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54 range was increased to 256. This increase brought the interval Adequate, accessible, and safe side spot or shoulder loca- of frequency analysis down to 11.7 Hz, ensuring the existence tion for placing the microphone array at a distance of 20 ft of an adequate number of frequency samples in each one- (6 m) from the edge of the nearest driving lane, as well as third octave band used for array beamforming. A-weighting for parking a minivan with the data acquisition system. factors were then applied to each band sound level. To com- The setup should be located on a flat surface at the road ele- pensate for the increased sample time of the FFT processing, vation, with no buildings or other reflective surfaces behind. the number of ensembles for analysis was also reduced from that used earlier. Based upon the site review and evaluation, the site on The additional code development in this second phase also US 301 at MD 405 Price Station Road was selected for the provided the vertical distribution of the sound received at the roadside testing as meeting all of the criteria. At this location, array, to infer the vertical distribution of apparent sources the opposite directions of travel on the four-lane highway split above and below the road surface. The processing that was forming a wide median with trees, while the northbound lanes developed consisted of applying the array's beamformer algo- remain straight, generally flat, and provide a wide, paved turn- rithm to each truck passby, but with steering limited only to ing lane with a shoulder convenient for the measurement points directly in front of the array and steered to points along array installation. The road pavement in the vicinity of the site a vertical line extending both above and below the surface of was dense-graded asphaltic concrete (DGAC). The posted the road. These vertical distributions are consistent with the speed limit at the site was 55 mph. two-dimensional beamformed images, as confirmed by eval- uations of distributions with the loudspeaker source. A network of codes provided a sequence of steps used in 3.6.4 Roadside Measurement Setup resolving the acoustic images of the passby: display the time The microphone array sections with pre-assembled micro- records of the passby; truncate the record to roughly 2 s; phones, the data acquisition system, and other equipment were develop the image functions at 11.7 Hz intervals; and calcu- transported in a minivan to the US 301 site selected for the test- late A-weighted one-third octave band levels of image values, ing on April 9, 2008. The array was assembled in the field and reference microphone levels, and vertical distributions of placed on a shoulder at a distance of 20 ft (6 m) from the edge source levels. of the nearest northbound driving lane. The data acquisition system was located in the minivan. The overall measurement 3.6.3 Test Site Selection setup is shown in Figure 67. Each measurement channel of the system was calibrated Five candidate sites, all on Maryland highways, were consid- using a Brel and Kjr Type 4231 acoustic calibrator. To track ered for the roadside truck noise measurements: a vehicle and determine its speed at the test site, the same two I-70 rest stop north of Frederick photocells that were used for the proof-of-concept testing US 15 near the Pennsylvania border, at a southbound rest were again installed on tripods near the microphone array, stop (Mason-Dixon Discovery Center) near Emmitsburg with their signals fed into the data acquisition system. Still US 301 near the Delaware border, at a side spot just south photographs of vehicles passing the array were taken in order of Wilson Street/Strawberry Lane to later relate the vehicle geometry to the sound distribution US 301 at MD 405 Price Station Road images. The measurement session was recorded with a video I-895 Harbor Tunnel Throughway in the vicinity of camera for later references as necessary. The results of these Halethorpe measurements are presented and discussed in the next section. All of the sites were visited prior to the testing, and their suitability for the measurements was evaluated. The follow- ing conditions were considered in this evaluation: Sufficient number of random traveling trucks in the range of 15 to 20 vehicles per hour, on average, to provide ade- quate time separation and discrimination among passbys. Sufficient space separation between the opposite directions of travel to minimize overlapping passbys and noise inter- ference from the opposite traffic lanes. No road curvatures and no grades in the direction of travel that would cause vehicles to decelerate or accelerate. Figure 67. Roadside measurement setup.