CHAPTER 6: ANALYSIS OF THE INFLUENCING FACTORS ON TIRE/ROAD NOISE OF POROUS PAVEMENT
6.2 Effect of Influencing Factors on Porous Pavement Tire/Road Noise
6.2.3 Influence of Porous Layer Thickness
In addition to porosity, the thickness of porous surface layer is also a critical factor in porous pavement design that can affect the acoustical characteristics of a porous pavement system. At least two effects are brought forth through variations in porous layer thickness. Firstly, a thicker porous course may be capable to dissipate more acoustic energy in the sound propagation process, since the sound wave has travel a longer distance within the porous medium. Secondly, the difference in porous layer thickness may alter the spectrum shape of acoustic absorption coefficient, making the absorption peak appear at a different frequency or changes the number of peaks in the 300 to 2500 Hz frequency range (Wai and Kai, 2004; Losa and Leandri, 2012). Therefore, it is possible to adjust porous layer thickness during the design phase to optimize traffic noise reduction. For this purpose, the influence of porous layer thickness on tire/road noise performance of porous pavements is analyzed using the developed numerical simulation model. Similar to the previous sections, research efforts are also focused on estimating the surface acoustic impedance of various porous surfaces and predicting the rolling-tire noise levels on each surfaces using the derived acoustic properties.
Figure 6.12 illustrates the absorption coefficients of porous pavements with various surface layer thicknesses, at three porosity levels. It is seen from the figure that porous layer thickness significantly affects peak absorption frequency. This may
235 be due to the interferences and phase differences between the sound wave traveling directly from source to receiver, wave reflected by pavement surface and wave penetrating into porous layer and reflected by the dense-grade base (Abbott et al., 2010). The primary absorption peak moves towards lower frequency due to the increase in porous layer thickness. The peak absorption occurs at about 2200 Hz on a 25 mm thick porous pavement surface and appears at about 700 Hz on a 100 mm thick porous layer. Along with the primary absorption peak shifting to lower frequency, a secondary peak may appear in the interested frequency range from the higher frequency side. Such a secondary peak can be observed on 100 mm porous layer at about 2200 Hz. Furthermore, porous layer thickness also affects the magnitude of absorption coefficient. In the examined range of porous layer thickness, the maximum acoustic absorption coefficient increases with the increase of porous layer thickness. The relative increase in primary absorption peak is about 20% when porous layer thickness increases from 25 mm to 100 mm. This is expected because more acoustic energy will dissipate when the sound wave travels a longer distance in the porous pavement layer.
Figure 6.13 illustrates the simulated noise spectra on porous pavements with various surface layer thicknesses and porosity values, subjected to the identical Type II texture level as shown in Figure 6.8. The acoustical performance of a perfectly acoustic hard surface (i.e. Case III in Table 6.1) is also shown in the figure. It can be seen that porous pavements may generate very different tire rolling noise spectra if porous layers with different thicknesses are used in pavement construction or rehabilitation, despite the same porosity level. Taking the case of 20% porosity [see Figure 6.13 (b)] as an example, the sound spectrum shapes differ from each other with the maximum noise reduction occurring at different frequencies. The primary noise reduction peak moves towards the lower frequency with the increase of porous layer thickness, appearing at 2500 Hz, 1600 Hz, 1000 Hz and 800 Hz, respectively, for porous layer with a 25 mm, 50 mm, 75 mm and 100 mm thickness. As previously
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observed, the maximum noise reduction may result from the peak sound absorbing capacity of a porous pavement. From Figure 6.12 (b), the peak absorption frequency is observed to be around 2200 Hz, 1400 Hz, 900 Hz and 700 Hz for porous surfaces with 25, 50, 75 and 100 mm thicknesses respectively. Taking the frequency-shift correction factor into consideration, the frequencies of noise reduction peaks agree with the frequencies of sound absorption peaks. Sound pressure level differences on various porous surfaces are insignificant at frequencies below 630 Hz, but are large at higher frequencies [up to 10 dB(A) for some particular frequencies]. Similar observations can be made on porous surfaces with 15% and 25% porosities, with slight differences in sound level magnitudes.
Figure 6.14 compares the overall noise levels emitted from a slick tire rolling on different porous pavement surfaces. Noise level on the perfectly acoustic hard surface is also shown in the figure. The overall noise levels are derived from the simulated spectra in Figure 6.13. It is seen that, for the same porosity level, the overall noise level decreases with an increase in porous layer thickness. The noise reduction performance of porous pavement is superior with thicker porous layer. For pavements with a 15% porosity, the overall noise level is 89.08 dB(A) when the porous layer thickness is 25 mm, and 85.02 dB(A) when the thickness is 100 mm.
The overall noise levels at 25 and 100 mm porous course thickness are 89.77 and 85.58 dB(A) for 20% porosity and 90.45 and 86.14 dB(A) for 25% porosity, respectively. The difference in overall noise levels due to porous layer thickness variations is more than 4 dB(A) in the examined thickness range. This is much more significant than the influence of porosity on tire/road noise. The dependency of noise reduction on porous layer thickness is nonlinear. The noise level difference between 50 mm and 75 mm is about 0.8 dB(A), while it is around 1.5 dB(A) when porous layer thickness increase from 25 to 50 mm or from 75 to 100 mm. This indicates that 50 mm thick porous layer may be the most cost effective for tire/road noise reduction.
237 100 mm porous layer could be used to enhance the noise reduction only when the additional material cost is reasonable.
The above observations indicate that tire/road noise on porous surfaces decreases with an increase in porous layer thickness with a nonlinear trend when porosity and texture are maintained identical. Porous layer thickness significantly affects the noise spectrum shape, making the noise reduction peak move towards a lower frequency as the porous layer thickness increases. Porous pavement design with a thicker porous surface layer could be adopted in the perspective of tire/road noise reduction if material cost is acceptable.