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The highway traffic noise prediction model the Danes use is NORD2000, Scandinavian prediction code written from 1996 to 2001. NORD2000 software has a new source model developed by measuring 4,000 vehicles on 21 streets traveling from 30 to 130 km/h. The noise emission levels in NORD2000 account for vehicle category, speed, age of vehicle, size of engine, engine type, and surface texture. Plans call for incorporating quiet pavements into the model, but this will not be available for many years until the reliability of the noise-reduction effect can be verified. Meanwhile, predictions are based on a reference pavement, where supporting documentation includes a table with values to adjust for different pavements.
The metric used in the standard noise program is A-weighted 24-hour equivalent sound level (L Aeq24h). The EU Environmental Noise Directive, however, will require the use of a different metric, the day-evening-night sound level (L den), which applies penalties to evening and nighttime hours.
For quiet pavements, the Danes use statistical pass-by (SPB) methodology (ISO 11819-1) to determine the noise-reduction benefit and report the statistical pass-by index (SPBI). In some cases, the methodology has been modified to best represent the traffic and data collected at the measurement sites. The data is also presented in terms of vehicle type, looking at a regression through the maximum sound levels, and recommended spectral data analysis. As part of determining noise reduction, a perceived-effect survey has been conducted in communities adjacent to pavement test sections. Close-proximity methodology (CPX) (ISO 11819-2) is used for pavement categorization. It is not used to determine noise-reduction benefits of pavements because it does not account for different vehicle types.
The only quiet pavements being tested are for research purposes, so no noise maintenance program is in place and monitoring of standard pavements is not conducted. For research purposes, the quiet pavements are tested using the SPB methodology two times a year. To date, wet pavement has not been tested.
None of the noise-abatement measures except barriers and insulation has achieved more than a 5-dB reduction. The greatest noise reduction achieved with quiet pavements is about 5 dB, for TLPA. Noise reductions are compared to their reference pavement, dense-grade asphalt (DGA) AB12.
The Danes indicated that the most important frequency range to address with quiet pavements is 800 to 1,500 Hz for light vehicles, and more broadband—peaking at a lower frequency—for heavy vehicles. The objective of the quiet pavement program is to create smooth, porous pavement with small aggregate size in a configuration that will stay clean.
The Danes have found that quiet pavements get much louder after 8 years. The noise-reduction benefit is characterized to last about 6 years, although some pavements have shown a 12-year lifetime. In the first year, the reference pavement benefit decreased 1.0 to 1.5 dBA.
The prediction model the Dutch use is SRM2, although this will change to harmonize with that used by the EU. The new prediction model may be called HARMONOISE.
The prediction model accounts for quiet pavements in two ways:
The noise metric now applied for impact purposes is an A-weighted equivalent sound level (L Aeq), measured for 24 hours. A L Aeq value is obtained for daytime and nighttime hours, in which case a nighttime penalty of 10 dB is applied. The final step in determining the sound level is to take the highest of the day and night levels.
Typically, measurements have been performed using the SPB method to determine the noise-reduction benefit, although CPX and coast-by methods have also been used. In addition to the SPB index, light vehicles and heavy vehicles are examined separately. (Measurements are taken annually under similar meteorological conditions and corrected for the air/pavement temperature.) The correlation between CPX and SPB results is being investigated and consideration is being given to using just the CPX method for future determinations of noise reductions. So far, good correlation has been seen for light vehicles, but not for heavy vehicles. For the various projects in the Netherlands, different measurement methodologies are being applied. For example, the Silenda Via (SILVIA, Sustainable Road Surfaces for Traffic Noise Control) project applies both the SPB and CPX methodologies with restrictions based on conformity of performance of pavements. Sound absorption of pavements is measured using an impedance tube method and in situ testing with a loudspeaker. Both measure normal incidence sound absorption (see figure 6).
Figure 6. Danish noise-measurement trailer.
The first research was done in the 1970s, when a reduction of 3 dB was seen for passenger cars on PA. No reduction was seen for heavy trucks.
More recent research has shown that TLPA performs better at higher highway speeds and can achieve the same or more noise reduction for heavy vehicles as for light vehicles. Thin layers perform better at lower speeds (urban roadways), but do not achieve as much noise reduction for heavy vehicles as for light vehicles. Study results for specific pavements are stated below, where noise-reduction values are compared to their reference pavement, dense asphalt concrete (DAC).
Figure 7.Twinlay mixture (twin layers of porous mixture).
