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

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 $L_{Aeq}$ 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 $L_{Aeq}$ difference behind noise barrier No. 2 was 18.6 dB(A). Taking in to account only low frequencies $L_{eq(16-200Hz)}$ the difference respectively was 13.7 dB and 11.7 dB (Table 1).

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:

For ½ of cylindrical domain:

where: $L_w$ – Sound power level of linear source in dB per length unit, $P$ – Sound power level of linear source in W per length unit, $R$ – a distance to linear source, m.

Calculated sound power of linear source $L_W$ 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 $\rho$ 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.

Examples of calculation in free field conditions, with simulated simplified existing noise barrier and enhanced noise barrier are presented in Fig. 3 (Appendix).

Fig. 2Simulated 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.