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1 The primary purpose of this research was to determine the height distributions and spectral content for heavy trucks, building on earlier research sponsored by the California Department of Transportation (Caltrans) and NCHRP. With the use of acoustic beam- forming technology, the noise source regions for 1,289 heavy trucks were mapped during vehicle pass-bys on public highways at 20 different measurement sites in two different states. The sites covered a range of operating conditions, pavement types, and vehicle speeds. These acoustical source maps indicated that tire/pavement noise at ground level was the predominant source, with engine-related noise being a secondary source. Noise from elevated exhaust outlets was very rarely indicated as a source. Tire/pavement noise typically split equally between the drive axle tires and the trailer tires. From the source maps, verti- cal profiles of noise versus height were calculated. The researchers found that profiles were essentially independent of vehicle speed, pavement, operating conditions, and region, even though the statistical isolated pass-by (SIP) levels were dependent on these parameters. The average of the vertical profiles was used to explore simpler source distributions that could be used in traffic noise modeling. The researchers found that a two-point source distribu- tion could adequately represent the profile and yield similar barrier insertion loss values. One source needed to be located at ground level for all frequency bands, which is the same as that used in the FHWA Transportation Noise Model (TNM®). The upper source height of 12 feet (ft) (3.7 meters [m]) used in TNM was not found to be viable in attempting to match the average heavy truck profile. To match the profile, the upper source height was 3.3ft (1m) at low frequency and 1ft (0.3m) at high frequency, with heights of 2.3 and 1.6ft (0.7 and 0.5m) used for frequency bands in between. Barrier insertion loss using the TNM distribution was also found to be less than (by 3 to 6 decibels [dB]) that calculated for the average truck profile. A second purpose of this research was to expand the database of heavy truck pass-by levels for use in future modeling. To accomplish this, the levels of each truck pass-by were captured at each site in addition to beamforming measurements for comparison with the FHWA Reference Energy Mean Emission Levels (REMELs) database from 1994. Above about 50 miles per hour (mph), the levels from this research determined a regression curve matched the REMELs quite well (within 1 dB or less) for both cruise and interrupted/ grade conditions. Below 50 mph, the pass-by results were about 2 to 5 dB lower than the REMELs for interrupted/grade conditions. These results indicated that, although the levels for higher speeds are adequately represented by the current database, additional data for lower speed conditions is needed. As part of the project, beamforming and SIP data were also acquired and processed for 83 medium trucks. Given the limited numbers and the higher variability than for heavy S U M M A R Y Mapping Heavy Vehicle Noise Source Heights for Highway Noise Analysis
2 Mapping Heavy Vehicle Noise Source Heights for Highway Noise Analysis truck pass-by noise, there were no definitive conclusions from this data, other than that more data is needed to develop proper source distributions and average pass-by levels. In general, tire/pavement noise was again seen to be the predominate source, with engine noise also present in most cases. It was also indicated that the average profile should be similar to the heavy truck profile, except with less noise for the upper heights, and that source height distribution would be different than that used in TNM. More data is required to deter- mine how well current medium truck pass-by levels compare with the REMELs curves; however, there is some indication that todayâs levels are lower.
