineering PROCEDIA

  © JVE INTERNATIONAL LTD. VIBROENGINEERING PROCEDIA. OCTOBER 2014. VOLUME 3. ISSN 2345-0533 313 
Low frequency traffic noise prediction and modelling of 
noise barriers using FEM 
Darius Saliunas1, Vitalijus Volkovas2 
Technological System Diagnostic Institute, Kaunas University of Technology, Lithuania 
1Corresponding author 
E-mail: 1darius.saliunas@ktu.edu, 2vitalijus.volkovas@ktu.lt 
(Accepted 22 September 2014) 
Abstract. The paper is a part of investigation near one of the busiest Lithuanian roads with big 
percentage of heavy traffic where the spectrum of traffic noise and efficiency of existing noise 
screens also enhancement of them have been studied. The paper presents use of finite element 
method (FEM) for traffic noise prediction at low (16-200 Hz) frequencies also for modelling of 
noise barriers and enhancement of them by adding tops (heads). Acoustic field transformation 
simulation model is prepared within COMSOL software. Model is verified according 
investigation in situ (measurements) and deals with divergence loss, diffraction from obstacles, 
absorption and reflection from surfaces of cylindrical sound waves solving Helmholtz equation. 
Numerical prediction showed that investigated additional tops can improve effectiveness of noise 
barrier either at low frequencies ( ܮ௘௤(ଵ଺ିଶ଴଴ ୌ୸) ) up to ~2 dB and up to ~7 dB at discrete 
frequencies. 
Keywords: low frequency noise, traffic noise, noise barriers, noise impact assessment, noise 
screen enhancement. 
1. Introduction 
Road traffic noise and noise abatement measures are investigated comprehensively applying 
different models and calculation methods. Methods for traffic noise prediction can be classified 
to most often used empirical and ray tracing methods or numerical methods. Empirical methods 
can predict noise spreading in big areas but they have a weakness solving acting of complex-shape 
noise barriers meanwhile numerical methods (despite of big computer resources demand) are more 
accurate and are perfect for solving such tasks. Furthermore empirical methods as rule deal with 
overall criteria (e.g. ܮ஺௘௤) whereas numerical methods deal with single (discrete) frequencies.  
According measurements, which were made near highway with 43.3-49.8 % of heavy traffic 
(high amount of heavy traffic generates high levels of low frequencies) and existing two noise 
barriers, theoretical simplified numerical model was created and simulation of noise barriers 
enhancement by adding different types of tops using finite element method (FEM) have been 
proceeded. 
Under national legislation most countries considered that low frequency sounds are up to 
200 Hz (some countries 250 Hz), whereas infrasound frequencies are below 20 Hz (or 16 Hz) [1], 
therefore 16-200 Hz frequency spectrum was analysed in a paper. Comparing to overall noise 
criteria (which covers whole 6,3-20000 Hz range of frequencies) low frequency noise is less 
investigated – considering in major to stationary noise sources (e.g. wind turbines) and adverse 
impact on health (e.g. [2-3]), or investigations are made as a part of analysis of whole noise 
spectrum of road traffic noise and barrier performance (e.g. [4]). Consequently, as road traffic is 
one of the most spread noise sources, more attention should be paid to low frequency traffic noise 
and abatement measures investigations. 
The investigation had 3 aims:  
1) To identify a noise spectrum of busy road with big percentage of lorries; 
2) To measure effectiveness of existing noise barriers at different frequencies; 
3) To consider a possibility of noise barriers effectiveness improvement at low frequencies.  
To achieve aims above the main tasks have been drawn: 
1) To make noise measurements at free field conditions and behind existing noise barriers to 
LOW FREQUENCY TRAFFIC NOISE PREDICTION AND MODELLING OF NOISE BARRIERS USING FEM.  
DARIUS SALIUNAS, VITALIJUS VOLKOVAS 
314 © JVE INTERNATIONAL LTD. VIBROENGINEERING PROCEDIA. OCTOBER 2014. VOLUME 3. ISSN 2345-0533  
clarify road traffic induced sound pressure level (SPL) and spectrum either insertion loss of 
existing noise barriers. 
2) To create simplified acoustic field transformation simulation model (verified model) 
regarding measurements results. Smaller pieces of work: 
• Conviction about suitability to use Helmholtz equation for simulation of acoustic field 
transformations; 
• Definition of sound power levels for linear sound source (road traffic) at different frequencies 
and definition of acoustical specifications for surrounding; 
• Definition of simplified model and properties of noise barrier to be calculated.  
3) To simulate acoustic field transformations at low frequencies influenced by existing noise 
barriers modifications adding different types of tops. 
2. Measurements 
2.1. Equipment and weather conditions 
Measurements of traffic noise and insertion loss of noise barriers have been made using Brüel 
and Kjær sound level meter Type 2260 Investigator™ (calibrator Brüel and Kjær 4231). 
Measurements conditions: microphone height 1.5 m; air temperature ~20 °C, wind speed < 5 m/s.  
The SPL reduction of two noise barriers have been investigated. Both noise barriers are 3 m 
height with mineral wool inside (width 10 cm). Barrier No. 1 made from wood and wooden planks 
with 2 cm wide parallel gaps in front panel (steel struts are behind screen). Barrier No. 2 made 
from perforated plastic planks with mineral wool inside.  
2.2. Location of the measurements 
Noise measurements were executed near main road A5 Kaunas – Marijampole – Suwalki that 
is part of the transport corridor E67 Helsinki – Tallinn – Riga – Panevezys – Kaunas – Warsaw – 
Wroclaw – Prague. Chosen measurement locations are situated near Garliava and Kaunas city 
with annual average traffic respectively 19264 veh./day and 311914 veh./day (heavy vehicles 
comprise 49.8 percent and 43.3 percent) [5].  
For verification of created numerical model measurements in free field conditions in the 
distance of 3, 10 and 20 m from the nearest driving lane (or 6, 13, and 23 m from the nearest 
driving lane axis) proceeded. For model suitability to calculate insertion loss of noise barriers, 
measurements have been carried out in a row with noise barriers (~3 m from the nearest driving 
lane) and right beyond the barrier (4 m from barrier) and few meters farther (10 m from barrier). 
Measurements periodicity: 3 times at every measurement point (10 minutes each).  
2.3. Results of the measurements 
In spite of non-homogeneous traffic intensity and content, measurements in free field 
conditions and near driving lane beside noise barriers (also free field conditions) showed that 
spectrum of heavy duty busy road has 2 peaks of sound pressure level (SPL): at 63 Hz and 
800-1600 Hz frequencies (Fig. 1). 
Considering measurement results near noise barriers, difference between ܮ஺௘௤ at distance of 
3 m from driving lane (in a row with barrier) and 4 m behind noise barrier No. 1 was 20.6 dB(A); 
respectively ܮ஺௘௤ difference behind noise barrier No. 2 was 18.6 dB(A). Taking in to account only 
low frequencies ܮ௘௤(ଵ଺ିଶ଴଴ு௭) the difference respectively was 13.7 dB and 11.7 dB (Table 1). 
3. Simulation of acoustical field 
In order to simulate acoustical field transformations predicting acting of noise barriers with or 
LOW FREQUENCY TRAFFIC NOISE PREDICTION AND MODELLING OF NOISE BARRIERS USING FEM.  
DARIUS SALIUNAS, VITALIJUS VOLKOVAS 
 © JVE INTERNATIONAL LTD. VIBROENGINEERING PROCEDIA. OCTOBER 2014. VOLUME 3. ISSN 2345-0533 315 
without enhancements 3D FEM ½ cylinder shape model was created in Acoustic-Solid Interaction 
Frequency Domain within COMSOL 4.4 software. FEM model predicts cylindrical sound wave 
propagation solving Helmholtz equation. Free field simulation results matched measurements. 
The results of 2 noise barriers efficiency (insertion loss) measurements at low frequencies were 
similar; therefore, in conformity to measurements results (at measurement point), one model with 
3 m height noise barrier was created.  
 
