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12 CHAPTER 3 Research Findings 3.1 Review of Beamforming its associated with the system being general purpose, rather for Vehicle Noise Source than optimized for the specific type of measurements needed Identification for moving trucks. In general, however, the circular spiral arrays (2428) offer A recent development in microphone array application for adequate side lobe suppression with a minimum number of localization of sound sources of moving motor vehicles (21) sensors. As such, this class of arrays can be used to provide showed that 124 microphones arranged in a two-dimensional affordable performance over a broad frequency range. The cir- snowflake array made it possible to locate sound sources dur- cular spiral microphone array was used as the baseline for the ing car passbys with a resolution of 1 to 1.3 ft (0.3 to 0.4m) in beamforming design in the current study. a frequency range of 500 to 4500 Hz. With an overall array size of approximately 12 by 12 ft (3.7 m by 3.7 m) and a dis- tance of 8 ft (2.4 m) between the array and test object, the 3.2 Development of two-dimensional distributions of the sound pressure level were Experimental Design measured during passbys of an automobile at speeds from 50 3.2.1 Noise Mapping to 110 mph (80 to 180 km/h). In the frequency range between Technique Development 300 and 500 Hz, however, the array's spatial resolution was worsened by a factor of 2. Also, even with a simple, stationary Specific parameters that were considered relative to the point source in front of the center of the array, the false sources beamforming technique developed in this study included the of lower magnitude, so-called ghost images, begin to appear in system requirements for heavy trucks (e.g., frequency range of the resulting source maps at frequencies of 1000 Hz and above interest, horizontal and vertical spatial resolution required, despite the array side lobes suppressed by the 10 to 15 dB array source to array distance) and design parameters for beamform- gain. Barsikow suggests that the frequency range of the array ing measurement application (e.g., number of microphones, could be extended by increasing the number of microphones array size and geometry, sample size and averaging time, pro- to a total of about 160 (21). cessing software). A factor considered, both as a requirement Commercial beamforming wheel arrays, instrumentation, and a design parameter, was the frequency resolution. One and software packages were developed by Brel & Kjr (22, final factor is that, for highway barrier design purposes, verti- 23). The wheel arrays consist of a number of spokes (from 7 to cal resolution is much more important than horizontal. 15), each holding up to six microphones. Three typical wheel- The design of candidate array options was built on lessons array designs of 42, 66, and 90 channels with different perfor- learned during the array-based demonstration tests of truck mance levels cover different applications from general purpose noise source localization conducted by Blake and Donavan to automotive components to entire vehicle. The geometry of under Caltrans sponsorship in 2005 (20). each array is optimized for minimum side lobe levels over a The current design study had four major objectives: certain frequency range. A 90-channel Brel & Kjr wheel array with a PULSE Extend the directivity gain to lower frequencies than used beamforming software package were used by I&R for truck in the Caltrans demonstration of 2005. noise testing on IT's endurance track performed for Caltrans Meet or exceed the array gain performance of the Brel & in 2005 (20). Useful results were obtained, but there were lim- Kjr array used in the 2005 test.