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53 3.6 Roadside Testing The beamforming noise mapping technique developed in this project and validated through the proof-of-concept test- ing as described in Sections 3.1 through 3.5 of this chapter was further applied to quantify noise sources for a wide range of trucks under actual road conditions. This second experi- mental task of the project was accomplished through the roadside truck noise measurements using the developed measurement system on an in-service highway. Four related issues are discussed in the following subsections: (1) micro- phone array modifications for the roadside testing, (2) mod- ifications to the test data post-processing algorithm, (3) site selection for the roadside testing, and (4) roadside measure- ment setup. The roadside test results are presented and dis- cussed in Section 3.7. Figure 66. Microphone cable junction box. 3.6.1 Microphone Array Modifications The results of the proof-of-concept testing presented previ- 3.6.2 Data Post-Processing ously confirmed that the microphone array, data acquisition Algorithm Modifications system, and beamforming software developed in the course of The data analysis relies heavily on a time-gated FFT tech- the study performed generally as expected and required no nique that provides a detailed frequencytime decomposition major adjustments. Based on the field experience obtained of the truck's sound levels and frequency content during its during the testing, certain improvements to the system were passby. The sound pressure spectra are measured in 1 Hz fre- implemented prior to the roadside testing of the array to sim- quency bands evenly distributed from 0 Hz to the selected plify its setup and tear down. A few minor modifications for the upper limit (generally taken to be 3000 Hz). Numerical effi- microphone array were necessary to speed up assembly of the ciency and limitations on the processing time provide a con- array and improve handling of multiple microphone cables straint on the number of frequencies analyzed and the number between the array and data acquisition system in the field. of times for which spectra are calculated. This was discussed For the previous proof-of-concept testing, individual 50 ft in detail in Section 3.5.4. For the earlier proof-of-concept tests, (15 m) long microphone cables were connected directly the settings were used that provided a frequency range from 0 between the microphones and data acquisition system, as to 3000 Hz in 64 steps for each of 15 to 20 time segments. The could be seen in Figure 10. While functional, that setup was actual measurement bandwidth used at that time was about laborious for the large number of microphones employed. 47 Hz (3000/64) at all frequencies, with occasionally broader To make setup more efficient, microphones at each of the bandwidths. Once these spectra were obtained, using a simple three frame sections of the array were pre-wired with a cable arithmetic conversion formula the source levels were pre- junction panel, as shown in Figure 66 for the lower section. sented as equivalent A-weighted one-third octave band levels When mechanical assembly of the frame sections of the of sound pressure relative to 20 µPa at each indicated fre- array was complete in the field, three cable bundles similar quency. This was done to provide levels that could be com- to professional audio cable snakes, also seen in Figure 66, pared approximately to classically processed one-third octave were connected between the three junction panels and the band levels, which were also collected during the passbys. data acquisition system. For measurements, the total frame This use of approximate one-third octave band equivalents, and cable connections could now be set up in approximately though convenient, was not ideal because of its fundamental 30 min compared to several hours previously. inability to accurately present one-third octave band levels that The array dimensions and microphone locations remained contain tones, such as tire tones. Thus, a wrapper code for use unchanged from the design described in Section 3.3.1. Each in batch-processing and saving the spectral files with frequency of the 14 radial spokes had 5 microphones, and 7 additional intervals much smaller than 47 Hz was developed for the analy- microphones were mounted vertically in the center, such that sis of the roadside recordings. Using a frequency range of the total of 77 microphones was used for the roadside testing. 200 to 2500 Hz for the desired one-third octave band spectra, The microphone coordinates in the array are provided in and the need for at least 3 frequency samples in each band, the Appendix A. number of digital frequency samples within the 0 to 3000 Hz