Fig. 1. Results of the noise measurements (unweighted SPL) in free field conditions 
Table 1. Low frequency SPL measured at noise barriers No. 1 and No. 2  
1/3 octave band 
centre 
frequencies, Hz 
16 20 25 31.5 40 50 63 80 100 125 160 200 
Noise barrier No. 1 
SPL 3 m from 
driving lane, dB 62.6 61.3 60.2 60.1 62.1 68.1 71.7 65.9 65.4 66.8 64.4 64.1 
SPL 4 m from 
noise barrier, 
dB 
58.6 57.0 56.2 55.5 56.7 59.2 64.8 58.7 53.5 52.0 46.8 44.8 
Difference, dB  4.0 4.3 4.1 4.7 5.3 8.9 6.8 7.1 11.9 14.8 17.6 19.3 
Noise barrier No. 2 
SPL 3 m from 
driving lane, 
dB  
66.9 65.4 63.5 63.9 64.6 68.3 74.9 73.6 69.9 69.5 68.2 68.4 
SPL 4 m from 
noise barrier, 
dB 
62.7 61.4 59.0 58.1 59.4 61.8 66.0 65.2 60.2 58.9 55.8 53.0 
Difference, dB  4.2 4.0 4.5 5.8 5.2 6.5 8.9 8.4 9.7 10.7 12.4 15.4 
3.1. Equations, boundary conditions and initial values 
To assign sound power level for linear sound source, the highest measured sound pressure 
level 3 m from driving lane was taken (72.5 dB at 63 Hz 1/3 octave band centre frequency) and 
calculated according to Eq. (1) and (2). 
Linear noise source sound power levels were calculated according equations [6]. For cylindrical 
domain: 
ܮ௪ ൌ ܮ௣ ൅ 10 logଵ଴(ܴ) ൅ 6 െ 10 logଵ଴ ቀ
ߩܿ
400ቁ dB/m. (1) 
For ½ of cylindrical domain: 
ܲ ൌ ( ଴ܲ ∙ 10௅ೢ భబ⁄ )/2 W/m, (2) 
LOW FREQUENCY TRAFFIC NOISE PREDICTION AND MODELLING OF NOISE BARRIERS USING FEM.  
DARIUS SALIUNAS, VITALIJUS VOLKOVAS 
316 © JVE INTERNATIONAL LTD. VIBROENGINEERING PROCEDIA. OCTOBER 2014. VOLUME 3. ISSN 2345-0533  
where: ܮ௪ – Sound power level of linear source in dB per length unit, ܲ – Sound power level of 
linear source in W per length unit, ܴ – a distance to linear source, m. 
Calculated sound power of linear source ܮௐ is 0.0002053 W/m. Boundary of ½ of cylinder 
was assigned as cylindrical wave radiation in a model. For definition of road surface Sound Hard 
Boundary conditions were selected. Grass covered ground surface has Impedance of 3 kPa·s·m−1 
(according [7-8]) boundary conditions. All expressions of equations can be found in [9].  
Considering that existing barriers have absorbing part as well as structure elements, the 
simplified model was designed to simulate acoustic field transformation influenced by absorbing 
material and sound – solid interaction. The noise barrier model was designed from 10 cm width 
macroscopic porous material part and 2 cm width solid part. Porous material (domain) modelled 
as an equivalent fluid, using empirical Delany-Bazley-Miki model [10, 11] with properties: flow 
resistivity – 20 kPa·s/m2, speed of sound and density ߩ  values are taken from material (air 
properties). Noise barrier solid part – wood, with basic acoustic properties: 1150 kg/m3 density 
and 3500 m/s speed of sound. Within Comsol Acoustic-structure Interaction interface, fluid’s 
pressure loads solid domain, and the structural acceleration affects the fluid domain as a normal 
acceleration across the fluid-solid boundary [9]. All tops were modelled describing only as porous 
material.  
Considering analysed frequencies and construction of modelled noise barriers, model mesh 
was calibrated for general physics, defining maximum element size 0.25 m, minimum element 
size 0.002 m, with maximum element growth rate 1.3. Parameters of mesh weren’t changed for 
different frequencies. 
3.2. Results of acoustic simulation 
It should be noted that SPL at low frequencies very depended (differs) on evaluation point 
place (height and distance from noise barrier), therefore to assess sound level reduction by 
enhancing noise barriers with different tops assessment at all spectrum (16-200 Hz) average sound 
pressure level was calculated at 4×9 m rectangle 4 m behind the noise barrier Figs. 2-3.  
Table 2. Simulation results of average SPL behind noise barrier, (in 4×9 m rectangle), dB  
1/3 octave band 
centre 
frequencies, Hz 
16
 