At 110 km/h, the reductions achieved are about 2 dB for the 50-mm porous asphalt, 4.5 dB for the two-layer PA Twinlay (figure 7), and 6 dB for the two-layer PA Twinlay M. At 130 km/h, an approximate 8-dB reduction was achieved with the two-layer PA Twinlay M. The two-layer PA has better reduction around 600 Hz, which allows for more absorption of truck tire noise.
For the Zebra test sections, the average two-layer reduction was about 6 dB (light and heavy vehicles), although the reduction for the eight test sections varied from about 4 to 7 dB. Listed below are the pavements and their corresponding reduction from the reference DAC:
These sound level values were obtained using SPB methodology on pavement aged about 2 months. The oldest section is 2 years old, where durability has been the main issue over time, not noise reduction. Measurements will continue for 7 years.
The model the French now use contains vehicle noise emission levels from 1980, but new vehicle noise emission levels are being implemented. The new emission levels consider noise contributions from two sources, engine and tire/pavement interaction. The work for the tire/pavement interaction noise has been completed, and the engine noise work is scheduled to be completed in 2004. The process of determining how to include the various pavements in the model and the effects for aging pavement is in progress. Calculations are performed on an octave band basis. However, it is envisioned that one-third octave band calculations will be implemented in the future. For the tire/pavement interaction noise, sound level curves have been produced as a function of speed for three different pavement categories:
France is also modeling the relationship between pavement parameters and noise. The parameters examined include acoustic absorption and impedance, porosity, specific flow resistance, tortuosity, and porous layer thickness.
For policy purposes and model validation, the A-weighted equivalent sound level is measured for daytime hours (L Aeq, 6h to 22h) and nighttime hours (L Aeq, 22h to 6h) at the building façade. The French are not in favor of using L Amax in determining noise impacts.
To determine the noise-reduction benefits of pavement, the SPB method (ISO 11819-1) is applied. The CPB method (ISO 11819-1) is also applied (with light vehicles only). On roads with heavy traffic, the French will close the road to perform CPB measurements. With these methods, the drawbacks are that the noise level is measured at only one location, not for the whole length of the road, and site conditions for performing the measurements are stringent. L Aeq (time-averaged) measurements of existing traffic are performed, as is done for policy purposes, to determine noise-reduction values resulting from quiet pavements.
Although a modified (microphones mounted on the vehicle) CPX method (ISO 11819-2) is still being developed for use in France, the French would like to use the method to determine pavement benefits. The correlation between CPB or SPB and CPX has not been determined fully. Good correlation is seen with light vehicles, but not yet with heavy vehicles. The method of instrumentation, not as a trailer but with microphones mounted on a light vehicle, is being developed. Pavement sound absorption is measured using core samples in a tube (ISO 10534-1) and the impulse multiple load simulator (MLS) technique (ISO DIS 13472-1). The latter is used in the field for both stationary and moving applications, and is the preferred method because it is nonintrusive.
Applying the SPB method compared to the reference DA pavement (BBSG 0/10), the following noise reductions are achieved:
The influence of pavement type has not been as much for heavy vehicles as for light vehicles, especially at lower speeds. For Colsoft, L Aeq values are showing a 4.6-dB benefit during daytime hours and a 5.8-dB benefit during nighttime hours.
In observing spectral data for light and heavy vehicles, the following was observed:
France showed data results for pavement aged 1 to 5 years, with limited data out to 6 and 7 years. There was not much difference with most pavements tested, about 1 dB over 5 years. The noise for dense asphalt BBSG 0/10 did increase. The BBTM 0/6 Type 2 (18 percent voids) showed a 3 dB increase in sound after 6 years, and one data point at 7 years showed an additional 0.8 dB increase. More research is needed on aging. Long-term observations will be made over the next 10 years.
France has identified the following parameters to consider for quiet pavements:
French officials believe that quiet pavements have not been defined sufficiently yet, so they have a tendency to choose traditional solutions (noise walls, insulation, etc.) for noise abatement.
Quiet pavements are measured on test sections using SPB or CPB methods and are used in the emissions model of the predictions. Italy participates in the HARMONOISE and IMAGINE projects and plans to use the prediction code when it is available.
Several acoustic models are part of the SIRUUS project: tire noise generation, sound absorption, vehicle emissions, and structural behavior of silent pavements.
For noise impact determinations, the Italians measure both a daytime and nighttime equivalent sound level (L eq), and plan to switch to the day-evening-night sound level (L den).
SPB is used when possible to measure pavement noise reduction, where measurements occur once each year. When SPB is not possible, the Italians use their own version of a controlled pass-by method, with at least one light and one heavy vehicle. At least one study was performed by measuring the SEL, in which noise benefits were examined according to height above the ground. Autostrade is in the process of developing a modified CPX method, in which microphones are mounted directly on the vehicle.