20
 
25
 
31
.5 40
 
50
 
63
 
80
 
10
0 
12
5 
16
0 
20
0 
ܮ ௘
௤(
ଵ଺
ିଶ
଴଴
ு௭
) 
Free field SPL 
transformation 79.9 78.9 77.9 76.8 75.7 74.7 73.7 72.6 71.5 70.6 69.4 68.4 86.4 
Existing noise 
barrier 79.7 78.5 76.8 74.5 71.4 68.7 66.7 64.4 60.8 59.8 56.7 54.7 84.3 
Noise barrier 
with T top 78.7 77.0 74.8 72.0 68.8 66.6 65.0 57.2 56.8 53.2 54.7 54.1 82.7 
Noise barrier 
with 45º T top 79.1 77.6 75.6 72.9 69.8 67.4 65.2 60.8 58.3 55.6 53.8 52.2 83.3 
Noise barrier 
with quadratic 
top 
78.7 76.8 74.5 71.5 68.3 66.0 64.1 58.8 56.9 56.1 55.9 53.5 82.5 
Noise barrier 
with octagon top 78.9 77.1 74.8 71.9 68.9 66.3 64.4 59.5 56.9 55.9 54.0 52.1 82.8 
Noise barrier 
with complex 
shape top 
78.8 77.0 74.6 71.7 68.5 66.2 63.9 57.9 56.6 54.4 54.6 52.9 82.7 
Noise barrier 
with V top 78.9 77.2 75.1 72.4 69.3 67.0 64.6 59.9 57.7 54.3 52.3 50.1 83.0 
Examples of calculation in free field conditions, with simulated simplified existing noise 
barrier and enhanced noise barrier are presented in Fig. 3 (Appendix).  
LOW FREQUENCY TRAFFIC NOISE PREDICTION AND MODELLING OF NOISE BARRIERS USING FEM.  
DARIUS SALIUNAS, VITALIJUS VOLKOVAS 
 © JVE INTERNATIONAL LTD. VIBROENGINEERING PROCEDIA. OCTOBER 2014. VOLUME 3. ISSN 2345-0533 317 
 
Fig. 2. Simulated additional of averaged sound pressure level behind noise barrier  
by enhancing with additional top  
Numerical prediction results show that noise barrier enhancement by adding different tops 
gives 0.6-1.7 dB additional SPL reduction at 16-20 Hz frequency range, 1.3-3 dB additional SPL 
reduction at 15-63 Hz frequency range and 0.6-7.2 dB additional SPL reduction at 80-200 Hz 
frequency range. Different tops plays the best at different discrete frequencies, for example T top 
is most efficient at 80 Hz frequency and not effective at 200 Hz frequency, meanwhile 
effectiveness of V top is similar in all 80-200 Hz range.  
4. Summary and conclusions 
Measurements results showed that that spectrum of heavy traffic busy road has 2 peaks of SPL: 
at 63 Hz and 800-1600 Hz frequencies. Measurements results also showed, that effectiveness of 
existing noise barriers are better in mid and high frequencies and less at low frequencies. 
Numerical prediction showed that enhancement of existing noise barrier can improve 
effectiveness of noise barrier either at low frequencies. According calculation results, which where 
proceeded using finite element method, investigated tops can give such additional SPL reduction 
at low frequencies:  
• 0.6-1.7 dB reduction at 16-20 Hz frequency range; 
• 1.3-3 dB reduction at 15-63 Hz frequency range; 
• 0.6-7.2 dB reduction at 80-200 Hz frequency range.  
Looking to ܮ௘௤(ଵ଺ିଶ଴଴ு௭) of simulation results at Table 2, the conclusion can be drawn that all 
investigated tops additionally reduces SPL at low frequencies by 1.0-1.8 dB however for discrete 
frequencies higher reduction can be achieved.  
According to measurements results, regulated values and destination (reached to mitigate) 
frequencies various tops can be selected. In our case if we’d like to reduce noise at existing peak 
(SPL peak at 63 Hz frequencies) quadratic, octagon and complex shape tops would give best 
results.  
References 
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Calculations of Impact Range According to HN 30:2009. Environmental Research, Engineering and 
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LOW FREQUENCY TRAFFIC NOISE PREDICTION AND MODELLING OF NOISE BARRIERS USING FEM.  
DARIUS SALIUNAS, VITALIJUS VOLKOVAS 
318 © JVE INTERNATIONAL LTD. VIBROENGINEERING PROCEDIA. OCTOBER 2014. VOLUME 3. ISSN 2345-0533  
[5] Lithuanian Road Administration traffic intensity database, http://lakis.lakd.lt/vmpei/. 
[6] Bies D. A., Hansen C. H. Engineering Noise Control – Theory and Practice. Abingdon, Spon Press, 
Fourth Edition, 2009. 
[7] Embleton T. F. W., Piercy J. E., Daigle G. A. Effective flow resistivity of ground surfaces 
determined by acoustical measurements. Journal of the Acoustical Society of America, Vol. 74, 
Issue 4, 1983, p. 1239-1244. 
[8] Hutchins D. A., Jones H. W., Russell L. T. Model studies of acoustic propagation over finite 
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[9] Comsol 4.4. Acoustics Module User’s Guide, 2011, p. 53. 
[10] Komatsu T. Improvement of the Delany-Bazley and Miki models for fibrous sound-absorbing 
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2009. 
Appendix 
Fig. 3. Examples of calculation in free field conditions, with simulated simplified existing noise barrier and 
enhanced noise barriers. Acoustic field transformations at 200 Hz frequency 


Low frequency traffic noise prediction and modelling of noise barriers using FEM

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Low frequency traffic noise prediction and modelling of noise barriers using FEM

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