The absorption coefficient for pavement is measured on core samples using traditional standing wave methods or impulse methods using a speaker located above the actual pavement. The latter is used in the field for both stationary and moving applications. The impulse method has been a challenge for measuring frequencies below 400 Hz.
Noise-reduction results for various pavements from the SIRUUS research project are shown in the research section of this chapter.
The prediction method used is calculation of road traffic noise (CRTN) (ISBN 0 11 550847 3), which has been implemented in privately developed computer models. Noise predictions are based on traffic flows expected 15 years after the new construction is open. Surface correction for PA is defined in CRTN as -3.5 dB (long-term). For standard quiet pavement the correction is taken as -2.5 dB (as defined in specification). EAC can be taken as having a -1.5-dB correction. These corrections are made from the HRA reference pavement (20-mm aggregate size) and are made at the source, on the overall A-weighted level (not spectrally). TRL has developed a conversion from L 10 (sound level exceeded 10 percent of the time) to L den for use with CRTN. However, as part of the EU, the United Kingdom will implement the use of computer models based on the outputs from the HARMONOISE and IMAGINE projects. The latter is intended to provide an engineering model version for the purpose of mapping, while the former was developed for validation purposes.
For noise impacts/intervention purposes (levels exceeding 68 dB), the metric L 10 is determined for the hours 6 a.m. to midnight. For associated noise measurements, the microphone is placed 1 m from the façade.
HAPAS determines the noise relationship of the various quiet pavement systems (the method, which also includes SPB measurements, is described in a World Road Association (PIARC) paper). Tests qualify which types of pavements are allowed for thin layers. A reduction of 2.5 dB or greater qualifies as a quiet pavement. Testing is conducted at two sites for 2 years for each thin layer. CPX (using the Transport Research Laboratory’s TRITON mobile research laboratory) and coast-by methodologies (light vehicles only) are also used.
The British perform both static and dynamic noise-absorption testing. They have implemented the MLS system on a rolling vehicle, and are investigating relationships by which absorption measurements combined with CPX measurements can be used to predict SPB sound levels (see figure 8).
Figure 8. TRL noise test trailer.
Standardized measurement methods that examine only one lane of traffic (SPB, CPX, etc.) may not be best for measuring the pavement noise benefits from multiple lanes of the pavement. Also, it is important to measure real traffic, since noise benefits have been seen to be less for heavy vehicles than for light vehicles.
In the 1980s, the United Kingdom began to recognize the noise benefits of PA. In the mid-1990s, researchers began to recognize the benefit of thin surface overlays. Tire/pavement noise measurement results, some showing comparisons to the reference pavement (HRA), are described below.
Exposed aggregate concrete was investigated in TRL576. In the first 12 months, the noise levels (SPB) from the 6/10-mm aggregate EAC were lower than the HRA, an average of 1.7 dBA for light vehicles and 1.3 dBA for heavy vehicles. The 8/14-mm aggregate EAC was similar to the HRA. Over time (in some cases, out to 82 months), larger increases in noise are seen with HRA (in the 500-to-1,250-Hz range for light and heavy vehicles) than with EAC (in the 1,000-to-3,000-Hz range for light vehicles and 800-to-1,250 Hz range for heavy vehicles). Additional data will be collected to support this conclusion. For SILVIA, some experiments have shown that for 20-mm aggregate PA, a 5-to-6-dB initial reduction and a 3-dB, 8-year reduction is being seen compared to 20-mm aggregate HRA.
After rainfall, traffic noise levels measured alongside both Masterpave and PA surfaces increased by 3.2 dBA and 3.5 dBA, respectively, when compared with dry surfaces. There was no increase in the noise for HRA.
Researchers in the United Kingdom also commented on annoyance in communities and clogging of porous pavement. Studies show that a change in the road does not follow the steady-state relationship between noise and proportion of people highly annoyed. For example, for a new road, annoyance is higher than the steady-state yields, and for a case in which a bypass was constructed (much less traffic on the existing road), the annoyance is lower than the steady-state level measurement would predict. For clogging of porous pavements, even though higher-speed roadways are self-cleaning, it is thought that most of the cleaning occurs in the tire tracks. In other locations, the pavement can clog, which causes a reduction in the noise benefit that occurs from absorption of propagating sound.
Officials generalized that for macro texture larger aggregates cause more noise, and for micro texture smaller aggregates cause more noise.
For a quiet pavement there is a reduction in block snap and air pumping mechanisms, plus absorption across porous surfaces. This last characteristic tends to deteriorate with age (about 50 percent loss of benefit overall after 5 to 6 years).
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